Patent Publication Number: US-8532779-B2

Title: Implantable medical device crosstalk evaluation and mitigation

Description:
This application claims the benefit of U.S. Provisional Application No. 61/110,239, entitled, “IMPLANTABLE MEDICAL DEVICE CROSSTALK EVALUATION AND MITIGATION,” and filed on Oct. 31, 2008, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to therapy systems, and, more particularly, therapy systems including at least two therapy delivery modules. 
     BACKGROUND 
     A wide variety of implantable medical devices that deliver a therapy or monitor a physiologic condition of a patient have been clinically implanted or proposed for clinical implantation in patients. Some implantable medical devices may employ one or more elongated electrical leads and/or sensors. Such implantable medical devices may deliver therapy or monitor the heart, muscle, nerve, brain, stomach or other organs. In some cases, implantable medical devices deliver electrical stimulation therapy and/or monitor physiological signals via one or more electrodes or sensor elements, at least some of which may be included as part of one or more elongated implantable medical leads. Implantable medical leads may be configured to allow electrodes or sensors to be positioned at desired locations for delivery of stimulation or sensing electrical depolarizations. For example, electrodes or sensors may be located at a distal portion of the lead. A proximal portion of the lead may be coupled to an implantable medical device housing, which may contain electronic circuitry such as stimulation generation and/or sensing circuitry. In some cases, electrodes or sensors may be positioned on an IMD housing as an alternative or in addition to electrodes or sensors deployed on one or more leads. 
     For example, implantable cardiac devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic electrical stimulation to the heart by delivering electrical therapy signals such as pulses or shocks for pacing, cardioversion or defibrillation pulses via electrodes of one or more implantable leads. In some cases, an implantable cardiac device may sense intrinsic depolarizations of the heart, and control the delivery of therapeutic stimulation to the heart based on the sensing. When an abnormal rhythm of the heart is detected, such as bradycardia, tachycardia or fibrillation, an appropriate electrical therapy (e.g., in the form of pulses) may be delivered to restore the normal rhythm. For example, in some cases, an implantable medical device may deliver pacing, cardioversion or defibrillation signals to the heart of the patient upon detecting ventricular tachycardia, and deliver cardioversion or defibrillation therapy to a patient&#39;s heart upon detecting ventricular fibrillation. Some medical device systems that include a neurostimulator in addition to implantable cardiac device have also been proposed. 
     SUMMARY 
     In general, the disclosure is directed toward therapy systems that deliver electrical stimulation therapy to a tissue site within a patient and cardiac rhythm management therapy to a heart of a patient. The tissue site for the electrical stimulation therapy may be, for example, a nonmyocardial tissue site or nonvascular cardiac tissue site (e.g., a cardiac fat pad). In some examples, the therapy system may include a first implantable medical device (IMD) that delivers electrical stimulation to a tissue site within a patient, such as proximate a nerve (e.g., a vagus nerve or a spinal cord) or another nonmyocardial tissue site, and a second implantable medical device (IMD) that delivers cardiac rhythm management therapy, such as at least one of pacing, cardioversion or defibrillation therapy to a heart of the patient. The first IMD may be referred to as an implantable neurostimulator (INS) or an electrical stimulator, and the second IMD may be referred to as an implantable cardiac device (ICD). The INS may deliver electrical stimulation to nonmyocardial tissue sites other than sites adjacent nerves, and the ICD may deliver any combination of pacing, cardioversion, and defibrillation pulses. In other examples, the therapy system may include an implantable medical device that includes a first therapy module that delivers stimulation therapy to a nonmyocardial tissue site within a patient and a second therapy module that delivers at least one of pacing, cardioversion or defibrillation therapy to the heart of the patient, where the first and second therapy modules are disposed in a common housing. 
     Techniques for minimizing interference between the INS and the ICD or between the different therapy modules of a common medical device are described herein. In some examples, the therapy parameter values that define the electrical stimulation delivered by the INS may be modified in order to reduce the possibility that the ICD senses the electrical stimulation signals delivered by the INS and mischaracterizes the sensed signals as cardiac signals. In other examples, the INS may switch therapy programs that define the electrical stimulation signals generated and delivered by the INS upon the detection of an arrhythmia by the ICD, the INS or another device. In addition to or instead of modifying the operation of the INS, some examples described herein modify one or more sensing parameter values of an ICD order to reduce the possibility that the ICD senses the electrical stimulation signals delivered by the INS and mischaracterizes the sensed signals as cardiac signals. 
     In addition, the disclosure describes techniques for evaluating the amount of interference (or “crosstalk”) between an INS and ICD implanted within a patient. The measured interference may be used to modify operation of the INS or ICD, and, in some cases, may be recorded for later analysis by a clinician. 
     In one aspect, the disclosure is directed to a method comprising determining a characteristic of a first electrical signal sensed by a first IMD while a second IMD is delivering a second electrical signal to a tissue site within a patient, wherein the first and second IMDs are implanted within the patient, comparing the characteristic of the first electrical signal to a threshold value, and modifying at least one of a stimulation parameter with which the second IMD generates electrical stimulation that is delivered to the tissue site or a sensing parameter with which the first IMD senses the first electrical signal based on the comparison. 
     In another aspect, the disclosure is directed to a system comprising a first IMD that senses a first electrical signal within a patient, a second IMD that delivers a second electrical signal to a tissue site within patient, wherein the first IMD senses the first electrical signal while the second IMD delivers the second electrical signal to the patient, and a processor that determines a characteristic of the first electrical signal, compares the characteristic to a threshold value, and modifies at least one of a stimulation parameter with which the second IMD generates electrical stimulation therapy or a sensing parameter with which the first IMD senses the first electrical signal based on the comparison. 
     In another aspect, the disclosure is directed to a system comprising means for determining a characteristic of a first electrical signal sensed by a first IMD while a second IMD is delivering a second electrical signal to a tissue site within a patient, wherein the first and second IMDs are implanted within the patient, means for comparing the characteristic of the first electrical signal to a threshold value, and means for modifying at least one of a stimulation parameter with which the second IMD generates electrical stimulation that is delivered to the tissue site or a sensing parameter with which the first IMD senses the first electrical signal based on the comparison. 
     In another aspect, the disclosure is directed to a computer-readable medium comprising instructions that cause a programmable processor to determine a characteristic of a first electrical signal sensed by a first IMD while a second IMD is delivering a second electrical signal to a tissue site within a patient, wherein the first and second IMDs are implanted within the patient, compare the characteristic of the first electrical signal to a threshold value, and modify at least one of a stimulation parameter with which the second IMD generates electrical stimulation that is delivered to the tissue site or a sensing parameter with which the first IMD senses the first electrical signal based on the comparison. 
     In one aspect, the disclosure is directed to a method comprising evaluating crosstalk between a first IMD implanted within a patient and a second IMD implanted within the patient with an external device. The external device determines a characteristic of a first electrical signal sensed by the first IMD while a second IMD is delivering a second electrical signal to a tissue site within a patient, compares the characteristic of the first electrical signal to a threshold value, and modifies at least one of a stimulation parameter with which the second IMD generates electrical stimulation that is delivered to the tissue site or a sensing parameter with which the first IMD senses the first electrical signal based on the comparison. 
     In one aspect, the disclosure is directed to a method comprising determining a characteristic of a first electrical signal sensed by a first IMD while a second IMD delivers a second electrical signal to a tissue site within a patient, wherein the first and second IMDs are implanted within the patient, determining an electrical noise status based on the characteristic of the first electrical signal, and generating a notification based the noise status 
     In another aspect, the disclosure is directed to a system comprising a first IMD that senses a first electrical signal within a patient, a second IMD that delivers a second electrical signal to a patient, wherein the first IMD senses the first electrical signal while the second IMD delivers the second electrical signal to the patient, and a processor that determines a characteristic of the first electrical signal, determines an electrical noise status based on the characteristic of the first electrical signal, and generates a notification based the noise status. 
     In another aspect, the disclosure is directed to a system comprising means for determining a characteristic of a first electrical signal sensed by a first IMD while a second IMD delivers a second electrical signal to a tissue site within a patient, wherein the first and second IMDs are implanted within the patient, means for determining an electrical noise status based on the characteristic of the first electrical signal, and means for generating a notification based the noise status. 
     In another aspect, the disclosure is directed to a computer-readable medium comprising instructions that cause a programmable processor to determine a characteristic of a first electrical signal sensed by a first IMD while a second IMD is delivering a second electrical signal to a tissue site within a patient, wherein the first and second IMDs are implanted within the patient, determine an electrical noise status based on the characteristic of the first electrical signal, and generate a notification based the noise status. 
     In one aspect, the disclosure is directed to a method comprising evaluating crosstalk between a first IMD implanted within a patient and a second IMD implanted within the patient with an external device. The external device determines a characteristic of a first electrical signal sensed by the first IMD while a second IMD delivers a second electrical signal to a tissue site within a patient, determines an electrical noise status based on the characteristic of the first electrical signal, and generates a notification based the noise status. 
     In one aspect, the disclosure is directed to a method comprising determining a first electrical parameter value indicative of an impedance of an electrical path coupled to a first IMD, activating delivery of electrical stimulation to a patient by a second IMD after determining the first electrical parameter value, determining a second electrical parameter value indicative of the impedance of the electrical path while the second IMD delivers electrical stimulation, determining whether the first and second electrical parameter values are within a threshold range of each other, and generating an indication if the first and second electrical parameter values are not within the threshold range of each other. 
     In another aspect, the disclosure is directed to a system comprising a first IMD implanted within a patient, a second IMD implanted within the patient, and a processor. The processor determines a first electrical parameter value indicative of an impedance of an electrical path coupled to the first IMD, determines a second electrical parameter value indicative of the impedance of the electrical path while the second IMD delivers electrical stimulation to the patient, determines whether the first and second electrical parameter values are within a threshold range of each other, and generates an indication if the first and second electrical parameter values are not within the threshold range of each other. 
     In another aspect, the disclosure is directed to a system comprising means for determining a first electrical parameter value indicative of an impedance of an electrical path coupled to a first IMD, means for activating delivery of electrical stimulation to a patient by a second IMD after determining the first electrical parameter value, means for determining a second electrical parameter value indicative of the impedance of the electrical path while the second IMD delivers electrical stimulation, means for determining whether the first and second electrical parameter values are within a threshold range of each other, and means for generating an indication if the first and second electrical parameter values are not within the threshold range of each other. 
     In another aspect, the disclosure is directed to a computer-readable medium comprising instructions that cause a programmable processor to determine a first electrical parameter value indicative of an impedance of an electrical path coupled to a first IMD, activate delivery of electrical stimulation to a patient by a second IMD after determining the first electrical parameter value, determine a second electrical parameter value indicative of the impedance of the electrical path while the second IMD delivers electrical stimulation, determine whether the first and second electrical parameter values are within a threshold range of each other, and generate an indication if the first and second electrical parameter values are not within the threshold range of each other. 
     In one aspect, the disclosure is directed to a method comprising determining a first characteristic of a first electrical cardiac signal of a patient sensed by a first IMD, determining a second characteristic of a second electrical cardiac signal of the patient sensed by the first IMD while a second IMD delivers electrical stimulation to a tissue site within the patient, wherein the first and second IMDs are implanted within the patient, determining an electrical noise status based on the first and second characteristics, and generating a notification based the electrical noise status. 
     In another aspect, the disclosure is directed to a system comprising a first IMD that senses first and second electrical cardiac signals of a heart of a patient, a second IMD that delivers a second electrical signal to a patient, wherein the first IMD senses the second electrical cardiac signal while the second IMD delivers electrical stimulation to a tissue site within patient, and a processor that determines a first characteristic of the first electrical signal and a second characteristic of a second electrical cardiac signal, determines an electrical noise status based on the first and second characteristics, and generates a notification based the electrical noise status. 
     In another aspect, the disclosure is directed to a system comprising means for determining a first characteristic of a first electrical cardiac signal of a heart of a patient sensed by a first IMD, means for determining a second characteristic of a second electrical cardiac signal of the heart sensed by the first IMD while a second IMD delivers electrical stimulation to a tissue site within the patient, wherein the first and second IMDs are implanted within the patient, means for determining an electrical noise status based on the first and second characteristics, and means for generating a notification based the electrical noise status. 
     In another aspect, the disclosure is directed to a computer-readable medium comprising instructions that cause a programmable processor to determine a first characteristic of a first electrical cardiac signal of a heart of a patient sensed by a first IMD, determine a second characteristic of a second electrical cardiac signal of the heart sensed by the first IMD while a second IMD delivers electrical stimulation to a tissue site within the patient, wherein the first and second IMDs are implanted within the patient, determine an electrical noise status based on the first and second characteristics, and generate a notification based the electrical noise status. 
     In another aspect, the disclosure is directed to a method comprising evaluating crosstalk between a first IMD implanted within a patient and a second IMD implanted within the patient with an external device, wherein the external device, determines a first characteristic of a first electrical cardiac signal of a heart of a patient sensed by the first IMD, determines a second characteristic of a second electrical cardiac signal of the heart sensed by the first IMD while a second IMD delivers electrical stimulation to a tissue site within the patient, determines an electrical noise status based on the first and second characteristics, and generates a notification based the electrical noise status. 
     In another aspect, the disclosure is directed to a computer-readable medium comprising instructions. The instructions cause a programmable processor to perform any part of the techniques described herein. 
     The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the example statements provided below. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example therapy system including an implantable cardiac device (ICD) and an implantable neurostimulator (INS). 
         FIG. 2  is a conceptual diagram illustrating another example therapy system that includes the ICD and the INS. 
         FIG. 3  is a conceptual diagram illustrating the ICD of  FIGS. 1 and 2  and the respective leads in greater detail. 
         FIG. 4  is a conceptual diagram illustrating another example of the ICD of  FIGS. 1 and 2  and the respective leads in greater detail. 
         FIG. 5  is a conceptual diagram illustrating another example therapy system that includes an ICD and an INS. 
         FIG. 6  is a functional block diagram of an example ICD that generates and delivers electrical stimulation to a heart of a patient. 
         FIG. 7  is a functional block diagram of an example INS that generates and delivers electrical stimulation signals to a tissue site within the patient. 
         FIG. 8  is a functional block diagram of an example medical device programmer. 
         FIG. 9  is a flow diagram illustrating an example technique for modifying electrical stimulation therapy delivered by an INS. 
         FIG. 10  is a flow diagram illustrating another example technique for modifying electrical stimulation therapy delivered by an INS. 
         FIGS. 11A-11D  are flow diagrams illustrating another example technique for modifying electrical stimulation therapy delivered by an INS. 
         FIGS. 12A and 12B  are flow diagrams illustrating example techniques for delivering electrical stimulation therapy to a patient. 
         FIGS. 13A-13I  are conceptual illustrations of example waveforms for electrical stimulation therapy. 
         FIGS. 14A and 14B  are conceptual illustrations of example electrode combinations that may be used to deliver a biphasic stimulation signal. 
         FIGS. 15A-15F  are conceptual illustrations of example electrode combinations that may be used to help focus a stimulation field generated by the delivery of electrical stimulation by an INS. 
         FIG. 16  is a flow diagram illustrating an example technique that an ICD may implement in order to detect an arrhythmia while an INS is delivering electrical stimulation. 
         FIGS. 17A and 17B  are conceptual illustrations of sensed electrocardiogram (ECG) signals prior to and after a neurostimulation signal artifact is at least partially removed from the sensed ECG signal. 
         FIGS. 18A and 18B  are conceptual illustrations of sensed ECG during a ventricular tachycardia prior to and after a neurostimulation signal artifact is at least partially removed from the sensed ECG signal. 
         FIGS. 19A and 19B  are flow diagrams illustrating example techniques that an ICD may implement in order to detect an arrhythmia while an INS is delivering electrical stimulation. 
         FIG. 20  is a flow diagram illustrating an example technique for evaluating the crosstalk between an INS and an ICD implanted within a patient. 
         FIG. 21  is a flow diagram illustrating an example technique that may be used to evaluate the extent of the crosstalk between an INS and ICD implanted within a patient and minimize the crosstalk if the crosstalk exceeds a threshold level. 
         FIG. 22  is a conceptual illustration of a programmer, which may display various signals indicative of the extent of crosstalk between an ICD and an INS implanted within a patient. 
         FIG. 23  is a flow diagram of an example technique for categorizing sensed crosstalk between ICD and INS into different categories. 
         FIG. 24  is a flow diagram of an example technique for extracting data from a waveform of an artifact present in an electrical signal sensed by an ICD. 
         FIG. 25  is a flow diagram illustrating an example technique that may be used to evaluate the extent of the crosstalk between an INS and an ICD. 
         FIG. 26  is a flow diagram illustrating another example technique that may be used to evaluate the extent of the crosstalk between an INS and an ICD. 
         FIG. 27  is a flow diagram of an example technique for determining whether the crosstalk between an ICD and an INS may be adversely affecting the impedance measurements taken by the ICD. 
         FIG. 28  is a flow diagram illustrating an example technique for modifying an electrical stimulation signal generated and delivered by an INS to mitigate the affect on impedance measurements of electrical paths taken by an ICD. 
         FIG. 29  is a flow diagram of an example technique for determining whether the crosstalk between an ICD and an INS may be adversely affecting the impedance measurements taken by the INS. 
         FIG. 30  is a flow diagram illustrating an example technique for evaluating the integrity of a therapy system. 
         FIG. 31  is a functional block diagram of an example implantable medical device that includes a neurostimulation module that generates and delivers electrical stimulation to a tissue site within a patient and a cardiac therapy module that generates and delivers electrical stimulation to a heart of the patient. 
         FIG. 32  is a block diagram illustrating an example system that includes an external device, such as a server, and one or more computing devices that are coupled to the INS, ICD, and programmer shown in  FIG. 1  via a network. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a conceptual diagram illustrating an example therapy system  10  that provides therapy to patient  12 . Therapy system  10  includes implantable cardiac device (ICD)  16 , which is connected to leads  18 ,  20 , and  22 , and programmer  24 . ICD  16  may be, for example, a device that provides cardiac rhythm management therapy to heart  14 , and may include, for example, an implantable pacemaker, cardioverter, and/or defibrillator that provide therapy to heart  14  of patient  12  via electrodes coupled to one or more of leads  18 ,  20 , and  22 . In some examples, ICD  16  may deliver pacing pulses, but not cardioversion or defibrillation pulses, while in other examples, ICD  16  may deliver cardioversion or defibrillation pulses, but not pacing pulses. In addition, in further examples, ICD  16  may deliver pacing, cardioversion, and defibrillation pulses. 
     In some examples, ICD  16  may not deliver cardiac rhythm management therapy to heart  14 , but may instead only sense electrical cardiac signals of heart  14  and/or other physiological parameters of patient  12  (e.g., blood oxygen saturation, blood pressure, temperature, heart rate, respiratory rate, and the like), and store the electrical cardiac signals and/or other physiological parameters of patient  12  for later analysis by a clinician. In such examples, ICD  16  may be referred to as a patient monitoring device. Examples of patient monitoring devices include, but are not limited to, the Reveal Plus Insertable Loop Recorder, which is available from Medtronic, Inc. of Minneapolis, Minn. For ease of description, ICD  16  will be referred to herein as a cardiac rhythm management therapy delivery device. 
     Therapy system  10  further comprises implantable electrical stimulator  26 , which is coupled to lead  28 . Electrical stimulator  26  may also be referred to as an implantable neurostimulator (INS)  26 . INS  26  may be any suitable implantable medical device (IMD) that includes a signal generator that generates electrical stimulation signals that may be delivered to a tissue site of patient  12 , e.g., tissue proximate a vagus nerve, a spinal cord or heart  14  of patient  12 . 
     In some examples, the tissue site may include at least one of a nonmyocardial tissue site or a nonvascular cardiac tissue site. A nonmyocardial tissue site may include a tissue site that does not include cardiac muscle (e.g., the myocardium). For example, a nonmyocardial tissue site may be proximate a muscle other than cardiac muscle, an organ other than the heart, or neural tissue. A tissue site proximate a nerve may be a neural tissue site to which delivery of electrical stimulation may activate the nerve. In some examples, a tissue site proximate a nerve may be in a range of about zero centimeters to about ten centimeters from the nerve, although other distance ranges are contemplated and may depend upon the nerve. The nonmyocardial tissue site may include extravascular tissue sites or intravascular tissue sites. A nonvascular cardiac tissue site may include, for example, a cardiac fat pad. 
     In some examples, delivery of electrical stimulation to a tissue site proximate a nerve or a nonmyocardial tissue site that may not be proximate a nerve may help modulate an autonomic nervous system of patient  12 . In some examples, INS  26  may deliver electrical stimulation therapy to a nerve of patient  12  via a lead implanted within vasculature (e.g., a blood vessel) of patient  12 . In some examples, INS  26  may deliver electrical stimulation that is delivered to peripheral nerves that innervate heart  14 , or fat pads on heart  14  that may contain nerve bundles. The fat pads may be referred to as a nonvascular cardiac tissue site. 
     In the example shown in  FIG. 1 , electrodes of lead  28  are positioned outside the vasculature of patient  12  and positioned to deliver electrical stimulation to a vagus nerve (not shown) of patient  12 . Stimulation may be delivered to extravascular tissue sites, for example, when lead  28  is not implanted within vasculature, such as within a vein, artery or heart  14 . In other examples, stimulation may be delivered to a nonmyocardial tissue site via electrodes of an intravascular lead that is implanted within vasculature. 
     In the example shown in  FIG. 1 , the components of ICD  16  and INS  26  are enclosed in separate housings, such that ICD  16  and INS  26  are physically separate devices. In other examples, as described with respect to  FIG. 31 , the functionality of ICD  16  and INS  26  may be performed by an IMD that includes both a cardiac therapy module that generates and delivers at least one of pacing, cardioversion or defibrillation therapy to patient  12  and an electrical stimulation therapy module that generates and delivers electrical stimulation to a target tissue site within patient  12 , which may be proximate a nerve or may be an extravascular tissue site that is not proximate a nerve. 
     Leads  18 ,  20 ,  22  extend into the heart  14  of patient  12  to sense electrical activity of heart  14  and/or deliver electrical stimulation to heart  14 . In the example shown in  FIG. 1 , right ventricular (RV) lead  18  extends through one or more veins (not shown), the superior vena cava (not shown), and right atrium  30 , and into right ventricle  32 . Left ventricular (LV) coronary sinus lead  20  extends through one or more veins, the vena cava, right atrium  30 , and into the coronary sinus  34  to a region adjacent to the free wall of left ventricle  36  of heart  14 . Right atrial (RA) lead  22  extends through one or more veins and the vena cava, and into the right atrium  30  of heart  14 . As described in further detail with reference to  FIG. 5 , in other examples, an ICD may deliver stimulation therapy to heart  14  by delivering stimulation to a nonmyocardial tissue site in addition to or instead of delivering stimulation via electrodes of intravascular leads  18 ,  20 ,  22 . 
     ICD  16  may sense electrical signals attendant to the depolarization and repolarization of heart  14  via electrodes (not shown in  FIG. 1 ) coupled to at least one of the leads  18 ,  20 ,  22 . In some examples, ICD  16  provides pacing pulses to heart  14  based on the electrical signals sensed within heart  14 . These electrical signals sensed within heart  14  may also be referred to as cardiac signals or electrical cardiac signals. The configurations of electrodes used by ICD  16  for sensing and pacing may be unipolar or bipolar. ICD  16  may also provide defibrillation therapy and/or cardioversion therapy via electrodes located on at least one of the leads  18 ,  20 ,  22 . ICD  16  may detect arrhythmia of heart  14 , such as fibrillation of ventricles  32  and  36 , and deliver defibrillation therapy to heart  14  in the form of electrical pulses. In some examples, ICD  16  may be programmed to deliver a progression of therapies, e.g., pulses with increasing energy levels, until a fibrillation of heart  14  is stopped. ICD  16  may detect fibrillation employing one or more fibrillation detection techniques known in the art. 
     In the example of  FIG. 1 , INS  26  has been implanted in patient  12  proximate to an nonmyocardial target stimulation site  40 , such as a tissue site proximate a vagus nerve. For example, INS  26  may be subcutaneously or submuscularly implanted in the body of a patient  12  (e.g., in a chest cavity, lower back, lower abdomen, or buttocks of patient  12 ). INS  26  provides a programmable stimulation signal (e.g., in the form of electrical pulses or a continuous signal) that is delivered to target stimulation site  40  by implantable medical lead  28 , and more particularly, via one or more stimulation electrodes carried by lead  28 . Proximal end  28 A of lead  28  may be both electrically and mechanically coupled to connector  42  of INS  26  either directly or indirectly (e.g., via a lead extension). In particular, conductors disposed in the lead body may electrically connect stimulation electrodes (and sense electrodes, if present) of lead  28  to INS  26 . 
     INS  26  may also be referred to as a signal generator. In some examples, lead  28  may also carry one or more sense electrodes to permit INS  26  to sense electrical signals from target stimulation site  40 . Furthermore, in some examples, INS  26  may be coupled to two or more leads, e.g., for bilateral or multi-lateral stimulation. 
     Delivery of electrical stimulation by INS  26  to one or more target tissues sites proximate to a nerve, nerve site, cardiac fat pad, or an extravascular target tissue site that is not proximate a nerve may provide cardioprotective benefits to patient  12 . An extravascular tissue site may be outside of heart  14  and outside of arteries, veins, or other vasculature of patient  12 . For example, delivery of electrical stimulation to a tissue site proximate a nerve of patient  12  may help treat heart failure. In addition, delivery of electrical stimulation to a tissue site proximate a nerve of patient  12  to modulate an autonomic nervous system of patient  12  may help reduce or eliminate cardiovascular conditions such as bradycardia, tachycardia, unhealthy cardiac contractions, ischemia, inefficient heart pumping, inefficient collateral circulation of heart  14  or cardiac muscle trauma. Delivery of electrical stimulation by INS  26  may compliment antitachycardia therapy (e.g., antitachycardia pacing, cardioversion or defibrillation) by ICD  16  or provide back-up therapy to the cardiac rhythm therapy provided by ICD  16 . For example, if ICD  16  is unavailable to provide therapy to patient  12 , e.g., due to a low power level, INS  26  may deliver therapy to patient  12  to help terminate or prevent a cardiac event (e.g., tachycardia). 
     In some examples, INS  26  delivers electrical stimulation to peripheral nerves that innervate heart  14 , or fat pads on heart  14  that may contain nerve bundles. In the example shown in  FIG. 1 , electrodes of lead  28  are positioned to deliver electrical stimulation to a vagus nerve (not shown) of patient  12 . Although INS  26  is referred to throughout the remainder of the disclosure as a “neurostimulator” and as delivering neurostimulation pulses, in other examples, INS  26  may deliver electrical stimulation to any suitable nonmyocardial tissue site within patient  12 , which may or may not be proximate a nerve. 
     In the example shown in  FIG. 1 , INS  26  provides electrical stimulation therapy of a parasympathetic nerve, such as a vagus nerve, of patient  12 . Stimulation of a parasympathetic nerve of patient  12  may help slow intrinsic rhythms of heart  14 , which may facilitate antitachyarrhythmia therapy (e.g., antitachycardia pacing, cardioversion or defibrillation) delivered by ICD  16 . In this way, neurostimulation by INS  26  may help control a heart rate of patient  12  or otherwise control cardiac function. 
     In other examples, electrodes of lead  28  may be positioned to deliver electrical stimulation to any other suitable nerve, organ, muscle or muscle group in patient  12 , which may be selected based on, for example, a therapy regimen selected for a particular patient. In some examples, INS  26  may deliver electrical stimulation to other parasympathetic nerves, baroreceptors, the carotid sinus or a cardiac branch of the vagal trunk of patient  12  in order to compliment the delivery of therapy by ICD  16 . 
     The electrical stimulation signals generated and delivered by INS  26  may be referred to as neurostimulation signals. However, in some examples, INS  26  may deliver electrical stimulation to a target tissue site  40  that is not proximate to a nerve. For example, in some examples, INS  26  may deliver electrical stimulation to a peripheral nerve field site, whereby electrodes  124  ( FIG. 7 ) are implanted in a region where patient  12  experiences pain. The pain may be related to stimulation delivered by ICD  16  or a patient condition, such as angina or chronic back pain. As other examples, INS  26  may deliver electrical stimulation to a muscle, muscle group, organ, or other sites that may not be proximate a nerve. Thus, while “neurostimulation” signals are primarily referred to herein, the disclosure is also applicable to examples in which INS  26  delivers electrical stimulation to other tissue sites. 
     As another example, as shown in  FIG. 2 , INS  26  may be positioned to deliver electrical stimulation to spinal cord  44  of patient  12 . Stimulation of spinal cord  44  or nerves branching therefrom by INS  26  may help prevent or mitigate occurrences of tachyarrhythmias and may facilitate reduction of the level of aggressiveness of the cardiac therapy, such as pacing, cardioversion or defibrillation therapy, delivered by ICD  16 . In this way, ICD  16  and INS  26  may operate in conjunction with each other to help prevent arrhythmias of heart  14  of patient  12 , as well as to terminate detected arrhythmias. 
     In some examples, depending upon the neurostimulation target, the delivery of electrical stimulation by INS  26  may also mitigate perceptible discomfort generated from the delivery of pacing pulses or cardioversion/defibrillation shocks by ICD  16 . For example, if INS  26  delivers electrical stimulation to spinal cord  44  of patient  12 , the neurostimulation may produce paresthesia, which may help reduce the discomfort felt by patient  12  from the delivery of stimulation by ICD  16 . 
     In the example shown in  FIG. 2 , in therapy system  11 , INS  26  is coupled to two leads  28 ,  29  to provide bilateral stimulation of spinal cord  44 . Leads  28 ,  29  may be introduced into spinal cord  44  in the thoracic region, as shown in  FIG. 2 . In other examples, leads  28 ,  29  may be introduced into spinal cord  44  in the cervical or lumbar regions. Electrodes of leads  28 ,  29  may be positioned within an intrathecal space or epidural space of spinal cord  44 , or, in some examples, adjacent nerves that branch off of spinal cord  44 . In some examples, leads  28 ,  29  are implanted within patient  12  and positioned such that electrodes of leads  28 ,  29  deliver electrical stimulation to locations proximate to the T1 to T6 thoracic vertebrae of the patient&#39;s vertebral column. For example, electrodes of at least one of the leads  28 ,  29  may span the T3 to T6 thoracic vertebrae or deliver electrical stimulation to a tissue site proximate at least one of the T3 to T6 thoracic vertebrae. In other examples, leads  28 ,  29  may be implanted to deliver electrical stimulation to other regions proximate or within spinal cord  44 , such as over or near other vertebrae. 
     In some examples, INS  26  delivers therapy to patient  12  with a voltage amplitude of about 0.2 volts to about 12 volts, a pulse duration of about 40 microseconds (μs) to about 600 μs, such as about 50 μs to about 500 μs), and a pulse rate of about 1 Hz to about 1 kilohertz (e.g., about 10 Hz to about 100 Hz). However, other stimulation parameter values for INS  26  are contemplated. INS  26  may deliver electrical stimulation to patient  12  substantially continuously or periodically. In some examples, INS  26  may deliver electrical stimulation to patient  12  based on the timing of electrical stimulation by ICD  16 , such as prior to the delivery of electrical stimulation (e.g., antitachycardia pacing or a defibrillation or cardioversion pulse) by ICD  16 , during the delivery of electrical stimulation by ICD  16 , subsequent to the delivery of electrical stimulation by ICD  16  or any combination of the aforementioned times. In addition, in some examples, INS  26  may deliver electrical stimulation to patient  12  based on a sensed event or, such as atrial or ventricular depolarization, or based on a sensed physiological condition. The event or physiological condition may be sensed by ICD  16 , INS  26  or another sensing device. 
     ICD  16  and INS  26  may communicate with each other in order for INS  26  to time the delivery of electrical stimulation based on the delivery of stimulation pulses by ICD  16 , where the stimulation pulses may be pacing pulses or cardioversion/defibrillation pulses. ICD  16  and INS  26  may communicate directly or indirectly (e.g., via an intermediate device, such as programmer  24 ) using any suitable communication technique. Examples communication techniques that may be implemented to facilitate communication between ICD  16  and INS  26  may include, for example, radiofrequency (RF) communication techniques, optical communication techniques, ultrasonic communication techniques, and the like. Communication between ICD  16  and INS  26  may be periodic, e.g., according to a regular schedule, or on an as-needed basis, e.g., when INS  26  delivers electrical stimulation to patient  12  or when excessive crosstalk is detected by ICD  16 , programmer  24 , INS  26  or another device. Example techniques for evaluating the crosstalk between ICD  16  and INS  26  are described below with reference to  FIGS. 20 ,  21 , and  23 - 30 . 
     In other examples, INS  26  may deliver electrical stimulation to patient  12  independently of the cardiac rhythm therapy delivered by ICD  16 . For example, INS  26  may be programmed to deliver electrical stimulation to patient  12  according to a schedule that is determined independently of the actual delivery of stimulation pulses by ICD  16 . The schedule may be determined, for example, by a clinician based on a trial stimulation period in which multiple therapy schedules for INS  26  are tested on patient  12 . The schedule may dictate when INS  26  actively delivers electrical stimulation to patient  12  and when INS  26  does not actively deliver electrical stimulation to patient  12 . For example, the schedule may include a mandatory sleep period for INS  26  during which INS  26  reverts to a relatively low-power sleep mode. During the sleep mode, INS  26  may not deliver therapy to patient  12  or may deliver a relatively minimal amount of electrical stimulation therapy to patient  12 . The sleep period may be, for example, when patient  12  is sleeping or otherwise has a relatively low activity level. The sleep period may be useful for conserving the power source of INS  26 . 
     In some examples, a stimulation schedule for INS  26  may comprise a first period of time in which stimulation is delivered to patient  12  substantially continuously or for brief durations (e.g., 0.1 seconds to about five seconds) and a second period of time during which no stimulation is delivered to patient  12 . The first and second periods of time may be on the order of seconds, minutes, hours or days. 
     Delivering stimulation to patient  12  via INS  26  periodically rather than substantially continuously may help elongate the useful life of therapy delivery by INS  26  or therapy delivery by INS  26  according to a particular set of stimulation parameter values. Patient  12  may adapt to stimulation provided by INS  26  over time. That is, a certain level of electrical stimulation provided to a target tissue site by INS  26  may be less effective over time. This phenomenon may be referred to as “adaptation.” As a result, any beneficial effects to patient  12  from the stimulation delivery by INS  26  may decrease over time. While the electrical stimulation levels (e.g., amplitude or frequency of the electrical stimulation signal) may be increased to overcome the adaptation, the increase in stimulation levels may consume more power, and may eventually reach undesirable or harmful levels of stimulation. Adaptation to therapy delivery by INS  26  may be reduced by decreasing the total amount of stimulation delivered to patient  12  by INS  26 , such as by delivering stimulation to patient  12  when needed (e.g., upon the detection of an arrhythmia) or according to a schedule in which therapy is turned off or minimized for a period of time. Moreover, noncontinuous therapy delivery to patient  12  by INS  26  may be more energy efficient. 
     In addition, delivering stimulation to patient  12  via INS  26  periodically rather than substantially continuously may help elongate the useful life of therapy delivery by INS  26  by extending the life of the power source of INS  26 . Increasing the amount of time between INS  26  recharge or power source replacement may be useful because the inconvenience to patient  12  from the recharge or battery placement may be minimized. 
     The values for the therapy parameters that define the electrical stimulation delivered by INS  26  may be organized into a group of parameter values referred to as a “therapy program” or “therapy parameter set.” “Therapy program” and “therapy parameter set” are used interchangeably herein. In the case of electrical stimulation, the therapy parameters may include an electrode combination, an amplitude, which may be a current or voltage amplitude, a slew rate, and a frequency, and, if INS  26  delivers electrical pulses, a pulse width, and a pulse rate for stimulation signals to be delivered to the patient. An electrode combination may include a selected subset of one or more electrodes of lead  28 , as well as lead  29  if INS  26  is connected to two leads  28 ,  29 . The electrode combination may also refer to the polarities of the electrodes in the selected subset. By selecting particular electrode combinations, a clinician may target particular anatomic structures within patient  12 . In some cases, INS  26  may deliver stimulation to patient  12  according to a program group that includes more than one therapy program. The stimulation signals according to the different therapy programs in a therapy group may be delivered on a time-interleaved basis or substantially simultaneously. 
     The electrical stimulation parameters may also include a duty cycle of stimulation signals, a timing of the delivery of the electrical stimulation relative to a cardiac cycle of heart  14  of patient  12 , and a waveform shape or a signal envelope of the electrical stimulation signal. A signal envelope may generally traces the outline of the amplitude of a stimulation signal for a given period of time. The signal envelope may characterize the amplitude ramp-up and ramp-down times, which may be gradual or abrupt. 
     If INS  26  delivers therapy to patient  12  according to two or more electrode combinations, e.g., according to a therapy program group including two or more therapy programs defining at least two different electrode combinations, time-interleaving the stimulation signals defined each of the therapy programs may result in stimulation that is sequentially applied to different electrodes. Varying the tissue site at which INS  26  delivers stimulation by delivering therapy according to different electrode combinations may also help reduce the patient&#39;s adaptation to therapy delivery by INS  26 . For example, sequentially delivering stimulation via different electrode combinations may help reduce the amount of time that a particular tissue site is stimulated. 
     In some examples, the therapy parameter values with which INS  26  generates electrical stimulation therapy for patient  12  may be selected based on an effect the stimulation has on heart  14 . For example, INS  26  may deliver stimulation to a nonmyocardial tissue site within patient  12  according to a first therapy program defining values for a set of therapy parameters, and ICD  16  may assess the response of heart  14  or other portions of the cardiovascular system to the delivery of stimulation by INS  26 . For example, ICD  16  may sense cardiac activity via electrodes of leads  18 ,  20 ,  22 . Example responses of heart  14  include, for example, proarrhythmic effects. The therapy program may be analyzed based on a positive or negative response of heart  14  or other portions of the cardiovascular system to the delivery of stimulation by INS  26 . The therapy program may be selected for storage in INS  26 , e.g., for chronic therapy delivery if the test stimulation via the therapy program evoked a positive response by heart  14  and/or other portions of the patient&#39;s cardiovascular system. 
     Stimulation delivered by INS  26  may have a carryover effect on patient  12 . A carryover effect generally refers to a physiological effect generated in response to the delivery of an electrical stimulation signal, where the effect persists after termination of the stimulation signal. The carryover effect may be at least partially attributable to, for example, neurochemicals that are by the patient&#39;s body that have an ongoing effect on the patient&#39;s physiological condition after the termination of a stimulation signal. Neurochemicals may provide the benefits of electrical stimulation therapy that continue for a period of time, such as seconds, minutes, hours or days, after the delivery of a stimulation signal by INS  26 . If INS  26  delivers electrical stimulation to one or more nerves of patient  12 , the carryover effect may also be at least partially attributable to nerves maintaining a self-stimulating mode following the delivery of a stimulation signal by INS  26 . Nerves may continue to fire after the termination of a stimulation signal, which may also provide on-going benefits of INS  26  to patient  12  that continue for a period of time, such as seconds, minutes, hours or days, following the termination of a stimulation signal. 
     In some examples, the stimulation schedule or therapy program (e.g., frequency of stimulation signals) for INS  26  may be selected based on the carryover effect of the stimulation delivery by INS  26  on patient  12 . For example, the interval at which INS  26  delivers stimulation signals to patient  12  may be substantially equal to or less than a duration of a carryover effect from the delivery of a stimulation signal. The carryover effect may differ between patients and/or based on the type of stimulation signals, and, thus, a clinician may test patient  12  to determine the duration of a carryover effect. For example, if delivery of electrical stimulation therapy by INS  26  causes paresthesia that patient  12  perceives, the clinician may control INS  26  to deliver a stimulation signal and then measure the duration of time required for the paresthesia to dissipate. This duration of time may be substantially equal to a duration of a carryover effect for that particular stimulation signal. 
     In some cases, ICD  16  may sense electrical noise and interpret the electrical noise as electrical cardiac signals (e.g., an electrocardiogram (ECG) or electrogram (EGM) signal). The misinterpretation of electrical noise may cause ICD  16  to oversense cardiac signals, and, in some cases, erroneously detect an arrhythmia. For example, a processor of ICD  16  may interpret electrical noise as a heart rhythm, and detect the presence of a tachyarrhythmia episode or event (e.g., a heart cycle measured between successive R-waves that has a duration less than a threshold value) based on the electrical noise. A tachyarrhythmia episode may include more than one tachyarrhythmia event. Depending on the source of the electrical noise, the electrical noise may present itself as a relatively fast rhythm, which the processor may interpret as one or more tachyarrhythmia events, which may then be used to detect a tachyarrhythmia episode. ICD  16  may detect the presence of a tachyarrhythmia episode by determining whether a certain number of intervals of a particular number of total intervals have a certain duration, e.g., whether a certain number of intervals are considered tachyarrhythmia events. 
     Oversensing heart rhythms may result in inappropriate withholding or delivery of electrical stimulation to heart  14 . For example, oversensing may cause ICD  16  to detect a tachycardia or fibrillation episode when heart  14  is in a normal sinus rhythm, which may result in the inappropriate delivery of a high voltage defibrillation shock. Thus, oversensing of heart rhythms by ICD  16  is generally undesirable. 
     Electrical noise that ICD  16  characterizes as heart rhythms may be attributable to different sources. In some cases, ICD  16  may sense the electrical stimulation signals (or “neurostimulation signals”) generated by and delivered to target tissue site  40  by INS  26 . The electrical stimulation signals generated by INS  26  and sensed by ICD  16  may be referred to as “electrical noise” or “interference,” and the presence of electrical noise between INS  26  and ICD  16  may be referred to as “crosstalk.” As previously indicated, ICD  16  may control the delivery of electrical stimulation to heart  14  based on electrical cardiac signals (e.g., EGM signals) sensed within heart  14 . A sensing integrity issue may arise when ICD  16  senses the electrical stimulation signals generated by INS  26  and mischaracterizes the stimulation signals as cardiac signals. For example, if ICD  16  detects an arrhythmia of heart  14  based on electrical signals generated by INS  26  rather than true electrical cardiac signals, ICD  16  may unnecessarily deliver electrical stimulation (e.g., pacing pulses or defibrillation/cardioversion shocks) to heart  14 . 
     Therapy system  10  may implement various techniques described herein to reduce the amount of crosstalk between INS  26  and ICD  16 . In some examples, one or more therapy parameter values of the electrical stimulation delivered by INS  26  may be modified in order to minimize the possibility that the electrical stimulation delivered by INS  26  and sensed by ICD  16  mimics cardiac signals, thereby minimizing the possibility that ICD  16  mischaracterizes the electrical stimulation delivered by INS  26  as cardiac signals. Modifying the one or more therapy parameter values with which INS  26  generates electrical stimulation signals may help modify one or more signal characteristics of the noise sensed by ICD  16 , such as the signal amplitude or frequency. 
     As described in further detail below with reference to  FIGS. 9-11D , in some examples, if ICD  16  detects an arrhythmia via electrodes of one or more leads  18 ,  20 ,  22  or a housing of ICD  16 , ICD  16  may determine whether the arrhythmia was detected based on noise attributable to the electrical stimulation delivered by INS  26 . ICD  16  may, for example, instruct INS  26  to temporarily stop delivery of electrical stimulation or reduce an intensity of stimulation, and ICD  16  may determine, while INS  26  is not delivering stimulation or delivering stimulation with a lower intensity, whether sensed cardiac signals still indicate an arrhythmia. An intensity of stimulation may be adjusted by modifying one or more stimulation parameter values, such as the current or voltage amplitude of the stimulation signal, the frequency, slew rate, duty cycle, and, if the stimulation signal comprises stimulation pulses, the pulse width and pulse rate. 
     If the cardiac signals detected within the suspend period of the INS  26 , i.e., the period during which INS  26  does not deliver electrical stimulation or during which INS  26  delivers electrical stimulation having a lower intensity, indicate that an arrhythmia is not present, ICD  16  may determine that the arrhythmia was detected based on noise from electrical stimulation delivered by INS  26 . In some examples, ICD  16  may control INS  26  to modify one or more stimulation parameter values in order to change the stimulation signal that is detected by ICD  16  and reduce the possibility that ICD  16  senses the stimulation signals generated by INS  26  and mischaracterizes the sensed stimulation signals as cardiac signals. 
     As described with reference to  FIGS. 12A and 12B , ICD  16  may control INS  26  to modify one or more stimulation parameter values by switching therapy programs. For example, INS  26  may switch from therapy delivery according to a first therapy program to therapy delivery according to a second therapy program upon detection of an arrhythmia by ICD  16 . The therapy programs may define electrical stimulation parameter values with which INS  26  may generate electrical stimulation signals. Therapy delivery by INS  26  according to the second therapy program may result in the generation and delivery of electrical signals that are not mischaracterized by ICD  16  as cardiac signals. For example, the second therapy program may define electrical stimulation signals that have a waveform that differs from a cardiac signal in at least one respect, such that ICD  16  does not mischaracterize the electrical stimulation delivered by INS  26  according to the second therapy program as cardiac signals. In other examples, INS  26  may switch from therapy delivery according to a first therapy program group to therapy delivery according to a second therapy program group upon detection of an arrhythmia by ICD  16 . The therapy program groups may include one or more therapy programs. 
     Programmer  24  may include a handheld computing device or a computer workstation. Programmer  24  may include a user interface that receives input from a user. The user interface may include, for example, a keypad and a display, which may for example, be a cathode ray tube (CRT) display, a liquid crystal display (LCD) or light emitting diode (LED) display. The keypad may take the form of an alphanumeric keypad or a reduced set of keys associated with particular functions. Programmer  24  can additionally or alternatively include a peripheral pointing device, such as a mouse, via which a user may interact with the user interface. In some examples, a display of programmer  24  may include a touch screen display, and a user may interact with programmer  24  via the display. 
     A user, such as a physician, technician, or other clinician, may interact with programmer  24  to communicate with ICD  16  and/or INS  26 . For example, the user may interact with programmer  24  to retrieve physiological or diagnostic information from ICD  16  and/or INS  26 . A user may also interact with programmer  24  to program ICD  16  and INS  26 , e.g., select values for operational parameters of ICD  16  and INS  26 , respectively. 
     For example, the user may use programmer  24  to retrieve information from ICD  16  regarding the rhythm of heart  14 , trends therein over time, or tachyarrhythmia episodes. As another example, the user may use programmer  24  to retrieve information from ICD  16  regarding other sensed physiological parameters of patient  12 , such as electrical depolarization/repolarization signals from the heart (referred to as “electrogram” or EGM), intracardiac or intravascular pressure, activity, posture, respiration, heart sounds, or thoracic impedance. As another example, the user may use programmer  24  to retrieve information from ICD  16  regarding the performance or integrity of ICD  16  or other components of system  10 , such as leads  18 ,  20 , and  22 , or a power source of ICD  16 . 
     The user may use programmer  24  to program a therapy progression, select electrodes used to deliver defibrillation pulses, select waveforms for the defibrillation pulse, or select or configure a fibrillation detection algorithm for ICD  16 . The user may also use programmer  24  to program aspects of other therapies provided by ICD  16 , such as cardioversion or pacing therapies. In some examples, the user may activate certain features of ICD  16  by entering a single command via programmer  24 , such as depression of a single key or combination of keys of a keypad or a single point-and-select action with a pointing device. 
     The user may also use programmer  24  to retrieve information from INS  26  regarding the performance or integrity of INS  26  or leads  28 ,  29  (if INS  26  is connected to more than one lead) or a power source of INS  26 . In addition, the user may use programmer  24  to program INS  26 . For example, with the aid of programmer  24  or another computing device, a user may select values for therapy parameters for controlling therapy delivery by INS  26 . The values for the therapy parameters may be organized into a group of parameter values referred to as a “therapy program” or “therapy parameter set.” “Therapy program” and “therapy parameter set” are used interchangeably herein. 
     In the case of electrical stimulation, the therapy parameters for INS  26  may include an electrode combination, and an amplitude, which may be a current or voltage amplitude, and, if INS  26  delivers electrical pulses, a pulse width, and a pulse rate for stimulation signals to be delivered to patient  12 . An electrode combination may include a selected subset of one or more electrodes located on implantable lead  28  coupled to INS  26 . By selecting particular electrode combinations, a clinician may target particular anatomic structures within patient  12 . In addition, by selecting values for amplitude, pulse width, and pulse rate, the physician can attempt to generate an efficacious therapy for patient  12  that is delivered via the selected electrode subset. 
     Programmer  24  may communicate with ICD  16  and INS  26  via wireless communication using any techniques known in the art. Examples of communication techniques may include, for example, low frequency or RF telemetry, but other techniques are also contemplated. In some examples, programmer  24  may include a programming head that may be placed proximate to the patient&#39;s body near the ICD  16  and INS  26  implant sites in order to improve the quality or security of communication between ICD  16  or INS  26 , respectively, and programmer  24 . 
       FIG. 3  is a conceptual diagram illustrating ICD  16  and leads  18 ,  20 ,  22  of therapy system  10  in greater detail. Leads  18 ,  20 ,  22  may be electrically coupled to a stimulation generator, a sensing module, or other modules ICD  16  via connector block  48 . In some examples, proximal ends of leads  18 ,  20 ,  22  may include electrical contacts that electrically couple to respective electrical contacts within connector block  48 . In addition, in some examples, leads  18 ,  20 ,  22  may be mechanically coupled to connector block  48  with the aid of set screws, connection pins or another suitable mechanical coupling mechanism. 
     Each of the leads  18 ,  20 ,  22  includes an elongated insulative lead body, which may carry a number of concentric coiled conductors separated from one another by tubular insulative sheaths. Other lead configurations are also contemplated, such as configurations that do not include coiled conductors. In the illustrated example, bipolar electrodes  40  and  42  are located proximate to a distal end of lead  18 . In addition, bipolar electrodes  54  and  56  are located proximate to a distal end of lead  20  and bipolar electrodes  58  and  60  are located proximate to a distal end of lead  22 . 
     Electrodes  50 ,  54 , and  58  may take the form of ring electrodes, and electrodes  52 ,  56 , and  60  may take the form of extendable helix tip electrodes mounted retractably within insulative electrode heads  62 ,  64 , and  66 , respectively. Each of the electrodes  50 ,  52 ,  54 ,  56 ,  58 , and  60  may be electrically coupled to a respective one of the conductors within the lead body of its associated lead  18 ,  20 ,  22 , and thereby coupled to respective ones of the electrical contacts on the proximal end of leads  18 ,  20  and  22 . 
     Electrodes  50 ,  52 ,  54 ,  56 ,  58 , and  60  may sense electrical signals attendant to the depolarization and repolarization of heart  14 . The electrical signals are conducted to ICD  16  via the respective leads  18 ,  20 ,  22 . In some examples, ICD  16  also delivers pacing pulses via electrodes  50 ,  52 ,  54 ,  56 ,  58 , and  60  to cause depolarization of cardiac tissue of heart  14 . In some examples, as illustrated in  FIG. 2 , ICD  16  includes one or more housing electrodes, such as housing electrode  68 , which may be formed integrally with an outer surface of hermetically-sealed housing  70  of ICD  16  or otherwise coupled to housing  70 . In some examples, housing electrode  68  is defined by an uninsulated portion of an outward facing portion of housing  70  of ICD  16 . Divisions between insulated and uninsulated portions of housing  70  may be employed to define two or more housing electrodes. In some examples, housing electrode  68  comprises substantially all of housing  70 . Any of the electrodes  50 ,  52 ,  54 ,  56 ,  58 , and  60  may be used for unipolar sensing or pacing in combination with housing electrode  68 . As described in further detail with reference to  FIG. 6 , housing  70  may enclose a stimulation generator that generates cardiac pacing pulses and defibrillation or cardioversion shocks, as well as a sensing module for monitoring the patient&#39;s heart rhythm. 
     Leads  18 ,  20 ,  22  also include elongated electrodes  72 ,  74 ,  76 , respectively, which may take the form of a coil. ICD  16  may deliver defibrillation pulses to heart  14  via any combination of elongated electrodes  72 ,  74 ,  76 , and housing electrode  68 . Electrodes  68 ,  72 ,  74 ,  76  may also be used to deliver cardioversion pulses to heart  14 . Electrodes  72 ,  74 ,  76  may be fabricated from any suitable electrically conductive material, such as, but not limited to, platinum, platinum alloy or other materials known to be usable in implantable defibrillation electrodes. 
     The configurations of therapy system  10  illustrated in  FIGS. 1-3  are merely examples. In other examples, a therapy system may include epicardial leads and/or patch electrodes instead of or in addition to the transvenous leads  18 ,  20 ,  22  illustrated in  FIG. 1 . Further, ICD  16  and INS  26  need not be implanted within patient  12 . In examples in which ICD  16  is not implanted in patient  12 , ICD  16  may deliver defibrillation pulses and other therapies to heart  14  via percutaneous leads that extend through the skin of patient  12  to a variety of positions within or outside of heart  14  or via external patch electrodes. In examples in which INS  26  is not implanted in patient  12 , INS  26  may deliver electrical stimulation to target tissue sites within patient  12  via external electrodes or via percutaneous leads that extend through the skin of patient  12 . 
     In other examples of therapy systems that provide electrical stimulation therapy to heart  14 , a therapy system may include any suitable number of leads coupled to ICD  16 , and each of the leads may extend to any location within or proximate to heart  14 . Other examples of therapy systems may include three transvenous leads located as illustrated in  FIGS. 1-3 , and an additional lead located within or proximate to left atrium  38 . Other examples of therapy systems may include a single lead that extends from ICD  16  into right atrium  30  or right ventricle  32 , or two leads that extend into a respective one of the right ventricle  32  and right atrium  30 . An example of this type of therapy system is shown in  FIG. 4 . 
       FIG. 4  is a conceptual diagram illustrating another example of therapy system  78 , which includes ICD  16  connected to two leads  18 ,  22 , rather than three leads as shown in  FIGS. 1-3 . Leads  18 ,  22  are implanted within right ventricle  32  and right atrium  30 , respectively. Therapy system  78  shown in  FIG. 4  may be useful for providing defibrillation and pacing pulses to heart  14 . Therapy system  78  may further include INS  26  (not shown in  FIG. 4 ), which is configured to deliver electrical stimulation therapy to modulate an autonomic nervous system of patient  12 , (e.g., via stimulation of a vagus nerve or within spinal cord  44 ) in order to help prevent or mitigate an arrhythmia of patient  12 . 
       FIG. 5  is a conceptual diagram of another example therapy system  80  that includes two medical devices to provide therapy to patient  12 . In addition to INS  26 , therapy system  80  includes ICD  82 , which delivers electrical stimulation to heart  14  without intravascular leads. ICD  82  is coupled to extravascular leads  83 ,  84 , which each include at least one electrode  85 ,  86 , respectively. Electrodes  85 ,  86  may be subcutaneous coil electrodes, which may be positioned within a subcutaneous tissue layer of patient  12 . In other examples, electrodes  85 ,  86  may comprise any other suitable type of extravascular electrode. For example, electrodes  85 ,  86  may include any other type of subcutaneous electrode, such as subcutaneous ring electrodes, subcutaneous plate electrodes, subcutaneous patch or pad electrodes, or any other type of extrathoracic electrode, such as a submuscular electrode, an epicardial electrode or an intramural electrode. 
     Electrodes  85  may be located within the thoracic cavity of patient  12  proximate to right ventricle  32  ( FIG. 1 ), on the patient&#39;s side or back, or any other portion of the body appropriate for providing electrical stimulation to heart  14 . Electrode  86  may be located within the thoracic cavity of patient  12  proximate left ventricle  36  ( FIG. 1 ), on the patient&#39;s side or back, or any other portion of the body appropriate for providing electrical stimulation to the heart. Similar extravascular electrodes are disclosed in commonly-assigned U.S. Pat. No. 5,261,400 to Bardy, which is entitled “DEFIBRILLATOR EMPLOYING TRANSVENOUS AND SUBCUTANEOUS ELECTRODES AND METHOD OF USE” and issued Nov. 16, 1993, and U.S. Pat. No. 5,292,338 to Bardy, which is entitled “ATRIAL DEFIBRILLATOR EMPLOYING TRANSVENOUS AND SUBCUTANEOUS ELECTRODES AND METHOD OF USE” and issued Mar. 8, 1994. U.S. Pat. Nos. 5,261,400 and 5,292,338 are incorporated herein by reference in their entireties. 
     Leads  83 ,  84  may be electrically coupled to stimulation modules, and, in some cases, sensing modules, that are enclosed within housing  87  of ICD  82 . As with housing  70  of ICD  16  ( FIG. 3 ), housing  87  may comprise a hermetic housing that substantially encloses the components of ICD  16 , such as a sensing module, stimulation generator, processor and the like. Components of an example ICD  16  or ICD  82  are described with respect to  FIG. 6 . ICD  82  may deliver electrical stimulation (e.g., pacing, cardioversion or defibrillation pulses) to heart  14  between electrodes  85 ,  86  e.g., in a bipolar configuration. In other examples, ICD  82  may deliver electrical stimulation to heart  14  between electrodes  85  and housing  87  (or an electrode attached to an outer surface of housing  87 ), or between electrode  86  and housing  87 , e.g., in a unipolar configuration. 
     Just as with ICD  16  ( FIG. 1 ) that delivers stimulation to heart  14  via intravascular electrodes, the delivery of electrical stimulation by INS  26  may interfere with the ability of ICD  82  to sense cardiac signals and deliver appropriate therapy upon the detection of an arrhythmia. ICD  82  may include a sensing module similar to that of ICD  16 . In some cases, the sensing module may sense the electrical stimulation delivered by INS  26  and mischaracterize the signals as cardiac signals, which may cause ICD  82  to deliver inappropriate therapy to heart  14  of patient  12 . 
     While the disclosure primarily refers to therapy system  10  including ICD  16  ( FIG. 1 ) and INS  26 , the description of the techniques, systems, and devices herein are also applicable to therapy system  80  including ICD  82  and INS  26 . 
       FIG. 6  is a functional block diagram of an example configuration of ICD  16  ( FIG. 1 ), which includes processor  90 , memory  92 , stimulation generator  94 , sensing module  96 , telemetry module  98 , and power source  100 . The block diagram shown in  FIG. 6  may also illustrate an example configuration of ICD  82  ( FIG. 5 ). Memory  92  includes computer-readable instructions that, when executed by processor  90 , cause ICD  16  and processor  90  to perform various functions attributed to ICD  16  and processor  90  herein. Memory  92  may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital media. 
     Processor  90  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. In some examples, processor  90  may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to processor  90  herein may be embodied as software, firmware, hardware or any combination thereof. Processor  90  controls stimulation generator  94  to deliver stimulation therapy to heart  14  according to a selected one or more of therapy programs, which may be stored in memory  92 . Specifically, processor  44  may control stimulation generator  94  to deliver electrical pulses with the amplitudes, pulse widths, frequency, or electrode polarities specified by the selected one or more therapy programs. 
     Stimulation generator  94  is electrically coupled to electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and  76 , e.g., via conductors of the respective lead  18 ,  20 ,  22 , or, in the case of housing electrode  68 , via an electrical conductor disposed within housing  70  of ICD  16 . Stimulation generator  94  is configured to generate and deliver electrical stimulation therapy to heart  14  to manage a rhythm of heart  14 . For example, stimulation generator  94  may deliver defibrillation shocks to heart  14  via at least two electrodes  68 ,  72 ,  74 ,  76 . Stimulation generator  94  may deliver pacing pulses via ring electrodes  50 ,  54 ,  58  coupled to leads  18 ,  20 , and  22 , respectively, helical electrodes  52 ,  56 , and  60  of leads  18 ,  20 , and  22 , respectively, and/or housing electrode  68 . In some examples, stimulation generator  94  delivers pacing, cardioversion or defibrillation therapy in the form of electrical pulses. In other examples, stimulation generator  94  may deliver one or more of these types of therapy in the form of other signals, such as sine waves, square waves, or other substantially continuous time signals. 
     In some examples, stimulation generator  94  may include a switch module (not shown in  FIG. 6 ) and processor  90  may use the switch module to select, e.g., via a data/address bus, which of the available electrodes are used to deliver defibrillation pulses or pacing pulses. The switch module may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple stimulation energy to selected electrodes. In other examples, however, stimulation generator  94  may independently deliver stimulation to electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and  76  or selectively sense via one or more of electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and  76  without a switch matrix. 
     Sensing module  96  monitors signals from at least one of electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and  76  in order to monitor electrical activity of heart  14 , e.g., via an EGM signal. Sensing module  96  may also include a switch module (not shown in  FIG. 6 ) to select a particular subset of available electrodes to sense the heart activity. In some examples, processor  90  may select the electrodes that function as sense electrodes via the switch module within sensing module  96 , e.g., by providing signals via a data/address bus. In some examples, sensing module  96  includes one or more sensing channels, each of which may comprise an amplifier. In response to the signals from processor  90 , the switch module of sensing module  96  may couple the outputs from the selected electrodes to one of the sensing channels. 
     In some examples, sensing module  96  may include a plurality of channels. One channel of sensing module  96  may include an R-wave amplifier that receives signals from electrodes  50  and  52 , which are used for pacing and sensing in right ventricle  32  of heart  14 . Another channel may include another R-wave amplifier that receives signals from electrodes  54  and  56 , which are used for pacing and sensing proximate to left ventricle  36  of heart  14 . In some examples, in one operating mode of sensing module  96 , the R-wave amplifiers may take the form of an automatic gain controlled amplifier that provides an adjustable sensing threshold as a function of the measured R-wave amplitude of the heart rhythm. 
     In addition, in some examples, one channel of sensing module  96  may include a P-wave amplifier that receives signals from electrodes  58  and  60 , which are used for pacing and sensing in right atrium  30  of heart  14 . In some examples, in one operating mode of sensing module  96 , the P-wave amplifier may take the form of an automatic gain controlled amplifier that provides an adjustable sensing threshold as a function of the measured P-wave amplitude of the heart rhythm. Examples of R-wave and P-wave amplifiers are described in U.S. Pat. No. 5,117,824 to Keimel et al., which issued on Jun. 2, 1992 and is entitled, “APPARATUS FOR MONITORING ELECTRICAL PHYSIOLOGIC SIGNALS,” and is incorporated herein by reference in its entirety. Other amplifiers may also be used. Furthermore, in some examples, one or more of the sensing channels of sensing module  96  may be selectively coupled to housing electrode  68 , or elongated electrodes  72 ,  74 , or  76 , with or instead of one or more of electrodes  50 ,  52 ,  54 ,  56 ,  58  or  60 , e.g., for unipolar sensing of R-waves or P-waves in any of chambers  30 ,  32 , or  36  of heart  14 . 
     In some examples, sensing module  96  includes a channel that comprises an amplifier with a relatively wider pass band than the R-wave or P-wave amplifiers. Signals from the selected sensing electrodes that are selected for coupling to this wide-band amplifier may be provided to a multiplexer, and thereafter converted to multi-bit digital signals by an analog-to-digital converter for storage in memory  92  as an EGM. In some examples, the storage of such EGMs in memory  92  may be under the control of a direct memory access circuit. Processor  90  may employ digital signal analysis techniques to characterize the digitized signals stored in memory  92  to detect and classify the patient&#39;s heart rhythm from the electrical signals. Processor  90  may detect and classify the heart rhythm of patient  12  by employing any of the numerous signal processing methodologies known in the art. 
     If ICD  16  is configured to generate and deliver pacing pulses to heart  14 , processor  90  may include pacer timing and control module, which may be embodied as hardware, firmware, software, or any combination thereof. The pacer timing and control module may comprise a dedicated hardware circuit, such as an ASIC, separate from other processor  90  components, such as a microprocessor, or a software module executed by a component of processor  90 , which may be a microprocessor or ASIC. The pacer timing and control module may include programmable counters which control the basic time intervals associated with DDD, VVI, DVI, VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and other modes of single and dual chamber pacing. In the aforementioned pacing modes, “D” may indicate dual chamber, “V” may indicate a ventricle, “I” may indicate inhibited pacing (e.g., no pacing), and “A” may indicate an atrium. The first letter in the pacing mode may indicate the chamber that is paced, the second letter may indicate the chamber in which an electrical signal is sensed, and the third letter may indicate the chamber in which the response to sensing is provided. When a pacing code includes “D” as the third letter in the code, it may indicate that the sensed signal is used for tracking purposes. 
     Intervals defined by the pacer timing and control module within processor  90  may include atrial and ventricular pacing escape intervals, refractory periods during which sensed P-waves and R-waves are ineffective to restart timing of the escape intervals, and the pulse widths of the pacing pulses. As another example, the pace timing and control module may define a blanking period, and provide signals from sensing module  96  to blank one or more channels, e.g., amplifiers, for a period during and after delivery of electrical stimulation to heart  14 . The durations of these intervals may be determined by processor  90  in response to stored data in memory  92 . The pacer timing and control module of processor  90  may also determine the amplitude of the cardiac pacing pulses. 
     During pacing, escape interval counters within the pacer timing/control module of processor  90  may be reset upon sensing of R-waves and P-waves. Stimulation generator  94  may include pacer output circuits that are coupled, e.g., selectively by a switching module, to any combination of electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and  76  appropriate for delivery of a bipolar or unipolar pacing pulse to one of the chambers of heart  14 . Processor  90  may reset the escape interval counters upon the generation of pacing pulses by stimulation generator  94 , and thereby control the basic timing of cardiac pacing functions, including anti-tachyarrhythmia pacing. 
     The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used by processor  90  to measure the durations of R-R intervals, P-P intervals, P-R intervals and R-P intervals, which are measurements that may be stored in memory  92 . Processor  90  may use the count in the interval counters to detect a tachyarrhythmia event, such as ventricular fibrillation event or ventricular tachycardia event. Upon detecting a threshold number of tachyarrhythmia events, processor  90  may identify the presence of a tachyarrhythmia episode, such as a ventricular fibrillation episode, a ventricular tachycardia episode, or a non-sustained tachycardia (NST) episode. Examples of tachyarrhythmia episodes that may qualify for delivery of responsive therapy include a ventricular fibrillation episode or a ventricular tachyarrhythmia episode. In the case of a NST, however, the count in the interval counters may not meet the requirements for triggering a therapeutic response. 
     In some examples, processor  90  may operate as an interrupt driven device, and is responsive to interrupts from pacer timing and control module, where the interrupts may correspond to the occurrences of sensed P-waves and R-waves and the generation of cardiac pacing pulses. Any necessary mathematical calculations to be performed by processor  90  and any updating of the values or intervals controlled by the pacer timing and control module of processor  90  may take place following such interrupts. A portion of memory  92  may be configured as a plurality of recirculating buffers, capable of holding series of measured intervals, which may be analyzed by processor  90  in response to the occurrence of a pace or sense interrupt to determine whether heart  14  of patient  12  is presently exhibiting atrial or ventricular tachyarrhythmia. 
     In some examples, an arrhythmia detection method may include any suitable tachyarrhythmia detection algorithms. In one example, processor  90  may utilize all or a subset of the rule-based detection methods described in U.S. Pat. No. 5,545,186 to Olson et al., entitled, “PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND TREATMENT OF ARRHYTHMIAS,” which issued on Aug. 13, 1996, or in U.S. Pat. No. 5,755,736 to Gillberg et al., entitled, “PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND TREATMENT OF ARRHYTHMIAS,” which issued on May 26, 1998. U.S. Pat. No. 5,545,186 to Olson et al. and U.S. Pat. No. 5,755,736 to Gillberg et al. are incorporated herein by reference in their entireties. However, other arrhythmia detection methodologies may also be employed by processor  90  in other examples. 
     In the examples described herein, processor  90  may identify the presence of an atrial or ventricular tachyarrhythmia episode by detecting a series of tachyarrhythmia events (e.g., R-R or P-P intervals having a duration less than or equal to a threshold) of an average rate indicative of tachyarrhythmia or an unbroken series of short R-R or P-P intervals. The thresholds for determining the R-R or P-P interval that indicates a tachyarrhythmia event may be stored within memory  92  of ICD  16 . In addition, the number of tachyarrhythmia events that are detected to confirm the presence of a tachyarrhythmia episode may be stored as a number of intervals to detect (NID) threshold value in memory  92 . In some examples, processor  90  may also identify the presence of the tachyarrhythmia episode by detecting a variability of the intervals between tachycardia events. For example, if the interval between successive tachyarrhythmia events varies by a particular percentage or the differences between the coupling intervals are higher than a given threshold over a predetermined number of successive cycles, processor  90  may determine that the tachyarrhythmia is present. 
     If processor  90  detects an atrial or ventricular tachyarrhythmia based on signals from sensing module  96 , and an anti-tachyarrhythmia pacing regimen is desired, timing intervals for controlling the generation of anti-tachyarrhythmia pacing therapies by stimulation generator  94  may be loaded by processor  90  into the pacer timing and control module to control the operation of the escape interval counters therein and to define refractory periods during which detection of R-waves and P-waves is ineffective to restart the escape interval counters. 
     If ICD  16  is configured to generate and deliver defibrillation pulses to heart  14 , stimulation generator  94  may include a high voltage charge circuit and a high voltage output circuit. In the event that generation of a cardioversion or defibrillation pulse is required, processor  90  may employ the escape interval counter to control timing of such cardioversion and defibrillation pulses, as well as associated refractory periods. In response to the detection of atrial or ventricular fibrillation or tachyarrhythmia requiring a cardioversion pulse, processor  90  may activate a cardioversion/defibrillation control module, which may, like pacer timing and control module, be a hardware component of processor  90  and/or a firmware or software module executed by one or more hardware components of processor  90 . The cardioversion/defibrillation control module may initiate charging of the high voltage capacitors of the high voltage charge circuit of stimulation generator  94  under control of a high voltage charging control line. 
     Processor  90  may monitor the voltage on the high voltage capacitor, e.g., via a voltage charging and potential (VCAP) line. In response to the voltage on the high voltage capacitor reaching a predetermined value set by processor  90 , processor  90  may generate a logic signal that terminates charging. Thereafter, timing of the delivery of the defibrillation or cardioversion pulse by stimulation generator  94  is controlled by the cardioversion/defibrillation control module of processor  90 . Following delivery of the fibrillation or tachycardia therapy, processor  90  may return stimulation generator  94  to a cardiac pacing function and await the next successive interrupt due to pacing or the occurrence of a sensed atrial or ventricular depolarization. 
     Stimulation generator  94  may deliver cardioversion or defibrillation pulses with the aid of an output circuit that determines whether a monophasic or biphasic pulse is delivered, whether housing electrode  68  serves as cathode or anode, and which electrodes are involved in delivery of the cardioversion or defibrillation pulses. Such functionality may be provided by one or more switches or a switching module of stimulation generator  94 . 
     Telemetry module  98  includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as INS  26  or programmer  24  ( FIG. 1 ). Under the control of processor  90 , telemetry module  98  may receive downlink telemetry from and send uplink telemetry to programmer  24  with the aid of an antenna, which may be internal and/or external. Processor  90  may provide the data to be uplinked to programmer  24  and the control signals for the telemetry circuit within telemetry module  98 , e.g., via an address/data bus. In some examples, telemetry module  98  may provide received data to processor  90  via a multiplexer. 
     In some examples, processor  90  may transmit atrial and ventricular heart signals (e.g., ECG signals) produced by atrial and ventricular sense amp circuits within sensing module  96  to programmer  24 . Programmer  24  may interrogate ICD  16  to receive the heart signals. Processor  90  may store heart signals within memory  92 , and retrieve stored heart signals from memory  92 . Processor  90  may also generate and store marker codes indicative of different cardiac episodes that sensing module  96  detects, and transmit the marker codes to programmer  24 . An example pacemaker with marker-channel capability is described in U.S. Pat. No. 4,374,382 to Markowitz, entitled, “MARKER CHANNEL TELEMETRY SYSTEM FOR A MEDICAL DEVICE,” which issued on Feb. 15, 1983 and is incorporated herein by reference in its entirety. 
     The various components of ICD  16  are coupled to power source  100 , which may include a rechargeable or non-rechargeable battery. A non-rechargeable battery may be selected to last for several years, while a rechargeable battery may be inductively charged from an external device, e.g., on a daily or weekly basis. Examples of a rechargeable battery include, but are not limited to, a lithium ion battery, a lithium polymer battery or a supercapacitor. 
     In some examples, data from sensing module  96  may be uploaded to a remote server, from which a clinician or another user may access the data to determine whether a potential sensing integrity issue exists. An example of a remote server includes the CareLink Network, available from Medtronic, Inc. of Minneapolis, Minn. An example of a system that includes an external device, such as a server, and one or more computing devices that are coupled to ICD  16  and programmer  24  via a network is described below with respect to  FIG. 32 . 
     Telemetry module  98  may also be useful for communicating with INS  26 , which may also include a telemetry module as described with respect to  FIG. 7 . In some examples, INS  26  and ICD  16  may communicate with each other by way of RF communication techniques supported by the respective telemetry modules. In addition to or instead of the RF communication techniques, INS  26  and ICD  16  may communicate with each other by generating electrical communication signals that are sensed via the other device. For example, as described in U.S. Provisional Patent Application No. 61/110,117 by Burnes et al., which is entitled, “INTERDEVICE IMPEDANCE” and was filed on Oct. 31, 2008, and U.S. patent application Ser. No. 12/362,895 by Burnes et al., which is entitled “INTERDEVICE IMPEDANCE” and was filed on Jan. 30, 2009, in order to transmit information to INS  26 , ICD  16  may generate an electrical signal between two or more electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 ,  76  electrically connected to ICD  16 , and INS  26  may sense the electrical signal and retrieve information from the sensed electrical signal. The electrical signal may or may not provide therapeutic benefits to patient  12 . The entire contents of U.S. Provisional Patent Application No. 61/110,117 by Burnes et al. and U.S. patent application Ser. No. 12/362,895 by Burnes et al. are incorporated herein by reference. U.S. patent application Ser. No. 12/362,895 published as U.S. Patent Application Publication No. 2010/0114204 on May 6, 2010. 
     As another example, as described in U.S. patent application Ser. No. 12/362,895 by Burnes et al., in order to transmit information to ICD  16 , INS  26  may generate an electrical signal between two or more electrodes  124  electrically connected to INS  26  and INS  26  may sense the electrical signal and retrieve information therefrom. Again, the electrical signal may or may not provide therapeutic benefits to patient  12 . In either example, ICD  16  or INS  26  may modulate one or more characteristics of the electrical signal (e.g., an amplitude of frequency of the signal) in order to exchange information with the other device INS  26  or ICD  16 , respectively. 
     Another example of a suitable communication technique for exchanging information between ICD  16  and INS  26  is described in commonly-assigned U.S. Pat. No. 4,987,897 to Funke, which is entitled, “BODY BUS MEDICAL DEVICE COMMUNICATION SYSTEM,” and issued on Jan. 29, 1991. U.S. Pat. No. 4,987,897 to Funke is 
       FIG. 7  is a functional block diagram of an example INS  26 . INS  26  includes processor  110 , memory  112 , stimulation generator  114 , switching module  116 , telemetry module  118 , and power source  120 . In the example shown in  FIG. 7 , processor  110 , memory  112 , stimulation generator  114 , switching module  116 , telemetry module  118 , and power source  120  are enclosed within housing  122 , which may be, for example a hermetic housing. As shown in  FIG. 7 , stimulation generator  114  is coupled to lead  28  either directly or indirectly (e.g., via a lead extension). Alternatively, stimulation generator  114  may be coupled to more than one lead directly or indirectly (e.g., via a lead extension, such as a bifurcating lead extension that may electrically and mechanically couple to two leads) as needed to provide neurostimulation therapy to patient  12 . 
     In the example illustrated in  FIG. 7 , lead  28  includes electrodes  124 A- 124 D (collectively referred to as “electrodes  124 ”). Electrodes  124  may comprise ring electrodes. In other examples, electrodes  124  may be arranged in a complex electrode array that includes multiple non-contiguous electrodes at different angular positions about the outer circumference of lead  28 , as well as different levels of electrodes spaced along a longitudinal axis of lead  28 . The configuration, type, and number of electrodes  124  illustrated in  FIG. 7  are merely exemplary. In other examples, INS  26  may be coupled to any suitable number of leads with any suitable number and configuration of electrodes. Moreover, lead  28  may comprise a shape other than a cylindrical shape. As an example, lead  28  may comprise a paddle-shaped portion that carries electrodes  124 . 
     Memory  112  includes computer-readable instructions that, when executed by processor  110 , cause INS  26  to perform various functions. Memory  112  may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a RAM, ROM, NVRAM, EEPROM, flash memory, or any other digital media. Memory  112  may store therapy programs, which may be stored in therapy program groups, and operating instructions. The therapy programs may define a particular program of therapy in terms of respective values for electrical stimulation parameters, such as electrode combination, electrode polarity, current or voltage amplitude, pulse width and pulse rate. A program group may comprise a plurality of therapy programs that may be delivered together on an overlapping or non-overlapping basis. The stored operating instructions may guide the general operation of INS  26  under control of processor  110 , and may include instructions for measuring the impedance of electrodes  124 . 
     Stimulation generator  114  generates stimulation signals, which may be pulses as primarily described herein, or continuous signals, such as sine waves, for delivery to patient  12  via selected combinations of electrodes  124 . Processor  110  controls stimulation generator  114  according to stored therapy programs and/or program groups in memory  112  to apply particular stimulation parameter values specified by one or more of programs, such as amplitude, pulse width, and pulse rate. Processor  110  may include any one or more microprocessors, controllers, a DSPs, ASICs, FPGAs, or equivalent discrete or integrated digital or analog logic circuitry, and the functions attributed to processor  110  herein may be embodied as software, firmware, hardware or any combination thereof. 
     Processor  110  may also control switching module  116  to apply the stimulation signals generated by stimulation generator  114  to selected combinations of electrodes  124 . In particular, switching module  116  couples stimulation signals to selected conductors within lead  28  which, in turn, deliver the stimulation signals across selected electrodes  124 . Switching module  116  may be a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple stimulation energy to selected electrodes. Hence, stimulation generator  114  is coupled to electrodes  124  via switching module  116  and conductors within lead  28 . In some examples, INS  26  does not include switching module  116 . 
     Stimulation generator  114  may be a single or multi-channel stimulation generator. In particular, stimulation generator  114  may be capable of delivering a single stimulation pulse, multiple stimulation pulses, or a continuous signal at a given time via a single electrode combination or multiple stimulation pulses at a given time via multiple electrode combinations. In some examples, however, stimulation generator  114  and switching module  116  may be configured to deliver multiple channels on a time-interleaved basis. In this case, switching module  116  serves to time division multiplex the output of stimulation generator  114  across different electrode combinations at different times to deliver multiple programs or channels of stimulation energy to patient  12 . 
     Telemetry module  118  supports wireless communication between INS  26  and an external programmer  24  ( FIG. 1 ) or another computing device, and, in some examples, between INS  26  and ICD  16  under the control of processor  110 . Processor  110  of INS  26  may receive, as updates to programs, values for various stimulation parameters such as amplitude and electrode combination, from programmer  24  via telemetry module  118 . The updates to the therapy programs may be stored within memory  112 . 
     The various components of INS  26  are coupled to power source  120 , which may include a rechargeable or non-rechargeable battery. A non-rechargeable battery may be selected to last for several years, while a rechargeable battery may be inductively charged from an external device, e.g., on a daily or weekly basis. In other examples, power source  120  may be powered by proximal inductive interaction with an external power source carried by patient  12 . 
       FIG. 8  is block diagram of an example programmer  24 . As shown in  FIG. 6 , programmer  24  includes processor  130 , memory  132 , user interface  134 , telemetry module  136 , and power source  138 . Programmer  24  may be a dedicated hardware device with dedicated software for programming of ICD  16  and INS  26 . Alternatively, programmer  24  may be an off-the-shelf computing device running an application that enables programmer  24  to program ICD  16  and INS  26 . In some examples, separate programmers may be used to program ICD  16  and INS  26 . However, a common programmer  24  that is configured to program both ICD  16  and INS  26  may provide a more streamlined programming process for a user, such as a clinician or patient  12 . 
     A user may use programmer  24  to select therapy programs (e.g., sets of stimulation parameters), generate new therapy programs, modify therapy programs through individual or global adjustments or transmit the new programs to a medical device, such as ICD  16  or INS  26  ( FIG. 1 ). The clinician may interact with programmer  24  via user interface  134 , which may include display to present graphical user interface to a user, and a keypad or another mechanism for receiving input from a user. 
     Processor  130  can take the form one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, and the functions attributed to processor  102  herein may be embodied as hardware, firmware, software or any combination thereof. Memory  132  may store instructions that cause processor  130  to provide the functionality ascribed to programmer  24  herein, and information used by processor  130  to provide the functionality ascribed to programmer  24  herein. Memory  132  may include any fixed or removable magnetic, optical, or electrical media, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM, or the like. Memory  132  may also include a removable memory portion that may be used to provide memory updates or increases in memory capacities. A removable memory may also allow patient data to be easily transferred to another computing device, or to be removed before programmer  24  is used to program therapy for another patient. Memory  132  may also store information that controls therapy delivery by ICD  16  and INS  26 , such as stimulation parameter values. 
     Programmer  24  may communicate wirelessly with ICD  16  and INS  24 , such as using RF communication or proximal inductive interaction. This wireless communication is possible through the use of telemetry module  136 , which may be coupled to an internal antenna or an external antenna. An external antenna that is coupled to programmer  24  may correspond to the programming head that may be placed over heart  14 , as described above with reference to  FIG. 1 . Telemetry module  136  may be similar to telemetry module  98  of ICD  16  ( FIG. 6 ) or telemetry module  118  of INS  26  ( FIG. 7 ). 
     Telemetry module  136  may also be configured to communicate with another computing device via wireless communication techniques, or direct communication through a wired connection. Examples of local wireless communication techniques that may be employed to facilitate communication between programmer  24  and another computing device include RF communication according to the 802.11 or Bluetooth specification sets, infrared communication, e.g., according to the IrDA standard, or other standard or proprietary telemetry protocols. In this manner, other external devices may be capable of communicating with programmer  24  without needing to establish a secure wireless connection. 
     Power source  138  delivers operating power to the components of programmer  24 . Power source  138  may include a battery and a power generation circuit to produce the operating power. In some examples, the battery may be rechargeable to allow extended operation. Recharging may be accomplished by electrically coupling power source  138  to a cradle or plug that is connected to an alternating current (AC) outlet. In addition or alternatively, recharging may be accomplished through proximal inductive interaction between an external charger and an inductive charging coil within programmer  24 . In other examples, traditional batteries (e.g., nickel cadmium or lithium ion batteries) may be used. In addition, programmer  24  may be directly coupled to an alternating current outlet to power programmer  24 . Power source  138  may include circuitry to monitor power remaining within a battery. In this manner, user interface  134  may provide a current battery level indicator or low battery level indicator when the battery needs to be replaced or recharged. In some cases, power source  138  may be capable of estimating the remaining time of operation using the current battery. 
     As previously indicated, in some cases, electrical stimulation signals generated and delivered to patient  12  by INS  26  may be sensed by ICD  16  and ICD  16  may mischaracterize the sensed electrical stimulation signals as cardiac signals.  FIG. 9  is a flow diagram illustrating an example technique that therapy system  10  may implement in order to minimize the possibility that ICD  16  delivers electrical stimulation to heart  14  in response to detecting electrical signals generated by INS  26  that resemble an arrhythmic cardiac signal. While the techniques shown in  FIGS. 9-12B ,  16 ,  19 - 21 , and  23 - 30  are primarily described as being performed by one or more of processors  90 ,  110 ,  130  of ICD  16 , INS  26 , and programmer  24 , respectively, any one or more parts of the techniques described herein may be implemented by a processor of one of the devices  16 ,  24 ,  26 , alone or in combination with each other. 
     INS  26  may deliver neurostimulation to patient  12  ( 140 ) and ICD  16  may sense cardiac signals ( 142 ). As described above, stimulation generator  114  of INS  26  ( FIG. 7 ) may generate electrical stimulation signals according to therapy parameter values defined by a therapy program or a therapy program group, and deliver the signals to patient  12  via a selected subset of electrodes  124  ( FIG. 7 ) of lead  28 . ICD  16  may sense cardiac signals of heart  14  via any subset of electrodes  50 ,  52 ,  54 ,  56 ,  58 , and  60  of leads  18 ,  20 ,  22  ( FIGS. 2 and 3 ) and electrode  68  of housing  70 . True electrical cardiac signals are generated as heart  14  depolarizes and repolarizes. 
     Processor  90  of ICD  16  may detect a potential arrhythmia based on the sensed cardiac signals ( 144 ). The potential arrhythmia may be, for example, a suspected bradycardia or a suspected tachyarrhythmia. The cardiac signals sensed by ICD  16  may appear to indicate that heart  14  of patient  12  is in an arrhythmia, but, as described herein, ICD  16  may sense noise from delivery of stimulation by INS  26  in addition to the true cardiac signals. The noise (also referred to as crosstalk) may mask the true cardiac activity of heart  14 , and, therefore, the detected arrhythmia may be referred to as a potential arrhythmia. 
     Processor  90  may implement any suitable technique to detect a potential arrhythmia of heart  14  ( 144 ). Processor  90  of ICD  16  may detect a potential arrhythmia by detecting a threshold number of arrhythmia events or an arrhythmia episode, which includes a predetermined number of arrhythmia events. In some examples, an arrhythmia event may comprise a tachyarrhythmia event, which includes a cardiac cycle that has an R-R interval that is less than a predetermined threshold value. If desired, processor  90  may characterize the arrhythmia event as a ventricular fibrillation event, a ventricular tachycardia event or a fast ventricular tachycardia event, where different threshold values may be used to characterize the cardiac cycle as the different types of events, e.g., based on the duration of the cardiac cycles. In other examples, the arrhythmia event may comprise a bradycardia event, which includes a cardiac cycle that has an R-R interval that is greater than a predetermined threshold. 
     The threshold duration values for determining whether an R-R interval qualifies a cardiac cycle as an arrhythmia event may be stored by memory  92  of ICD  16  ( FIG. 6 ). In addition, the threshold number of arrhythmia events that are characterized as a potential arrhythmia or the number of arrhythmia events that constitute an arrhythmia episode may be stored by memory  92  of ICD  16  ( FIG. 6 ) or a memory of another device (e.g., INS  26  or programmer  24 ). In some examples, the threshold number may be about two to about five arrhythmia events, such that processor  90  may detect a potential arrhythmia after about two to about five arrhythmia events are detected. However, processor  90  may use any suitable threshold number of arrhythmia events to detect a potential arrhythmia. In other examples, other techniques for detecting a potential arrhythmia may be used. 
     If processor  90  of ICD  16  does not detect a potential arrhythmia ( 144 ), processor  90  may not take any action to modify INS  26  and INS  26  may continue delivering electrical stimulation therapy to patient  12  ( 140 ) according to the current therapy program or program group. On the other hand, if processor  90  of ICD  16  detects a potential arrhythmia based on the sensed cardiac signals ( 144 ), processor  90  may determine that modification to the neurostimulation signals delivered by INS  26  are desirable in order to, for example, reduce the crosstalk between ICD  16  and INS  26 . Thus, processor  90  may initiate the modification to the neurostimulation signals delivered by INS  26  ( 146 ). 
     In some examples, processor  90  of ICD  16  initiates the modification to the electrical stimulation signals generated and delivered by INS  26 . For example, processor  90  of ICD  16  may provide INS  26  with a control signal via the respective telemetry modules  98 ,  118 , where the control signal causes processor  110  of INS  26  to modify one or more electrical stimulation parameter values of the electrical stimulation generated and delivered by INS  26 . In other examples, processor  90  of ICD  16  may modify the one or more electrical stimulation parameter values and transmit the modified parameter values to INS  26 . The stimulation parameter values that may be modified include, but are not limited to, a current amplitude, a voltage amplitude, a pulse width, a slew rate, a pulse rate, a continuous waveform frequency, a duty cycle, an electrode combination, a timing of the delivery of the electrical stimulation relative to a cardiac cycle of the heart of the patient, a waveform shape, and a signal envelope of the electrical stimulation signal. 
     Modifying the one or more electrical stimulation parameter values that define the electrical stimulation signals generated and delivered by INS  26  may help change the characteristics of the electrical signal delivered by INS  26  and sensed by ICD  16 . The modified neurostimulation signal generated and delivered by INS  26  may no longer resemble cardiac signals, thereby minimizing the possibility that ICD  16  senses the neurostimulation signals and mischaracterizes the signals as cardiac signals. For example, the modified neurostimulation signal may have a frequency component that falls outside of a sensing bandpass filter used by ICD  16  to sense cardiac signals. In this way, ICD  16  may “ignore” the modified neurostimulation signals. 
     The current or voltage amplitude or the frequency of the electrical signal that ICD  16  senses may change after the electrical stimulation parameter values for INS  26  are modified ( 146 ). As an example, if the current or voltage amplitude of the electrical stimulation signals delivered by INS  26  is modified, the current or voltage amplitude of the modified neurostimulation signals may no longer resemble cardiac signals and ICD  16  may no longer sense the electrical stimulation signals or mischaracterize the electrical stimulation signals as cardiac signals. As another example, the current or voltage amplitude of the modified neurostimulation signals may below the current or voltage amplitude threshold used by ICD  16  to identify cardiac signals. As another example, if the frequency of the electrical stimulation signal delivered by INS  26  is modified, the frequency of the signal sensed by ICD  16  may no longer have the required frequency or morphology to resemble an arrhythmic cardiac signal (e.g., the neurostimulation signals may no longer resemble a cardiac signal having short R-R intervals that characterize the signals as ventricular fibrillation cardiac signals). 
     In some examples, processor  110  of INS  26  may modify the combination of electrodes that INS  26  uses to deliver stimulation to patient  12 . That is, processor  110  may select a different subset of electrodes  124  of lead  28  ( FIG. 7 ) that are activated or modify the polarity of the selected electrodes. Modifying the electrode combination that is used to deliver neurostimulation may help reduce the amount of noise detected by ICD  16  from the delivery of electrical stimulation by INS  26 . For example, modifying the electrode combination with which INS  26  delivers electrical stimulation signals may help steer the stimulation field away from the sensing field of ICD  16  or at least reduce the amount of stimulation field that is sensed by ICD  16 . 
     In addition, modifying the electrode combination that is used to deliver neurostimulation may help reduce the amount of noise detected by ICD  16  by changing the nature of the noise detected by ICD  16 . For example, modifying the neurostimulation electrode combination may change the vector between the electrodes with which the neurostimulation signal is delivered to tissue of patient  12  and the sensing electrodes of ICD  16  that are used to sense a cardiac signal. Changing the relative vector with which the sensing electrodes of ICD  16  may sense electrical signals delivered by INS  26  may help change the characteristics of the electrical signals delivered by INS  26  and sensed by ICD  16 , such as the current or voltage amplitude of the neurostimulation signals sensed by ICD  16 , the frequency of the signals, and the like. Other types of modifications to the neurostimulation signals generated and delivered by INS  26  are also contemplated. 
     After modifying the one or more electrical stimulation parameter values that define the electrical stimulation signals generated and delivered by INS  26  ( 146 ), INS  26  may deliver stimulation to patient  12  via the modified electrical stimulation parameter values ( 147 ). After INS  26  begins delivering stimulation to patient  12  with the modified electrical stimulation signals, processor  90  of ICD  16  may confirm the presence of the arrhythmia ( 148 ). Processor  90  may confirm the presence of the arrhythmia using any suitable technique, such as the techniques that were used to detect the arrhythmia ( 144 ). In some examples, if processor  90  confirms that the arrhythmia is present, processor  90  of ICD  16  or processor  110  of INS  26  may modify the neurostimulation signals generated and delivered by INS  26  at least one more time in an attempt to reduce the electrical noise attributable to  26  ( 144 ). 
     In other examples, if, after modifying the neurostimulation signals delivered by INS  26 , processor  90  confirms that the arrhythmia is present, processor  90  of ICD  16  may determine that the arrhythmia is a true arrhythmia. In response, processor  90  may characterize a type of true arrhythmia detected. For example, based on the R-R interval of the sensed cardiac signals upon which the true arrhythmia was detected, processor  90  may determine whether the arrhythmia is a ventricular fibrillation, a bradycardia event, a supraventricular tachycardia, and the like. The type of true arrhythmia may be identified in order to select the appropriate cardiac rhythm therapy. Processor  90  may select a therapy program from memory  92  ( FIG. 6 ) of ICD  16  or a memory of another device based on the type of true arrhythmia that is detected. For example, a plurality of therapy programs for a plurality of different types of arrhythmia may be stored by memory  92 . After selecting a cardiac rhythm therapy based upon the type of true arrhythmia that is detected, processor  90  may control stimulation generator  94  ( FIG. 6 ) to deliver electrical stimulation to heart  14  based on the selected therapy in order to terminate the arrhythmia. 
     In some examples, INS  26  may deliver electrical stimulation therapy to patient  12  according to the modified neurostimulation signals ( 147 ) for a finite period of time (rather than substantially indefinitely) and then revert back to the prior electrical stimulation parameter values after the finite period of time. In some examples, the finite period of time may be selected by a clinician and stored by memory  92  of ICD  16  or a memory of another device, such as INS  26 . 
     In some examples of the technique shown in  FIG. 9 , as well as the other techniques described herein for modifying therapy delivery by INS  26  to minimize crosstalk with ICD  16  (e.g., FIGS.  10  and  11 A- 11 D), INS  26  may deliver neurostimulation to patient  12  ( 140 ) for a test period of time, e.g., for a certain number of cardiac cycles of patient  12 , and processor  90  of INS  26  may determine if the arrhythmia is detected during the delivery of electrical stimulation by INS  26 . For example, INS  26  may deliver stimulation to patient  12  for about ten to about twenty cardiac cycles (e.g., as indicated by heart beats), and during that time, ICD  16  may sense cardiac signals ( 142 ) and processor  90  may determine whether a potential arrhythmia is detected ( 144 ). The test electrical stimulation delivered by INS  26  may provide therapeutic benefits to patient  12 . In some examples, if the potential arrhythmia is detected during the delivery of the test neurostimulation to patient  12 , processor  90  may determine that the neurostimulation may be interfering with the detection of true cardiac signals by ICD  16 . Thus, in some examples, processor  90  may initiate the modification to the neurostimulation signals delivered by IND  26  ( 146 ), as described above with respect to  FIG. 9 . 
       FIG. 10  is a flow diagram of an example technique that may be implemented to determine whether an arrhythmia detected by ICD  16 , INS  26  or another device may have been attributable to noise from neurostimulation delivered by INS  26 . According to the example technique shown in  FIG. 10 , INS  26  may deliver neurostimulation to a nonmyocardial tissue site (e.g., proximate a nerve) within patient  12  according to a therapy program or therapy program group ( 140 ) and ICD  16  may sense cardiac signals ( 142 ). Processor  90  of ICD  16  may detect a potential arrhythmia based on the sensed cardiac signals using any suitable technique, such as the techniques described above with respect to  FIG. 9  ( 144 ). If processor  90  does not detect a potential arrhythmia, INS  26  may continue delivering neurostimulation to patient  12  according to the therapy program or therapy program group ( 140 ). 
     If processor  90  of ICD  16  detects a potential arrhythmia ( 144 ), processor  90  may adjust the delivery of neurostimulation by INS  26  ( 150 ). In one example, processor  90  of ICD  16  may generate and deliver a control signal to INS  26  via the respective telemetry modules  98 ,  118 . Upon receiving the control signal, processor  110  of INS  26  may temporarily adjust the delivery of stimulation, such as by suspending the active delivery of electrical stimulation to patient  12  or reducing an intensity of a stimulation signal delivered to patient  12 . The control signal may indicate how long INS  26  should deliver therapy according to the adjusted parameters or may only indicate that INS  26  should adjust the delivery of neurostimulation. For example, the control signal may indicate how long INS  26  should suspend the delivery of neurostimulation. In some examples, processor  110  of INS  26  may refer to instructions stored within memory  112  of INS  26  that indicate the duration of time for which INS  26  should suspend or otherwise adjust the delivery of neurostimulation in response to receiving the control signal from ICD  16 . The stored instructions may also indicate other operating parameters for the suspension period. For examples, in some cases, rather than deactivating all electrical stimulation signals delivered by INS  26 , processor  110  may control stimulation generator  114  to deliver stimulation to patient  12  according to a different set of therapy parameters, such as a therapy program that defines electrical stimulation having a lower intensity (e.g., a lower amplitude or frequency). 
     After INS  26  suspends or otherwise adjusts the delivery of neurostimulation ( 150 ), processor  90  of ICD  16  may sense cardiac signals and determine whether the cardiac signals indicate a potential arrhythmia ( 152 ). If the cardiac signals indicate a potential arrhythmia after neurostimulation is suspended or otherwise adjusted, processor  90  of ICD  16  may determine that the arrhythmia was not detected based on crosstalk from the delivery of neurostimulation by INS  26 . Processor  90  may, for example, determine that the arrhythmia was detected based on true cardiac signals and that a true arrhythmia may be present. Thus, in the technique shown in  FIG. 10 , processor may generate an arrhythmia indication if the cardiac signals indicate a potential arrhythmia after neurostimulation is suspended or otherwise adjusted ( 154 ). The arrhythmia indication may be a value, flag, or signal that is stored or transmitted to indicate the detection of an arrhythmia. 
     The arrhythmia indication may be used to control different aspects of therapy system  10 . In some examples, processor  90  may control stimulation generator  94  ( FIG. 6 ) to generate and deliver at least one of pacing, cardioversion or defibrillation therapy to heart  14  upon the generation of the arrhythmia indication. In other examples, processor  90  may confirm the detection of the arrhythmia using physiological parameter of patient  12  other than electrical cardiac signals upon the generation of the arrhythmia indication. For example, processor  90  may confirm the detection of the arrhythmia based on pressure within heart  14 , as described in U.S. patent application Ser. No. 12/180,160 to Mayotte, which is entitled, “SENSING INTEGRITY DETERMINATION BASED ON CARDIOVASCULAR PRESSURE,” and was filed on Jul. 25, 2008. 
     In other examples, processor  90  may confirm the detection of the arrhythmia based on relative tissue perfusion values, blood oxygen saturation levels, blood pressure, heart sounds, cardiovascular pressure, respiratory rate, intrathoracic impedance, cardiac mechanical activity, body temperature, acoustic signals indicative of cardiac mechanical activity, and the like. A decrease in tissue perfusion or blood oxygen saturation levels may indicate the presence of an arrhythmia for which therapy delivery to heart  14  is desirable. For example, processor  90  may discriminate between hemodynamically tolerated arrhythmias and arrhythmias for which therapy delivery is desirable based on the blood oxygen saturation level associated with the detected arrhythmia. Processor  90  may also store the arrhythmia indication in memory  92  of ICD  16  or a memory of another device, such as programmer  24  ( FIG. 1 ) for later analysis by a clinician. 
     If the cardiac signals sensed by ICD  16  do not indicate a potential arrhythmia after neurostimulation is suspended or otherwise adjusted, processor  90  of ICD  16  may determine that the previous arrhythmia detection ( 144 ) was based on crosstalk from the delivery of neurostimulation by INS  26 . Accordingly, processor  90  may initiate the modification of the neurostimulation ( 146 ), as described with respect to  FIG. 9 . After the neurostimulation signal is modified, e.g., via modifying one or more stimulation parameter values, INS  26  may deliver neurostimulation to patient  12  via the modified neurostimulation signal and processor  90  may continue controlling sensing module  96  ( FIG. 6 ) of ICD  16  to sense cardiac signals ( 142 ). The technique shown in  FIG. 10  may then be repeated as necessary. 
       FIGS. 11A-11D  are flow diagrams illustrating a technique that may be implemented to modify the electrical stimulation signals generated and delivered by INS  26  in order to reduce the crosstalk between ICD  16  and INS  26 . Crosstalk may refer to the phenomenon in which an electrical stimulation signal generated and delivered by INS  26  interferes with the ability of ICD  16  to deliver cardiac therapy to heart  14  of patient  12  ( FIG. 1 ). For example, the technique shown in  FIGS. 11A-11D  may be used to minimize the possibility that ICD  16  detects the neurostimulation signals and mischaracterizes the signals as cardiac signals by modifying one or more characteristics of the neurostimulation signal (e.g., the signal frequency, signal amplitude, slew rate, duty cycle, electrode combination, waveform shape, signal envelope, pulse width, and the like). 
     Processor  90  of ICD  16  may receive cardiac signals sensed via any of electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and  76  ( FIG. 3 ) of leads  18 ,  20 ,  22  or housing  70  of ICD  16 . Processor  90  may detect a potential arrhythmia based on the sensed cardiac signals ( 160 ). For example, as described above with respect to  FIG. 9 , processor  90  may detect an arrhythmia event or an arrhythmia episode, which includes a predetermined number of arrhythmia events. In some examples, the arrhythmia event may comprise a tachyarrhythmia event, which includes a cardiac cycle that has an R-R interval that is less than a predetermined threshold value. The threshold value may be stored within memory  92  of ICD  16  ( FIG. 6 ). In other examples, the arrhythmia event may comprise a bradycardia event, which includes a cardiac cycle that has an R-R interval that is greater than a predetermined threshold value, which may also be stored in memory  92  of ICD  16 . The predetermined threshold number of arrhythmia events that processor  90  detects prior to determining that an arrhythmia episode is detected may be stored in memory  92  of ICD  16  ( FIG. 6 ) or a memory of another device. 
     Upon detecting the potential arrhythmia ( 160 ), processor  90  of ICD  16  may temporarily cause INS  26  to suspend or otherwise adjust the delivery of neurostimulation signals to patient  12 , e.g., by decreasing the intensity of stimulation ( 150 ), as described with respect to  FIG. 10 . If processor  90  detects the potential arrhythmia after INS  26  suspends or otherwise adjusts the delivery of stimulation signals to patient  12 , processor  90  may determine that the sensed cardiac arrhythmia is a true cardiac arrhythmia. Thus, processor  90  may control stimulation generator  94  ( FIG. 6 ) of ICD  16  to deliver cardiac therapy to patient  12  in order to try to terminate the arrhythmia ( 162 ). The cardiac therapy may be selected based on the type of arrhythmia that is detected. For example, if processor  90  detects a ventricular fibrillation, processor  90  may control stimulation generator  94  to generate and deliver defibrillation shocks electrical stimulation to heart  14  until the ventricular fibrillation of heart  14  is stopped. In other examples, processor  90  may confirm the cardiac arrhythmia based on physiological parameters of patient  12  other than sensed electrical cardiac signals, such as based on vascular pressure, prior to delivering the cardiac therapy to terminate the arrhythmia. 
     If processor  90  does not detect the potential arrhythmia after INS  26  stops actively delivering stimulation signals to patient  12 , processor  90  may determine that the arrhythmia may have been detected based on electrical stimulation signals delivered to tissue of patient  12  by INS  26 . That is, processor  90  may determine that the electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and  76  ( FIG. 3 ) that were used to sense the electrical cardiac signals sensed the neurostimulation signals and processor  90  mischaracterized the sensed neurostimulation signals as electrical cardiac signals. This may indicate that the amount of crosstalk between ICD  16  and INS  26  exceeds an acceptable amount. Accordingly, either processor  90  of ICD  16  or processor  110  of INS  110  may modify a stimulation parameter value used by INS  26  to generate the neurostimulation signals in order to help reduce the amount of crosstalk. 
     As previously indicated, modifying at least one stimulation parameter value that defines the electrical stimulation therapy provided by INS  26  may help change at least one characteristic of the electrical stimulation signal delivered by INS  26 , such that ICD  16  either ignores the signal (i.e., does not sense the signal) or senses the electrical stimulation signal delivered by INS  26  and recognizes that the sensed signal is not a true cardiac signal. 
     In the example shown in  FIGS. 11A-11D , processor  110  of INS  26  modifies one stimulation parameter value at a time. For example, processor  110  may modify the stimulation parameter value that least affects the efficacy of therapy delivery to patient  12  by INS  26  and/or most likely reduces the possibility that ICD  16  will mischaracterize the neurostimulation signal as a cardiac signal. In other examples, processor  110  may modify the stimulation parameter values in any order or may modify more than one stimulation parameter value at a time. While the description of  FIGS. 11A-11D  states that processor  110  of INS  26  modifies the stimulation parameter values of INS  26 , in other examples, processor  90  of ICD  16  or a processor of another device (e.g., programmer  24 ) may modify the stimulation parameter values and provide the modified values to processor  110  of INS  26  or processor  110  of INS  26  may otherwise act under the direction of processor  90  of ICD  16 . 
     In the example shown in  FIG. 11A , processor  110  may modify a stimulation parameter value by modifying a frequency of the neurostimulation signal and processor  110  may subsequently controls stimulation generator  114  of INS  26  ( FIG. 7 ) to generate and deliver electrical signals having the modified frequency ( 164 ). In some examples, processor  110  may store the modified frequency in memory  112  as a therapy program. 
     In some examples, processor  110  may modify the frequency based on a set of rules that are stored in memory  112  of INS  26  or another device, such as ICD  16  or programmer  24 . The rules may, for example, provide a range of frequency values that provide efficacious therapy to patient  12  and the increments with which processor  110  may modify the frequency ( 164 ). The range of frequency values that provide efficacious therapy to patient  12  may indicate the maximum frequency and the minimum frequency of stimulation signals that provide efficacious therapy to patient  12 . Thus, in some examples, the rules may prohibit processor  110  from modifying the frequency outside of the range of stored frequency values in order to prevent processor  110  from modifying the electrical stimulation therapy delivery provided by INS  26  such that the therapy does not provide therapeutic benefits to patient  12 . 
     In some examples, the rules may indicate the type of modification processor  110  may make to the stimulation parameter based on the type of arrhythmia that was detected by ICD  16 . For example, the rules may indicate that if a tachyarrhythmia is detected, the frequency of the neurostimulation signal generated by INS  26  should be decreased by a particular increment. Decreasing the frequency of the neurostimulation signal may decrease the possibility that ICD  16  will sense the stimulation signal and mischaracterize the neurostimulation signal as a cardiac signal. In some examples, decreasing the frequency of a neurostimulation signal may result in electrical noise that does not meet the requirements of a tachyarrhythmia (e.g., does not appear to have an R-R interval that is less than a predetermined threshold value). As another example, the rules may indicate that if a bradycardia is detected, the frequency of the neurostimulation signal generated by INS  26  should be increased by a particular increment. The neurostimulation signal having the increased frequency may no longer resemble a cardiac signal, or at least may no longer resemble a cardiac signal that indicates a bradycardia (e.g., does not appear to have an R-R interval that is greater than a predetermined threshold value). 
     After processor  110  of INS  26  modifies the frequency of the neurostimulation signal, processor  90  of ICD  16  may sense cardiac signals and determine whether an arrhythmia is still detected based on the sensed cardiac signals ( 144 ). If the arrhythmia is no longer detected, processor  90  of ICD  16  may determine that the prior-detected arrhythmia was detected based on neurostimulation signals delivered by INS  26  and that the modification to the frequency ( 164 ) successfully reduced the amount of crosstalk between INS  26  and ICD  16 . Thus, if the arrhythmia is no longer detected after modifying the frequency of the neurostimulation, processor  110  may not take any further action to modify the neurostimulation delivered by stimulation generator  114 . Stimulation generator  114  may continue generating and delivering neurostimulation to patient  12  at the modified frequency ( 166 ). 
     On the other hand, if processor  90  of ICD  16  detects a cardiac arrhythmia after the frequency of the neurostimulation signal was modified, processor  90  of ICD  16  may control INS  26  to temporarily suspends or adjusts the delivery of neurostimulation signals to patient  12  ( 150 ), as described with respect to  FIG. 10 . If processor  90  detects the potential arrhythmia after INS  26  suspends or otherwise adjusts the delivery of stimulation signals to patient  12 , processor  90  may determine that the detected arrhythmia was a true arrhythmia and control stimulation generator  94  ( FIG. 6 ) of ICD  16  to deliver cardiac therapy (e.g., at least one of pacing, cardioversion or defibrillation pulses) to patient  12  in order to try to terminate the arrhythmia ( 162 ). Again, in some examples, processor  90  may confirm the presence of the arrhythmia based on a physiological parameter of patient  12  other than the electrical cardiac signals prior to delivering the cardiac therapy. 
     If processor  90  does not detect the arrhythmia after INS  26  stops delivering neurostimulation to patient  12 , processor  90  may determine that the arrhythmia was detected based on noise, rather than true cardiac signals. The noise may be at least partially attributable to the crosstalk from INS  26 . Thus, if processor  90  does not detect the arrhythmia after INS  26  stops delivering neurostimulation to patient  12 , processor  90  may determine that the prior modification to the frequency of neurostimulation delivered by INS  26  was insufficient to reduce the noise and that ICD  16  is still sensing the neurostimulation signals and mischaracterizing the signals as cardiac signals. Accordingly, processor  110  of INS  26  may modify at least one more stimulation parameter value that defines the neurostimulation therapy delivered by INS  26 . 
     In the example shown in  FIG. 11B , processor  110  modifies an amplitude of the neurostimulation signal and delivers neurostimulation according to the modified amplitude ( 168 ). In other examples, processor  110  may modify one or more other types of stimulation parameter values, including the frequency of the neurostimulation signal. The amplitude that is modified may be a current amplitude or a voltage amplitude and may depend on, for example, the type of amplitude that is defined by the therapy program currently implemented by INS  26 . In some examples, processor  110  may modify the amplitude of the neurostimulation signal by modifying the amplitude or pulse width value of the therapy program used by stimulation generator  114  to generate the neurostimulation signals. In other examples, as described with respect to  FIGS. 12A and 12B , processor  110  may select a different therapy program from memory  112  in order to modify the amplitude. 
     As previously indicated, modifying an amplitude of the neurostimulation signal may result in a stimulation signal waveform that differs from a cardiac signal. For example, decreasing or increasing the amplitude of the neurostimulation signal may result in a signal that falls outside of the range of threshold amplitude values that INS  26  uses to detect a cardiac signal. Just as with the modification to the frequency ( 164 ), in some examples, processor  110  may modify the amplitude of the neurostimulation signal based on a set of rules that are stored in memory  112  of INS  26 . The rules may provide a range of amplitude values that provide efficacious therapy to patient  12  and the increments with which processor  110  may modify the neurostimulation signal amplitude ( 168 ). In some examples, the rules may control processor  110  to modify the amplitude values within the stored range of values and prevent processor  110  from selecting an amplitude value that falls outside of the stored range of efficacious amplitude values. In addition, as indicated above, in some examples, the rules may indicate the type of modification processor  110  may make to the amplitude based on the type of arrhythmia that was detected by ICD  16 . 
     After modifying the amplitude of the neurostimulation signal generated and delivered by INS  26 , processor  90  of ICD  16  may sense cardiac signals and determine whether an arrhythmia is detected ( 144 ). If the arrhythmia is no longer detected, processor  90  of ICD  16  may determine that the prior detected arrhythmia was detected based on neurostimulation signals delivered by INS  26  and sensed by ICD  16 , and that the modification to the neurostimulation signal amplitude ( 168 ) successfully changed a characteristic of the neurostimulation signal so that it no longer resembles a cardiac signal. Thus, if the arrhythmia is no longer detected after modifying the amplitude of the neurostimulation signal, processor  110  may not take any further action to modify the neurostimulation delivered by stimulation generator  114 . Stimulation generator  114  may continue generating and delivering neurostimulation to patient  12  at the modified frequency and the modified amplitude ( 170 ). 
     On the other hand, if processor  90  of ICD  16  detects a cardiac arrhythmia after the frequency and amplitude of the neurostimulation signal were modified, processor  90  of ICD  16  may cause INS  26  to temporarily stop delivering neurostimulation signals to patient  12  ( 150 ). If processor  90  detects the potential arrhythmia after INS  26  suspends or otherwise adjusts the delivery of stimulation signals to patient  12 , processor  90  may determine that the detected arrhythmia was a true arrhythmia and control stimulation generator  94  ( FIG. 6 ) of ICD  16  to deliver cardiac therapy to patient  12  in order to try to terminate the arrhythmia ( 162 ). 
     If processor  90  does not detect the arrhythmia after INS  26  suspends or otherwise adjusts the delivery of neurostimulation to patient  12 , processor  90  may determine that the arrhythmia was detected based on noise (e.g., from INS crosstalk), rather than true cardiac signals. Processor  90  may determine that the prior modifications to the frequency and amplitude of the neurostimulation signal generated and delivered by INS  26  were insufficient to reduce the crosstalk between ICD  16  and INS  26 . That is, processor  90  may determine that ICD  16  is still sensing the neurostimulation signals and mischaracterizing the signals as cardiac signals. Accordingly, processor  110  of INS  26  may modify another parameter of the neurostimulation signal, e.g., in response to a control signal transmitted to INS  26  from processor  90  of ICD  16 . In the example shown in  FIG. 11B , processor  110  may modify an electrode combination that is used to deliver the neurostimulation signal to patient  12  and deliver neurostimulation with the modified electrode combination ( 172 ). The electrode combination may be defined by a therapy program used by INS  26  to generate the neurostimulation signals. Processor  110  may modify the electrode combination by modifying the therapy program currently implemented by INS  26  or by selecting a second therapy program from memory  112 , whereby the second therapy program defines a different electrode combination. 
     An electrode combination defines the subset of electrodes  124  of lead  28  ( FIG. 7 ) coupled to INS  26  that are used to deliver stimulation therapy to patient  12 . The electrode combination may also refer to the polarities of the electrodes in the selected subset. Modifying the subset of electrodes  124  that are used to deliver stimulation therapy to patient  12  may change the amount of crosstalk between INS  26  and ICD  16  by changing the vector between the neurostimulation signal delivered by INS  26  and the electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and/or  76  ( FIG. 3 ) coupled to ICD  16  that are used to sense cardiac signals. For example, delivering the neurostimulation with a different subset of electrodes  124  may steer the electrical field generated by the delivery of neurostimulation to the patient&#39;s tissue in a different direction, which may change the intensity of the neurostimulation signal that is transmitted through the patient&#39;s body to the sense electrodes of ICD  16 . This may help reduce the possibility that ICD  16  senses the neurostimulation signal and mischaracterizes the signal as a cardiac signal. For example, delivering the neurostimulation with a different subset of electrodes  124  may change the amplitude or frequency of the neurostimulation signal that is sensed by ICD  16 , such that ICD  16  does not mischaracterize the neurostimulation signal as a cardiac signal. 
     Processor  110  may modify the electrode combination by, for example, modifying the quantity of electrodes that are selected to deliver neurostimulation to patient  12 , modifying the location of the selected electrodes, and/or modifying the spacing between the selected electrodes. In addition to or instead of the aforementioned modifications to the electrode combination, processor  110  may increase the size of a ground reference electrode area, such as by increasing the number of ground electrodes. In some examples, the ground electrode may comprise an anode electrode, while in other examples the ground electrode may comprise one or more cathode electrodes. By reducing the resistance of the one or more grounded electrodes, the noise sensed by ICD  16  from the neurostimulation may be reduced by reducing the common mode noise. 
     In some examples, lead  28  coupled to INS  26  may comprise segmented electrodes or partial ring electrodes that do not extend around the entire outer circumference of lead  28 . Segmented electrodes may be useful for directing neurostimulation in a specific direction to enhance therapy efficacy. In examples in which lead  28  comprises segmented or partial ring electrodes, processor  110  may modify the electrode combination by selecting segmented electrodes to deliver the neurostimulation in a different direction, such as a direction away from ICD  16  and its associated electrodes. Processor  110  may modify the direction of stimulation via the segmented electrodes in order to minimize the far field neurostimulation signal sensed by ICD  16 . 
     In some examples, processor  110  may modify the electrode combination used to deliver the neurostimulation signal based on a set of rules or a predetermined set of electrode combinations that are stored in memory  112  of INS  26 . The rules may indicate which electrodes may be activated or deactivated, and the order in which the activation and deactivation of particular electrodes may take place. For example, the delivery of stimulation via certain electrodes  124  ( FIG. 7 ) may substantially increase or decrease the efficacy of neurostimulation therapy. Thus, in some examples, the stored rules may indicate that some electrodes should not be deactivated, or at least should be deactivated after other electrodes are deactivated, and other electrodes are not preferred electrodes for delivering electrical stimulation to patient  12 . 
     In some examples, the rules may indicate the type of modifications processor  110  may make to the electrode combination, as well as the order in which the types of modifications may be made. For example, the rules may set forth a hierarchy of modifications, whereby the processor  110  may first modify the quantity of selected electrodes, followed by the selected electrodes, followed by the space between the selected electrodes. 
     After modifying the electrode combination used to deliver a neurostimulation signal by INS  26 , processor  90  of ICD  16  may sense cardiac signals and determine whether an arrhythmia is detected ( 144 ). If the arrhythmia is no longer detected, processor  90  of ICD  16  may determine that the prior detected arrhythmia was detected based on neurostimulation signals delivered by INS  26  and sensed by ICD  16 , and that the modification to the electrode combination ( 172 ) successfully reduced the crosstalk between INS  26  and ICD  16 . Thus, if the arrhythmia is no longer detected after modifying the electrode combination used to deliver the neurostimulation signal, processor  110  may not take any further action to modify the neurostimulation delivered by stimulation generator  114 . Stimulation generator  114  may continue generating and delivering neurostimulation signals having the modified frequency and amplitude with the modified electrode combination ( 174 ). 
     On the other hand, if processor  90  of ICD  16  detects a cardiac arrhythmia after the electrode combination used to deliver the neurostimulation signal was modified, processor  90  of ICD  16  may cause INS  26  to temporarily stop delivering neurostimulation signals to patient  12  ( 150 ). If processor  90  detects the potential arrhythmia after INS  26  suspends or otherwise adjusts the delivery of stimulation signals to patient  12 , processor  90  may determine that the detected arrhythmia was a true arrhythmia and control stimulation generator  94  ( FIG. 6 ) of ICD  16  to deliver cardiac therapy to patient  12  in order to try to terminate the arrhythmia ( 162 ), or may confirm the arrhythmia based on other physiological parameters of patient  12 . 
     If processor  90  does not detect the arrhythmia after INS  26  suspends or otherwise adjusts the delivery of neurostimulation via the modified electrode combination to patient  12 , processor  90  may determine that the arrhythmia was detected based on noise, rather than true cardiac signals. Processor  90  may determine that the prior modifications to the frequency and amplitude of the neurostimulation signal and the modification to the electrode combination used to deliver the neurostimulation signals were insufficient to reduce the crosstalk between ICD  16  and INS  26 . Accordingly, processor  110  of INS  26  may modify another parameter of the neurostimulation signal. In the example shown in  FIG. 11C , processor  110  may modify a duty cycle of the neurostimulation, and deliver neurostimulation with the modified duty cycle ( 176 ). 
     A duty cycle of neurostimulation may refer to the proportion of time during which a neurostimulation signal is actively delivered to patient  12 . For example, INS  26  may deliver electrical stimulation to patient  12  in a regular duty cycle, whereby the stimulation is delivered for a first duration of time (e.g., in a single pulse or signal or a burst of multiple pulses or signals) and off for a second duration of time, followed by the stimulation delivery for the first duration of time and so forth. The duty cycle may indicate the ratio between the first duration of time and the total cycle time (the first duration plus the second duration of time). Modifying the duty cycle of the neurostimulation may help reduce the duration of the neurostimulation, such that even if crosstalk between ICD  16  and INS  26  is present due to the delivery of neurostimulation by INS  26 , the neurostimulation signals may not achieve the required duration of a cardiac signal indicative of an arrhythmia. That is, as described above, processor  90  of ICD  16  may detect a potential arrhythmia by detecting a threshold number of arrhythmia events. If the duration of the neurostimulation is minimized by modifying the duty cycle of the neurostimulation, the neurostimulation signal may not resemble a cardiac signal comprising the threshold number of arrhythmia events. Thus, even if the neurostimulation signal resembles an potential arrhythmia event, processor  90  may not detect the threshold number of potential arrhythmia events based on the neurostimulation signals. 
     In some examples, processor  110  may modify the duty cycle of the neurostimulation signals based on a set of rules or by switching to another therapy program stored in memory  112  of INS  26 . The rules may indicate maximum and minimum duty cycle values for the neurostimulation therapy, where the maximum and minimum may define a range of duty cycle values that may be selected without adversely affecting the efficacy of neurostimulation therapy. In some examples in which stimulation generator  114  delivers electrical stimulation pulses to patient  12 , processor  110  may modify the pulse width of the pulses instead of or in addition to modifying the duty cycle of the neurostimulation. 
     After modifying the duty cycle of the neurostimulation signal delivered by INS  26 , processor  90  of ICD  16  may sense cardiac signals and determine whether an arrhythmia is detected ( 144 ). If the arrhythmia is no longer detected, processor  90  of ICD  16  may determine that the prior detected arrhythmia was detected based on neurostimulation signals delivered by INS  26  and sensed by ICD  16 , and that the modification to the duty cycle ( 176 ) successfully changed a characteristic of the neurostimulation signal so that it no longer resembles a cardiac signal. Thus, if the arrhythmia is no longer detected after modifying the electrode combination used to deliver the neurostimulation signal, processor  110  may not take any further action to modify the neurostimulation delivered by stimulation generator  114 . Stimulation generator  114  may continue generating and delivering electrical stimulation signals having the modified frequency, amplitude, and duty cycle, and with the modified electrode combination ( 178 ). 
     On the other hand, if processor  90  of ICD  16  detects a cardiac arrhythmia after the duty cycle of the neurostimulation signal was modified, processor  90  of ICD  16  may cause INS  26  to temporarily suspend delivering neurostimulation signals to patient  12  or otherwise reduce the intensity of stimulation ( 150 ). If processor  90  detects the potential arrhythmia after INS  26  suspends or otherwise adjusts the delivery of stimulation signals to patient  12 , processor  90  may determine that the detected arrhythmia was a true arrhythmia and control stimulation generator  94  ( FIG. 6 ) of ICD  16  to deliver cardiac therapy to patient  12  in order to try to terminate the arrhythmia or may confirm the arrhythmia based on other physiological parameters of patient  12  ( 162 ). 
     If processor  90  does not detect the arrhythmia after INS  26  suspends or otherwise adjusts the delivery of neurostimulation having the modified duty cycle, processor  90  may determine that the arrhythmia was detected based on noise, rather than true cardiac signals. Processor  90  may determine that the prior modification to the frequency, amplitude, and duty cycle of the neurostimulation signal and the modification to the electrode combination used to deliver the neurostimulation signals were insufficient to reduce the crosstalk between ICD  16  and INS  26 , such that ICD  16  is still sensing the neurostimulation signals and mischaracterizing the signals as cardiac signals. Accordingly, processor  110  of INS  26  may modify another therapy parameter of the neurostimulation therapy. In the example shown in  FIG. 11D , processor  110  may modify the timing between the delivery of neurostimulation signals relative to the cardiac cycle of heart  14  of patient  12  ( 180 ). Processor  110  may then control stimulation generator  114  to deliver neurostimulation signals to patient  12  at the modified times ( 180 ). 
     In some examples, processor  110  controls stimulation generator  114  to deliver neurostimulation signals to patient  12  during a blanking period of sensing module  96  of ICD  16 , and to withhold the delivery of neurostimulation signals outside of the blanking period. In other examples, processor  110  may control stimulation generator  114  to deliver neurostimulation signals to patient  12  during a blanking period of sensing module  96  and for a relatively short amount of time after the blanking period. The relatively short amount of time may include, for example, about 1 millisecond (ms) to about 100 ms, although other time ranges are contemplated. The blanking period may refer to a period of time during which sensing module  96  does not sense any cardiac signals. Therefore, sensing module  96  of ICD  16  may not inadvertently sense neurostimulation signals that are delivered during the blanking period. In some examples, the blanking period may be about 120 ms, although other blanking periods are contemplated. 
     In addition, in some examples, processor  110  may control stimulation generator  114  to deliver neurostimulation signals to patient  12  outside of the blanking period, but relatively early in a cardiac cycle. In some examples, sensing module  96  ( FIG. 6 ) of ICD  16  may include an automatically adjusting sense amplifier threshold. In some examples, INS  26  may deliver stimulation to patient  12  early in the automatic adjustment period of the sensing module  96  amplifier because the sense amplifier may be less sensitive to noise from delivery of neurostimulation by INS  26  early in the automatic adjustment period of the sensing module  96  amplifier, which may help decrease oversensing. 
     Processor  110  of INS  26  may time the delivery of neurostimulation signals during the blanking period of sensing module  96  using any suitable technique. In some examples, processor  90  of ICD  16  may notify INS  26  at the beginning of each blanking period, and, in some cases, the end of each blanking period. The notification may be in the form of a flag or another format that may be transmitted to INS  26  via a wired or wireless signal. In other examples, ICD  16  and INS  26  have substantially synchronized clocks and memory  112  ( FIG. 7 ) of INS  26  may store information that details the timing of the blanking period of sensing module  96 . In addition, in some examples, ICD  16  and INS  26  may periodically synchronize their respective internal clocks, e.g., by via the respective telemetry modules  98 ,  118 . For example, ICD  16  may instruct INS  26  to synchronize its clock to the clock of ICD  16 , or INS  26  may instruct ICD  16  to synchronize its clock to the clock of INS  26 . Synchronizing clocks may be useful for coordinating stimulation activity. In some examples, ICD  16  and INS  26  may each include a crystal controlled clock, with counters or other means to provide collaborative clocking or strobe or synchronizing of circuits. 
     After modifying the timing of neurostimulation such that it is delivered during a blanking period of sensing module  96  of ICD  16  ( 180 ), processor  90  of ICD  16  may sense cardiac signals and determine whether an arrhythmia is detected ( 144 ). If the arrhythmia is no longer detected, processor  90  of ICD  16  may determine that the prior detected arrhythmia was detected based on neurostimulation signals delivered by INS  26  and sensed by ICD  16 , and that the modification to the timing of the neurostimulation signal relative to the cardiac signal sufficiently reduced the crosstalk between INS  26  and ICD  16 . Thus, if the arrhythmia is no longer detected after modifying the timing of the delivery of the neurostimulation signals, processor  110  may not take any further action to modify the neurostimulation delivered by stimulation generator  114 . Stimulation generator  114  may continue generating and delivering neurostimulation to patient  12  via the modified timing ( 182 ). 
     On the other hand, if processor  90  of ICD  16  detects a cardiac arrhythmia after the timing of the neurostimulation delivery was modified, processor  90  of ICD  16  may control INS  26  to indefinitely suspend the delivery of electrical stimulation signals to patient  12  or deliver electrical stimulation signals according to the adjusted stimulation parameters ( 184 ). Either processor  90  of ICD  16  or processor  110  of INS  26  may generate an interference indication ( 186 ) and transmit the indication to programmer  24  ( FIG. 1 ) or store the interference indication in the respective memory  92 ,  112 . The interference indication may indicate that the crosstalk between INS  26  and ICD  16  was not reducible by modifying one or more stimulation parameter values of INS  26 . The clinician may later retrieve the stored interference indication and determine whether other measures may be taken in order to reduce the crosstalk between INS  26  and ICD  16 . For example, the clinician may determine whether repositioning lead  28  coupled to INS  26  within patient  12  may help reduce the crosstalk. 
     In some examples, other types of therapy parameter values may be modified in accordance with the technique described with reference to  FIGS. 11A-11D . For example, in other examples of the technique shown in  FIGS. 11A-11D , processor  110  may modify the waveform shape of the neurostimulation signal, the signal envelope (e.g., by adjusting the stimulation start and stop times), and the like. 
     For each of the adjustments to the therapy parameter values of INS  26  described above with reference to  FIGS. 11A-11D , the adjustments may be occur over several steps, rather than one step as described above. For example, the adjustments to the frequency of the neurostimulation signal may be made in several increments until a predetermined limit is reached. For example, processor  110  of INS  26  may modify the frequency in  5  Hz increments until the frequency is increased by a total of 50 Hz. Other increment and total limit values are contemplated. In some examples, a range of parameter values for a particular stimulation parameter may be implemented by processor  110  prior to modifying a different type of stimulation parameter value. 
     In addition, in some examples, two or more therapy parameter values of INS  26  may be adjusted in a single step, e.g., upon detecting a potential arrhythmia, rather than adjusting independent stimulation parameters in different steps as described above with reference to  FIGS. 11A-11D . For example, upon detecting a potential arrhythmia, processor  110  of INS  26  may modify both the frequency and amplitude of a neurostimulation signal. Other combinations of therapy parameter values may also be modified together. 
     In some examples, processor  110  of INS  26  may generate electrical stimulation signals according to a different therapy program (or program group) in order to modify one or more stimulation parameter values. For example, processor  110  may control stimulation generator  114  to generate electrical stimulation signals according to a first therapy program, and, upon the detection of an arrhythmia, processor  110  may control stimulation generator  114  to generate electrical stimulation signals according to a second therapy program that has at least one different stimulation parameter value than the first therapy program. The first and second therapy programs, as well as any number of other therapy programs may be stored in memory  112  of INS  26  or a memory of another device, such as ICD  16 . 
     As previously indicated, if ICD  16  detects an arrhythmia based on the electrical stimulation signals delivered by INS  26 , switching therapy programs with which INS  26  generates stimulation signals may change the characteristics of the neurostimulation signals, which may reduce the possibility that ICD  16  detects the arrhythmia based on the electrical signals from INS  26 . Thus, in some cases, if ICD  16  detects an arrhythmia after a therapy program of INS  26  is modified, ICD  16  may determine that the arrhythmia is a true arrhythmia or at least not detected based on electrical noise from the delivery of electrical stimulation signals by INS  26 . 
       FIG. 12A  is a flow diagram of an operating mode of therapy system  10  including ICD  16  and INS  26 . Processor  110  of INS  26  may control stimulation generator  114  to generate and deliver electrical stimulation according to a first operating mode to modulate a nerve of patient  12  or deliver electrical stimulation to a nonmyocardial tissue site of patient  12  that is not proximate a nerve ( 190 ). In the examples described herein, the first operating mode is defined by a first therapy program. As previously indicated, a therapy program defines values for the therapy parameters that define the electrical stimulation delivered by INS  26 . In the case of electrical stimulation, the therapy parameters may include an electrode combination, and an amplitude, which may be a current or voltage amplitude, and, if INS  26  delivers electrical pulses, a pulse width for stimulation signals. The therapy program may also indicate the timing of the stimulation signals relative to, e.g., cardiac signals. 
     ICD  16  may sense cardiac signals via at least one or more of electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and/or  76  ( 192 ). ICD  16  may determine whether the sensed cardiac signals, and, in some examples, one or more other physiological parameter values of patient  12  indicate an arrhythmia of heart  14  ( 194 ). If ICD  16  does not detect an arrhythmia ( 194 ), processor  110  of INS  26  may continue controlling stimulation generator  114  to generate and deliver neurostimulation to patient  12  according to the first operating mode. On the other hand, if ICD  16  detects an arrhythmia ( 194 ), processor  110  of INS  26  may control stimulation generator  114  of INS  26  to generate and deliver stimulation therapy according to a second operating mode that is different than the first operating mode ( 196 ). In the examples described herein, the second operating mode is defined by a second therapy program that is different than the first therapy program. The second therapy program may comprise at least one stimulation parameter value that differs from the first therapy program. In some examples, processor  90  of ICD  16  or another device (e.g., programmer  24 ) may instruct processor  110  of INS  26  to switch operating modes (e.g., switch therapy programs). In addition, in some examples, processor  90  of ICD  16  or another device may transmit the therapy parameter values of the second operating mode to INS  26 . 
     The therapy parameter values of the first therapy program may be selected to provide patient  12  with efficacious neurostimulation therapy. In some cases, the therapy parameter values of the first therapy program may be selected with little or no regard as to the impact of the crosstalk from the neurostimulation on the sensing of cardiac signals by ICD  16 . The second therapy program, on the other hand, may define therapy parameter values that minimize the possibility that ICD  16  senses the neurostimulation signals delivered by INS  26  and mischaracterizes the neurostimulation signals as cardiac signals. For example, the second therapy program may define a different frequency, current or voltage amplitude, pulse width or duty cycle than the first therapy program. 
     In some examples, the second therapy program defines a stimulation signal comprising a different waveform than the first therapy program. For example, processor  110  of INS  26  may select a second therapy program that defines a waveform that has a voltage or current amplitude that ramps up in amplitude and ramps down in amplitude over a longer period of time than a true cardiac signal (e.g., an EGM signal), such that ICD  16  does not mischaracterize the neurostimulation signal as a cardiac signal. The ramping up and down of a stimulation signal waveform may help reduce the amount of artifact imposed on the signal sensed by ICD  16  because the rise time of the neurostimulation signal may be less abrupt than a rise time of a true cardiac signal. In some examples, the waveforms defined by the second therapy program may comprise nonrectangular waveforms that gradually ramp up and gradually ramp down in amplitude over time. Example waveforms for stimulation signals defined by the second therapy program are shown and described with respect to  FIGS. 13A-13I . 
     Processor  110  of INS  26  may generate and deliver electrical stimulation signals according to the second therapy program for a limited period of time, which may be preset by a clinician or another individual, or may be based on a sensed physiological parameter of patient  12 . For example, processor  110  of INS  26  may generate and deliver electrical stimulation signals according to the second therapy program until ICD  16  no longer detects an arrhythmia or a predetermined amount of time following the detection of an arrhythmia, such as about thirty seconds to about ten minutes following the detection of an arrhythmia. In some examples, processor  110  of INS  26  may generate and deliver electrical stimulation signals according to the second therapy program until ICD  16  indicates that the detected arrhythmia has been terminated. ICD  16  may, for example, communicate with INS  26  via wireless communication techniques, as previously described. 
       FIG. 12B  is a flow diagram of another example technique that processor  110  may implement to control stimulation generator  114  of INS  26 . Just as in the technique shown in  FIG. 12A , processor  110  may control stimulation generator  114  to generate and deliver neurostimulation to a nonmyocardial tissue site of patient  12  according to a first operating mode ( 190 ). The first operating mode may be characterized by a first therapy program that defines a first set of stimulation parameter values with which stimulation generator  114  ( FIG. 7 ) of INS  26  generates electrical stimulation signals. ICD  16  may sense electrical cardiac signals of patient  12  via at least one or more of electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 , and/or  76  and determine whether the sensed electrical cardiac signals, and, in some examples, one or more other physiological parameter values of patient  12  indicate an arrhythmia of heart  14  ( 194 ). If ICD  16  does not detect an arrhythmia ( 194 ), processor  110  of INS  26  may continue controlling stimulation generator  114  to generate and deliver neurostimulation to patient  12  according to the first operating mode. 
     On the other hand, if ICD  16  detects a potential arrhythmia ( 194 ), processor  110  of INS  26  may control stimulation generator  114  of INS  26  to adjust the generation and delivery of electrical stimulation signals according to the first therapy mode ( 195 ). Processor  110  may adjust the delivery of electrical stimulation to patient  12  by suspending the delivery of stimulation or by decreasing the intensity of stimulation (e.g., modifying an amplitude, frequency, duty cycle, waveform, or another stimulation parameter). If, upon suspending or otherwise adjusting the generation and delivery of electrical stimulation signals according to the first therapy mode, processor  110  of INS  26  either detects a potential arrhythmia (e.g., based on sensed physiological signals or by receiving an indication that indicates a potential arrhythmia is detected) ( 194 ), processor  110  may determine that the potential arrhythmia was not detected based on the electrical stimulation signals from INS  26 . Accordingly, processor  110  of INS  26  or processor  90  of ICD  16  may generate an arrhythmia indication ( 154 ), as described with respect to  FIG. 10 . 
     If, upon suspending or otherwise adjusting the generation and delivery of electrical stimulation signals according to the first operating mode, processor  110  of INS  26  does not detect a potential arrhythmia or receive an indication that an arrhythmia is detected ( 194 ), processor  110  may determine that the arrhythmia may have been detected based on noise resulting from electrical signals delivered by INS  26 , rather than true cardiac signals. In order to mitigate the crosstalk between INS  26  and ICD  16  while still maintaining therapeutic benefits that may be provided by INS  26 , processor  110  may control stimulation generator  114  to generate and deliver electrical stimulation signals according to a second operating mode, e.g., a second therapy program ( 196 ). 
     In some examples, the second operating mode may define a therapy program in which no neurostimulation is delivered to patient  12 . Thus, when processor  110  controls stimulation generator  114  to generate and deliver electrical stimulation signals to patient  12  according to a second operating mode, INS  26  may suspend the delivery of stimulation to patient  12 . 
     Processor  110  of INS  26  may control stimulation generator  114  to deliver therapy to patient  12  according to the second operating mode for a predetermined period of time following the switch from the first operating mode to the second operating mode. After the period of time has expired, processor  110  may control stimulation generator  114  to switch therapy delivery from therapy according to the second operating mode to therapy according to the first operating mode. The period of time may be stored in memory  112  of INS  26  or a memory of another device. The period of time may be selected by a clinician, e.g., based on how much the clinician wishes to mitigate the possibility of inadvertent cardiac rhythm therapy by ICD  16 . In some examples, the period of time with which INS  26  delivers therapy to patient  12  according to the second operating mode is in a range of about 100 ms to about 24 hours or more. 
     Processor  110  of INS  26  may prohibit further delivery of therapy according to the first operating mode (e.g., first therapy program) based upon a number of times therapy delivery by INS  26  is switched from therapy according to the first operating mode to therapy according to the second operating mode. In some examples, processor  110  may prohibit stimulation generator  114  from delivering therapy according to the first operating mode if the therapy delivery is switched from the first to the second operating modes a threshold number of times within a predetermined period of time. The threshold number of therapy switches and predetermined period of time may be stored in memory  112  of INS  26  or a memory of another device (e.g., ICD  16  or programmer  24 ). 
     In the example shown in  FIG. 12B , processor  110  may track the number of times therapy delivery by INS  26  is switched from therapy according to the first operating mode to therapy according to the second operating mode with a counter. For example, upon switching operating modes (e.g., by switching therapy programs) of INS  26  in response to the detected arrhythmia, processor  110  of INS  26  may increment a counter ( 197 ) and determine whether the value of the counter is greater than or equal to a threshold value ( 198 ). The value of the counter may indicate the number of times that processor  110  switched operating modes in response to a detected arrhythmia event. In some examples, the counter may track the number of detected arrhythmias for a particular period of time, which may be programmed by a clinician and stored in memory  112 . After the period of time expires, processor  110  may reset the counter. 
     The threshold value may indicate the number of operating mode switches that are acceptable. The threshold value may be stored within memory  112  of INS  26  or a memory of another device, such as ICD  16  or programmer  24 . In some examples, the threshold value may be about two to about ten, such as about three, and a time period for tracking the number of operating mode switches may be about one hour to about one day, although other threshold values and time periods are contemplated. 
     In some examples, processor  110  may increment the counter by a number that is selected based on the type of arrhythmia that is detected. For example, if a ventricular tachyarrhythmia is detected ( 194 ), processor  110  may increment the counter by a greater number (e.g., two counts) than if a nonsustained tachyarrhythmia is detected. A nonsustained tachyarrhythmia may comprise fewer arrhythmia events (e.g., R-R intervals less than a threshold value) than the ventricular tachyarrhythmia. In addition, in some examples, ICD  16  may not deliver cardiac rhythm therapy to heart  14  if a nonsustained tachyarrhythmia is detected, but may deliver therapy if a ventricular tachyarrhythmia is detected. 
     If the number of times that processor  110  switched operating modes, i.e., the count, is not greater than or equal to the threshold value, processor  110  may continue delivering therapy according to first and second operating modes of INS  26 , as described with respect to  FIG. 12A . However, if the number of times that processor  110  switched operating modes is equal to or exceeds the threshold value, processor  110  may determine that the delivery of electrical stimulation according to the first operating mode results in excessive interference with the proper detection of cardiac signals by ICD  16 . Thus, if the number of times that processor  110  switched operating modes is greater than or equal to the threshold value, processor  110  may prohibit any further delivery of electrical stimulation signals generated according to the first therapy program ( 199 ). That is, processor  110  may indefinitely switch to the second operating mode of INS  26 . For example, processor  110  may control stimulation generator  114  to generate and deliver electrical stimulation therapy to patient  12  according to a second therapy program indefinitely, rather than continuing to switch between first and second therapy programs. 
     Processor  110  may prohibit the generation and delivery of electrical stimulation according to the first operating mode until user intervention is received, e.g., to assess the extent of crosstalk. The user intervention may comprise, for example, input from patient  12  or the clinician resetting the counter, such that INS  26  may deliver stimulation signals that are generated in accordance with the first therapy program. The input may be received via user interface  134  ( FIG. 8 ) of programmer  24  or a user interface of another computing device, which may transmit the user input to processor  110  via the respective telemetry modules  136  ( FIG. 8 ),  118  ( FIG. 7 ). In addition, in some examples, the user input may be received from a clinician at a remote location, e.g., via the system including a network that is described with respect to  FIG. 32 . 
     In some cases, processor  110  may generate an interference indication that is transmitted to patient  12  or a clinician, e.g., via programmer  24 . For example, processor  110  may transmit the interference indication to programmer  24  via telemetry module  118  ( FIG. 7 ) and programmer  24  may receive the indication via telemetry module  136  ( FIG. 8 ) and generate an interference indication to notify patient  12  or another person that clinician intervention may be necessary to mitigate crosstalk between ICD  16  and INS  26 . Processor  110  may generate the interference indication in response to the mode switch counter value exceeding the threshold. 
       FIGS. 13A-13I  are conceptual illustrations of example non-rectangular waveforms that may be defined by a second operating mode implemented by INS  26  to generate neurostimulation signals after the detection of an arrhythmia.  FIG. 13A  illustrates a ramped square waveform  200 , which includes a plurality of waves  202 . Each wave  202  includes a leading edge  204  that gradually increases in amplitude over time and a trailing edge  205  that follows the leading ledge  204  and gradually decreases in time. In some examples, leading edge  204  exhibits a substantially continuous increase in amplitude, such that leading edge  204  has a different amplitude at subsequent points in time. Similarly, trailing edge  205  may exhibit a substantially continuous decrease in amplitude, such that trailing edge  205  has a different amplitude at subsequent points in time. Although leading edge  204  and trailing edge  205  are illustrated as having substantially equal, but opposite slopes, in other examples, leading edge  204  and trailing edge  205  may have slopes of different magnitude. 
     In some examples, stimulation generator  114  of INS  26  may generate the ramped square wave by generating a square wave stimulation signal and modulating the amplitude by a relatively slow sine wave. For example, stimulation generator  114  may generate square wave signals having a frequency of about 80 Hz and a pulse duration of about 300 μs duration pulses, and modulate the amplitude of the square wave from about 0% to about 100% by an approximately 3 Hz sine wave. The resulting square wave signal may have a frequency of about 80 Hz and a signal envelope of about 3 Hz. 
       FIG. 13B  illustrates a stair step square waveform  206 , which includes a plurality of waves  207 . Each wave  207  includes a leading edge  208  and a trailing edge  209 . The lead edge  208  includes stepwise increases in amplitude over time, whereas the trailing edge  209  includes stepwise decreases in amplitude over time. Although  FIG. 13B  illustrates waves  207  in which leading edge  208  and trailing edge  209  increase and decrease, respectively, in substantially equal increments of amplitude, in other examples each step of leading edge  208  and trailing edge  209  may increase and decrease, respectively, in amplitude by different magnitudes. Moreover, the rising edge of each step in leading edge  208  may have a different absolute magnitude than other steps in leading edge  208 , such that some steps of leading edge  208  are larger than others. Similarly, each step of trailing edge  209  may have a different absolute magnitude than other steps in trailing edge  209 . 
       FIG. 13C  illustrates rounded square waveform  210 , which includes a plurality of waves  211 . Each wave  211  defines a leading edge  204  that gradually increases in amplitude over time and a trailing edge  205  that gradually decreases in time, as described with respect to ramped square waveform  200  in  FIG. 13A . In addition, waves  211  of rounded square waveform  210  includes rounded portion  212  between leading edge  204  and trailing edge  205 . Rounded portion  212  may help further distinguish neurostimulation waveform  210  from a sinus rhythm of heart  14  ( FIG. 1 ) because of the gradual increase and decrease in amplitude. In contrast, the sinus rhythm of heart  14  may exhibit a sharper increase and decrease in amplitude. 
     In some examples, stimulation generator  114  may generate rounded square waveform  210  shown in  FIG. 13C  by passing a square wave signal or a substantially square wave signal through a resistor-capacitor (RC) low pass filter with a cutoff frequency in a range of about 20 Hz to about 100 Hz, such as about 60 Hz. The RC low pass filter may help eliminate the relatively rapid rise time of the square wave, which may help reduce the stimulation signal artifact imposed on ICD  16  because the resulting rounded square wave may no longer resemble a true electrical cardiac signal, which may comprise a relatively rapid rise time. 
       FIG. 13D  illustrates trapezoidal waveform  214 , which includes a plurality of waves  215  comprising a substantially trapezoidal shape. Each trapezoidal wave  215  comprises leading edge  216  and trailing edge  217 , which follows leading edge  216  in time. Leading edge  216  may comprise a greater slope compared to lead edge  204  of ramped square wave  202  ( FIG. 13A ). Similarly, trailing edge  217  may comprise a smaller slope (or a greater absolute slope value) compared to trailing edge  205  of ramped square wave  202  ( FIG. 13A ). Although leading edge  216  and trailing edge  217  are illustrated as having substantially equal, but opposite slopes, such that the waves  215  define isosceles trapezoids, in other examples, leading edge  216  and trailing edge  217  may have slopes of different magnitude. 
       FIG. 13E  illustrates triangular waveform  218 , which includes a plurality of waves  219  defining a substantially triangular shape. Waves  219  each comprise leading edge  220  and trailing edge  221 , which follows leading edge  220  in time. Leading edge  220  and trailing edge  221  of each wave  219  may have slopes of substantially equal magnitude, or may have different slopes. In some examples, upon the detection of an arrhythmia, processor  110  of INS  26  may control stimulation generator  114  to generate and deliver electrical stimulation signals comprising a stair-step triangular waveform. Just as with the stair step square wave shown in  FIG. 13B , a leading edge of the stair step triangular wave may define stepwise increases in amplitude over time, and a trailing edge of the waveform may define stepwise decreases in amplitude over time. 
     In some examples, stimulation generator  114  of INS  26  may generate and deliver biphasic stimulation signals, as shown in  FIG. 13F .  FIG. 13F  illustrates biphasic triangular waveform  222 , which includes triangular waves  219  having a positive amplitude and triangular waves  223  having a negative amplitude. Biphasic triangular waveform  222  may include alternating positive amplitude triangular waves  219  and negative amplitude triangular waves  223 . Biphasic waveforms may also help distinguish neurostimulation signals from cardiac signals. Other types of biphasic waveforms are also contemplated, such as biphasic square waves. 
     Stimulation generator  114  of INS  26  may also generate and deliver neurostimulation to patient  12  via a sine waveform.  FIG. 13G  illustrates sine waveform  224 , which includes a periodic wave  225  defined by a sine function. In some examples, as shown in  FIG. 13H , stimulation generator  114  may also generate and deliver neurostimulation signals having a half sine waveform  226 , such as the positive half of a sine wave or a rectified sine wave. The half sine wave may have a duration of approximately 200 microseconds (μs), although other signal durations are contemplated. In other examples, stimulation generator  114  of INS  26  may generate and deliver neurostimulation signals comprising a stepwise half sine waveform  228 , as shown in  FIG. 13I   
     As previously described with respect to biphasic triangular waveform  222  in  FIG. 13F , in some examples, the second therapy program implemented by processor  110  of INS  26  after the detection of an arrhythmia may define a biphasic signal. That is, processor  110  may control stimulation generator  114  to deliver stimulation signals to selected electrodes  124  ( FIG. 7 ) of lead  28  such that the selected electrodes reverse polarity with each subsequent pulse or, in examples in which continuous wave signals are delivered, each subsequent half wave.  FIGS. 14A and 14B  provide a conceptual illustration of a configuration of electrode polarities that may be employed in order to for INS  26  to deliver a biphasic neurostimulation signal to patient  12  in the second operating mode.  FIGS. 13F and 13G  illustrate examples of biphasic waveforms that may be generated and delivered to patient  12 . 
       FIGS. 14A and 14B  illustrate lead  232  comprising a plurality of electrodes  234 A- 234 H, which may comprise ring electrodes, partial ring electrodes or segmented electrodes that extend around less than the full outer perimeter of lead  232 . In the example shown in  FIGS. 14A and 14B , lead  232  may comprise a cylindrical lead body with a circular cross-section (when the cross-section is take in a direction substantially orthogonal to a longitudinal axis of lead  232 ). Lead  232  may be coupled to stimulation generator  114  of INS  26  instead of or in addition to lead  28  and/or lead  29  ( FIG. 2 ). Although eight electrodes  234 A- 234 H are shown in  FIGS. 14A and 14B , in other examples, lead  232  may comprise any suitable number of electrodes, which may be greater than or fewer than eight. 
       FIG. 14A  illustrates a first example electrode combination that may be defined by a second therapy program that processor  110  of INS  26  may implement upon the detection of an arrhythmia. In the electrode configuration shown in  FIG. 14A , electrodes  234 A- 234 D are selected to be anodes and electrodes  234 E- 234 H are selected to be cathodes. Stimulation generator  114  may generate a first stimulation pulse or another type of stimulation signal and transmit the stimulation pulse or signal to electrodes  234 A- 234 H via the conductors within lead  232 . An electrical field may be generated through the patient&#39;s tissue as the electrical signal flows between the anode electrodes  234 A- 234 D and the cathode electrodes  234 E- 234 H. In other examples, a subset of electrodes  234 A- 234 H may be selected as part of the electrode combination. 
       FIG. 14B  illustrates a second electrode combination defined by the second therapy program in which electrodes  234 A- 234 D are selected to be cathodes and electrodes  234 E- 234 H are selected to be anodes. Thus, compared to the first electrode combination shown in  FIG. 14A , electrodes  234 A- 234 H have reversed polarity. Stimulation generator  114  may utilize the electrode combination shown in  FIG. 14B  to deliver a subsequent stimulation pulse or wave, i.e., subsequent to the pulse or wave delivered with the electrode combination shown in  FIG. 14A . An electrical field may be generated through the patient&#39;s tissue as the electrical signal flows between the anode electrodes  234 E- 234 H and the cathode electrodes  234 A- 234 D. 
     In accordance with an example of the second operating mode of INS  26 , stimulation generator  114 , e.g., with the aid of switching module  116  ( FIG. 7 ) may continue delivering alternating pulses with the electrode combinations shown in  FIGS. 14A and 14B . In some examples, stimulation generator  114  may deliver neurostimulation to patient  12  with the same electrode combination (e.g., the same polarity configuration) for two or more pulses or stimulation waves in a row and subsequently deliver neurostimulation to patient  12  to an electrode combination having reversed polarities. For example, in other examples, stimulation generator  114  may deliver two or more pulses with the electrode combination shown in  FIG. 14A  followed by two or more pulses with the electrode combination shown in  FIG. 14B . In addition, in other examples, INS  26  may deliver a biphasic neurostimulation signal to patient  12  using the electrodes of two or more leads, rather than one lead as shown in  FIGS. 14A and 14B . 
     Delivering neurostimulation signals to patient  12  via a biphasic signal (e.g., via electrode combinations with alternating polarity) may help reduce the neurostimulation artifact impact on ICD  16  or another physiological parameter monitoring device. The stimulation output net energy artifact effect sensed by ICD  16  may be approximately zero due to the rapid encounter of alternate polarity artifact that may cancel out the neurostimulation signal. In addition, delivering neurostimulation signals to patient  12  via a biphasic signal may help limit the bandwidth of the transmitted neurostimulation signal, and limiting the bandwidth may help increase the possibility that ICD  16  may filter out the neurostimulation signal, e.g., via a bandpass filter. Further, in some examples, sensing module  98  of ICD  16  may be configured to disregard or attenuate the alternating polarity neurostimulation signals. Thus, if INS  26  delivers biphasic neurostimulation signals, ICD  16  may not sense the neurostimulation signals and if ICD  16  senses the neurostimulation signals, ICD  16  may not mischaracterize the neurostimulation signals as cardiac signals. 
     In either or both the first and second operating modes of INS  26 , INS  26  may deliver electrical stimulation signals to patient  12  with an electrode combination that reduces the extent of the energy and/or electrical field that leaves the target tissue site  40 , thereby reducing the intensity of neurostimulation signal that traverses through the patient&#39;s body and is sensed by ICD  16 . The anodes and cathodes of the electrode combination may be selected such that the stimulation field generated by the delivery of neurostimulation via the anodes and cathodes (i.e., the selected electrodes) may be relatively focused within target tissue site  40  ( FIG. 1 ). 
       FIGS. 15A-15F  illustrate different examples of electrode combinations that may be used to deliver neurostimulation therapy to patient  12 . In the electrode combinations shown in  FIGS. 15A-15F , the anodes and cathodes of the electrode combination are positioned relative to each other to help reduce the extent of the size of the electrical field (or stimulation field) that is generated as a result of the delivery of neurostimulation signals by INS  26 . In some examples, the electrode combinations shown in  FIGS. 15A-15F  may be used to deliver a plurality of stimulation pulses with an interval of time between each pulse, or a plurality of bursts of electrical stimulation that are separated by an interval of time, where each burst includes a plurality of stimulation pulses. 
     In some examples, during a programming session in which a clinician selects the one or more electrode combinations for a second operating mode of INS  26 , the clinician may utilize a user interface that graphically represents the stimulation field generated by stimulation delivery with a particular subset of electrodes of the one or more leads coupled to INS  26 . An example of a user interface that may be used to select an electrode combination for the delivery of neurostimulation is described in commonly-assigned U.S. patent application Ser. No. 11/999,722 to Goetz et al., entitled, “USER INTERFACE WITH TOOLBAR FOR PROGRAMMING ELECTRICAL STIMULATION THERAPY,” which was filed on Dec. 6, 2007 and is incorporated herein by reference in its entirety. 
     As described in U.S. patent application Ser. No. 11/999,722 to Goetz et al., a user interface may display a representation of implanted electrical leads in conjunction with at least one menu with icons that the user can use to adjust the stimulation field of the stimulation therapy with one or more field shape groups. For example, one menu may be a field shape selection menu that provides field shapes to indicate the resulting stimulation field according to initial stimulation parameters. Another menu may be a manipulation tool menu that allows a user to perform certain actions on the field shapes to adjust the stimulation therapy. The user interface may be useful for selecting an electrode combination and other stimulation parameter values that focus the stimulation field within target tissue site  40  ( FIG. 1 ). Focusing the neurostimulation within the desired target tissue site  40  may help minimize the extent of the stimulation field that falls outside of target tissue site  40 , particularly in a direction towards heart  14  ( FIG. 1 ), and decrease the extent to which the stimulation field may be sensed by ICD  16 . 
       FIG. 15A  illustrates an example of a guarded cathode electrode combination  236  that may be selected during the second operating mode of INS  26  in order to help focus the neurostimulation delivered to patient  12 . In a guarded cathode arrangement, two or more anodes are positioned around a cathode of the electrode combination. In  FIG. 15A , electrode  234 D of lead  232  is selected as a cathode of the electrode combination and electrodes  234 C and  234 E are selected as anodes. In the example shown in  FIG. 15A , the anode electrodes  234 C,  234 E and cathode electrode  234 D are substantially linearly aligned along a longitudinal axis of lead  232 . The anode electrodes  234 C and  234 E surrounding the cathode electrode  234 D may be useful for focusing a stimulation field generated by the delivery of electrical stimulation via electrode combination  236 . In particular positioning anode electrodes  234 C and  234 E on opposite sides of cathode electrode  235 D may help limit the size of the stimulation field resulting from the delivery of stimulation via the electrode combination  236 . 
     In some examples, INS  26  may be coupled to two or more leads, directly or via one or more lead extensions, such as a bifurcated lead extension.  FIG. 15B  illustrates a configuration in which INS  26  is coupled to lead  232  including eight electrodes  234 A- 234 H, lead  240  including four electrodes  242 A- 242 D, and lead  244  including four electrodes  246 A- 246 D. Electrodes  232 A- 232 H,  242 A- 242 D,  246 A- 246 D of leads  232 ,  240 ,  244  may define a three-lead full guard electrode configuration. The three-lead full guard electrode combination utilizes electrodes on all three leads implanted within patient  12 . In the example shown in  FIG. 15B , electrode combination  248  includes cathode electrode  234 D on a middle lead  232 , where the cathode electrode  234 D is surrounded by two anode electrodes  234 C,  234 E on the same lead  232  and anode electrodes  242 B,  246 B on leads  240 ,  244  on either side of cathode electrode  234 D. Anode electrodes  234 C,  234 E,  242 B,  246 B of electrode combination  248  may define a stimulation field that activates only the tissue around cathode electrode  235 D while inhibiting the tissue on all sides of cathode electrode  235 D. 
       FIG. 15C  illustrates another example electrode combination  250  that processor  110  of INS  26  may select during the second operating mode of INS  26  in order to help focus the neurostimulation delivered to patient  12 . Electrode combination  250  is defined by electrodes  242 A- 242 D and  246 A- 246 D of two leads  240 ,  244 , respectively. In the example shown in  FIG. 15C , electrodes  242 A,  242 C,  246 B,  246 D are anode electrodes and electrodes  242 B,  242 D,  246 A,  246 D are cathode electrodes. By substantially surrounding cathode electrodes  242 B,  242 D,  246 A,  246 C with anode electrodes  242 A,  242 C,  246 B,  246 D, electrode combination  250  may shape a stimulation field that focuses stimulation to the area proximate leads  240 ,  244 . 
       FIG. 15D  illustrates another example electrode combination  256  that is defined by selected electrodes  242 A- 242 D,  232 A- 232 H,  246 A- 246 D of three leads  240 ,  232 ,  244 , respectively. In the example shown in  FIG. 15D , electrodes  242 A- 242 D,  246 A- 246 D,  234 A,  234 C,  234 E,  234 G are anode electrodes and electrodes  234 B,  234 D,  234 F,  234 H are cathode electrodes. Anode electrodes  242 A- 242 D,  246 A- 246 D on leads  240 ,  244  adjacent to lead  232 , which includes cathode electrodes  234 B,  234 D,  234 F,  234 H, are positioned to help limit the size of the stimulation field generated by the delivery of electrical stimulation via electrode combination  256 . By placing the cathode electrodes  234 B,  234 D,  234 F,  234 H along a center lead  232 , the stimulation field may be focused to the region of tissue proximate leads  232 ,  240 ,  244 . 
     Anode electrodes  242 A- 242 D,  246 A- 246 D,  234 B,  234 D,  234 F,  234 H may act as guard band electrodes that help focus a stimulation field to the region of tissue proximate leads  232 ,  240 ,  244 . In some examples, anode electrodes  242 A- 242 D may define a substantially continuous and contiguous anode electrode, rather than a plurality of discrete electrodes, as shown in  FIG. 15D . Similarly, in some examples, anode electrodes  246 A- 246 D may define a substantially continuous and contiguous anode electrode, rather than a plurality of discrete electrodes, as shown in  FIG. 15D . Anode electrodes  242 A- 242 D,  246 A- 246 D on opposing sides of cathode electrodes  234 B,  234 D,  234 F,  234 H may serve as a guard band that reduce the projection of a stimulation field beyond leads  240 ,  246 , which may help reduce the amount of the neurostimulation signal that reaches the sense electrodes coupled to ICD  16 . This may help reduce the stimulation artifact on the sensing of cardiac signals by ICD  16 . 
       FIG. 15E  illustrates another example electrode combination  260  that is defined by electrodes  242 A- 242 D,  232 A- 232 H,  246 A- 246 D on three leads  240 ,  232 ,  244 , respectively. In particular, cathode electrodes  234 A- 234 H are located on the middle (or central) lead  232 , and anode electrodes  242 A- 242 D,  246 - 246 D are positioned on leads  240 ,  244  on opposing sides of center lead  232 , which, in some examples, may be spatially centered between leads  240 ,  244 . Again, in some examples, anode electrodes  242 A- 242 D may define a substantially continuous and contiguous anode electrode and anode electrodes  246 A- 246 D may define a substantially continuous and contiguous anode electrode. 
     Anode electrodes  242 A- 242 D,  246 A- 246 D on opposing sides of cathode electrodes  234 A- 234 H may serve as a guard band that reduce the projection of a stimulation field beyond leads  240 ,  246 , which may help reduce the amount of the neurostimulation signal that reaches the sense electrodes coupled to ICD  16 . This may help reduce the stimulation artifact on the sensing of cardiac signals by ICD  16 . 
       FIG. 15F  illustrates another example electrode combination  262  that is defined by electrodes positioned on four leads  240 ,  244 ,  264 ,  266  that are coupled to INS  26 , either directly or indirectly with a lead extension (e.g., a bifurcated lead extension). Electrodes  242 A- 242 D of lead  240  may be anode electrodes and electrodes  246 A- 246 D of lead  244  may be cathode electrodes. Electrodes  268 A- 268 D of lead  264  and electrodes  270 A- 270 D of lead  266  may be neutral, or inactive, electrodes. For example, electrodes  268 A- 268 D may be electrically connected, e.g., shorted, to electrodes  270 A- 270 D. Electrodes  268 A- 268 D,  270 A- 270 D may limit the size (e.g., breadth) of the stimulation field generated by therapy delivery according to electrodes  242 A- 242 D,  246 A- 246 D, e.g., by absorbing energy from the stimulation field. Minimizing the size of the stimulation field may help limit the extent to which ICD  16  senses the stimulation field, and, therefore, may help minimize crosstalk between INS  26  and ICD  16 . 
     In addition to or instead of modifying one or more operating parameters of INS  26 , one or more operating parameters (e.g., sensing parameters) of ICD  16  may be modified in order to help prevent the inappropriate delivery of stimulation by ICD  16  based on a neurostimulation signal artifact present in a signal sensed by that ICD  16 . Modifying the sensing parameters of ICD  16  may help minimize the possibility that ICD  16  mischaracterizes a neurostimulation signal as an electrophysiological cardiac signal.  FIG. 16  is a flow diagram illustrating an example technique that ICD  16  may implement in order to detect an arrhythmia while INS  26  is delivering electrical stimulation to a tissue site  40  ( FIG. 1 ) within patient  12 . 
     Processor  90  of ICD  16  may determine whether INS  26  is delivering stimulation to the nonmyocardial tissue site  40  ( 271 ). In some examples, INS  26  may transmit a signal to ICD  16  to notify ICD  16  that INS  26  is actively delivering electrical stimulation to patient  12 , i.e., the delivery of stimulation by INS  26  is not in a suspended state. For example, INS  26  may transmit a signal with predetermined characteristics to ICD  16  via the respective telemetry modules  118 ,  98  prior to or substantially at the same time that INS  26  delivers a stimulation signal to patient  12  or at the beginning of a stimulation pulse train including more than one stimulation pulse. 
     As another example, ICD  16  may determine when INS  26  is delivering stimulation based on a known stimulation schedule. As previously indicated, in some examples, ICD  16  and INS  26  have substantially synchronized clocks. Memory  92  ( FIG. 6 ) of ICD  16  may store information that indicates when INS  26  is expected to be delivering stimulation to patient  12 . For example, ICD  16  may store a stimulation schedule for INS  26 , where the stimulation schedule indicates the times of day at which INS  26  is programmed to actively deliver stimulation to patient  12 . 
     If INS  26  is delivering stimulation to patient  12 , processor  90  of ICD  16  may implement a first sense mode in order to monitor cardiac activity of patient  12  ( 272 ). The first sense mode may define a first sensing threshold that is used by processor  90  (or sensing module  96 , in some examples) to detect a cardiac signal. ICD  16  may filter sensed signals with the aid of the sensing threshold voltage in order to discriminate cardiac signals from noise, which may be attributable to many external sources. Sensing module  96  may sense electrical signals via two or more of the electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  68 ,  72 ,  74 ,  76  connected to sensing module  96 . Processor  90  may only identify sensed signals that have a voltage amplitude greater than the sensing threshold value as electrical cardiac activity. For example, sensing module  96  may only transmit EGM signals above the sensing threshold value to processor  90  for timing analysis. As previously indicated, the timing analysis may include an analysis of the sensed EGM signal for R-R intervals, P-P intervals, and so forth. 
     If INS  26  is not delivering stimulation to patient  12 , e.g., because the delivery of stimulation by INS  26  is currently suspended, processor  90  may implement a second sense mode in order to sense cardiac signals ( 273 ). The second sense mode may define a second sensing threshold that is used by processor  90  (or sensing module  96 , in some examples) to detect a cardiac signal. In some examples, the second sensing threshold may be lower than the first sensing threshold defined by the first sense mode. In this way, the second sense mode may be more sensitive to electrical cardiac signals than the first sense mode. 
     In some examples, the first and second sense modes may also define different amplifier gains used by the sensing amplifiers of sensing module  96  to sense electrical cardiac signals. The first sense mode may have a lower amplifier gain than the second sense mode, which may result in less sensitivity to cardiac signals. 
     While the first sense mode of ICD  16  may be less sensitive to electrical cardiac signals of patient  12 , ICD  16  may monitor other physiological parameters of patient in order to detect an arrhythmia, thereby at least partially compensating for the decreased sensitivity to electrical cardiac signals. That is, in the first sense mode, in addition to sensing electrophysiological cardiac signals (e.g., EGM or ECG signals) of patient  12 , processor  90  may detect an arrhythmia based on other non-electrophysiological parameters that are indicative of cardiac activity of patient in order to detect an arrhythmia. In contrast, in the second sense mode, sensing module  96  may not detect an arrhythmia based on non-electrophysiological parameters of patient  12  or may detect an arrhythmia based on fewer non-electrophysiological parameters of patient  12  compared to the second sense mode. 
     Examples of non-electrophysiological parameters of patient  12  that may be indicative of an arrhythmia include, but are not limited to, cardiovascular pressure, tissue perfusion, blood oxygen saturation levels, heart sound signals, respiratory rate, thoracic impedance, cardiac mechanical activity (e.g., muscle movement monitored via an accelerometer), body temperature (e.g., metabolic rate may change with decreased cardiac function, which may affect body temperature), acoustic signals indicative of cardiac mechanical activity or other blood flow information. Sensing a greater number of non-electrophysiological parameters of patient  12  in the first sense mode may help prevent underdetecting an arrhythmia of patient  12  despite the less sensitive sensing threshold utilized to sense cardiac signals. 
     Cardiovascular pressure may include intracardiac pressure (i.e., pressure within a chamber of heart  14 ) or extravascular pressure sensed outside of the patient&#39;s vasculature. One or more characteristics of sensed cardiovascular pressure in either the time domain or frequency domain may indicate whether a detected arrhythmia is a true arrhythmia. Cardiovascular pressure may vary based on the mechanical contraction and relaxation of heart  14 . Thus, changes in cardiovascular pressure may indicate whether heart  14  is mechanically contracting and relaxing in a normal manner and, therefore, may indicate the presence of an arrhythmia. For example, an arrhythmia may be detected if the time domain cardiovascular pressure data indicates that the pressure within right ventricle  32  ( FIG. 3 ) of heart  14  has decreased by at least a particular amount or decreased below a threshold amount. As another examples, processor  90  of ICD  16  may detect an arrhythmia using the first sense mode of ICD  16  if the pressure within right ventricle  32  ( FIG. 3 ) or another chamber is less than its expected physiologic range. 
     Intracardiac pressure may be monitored with the aid of a pressure sensor coupled to at least one of leads  18 ,  20 ,  22 . Extravascular pressure may be monitored with the aid of a pressure sensor located outside of heart  14 . The pressure sensor may be mechanically coupled to or physically separate from ICD  16  and INS  26 . If physically separate from ICD  16  and INS  26 , the pressure sensor may transmit a signal indicative of pressure to ICD  16  and INS  26  via a wired or wireless connection. The sensed pressure may be, for example, a systolic pressure, diastolic pressure, a pulse pressure, a maximum and minimum derivative of sensed pressure(s), or any combination thereof. 
     Tissue perfusion and blood oxygen saturation levels of patient  12  may also vary based on the mechanical contraction and relaxation of heart  14 . Thus, changes in tissue perfusion or blood oxygen saturation levels or a decreasing trend in blood oxygen saturation or tissue perfusion may indicate that heart  14  is not mechanically contracting and relaxing in a normal manner and, therefore, may indicate the presence of an arrhythmia. Tissue perfusion and blood oxygen saturation levels of patient  12  may be with the aid of an optical sensor, which may or may not be mechanically coupled to ICD  16  or INS  26 . 
     As described in U.S. Patent Application Publication No. 2007/0239215 to Bhunia et al., entitled, “METHOD AND APPARATUS FOR USING AN OPTICAL HEMODYNAMIC SENSOR TO IDENTIFY AN UNSTABLE ARRHYTHMIA,” which was filed on Mar. 31, 2006 and is incorporated herein by reference in its entirety, an optical perfusion sensor may include a red light emitting diode (LED) and an infrared (IR) LED as light sources, and a detector. An increase in a red optical signal sensed by the detector, which may indicate the amount of red light from the red LED that was reflected by blood in the tissue proximate to the optical perfusion sensor, and a decrease in an IR signal sensed by the detector, which may indicate the amount of IR light form the IR LED that was reflected by blood in the tissue in blood-perfused tissue, may indicate the occurrence of a cardiac arrhythmia. According to U.S. patent application Ser. No. 11/394,477 to Bhunia et al., electrical signals generated by the detector of the optical perfusion sensor may experience a significant change in value following a hemodynamically unstable ventricular fibrillation. This change may be detected by sensing module  96  or a separate optical sensor in the second sense mode of sensing module  96  in order to detect an arrhythmia. 
     Another non-electrophysiological parameter of patient  12  that processor  90  may use to detect an arrhythmia in the first sense mode includes heart sound signals or acoustic signals indicative of mechanical contractions of heart  14 . Heart sounds or other acoustic signals may be sensed with a sensor, which may or may not be coupled to ICD  16  or INS  26 , such as an accelerometer or acoustic transducer. The heart sounds or acoustic vibrations may be generated as the heart valves open and close during a cardiac cycle or by turbulent flow during the fill phases in diastole. Changes in the heart sounds or acoustic vibrations, such as the lack of heart sounds or acoustic vibrations or a decrease in the frequency of the heart sounds or acoustic vibrations may indicate the presence of an arrhythmia. 
     In some examples, in either or both the first and second sense modes, sensing module  96  or processor  90  may filter out the neurostimulation signals from sensed electrical signals. For example, sensing module  96  may implement a front-end filter to filter out the neurostimulation signals delivered by INS  26  or processor  90  may implement digital signal processing to filter out the neurostimulation signals. Because the source of the artifact from the electrical signals generated and delivered by INS  26  is known, and the characteristics of the electrical signals are known, it may be relatively easy for processor  90  to filter out the electrical signals generated and delivered by INS  26 . For example, sensing module  96  may filter sensed signals on the basis of frequency content and eliminate frequency components of a sensed signal that falls outside of the range. The neurostimulation signal delivered by INS  26  may have a known signature, in terms of the signal frequency, duty cycle, signal envelope, and so forth. 
     ICD  16  may store the known signature in memory  92  or INS  26  may periodically provide the neurostimulation signal information to ICD  16 . For example, INS  26  may periodically transmit the therapy program defining the stimulation parameter values with which INS  26  generates electrical stimulation signals. In some examples, ICD  16  may sense cardiac signals while INS  26  is delivering stimulation signals to patient  12 , and processor  90  may determine the characteristics (e.g., patterns, amplitude, frequency, and the like) of the neurostimulation signal artifact present in the sensed signal. This may be done, for example, after ICD  16  and INS  26  are implanted within patient  12 , e.g., in the same session. In this way, processor  90  of ICD  16  may learn the characteristics of the neurostimulation signal artifact that may be present in a sensed signal. 
     Processor  90  of ICD  16  may use these known characteristics of the neurostimulation signal to filter the signal out of the electrical signals sensed by sensing module  96 . In some examples, processor  90  or sensing module  96  may include a notch filter to filter the neurostimulation signals generated by INS  26 . The notch filter may comprise a band-stop filter (or a band rejection filter) that attenuates frequencies in a specific frequency range. The frequency range of the notch filter may be selected based on the known frequency range of the neurostimulation signals generated and delivered by INS  26 . The notch filter may be dynamically adjustable based on, for example, the therapy program with which INS  26  generates the electrical stimulation signals. 
     In some examples, sensing module  96  of ICD  16  may apply different filters to sensed electrical signals in the first and second sense modes. In addition, in some examples, processor  90  of ICD  16  may apply different arrhythmia detection algorithms based on whether the first or second sense modes are applied by ICD  16 . The arrhythmia detection algorithms may define the rules with which processor  90  identifies a potential arrhythmia. For example, the arrhythmia detection algorithms may define the number of arrhythmia events that define an arrhythmia episode, or the R-R interval duration that defines an arrhythmia event. 
     Modifying the type of arrhythmia detection algorithms based on whether INS  26  is delivering stimulation to patient  12  may help compensate for the decrease in sensitivity to electrical cardiac signals in the first sense mode of ICD  16  compared to the second sense mode. For example, when ICD  16  is applying the first sense mode to sense electrical cardiac signals, processor  90  of ICD  16  may determine that an arrhythmia episode is observed when a fewer number of R-R intervals having a duration less than a stored threshold are detected compared to arrhythmia detection algorithm implemented during the second sense mode. In this way, processor  90  may compensate for the decrease in sensitivity to electrical cardiac signals by increasing the sensitivity to arrhythmia detection. 
     In some examples, the segment of an electrical cardiac signal that is observed to detect the arrhythmia may differ based on whether ICD  16  is applying the first or second sense modes. For example, in the first sense mode, processor  90  of ICD  16  may detect arrhythmia events based on a duration of an S-T segment of a sensed EGM, and in the second sense mode, processor  90  may detect arrhythmia events based on a different segment of a sensed EGM (e.g., the R-R segment or P-P segment). 
       FIG. 17A  provides a conceptual illustration of an ECG signal  274  sensed by a sensing device via subcutaneous electrodes on left and right sides of a human subject. ECG signal is an example of an electrical signal that is sensed prior to the application of a filter by a processor (e.g., processor  90  of ICD  16 ).  FIG. 17B  provides a conceptual illustration of filtered ECG signal  276  after a processor applies a filter to sensed ECG signal  274 . An artifact from delivery of neurostimulation is present in ECG signal  274 . As  FIG. 17B  demonstrates, sensed ECG signal  274  comprising the neurostimulation signal artifact exhibits a relatively fast heart rhythm, e.g., about 260 beats per minute. Signal processing ECG signal  274 , e.g., by applying a filter to ECG signal  274 , may help remove the relatively high frequency neurostimulation signal artifact from sensed ECG signal  274 . As  FIG. 17B  illustrates, the processed ECG signal  276  exhibits a relatively slower heart rhythm, such as about 92 beats per minute. 
     The processed ECG signal may be a more accurate and precise representation of true cardiac signals of the human subject. For example, while the heart rhythms indicated by signal  274  may indicate a ventricular tachycardia events, the processed signal  276  indicates a slower heart rhythm, which may not be associated with a ventricular tachycardia events. Accordingly, it may be useful for processor  90  to apply one or more filters or implement other signal processing techniques to a sensed signal in order to minimize the possibility of delivering inappropriate therapy to patient  12 . 
       FIG. 18A  provides a conceptual illustration of an ECG signal  278  sensed by a sensing device via subcutaneous electrodes on left and right sides of a human subject. An ischemia-inducted ventricular tachycardia was induced in the human subject.  FIG. 18B  provides a conceptual illustration of filtered ECG signal  280  after a processor applies a filter to sensed ECG signal  278 . An artifact from delivery of neurostimulation is present in ECG signal  278 . As  FIG. 18A  demonstrates, sensed ECG signal  278  comprising the neurostimulation signal artifact exhibits a relatively fast heart rhythm, e.g., about 470 beats per minute. Signal processing ECG signal  278 , e.g., by applying a filter to ECG signal  278 , may help remove the relatively high frequency neurostimulation signal artifact from sensed ECG signal  278 . As  FIG. 18B  illustrates, the processed ECG signal  280  exhibits a relatively slower heart rhythm, such as about 280 beats per minute.  FIGS. 18A and 18B  further demonstrate that a processed ECG signal  280  may be a more accurate and precise representation of true cardiac signals of the human subject. As  FIGS. 18A and 18B  demonstrate, at least partially filtering the neurostimulation signal artifact from a sensed electrical signal may be useful for determining a true heart rate, such as a true ventricular tachycardia rate. 
       FIG. 19A  is a flow diagram illustrating another example technique that processor  90  of ICD  16  may implement in order to change a cardiac signal sense mode based on whether INS  26  is actively delivering electrical stimulation. As with the technique shown in  FIG. 16 , processor  90  may determine whether INS  26  is delivering stimulation to patient  12  ( 271 ). If INS  26  is currently delivering stimulation to patient  12 , processor  90  may control sensing module  96  to sense cardiac signals via a first sense mode ( 272 ). On the other hand, if INS  26  is not delivering stimulation to patient  12 , e.g., because the delivery of stimulation by INS  26  is currently suspended, processor  90  may implement a second sense mode in order to sense cardiac signals ( 273 ), where the second sense mode comprises at least one different sensing parameter than the first sense mode. In addition, if processor  90  (or sensing module  96  under the control of processor  90 ) detects an arrhythmia via the first sense mode ( 284 ), processor  90  may control INS  26  to suspend the delivery of therapy to patient  12  ( 285 ) and processor  90  may control sensing module  96  to sense according to the second sense mode ( 273 ). 
     If processor  90  detects an arrhythmia while sensing cardiac activity via the second sensing mode ( 286 ), processor  90  may control stimulation generator  94  ( FIG. 6 ) to deliver the appropriate stimulation therapy to heart  14  ( 288 ), which may be, for example, any one or more of pacing, cardioversion or defibrillation pulses. As shown in  FIG. 19A , the second sense mode of ICD  16  may be used to confirm the detection of an arrhythmia detected via the first sense mode. The second sense mode of ICD  16  may be more specific to appropriately detecting electrical cardiac signals than the first sense mode, e.g., may be more likely to detect a cardiac arrhythmia based on the electrical cardiac signals compared to the first sense mode. This may be attributable to, for example, the lower cardiac signal sensing threshold defined by the second sense mode and/or the higher amplifier gain used to sense the signals. By decreasing the sensing threshold or increasing the amplifier gain, the sensitivity of ICD  16  to heart signals may increase because sensing module  96  may characterize more electrical signals as cardiac signals, and, therefore decrease the possibility of undersensing cardiac signals. 
     As previously indicated, although the first sense mode is less sensitive to cardiac signals, the first sense mode detects an arrhythmia based on other physiological parameters of patient. Detecting a potential arrhythmia based on physiological parameters in addition to electrical cardiac signals may compensate for the decrease in sensitivity to cardiac signals. 
     As shown in  FIG. 19A , the second sense mode may be a default sense mode when INS  26  is not actively delivering stimulation therapy to patient because crosstalk between ICD  16  and INS  26  may be negligible. Thus, the possibility that sensing module  96  may oversense cardiac signals in the second sense mode is reduced when INS  26  is not actively delivering stimulation therapy to patient  12 . 
     In some examples, the first and second sense modes may comprise different sense vectors. A sense vector may be defined by the subset of electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  70 ,  72 ,  74 , and  76  electrically coupled to ICD  16  that are used by sensing module  96  to sense electrical cardiac signals. A sensing vector may be modified by switching the electrodes with which sensing module  96  senses intracardiac electrical signals. ICD  16  may sense electrical cardiac signals via one or more external electrodes. In some examples, ICD  16  may sense electrical cardiac signals via external electrodes in the first sense mode and sense electrical cardiac signals via implanted electrodes in the second sense mode. 
     As another example of how a sensing vector may be modified by selecting different electrode, if sensing module  96  senses an intracardiac electrical signal via electrodes  50 ,  52  of lead  18 , which are positioned in right ventricle  32  ( FIG. 3 ), and processor  90  detects an arrhythmia ( 284 ) based on the sensed signals, processor  90  may control sensing module  96  to switch sense modes, and, therefore, switch sensing vectors and sense intracardiac electrical signals via electrodes  54 ,  56  of lead  20 , which is positioned in left ventricle  36  ( FIG. 3 ). 
     In some examples, sensing module  96  of ICD  16  may sense electrical cardiac signals within left ventricle  36  ( FIG. 3 ) and outside of right ventricle  32  ( FIG. 3 ) of heart  14  in the first sense mode. That is, in the first sense mode, sensing module  96  may not sense electrical cardiac signals via electrodes  50 ,  52 ,  72  ( FIG. 3 ) positioned within right ventricle  32 . In addition, in some examples, sensing module  96  of ICD  16  may sense electrical cardiac signals within right ventricle  32  and outside of left ventricle  36  of heart  14  in the first sense mode. That is, in the first sense mode, sensing module  96  may not sense electrical cardiac signals via electrodes  54 ,  56 ,  74  ( FIG. 3 ) positioned within left ventricle  36 . 
     As another example of how ICD  16  may switch sense vectors with which electrical cardiac signals of heart  14  of patient  12  are sensed, in the first sense mode, ICD  16  may sense electrical cardiac signals via two electrodes of one of leads  18 ,  20 ,  22  ( FIG. 3 ), and in the second sense mode, ICD  16  may sense electrical cardiac signals via at least one electrode carried by a lead  18 ,  20  and/or  22  and housing electrode  68  ( FIG. 3 ). In this way, in the second sense mode, ICD  16  may sense electrical cardiac signals across a greater span of heart  14  than in the first sense mode. 
     In some examples, in at least the first sense mode, ICD  16  may sense electrical cardiac signals via each of a plurality of sense vectors. If ICD  16  senses electrical cardiac signals via each of a plurality of sense vectors in the second sense modes, the sense vectors defined by the second sense mode may be different than the sense vectors defined by the first sense mode. Crosstalk from therapy delivery by INS  26  may have different strengths, depending on the vector with which ICD  16  senses electrical signals. Thus, sensing electrical cardiac signals with a plurality of sense vectors may help increase the possibility that ICD  16  senses a true electrical cardiac signal or at least an electrical cardiac signal that that does not have a large signal artifact from INS crosstalk. 
     In addition, if ICD  16  senses electrical cardiac signals via each of a plurality of sense vectors, ICD  16  may determine cardiac function of patient  12  based on a weighted sum of the electrical cardiac signals or at least based on a correlation of the electrical cardiac signals sensed via two or more sense vectors. In one example of weighing the electrical signals sensed by each of a plurality of sensing vectors, processor  90  may individually gain and sum the signals and detect cardiac episodes or events (e.g., a tachyarrhythmia) based on the summed signal. In another example, processor  90  may sum the absolute value of each sensed signal. In general, processor  90  sums the different sensed signals in order to combine the sensing information and attempt to filter out crosstalk noise, which may only be affecting only one or two of the sensing vectors. 
     In some examples, the electrical signals sensed via each of the sense vectors are each used to determine the timing of the R-waves or other signal characteristics, e.g., to detect an arrhythmia. Processor  90  of ICD  16  may determine whether the R-waves sensed via different sense vectors indicate that an arrhythmia is detected. If, for example, a threshold number (e.g., two or more) of the electrical signals sensed via different sense vectors indicate different R-R intervals, processor  90  may determine that the sensed electrical cardiac signals are not true electrical cardiac signals, but are at least partially attributable to delivery of electrical stimulation by INS  26 . 
     If processor  90  detects a potential arrhythmia based on intracardiac electrical signals sensed via a first sensing vector defined by the first sense mode ( 284 ), processor  90  may determine whether an arrhythmia is detected based on the intracardiac electrical signals sensed via a second sensing vector defined by the second sense mode ( 273 ,  284 ). The first and second sensing vectors may be defined by respective subsets of electrodes  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  70 ,  72 ,  74 , and  76 , where the first and second sensing vectors may include at least one different electrode. If processor  90  detects the potential arrhythmia based on the signals sensed via the new sensing vector, processor  90  may confirm the presence of the arrhythmia and, therefore, determine whether the detected arrhythmia was based on true cardiac signals, or at least not based on electrical stimulation signals from INS  26 . 
     In some cases, switching the sensing vector may help decrease the crosstalk that ICD  16  senses by, for example, changing the relative vector between the stimulation electrodes  124  connected to INS  26  and the sensing vector used by ICD  16  to sense cardiac signals. Thus, in some cases, the crosstalk sensed by the new sensing vector may change characteristics compared to the initial sensing vector, and, as a result, processor  90  may not mischaracterize the artifact generated by the delivery of electrical stimulation by INS  26  as true cardiac signals. 
     If the potential arrhythmia is not detected ( 286 ) when ICD  16  is sensing in the second sense mode, processor  90  may determine that the potential arrhythmia detected via the cardiac signals sensed via the first sense mode ( 284 ) was a false detection based on noise from INS  26 , rather than true cardiac signals. Thus, processor  90  may not provide any therapy to patient  12 , and processor  90  of ICD  16  may determine whether INS  26  is delivering electrical stimulation to patient  12  ( 271 ) and control sensing module  96  to sense electrical cardiac signals of patient  12  via the first sense mode ( 272 ) if INS  26  is delivering stimulation to patient  12  and control sensing module  96  to sense electrical cardiac signals of patient  12  via the second sense mode if INS  26  is not delivering stimulation to patient  12  ( 273 ). 
       FIG. 19B  is a flow diagram illustrating another example technique that processor  90  of ICD  16  may implement in order to change a cardiac signal sense mode based on whether INS  26  is actively delivering electrical stimulation. The technique shown in  FIG. 19B  is similar to that shown in  FIG. 19A . However, in the example shown in  FIG. 19B , if processor  90  (or sensing module  96  under the control of processor  90 ) detects an arrhythmia via the first sense mode ( 284 ), in the example shown in  FIG. 19B , processor  90  may control stimulation generator  94  ( FIG. 6 ) to deliver the appropriate stimulation therapy to heart  14  ( 289 ). In contrast, in the example shown in  FIG. 19A , processor  90  controlled INS  26  to suspend or otherwise adjust the delivery of stimulation to patient  12  ( 285 ) and then determined whether the potential arrhythmia was also detected when the patient&#39;s condition was sensed via the second sense mode of ICD  16  ( 273 ,  286 ). 
     In some cases, it may be desirable to evaluate the extent of the crosstalk between INS  26  and ICD  16 . For example, it may be desirable to evaluate the strength of the electrical stimulation signal generated by INS  26  and sensed by ICD  16 , i.e., evaluate one or more characteristics of an artifact present in a signal sensed by ICD  16  when INS  26  is delivering stimulation to patient  12 . The artifact may be referred to as a neurostimulation artifact, although the artifact may also be attributable to the delivery of stimulation other than neurostimulation by INS  26 . A clinician or patient  12  may evaluate the crosstalk between INS  26  and ICD  16  in order to determine if the crosstalk is excessive at various times, such as after implantation of ICD  16  and INS  26  in patient  12 , after programming the electrical stimulation parameters or sensing parameters of either ICD  16  or INS  26  or periodically throughout the use of therapy system  10 . 
     Crosstalk may be excessive if it hinders the intended operation of ICD  16 , such as the sensing of true cardiac signals by ICD  16 . As previously described, in some examples, ICD  16  may sense the electrical stimulation signal generated and delivered by INS  26  and mischaracterize the electrical stimulation signal as a cardiac signal. This mischaracterization of the electrical stimulation signal as a cardiac signal may result in a detection of a cardiac arrhythmia, which may result in the inappropriate delivery of a defibrillation shock or other electrical stimulation to heart  14 . In this way, the crosstalk between INS  26  and ICD  16  may affect the intended operation of ICD  16 . 
     In some examples, the crosstalk between INS  26  and ICD  16  may be excessive if a characteristic of a signal sensed by the ICD  16  while electrical stimulation is being delivered by INS  26  differs from a characteristic of a baseline signal by a threshold value. As described in further detail below, the characteristic of the electrical signals may be an amplitude value or a power level (or energy level) in one or more frequency bands. For example, the characteristic of the electrical signals may be an absolute amplitude value or a root mean square amplitude value. In addition, the amplitude value may comprise a mean or median amplitude value over a period of time or a maximum amplitude or an amplitude in a particular percentile of the maximum (e.g., an amplitude value that represents 95% of the maximum amplitude value). In some examples, as described in further detail below, the threshold value may be a percentage of a sensing threshold with which ICD  16  senses electrical cardiac signals. 
     If the crosstalk between INS  26  and ICD  16  is determined to be excessive, a clinician or a device (e.g., INS  26 , ICD  16  or programmer  24 ) may attempt to reduce the extent of the crosstalk. For example, ICD  16  or INS  26  may modify one or more stimulation parameter values of INS  26 , as described with respect to  FIGS. 9-12B  and/or modify one or more sensing parameter values of ICD  16 , as described with respect to  FIGS. 16 ,  19 A, and  19 B. 
     In some examples, an external device, such as medical device programmer  24  ( FIG. 1 ) may be used to evaluate the extent of crosstalk between INS  26  and ICD  16 . While programmer  24  is primarily referred to throughout the description of  FIG. 20 , in other examples, another device may be used to measure the amount of crosstalk between INS  26  and ICD  16 . The device may be an external device, such as multifunction computing device or may be a device dedicated to measuring the amount of crosstalk between INS  26  and ICD  16 , or one of the implanted medical devices  16 ,  26 . In addition, in some examples, ICD  16 , INS  26  or another implanted device may measure the amount of crosstalk between ICD  16  and INS  26 . The implanted device may store the information indicative of the amount of crosstalk between ICD  16  and INS  26  or may transmit information to an external device, such as programmer  24 . 
       FIG. 20  is a flow diagram illustrating an example technique for evaluating crosstalk between ICD  16  and INS  26 . The technique shown in  FIG. 20  may be implemented in order to determine a status of the electrical noise sensed by ICD  16  due to the delivery of stimulation by INS  26 . The status determination may be used to, for example, modify an stimulation parameter of INS  26  or a sensing parameter of ICD  16 , e.g., in accordance with the techniques described above with respect to  FIGS. 9-12 ,  16 ,  19 A, and  19 B. 
     As shown in  FIG. 20 , processor  130  of programmer  24  ( FIG. 8 ) may evaluate one or more characteristics of a signal sensed by ICD  16  when the neurostimulation artifact is present under the direction of a clinician or automatically, e.g., based on a schedule determined by a clinician. The schedule may define an evaluation frequency with processor  130  evaluates the neurostimulation signal artifact sensed by ICD  16 . For example, the artifact evaluation frequency may be in a range of about one to about ten times per minute, once per hour, or once per day, although other frequency ranges are contemplated. 
     In order to measure the magnitude of the neurostimulation artifact (or “crosstalk”) present in the electrical cardiac signal sensed by ICD  16 , processor  130  of programmer  24  may instruct processor  110  of INS  26  to suspend or otherwise adjust the delivery of neurostimulation ( 290 ). For example, processor  130  of programmer  24  may transmit a control signal to processor  110  via the respective telemetry modules  136  ( FIG. 8 ),  118  ( FIG. 7 ). The control signal may not only indicate whether INS  26  should suspend or otherwise adjust the delivery of neurostimulation to patient  12 , but, in some examples, may indicate how long INS  26  should suspend neurostimulation or deliver therapy according to the adjust parameters. In other examples, memory  112  ( FIG. 7 ) of INS  26  may store instructions for suspending or otherwise adjusting neurostimulation when processor  110  of INS  26  receives the control signal from processor  130  of programmer  24 . As another example, INS  26  may suspend or otherwise adjust delivery of stimulation without intervention from programmer  24 , e.g., according to schedule stored by memory  112 . 
     During the time in which neurostimulation is suspended or adjusted, processor  130  of programmer  24  may receive an electrical signal sensed by ICD  16  from ICD  16  ( 292 ). This electrical signal may represent a baseline artifact level present in the cardiac signal sensed by ICD  16 . Artifacts from sources other than the neurostimulation signals delivered by INS  26  may be present in the signal sensed by ICD  16 , such as from electromagnetic interference from electronics or electrical outlets in the patient&#39;s surroundings. The baseline electrical signal may indicate these other artifacts present in the signal sensed by ICD  16 . 
     In some examples, processor  130  of programmer  24  may instruct processor  90  of ICD  16  to sense a baseline electrical signal via a selected sensing channel of sensing module  96  ( FIG. 6 ) of ICD  16 . As described with respect to  FIG. 6 , in some examples, sensing module  96  may include a plurality of sensing channels, which may each include an amplifier. For example, sensing module  96  may include a sensing channel including an R-wave amplifier to sense R-waves within right ventricle  32  of heart  14  ( FIG. 3 ), a sensing channel including an R-wave amplifier to sense R-waves within left ventricle  36  of heart  14  ( FIG. 3 ), a sensing channel including a P-wave amplifier to sense P-waves within right atrium  30  of heart  14  ( FIG. 3 ), and/or a sensing channel including a wide band amplifier in order to generate an EGM representing the electrical activity of heart  14 . Processor  90  of ICD  16  may transmit the electrical signal sensed on the selected sensing channel of sensing module  96  to processor  130  of programmer  24  via the respective telemetry modules  98  ( FIG. 6 ),  136  ( FIG. 8 ). 
     After processor  130  of programmer  24  receives the baseline electrical signal from ICD  16  ( 292 ), processor  130  may control processor  110  of INS  26  to activate the delivery of electrical stimulation ( 294 ). For example, processor  130  may generate a control signal that is transmitted to processor  110  of INS  26  via the respective telemetry modules  136  ( FIG. 8 ),  118  ( FIG. 7 ). Upon receiving the control signal, processor  110  of INS  26  may control stimulation generator  114  to begin generating and delivering neurostimulation therapy, e.g., in accordance with a first operating mode of INS  26 . As described with respect to  FIG. 12A , a first operating mode may be defined by a therapy program that defines one or more stimulation parameter values for the electrical stimulation signals generated and delivered by INS  26 . In other examples, processor  110  of INS  26  may begin generating and delivering neurostimulation therapy based on a predetermined schedule that indicates the times at which processor  110  should suspend the delivery of neurostimulation and initiate the delivery of stimulation. 
     After INS  26  commences the delivery of neurostimulation to patient  12 , processor  130  of programmer  24  may receive an electrical signal sensed by the selected channel of sensing module  96  of ICD  16  ( FIG. 6 ) ( 296 ). This electrical signal that is sensed on the selected sensing channel during the delivery of neurostimulation by INS  26  may be referred to as a “second electrical signal” to distinguish it from the baseline electrical signal. Processor  130  of programmer  24  may receive the baseline electrical signal and the second electrical signal, for example, by periodically interrogating ICD  16 . In other examples, ICD  16  may periodically transmit the baseline electrical signal and second electrical signal to processor  130  of programmer  24  without being interrogated by programmer  24 . 
     Processor  130  of programmer  24  may determine the neurostimulation signal artifact on the selected sensing channel of ICD  16  based on the baseline electrical signal and the second electrical signal that was sensed while INS  26  was actively delivering neurostimulation to patient  12  ( 298 ). In some examples, processor  130  of programmer  24  may determine the neurostimulation signal artifact that is present on more than one sensing channel of sensing module  96  of ICD  16 . In addition, in some examples, processor  90  of ICD  16  may sense the neurostimulation signal artifact present in the signal sensed via one or more selected sensing channels during a quiet segment of the cardiac cycle. The quiet segment of a cardiac cycle may be when the intrinsic electrical signal of heart  14  is least active, such as during the S-T segment of a sinus rhythm of heart  14 . 
     In some examples, processor  130  of programmer  24  may determine the neurostimulation signal artifact on the selected sensing channel by determining a difference between one or more signal characteristics of the baseline electrical signal and the second electrical signal. In some examples, the signal characteristic may comprise a current or a voltage amplitude of the signal waveforms. For example, processor  130  of programmer  24  may determine a difference in the amplitude of the baseline electrical signal and a sensing threshold of sensing module  96  ( FIG. 6 ) of ICD  16 . This value may be referred to as the “first value” for ease of description. The amplitude may be a mean or median amplitude (e.g., a peak-to-peak amplitude), a highest amplitude (e.g., a greatest peak-to-peak amplitude), a root means square (RMS) amplitude, an amplitude that is equal to a certain percentage (e.g., about 95%) of the amplitude, and the like. A sensing threshold may indicate a threshold amplitude value above which processor  90  of ICD  16  characterizes a sensed electrical signal as an electrical cardiac signal. 
     Processor  130  may also determine a second value indicative of the difference in the amplitude of the second electrical signal and a sensing threshold of sensing module  96 . The amplitude may be a mean or median amplitude, a highest amplitude, a RMS amplitude, an amplitude that is equal to a certain percentage (e.g., about 95%) of the highest amplitude, and the like. In order to determine the neurostimulation signal artifact on the selected sensing channel, processor  130  of programmer  24  may determine a difference between the first and second values. If the difference is greater than or equal to a stored threshold value, which may be based on the sensing threshold amplitude of ICD  16 , processor  130  may determine that the crosstalk between ICD  16  and INS  26  due to the delivery of neurostimulation by INS  26  is unacceptable. On the other hand, if the difference between the first and second values is less than the stored threshold value, processor  130  may determine that the crosstalk between ICD  16  and INS  26  due to the delivery of neurostimulation by INS  26  is within acceptable ranges. In this way, processor  130  may evaluate the extent of the crosstalk between ICD  16  and INS  26  due to the delivery of neurostimulation by INS  26 . The threshold value may be, for example, selected by a clinician and stored by programmer  24 , ICD  16 , INS  26  or another device. 
     As another example, the signal characteristic may comprise a power level within a particular frequency band of an electrical signal. Processor  130  may determine the neurostimulation signal artifact by determining a first value indicative of the difference in energy levels in the selected frequency band of the baseline electrical signal and a stored energy level, and a second value indicative of the difference in energy levels in the selected frequency band of the second electrical signal and the stored energy level. The difference between the first and second values may be indicative of noise on a sensing channel of ICD  16  due to the delivery of neurostimulation by INS  26 . 
     Processor  130  of programmer  24  may display data indicative of the extent of crosstalk between ICD  16  and INS  26  on a display of user interface  134  ( FIG. 8 ) ( 299 ). For example, the data may include a graphical display of the waveform of the baseline electrical signal or a waveform of the second electrical signal. An example of a graphical display of different types of waveforms indicative of the crosstalk between ICD  16  and INS  26  is shown in  FIG. 21 , which is described below. 
     As patient  12  changes posture and/or activity level, the one or more leads  28 ,  29  ( FIGS. 1 and 2 ) connected to INS  26  may move within patient  12 . For example, in the example shown in  FIG. 2 , as patient  12  changes posture, leads  28 ,  29  may move relative to ICD  16  as spinal cord  44  moves. The amount of neurostimulation artifact that ICD  16  senses may change as a function of the position of leads  28 ,  29  within patient  12 . For example, in some patient postures, at least one of the leads  28 ,  29  may be closer to the sense electrodes coupled to ICD  16 , and, as a result, ICD  16  may sense a stronger neurostimulation signal. That is, as leads  28 ,  29  move closer to ICD  16 , the extent of crosstalk between INS  26  and ICD  16  may increase. Similarly, for increased levels of patient activity, leads  28 ,  29  may undergo more movement within patient  12 , which may also result in at least one of the leads  28 ,  29  moving closer to heart  14 . 
     In some examples, in order to better evaluate the neurostimulation artifact present in a signal sensed by ICD  16  when INS  26  is actively delivering stimulation, processor  130  of programmer  24  may evaluate the neurostimulation artifact while patient  12  is in different postures and/or activity levels. This may help processor  130  and/or the clinician evaluate the spectrum of crosstalk that may be present between ICD  16  and INS  26 . In some examples, processor  130  may present a display on user interface  134  ( FIG. 8 ) of programmer  24  that prompts patient  12  to undertake different postures or activities. The different patient postures may include, for example, standing, sitting, a prone position, bending forward while standing or bending backward at the waist while standing, and the like. Processor  130  may then evaluate the amount of crosstalk between INS  26  and ICD  16  while patient  12  is in each of the different postures or activities, e.g., using the technique shown in  FIG. 20 . For example, while patient  12  is in each of the different postures or activities, processor  130  of programmer  24  may receive and record both a baseline and a second electrical signal sensed by ICD  16 . 
     In other examples, processor  90  of ICD  16 , rather than processor  130  of programmer  24 , may determine the neurostimulation signal artifact on the selected sensing channel of ICD  16  based on the baseline electrical signal and the second electrical signal that was sensed while INS  26  was actively delivering neurostimulation to patient  12 . In this way, ICD  16  may provide real-time detection of crosstalk and switch sensing modes at a useful time, e.g., before inappropriately delivering a shock to patient  12 , or communicate to the INS  26  to adjust therapy delivery (e.g., adjust a stimulation parameter value or suspend neurostimulation). 
       FIG. 21  is a flow diagram illustrating an example technique that may be used to evaluate the extent of the crosstalk between INS  26  and ICD  16  and minimize the crosstalk if the crosstalk exceeds a threshold level. Processor  130  of programmer  24  may measure the crosstalk ( 300 ), e.g., using the technique described with respect to  FIG. 20 . Processor  130  may determine whether the extent of crosstalk exceeds a threshold level ( 302 ). In some examples, processor  130  may determine whether the extent of crosstalk exceeds the threshold level by determining whether the values of one or more signal characteristics (e.g., a voltage amplitude) of the second electrical signal differs from the respective signal characteristic values of the baseline electrical signal. The threshold level may indicate a percentage change or an absolute value change the in one or more signal characteristics. As discussed with respect to  FIG. 20 , the baseline electrical signal may represent the amount of artifact present on a selected sensing channel of ICD  16  when the delivery of neurostimulation by INS  26  is suspended and the second electrical signal may represent the amount of artifact present on the selected sensing channel when INS  26  is delivering neurostimulation therapy, e.g., in the ordinary course of neurostimulation therapy. The threshold level may be stored within memory  132  of programmer  24  ( FIG. 8 ), memory  92  of ICD  16 , memory  112  of INS  26  or a memory of another device. 
     In some examples, processor  130  may determine whether the extent of crosstalk exceeds a threshold level ( 302 ) by comparing a first value indicative of the difference between a voltage amplitude of the baseline electrical signal and a sensing threshold of sensing module  96  ( FIG. 6 ) of ICD  16  and a second value indicative of the difference between a voltage amplitude of the second electrical signal and a sensing threshold of sensing module  96 . The relevant voltage amplitudes of the baseline and second electrical signals may be the average or median amplitudes over a particular range of time, the amplitudes at a particular point in time, such as a greatest amplitude over a particular range of time or a percentage of the greatest amplitude. In addition, in some examples, the voltage amplitude may also comprise an absolute amplitude value or a root mean square voltage amplitude. In some examples, the sensing threshold may be the sensing threshold of sensing module  96  ( FIG. 6 ) of ICD  16  at the most sensitive setting or at the least sensitive setting. 
     If the first and second values do not differ from each other by at least the threshold value (or threshold level), processor  130  of programmer  24  may determine that the extent of the crosstalk between INS  26  and ICD  16  is within an acceptable range. That is, if the difference between the first and second is less than or equal to the threshold value, processor  130  of programmer  24  may determine that the possibility that ICD  16  may sense the neurostimulation signals delivered by INS  26  and mischaracterize the neurostimulation signals as cardiac signals is relatively low. Processor  130  may then determine that modifications to the operating parameters of INS  26  or the sensing parameters of ICD  16  are not necessary. Processor  130  of programmer  24  may then continue measuring crosstalk ( 300 ) and comparing it to a threshold value ( 302 ). 
     On the other hand, if the first and second values differ from each other by at least the threshold value (or threshold level), processor  130  of programmer  24  may determine that the crosstalk between INS  26  and ICD  16  exceeds an acceptable level. In some examples, the threshold level may be up to about 100% of the sensing threshold of ICD  16 , such as about 25% to about 50% of the sensing threshold. As previously indicated, the sensing threshold may be the sensing threshold of sensing module  96  ( FIG. 6 ) of ICD  16  at the most sensitive setting or at the least sensitive setting. Thus, in some examples, if the difference between the first and second values is greater than the sensing threshold of ICD  16 , processor  130  may determine that the crosstalk between INS  26  and ICD  16  exceeds an acceptable level. Other percentages or absolute value changes in voltage amplitudes that indicate an unacceptable level of neurostimulation signal artifact are contemplated. 
     In other examples, processor  130  may determine whether the extent of crosstalk exceeds a threshold level ( 302 ) by comparing the spectral content of the baseline electrical signal and the second electrical signal. For example, processor  130  may implement a fast Fourier transform algorithm in order to extract the frequency components of the baseline electrical signal and the second electrical signal. Processor  130  may compare one or more frequency components of the baseline electrical signal and the second electrical signal. The one or more frequency components may include, for example, a power level within one or more frequency bands, a trend in the power level within one or more frequency bands over time, a ratio of power levels between one or more frequency bands, and the like. Different frequency bands may be more revealing of the extent to which the second electrical signal includes an unacceptable level of neurostimulation signal artifact. A clinician may determine the revealing frequency bands during a trial phase in which INS  26  and ICD  16  are tested to determine the frequency bands are relatively revealing of a neurostimulation artifact that adversely affects the sensing of cardiac signals by ICD  16 . 
     If processor  130  of programmer  24  determines that the extent of the crosstalk between INS  26  and ICD  16  exceeds an acceptable level, processor  130  may initiate the modification to one or more stimulation parameter values with which stimulation generator  114  of INS  26  generates and delivers neurostimulation therapy to patient  12  or one or more sensing parameter values of ICD  16  ( 304 ). Processor  130  may initiate the modification to the one or more stimulation parameter values of INS  26  using any suitable technique. In one example, processor  130  may transmit a control signal to processor  110  of INS  26 , and processor  110  may initiate the modification to the one or more stimulation parameter values upon receiving the control signal from processor  130  of programmer  24 . For example, processor  110  may modify the one or more stimulation parameter values using a set of rules stored in memory  112 , as described with respect to  FIGS. 11A-11D . Examples of stimulation parameter values that processor  110  may modify include, but are not limited to, an electrode combination, voltage amplitude, current amplitude, pulse rate, pulse duration, and the like. As another example, processor  110  may modify the one or more stimulation parameter values by switching therapy programs, as described with respect to  FIGS. 12A and 12B . 
     In other examples, processor  130  of programmer  24  may provide processor  110  of INS  26  with a new therapy program defining one or more stimulation parameter values or provide processor  110  with specific instructions for modifying the one or more stimulation parameter values. For example, the instructions may indicate that processor  110  of INS  26  should decrease the frequency of the neurostimulation signal by a certain percentage or to a specific value. Other types of therapy parameter value modification instructions are contemplated. In other examples, processor  130  of programmer  24  may instruct processor  110  of INS  26  to modify one or more stimulation parameter values by switching therapy programs, as described with respect to  FIG. 12A . 
     Processor  130  may initiate the modification to the one or more sensing parameters of ICD  16  using any suitable technique. In one example, processor  130  may transmit a control signal to processor  90  of ICD  16 , and processor  90  may initiate the modification to the one or more sensing parameters upon receiving the control signal from processor  130  of programmer  24 . For example, processor  90  may modify the one or more sensing parameter values by switching sense modes, as described with respect to  FIGS. 16 ,  19 A, and  19 B. Examples of sensing parameters values that processor  90  may modify include, but are not limited to, a sensing threshold value, an amplifier gain, a sensing vector, and a type of filter used by sensing module  96  or processor  90  to filter noise out of a sensed signal. 
     After the one or more neurostimulation parameter values or ICD  16  sensing parameters are modified ( 304 ), processor  130  of programmer  24  may measure the crosstalk ( 306 ) and determine whether the extent of the crosstalk exceeds a threshold level ( 308 ), e.g., using the techniques described above. If processor  130  determines that the extent of the crosstalk does not exceed the threshold level, processor  130  may not take any further action to modify the one or more neurostimulation parameter values of INS  26 . Processor  130  may then continue periodically or continuously measuring the crosstalk ( 300 ) until a condition in which the crosstalk exceeds a threshold level ( 302 ) is detected. 
     On the other hand, if processor  130  determines that the extent of the crosstalk exceeds the threshold level ( 308 ), processor  130  may suspend the delivery of neurostimulation by INS  26  ( 310 ). In other examples, prior to suspending the delivery of neurostimulation, processor  130  may initiate the modification to one or more stimulation parameter values of INS  26  or sensing parameters of ICD  16  in an attempt to minimize the neurostimulation artifact on the signal sensed by ICD  16 . Processor  130  may repeat the steps shown in blocks  304 ,  306 , and  308  to attempt to reduce the neurostimulation artifact. The one or more stimulation parameter values or sensing parameters may be modified for one or more iterations prior to suspending the delivery of neurostimulation by INS  26 . As described with the technique shown in  FIGS. 11A-11D , in some examples, processor  130  may modify a different stimulation parameter value or sensing parameter during each iteration of the stimulation parameter value modification ( 304 ), may modify the same stimulation parameter or sensing parameter for at least two consecutive or nonconsecutive iterations or may modify more than one type of stimulation parameter value in the same iteration of INS  26  modification. 
     Processor  130  of programmer  24  may generate an interference indication if the extent of the crosstalk between INS  26  and ICD  16  exceeds a threshold level, despite the modification to one or more stimulation parameter values ( 312 ). Processor  130  may present the interference indication to a user (e.g., a clinician or patient  12 ) via a display user interface  134  or processor  130  may generate an audible or a somatosensory alert (e.g., a pulse vibration of programmer  24 ) via programmer  24 . In this way, programmer  24  may present a real-time interference alert to a user to notify the user that the stimulation delivered by INS  26  may be interfering with the sensing of cardiac signals by ICD  16 . 
     In some examples, a characteristic of the visual, auditory or somatosensory alert may change in response to the amount of crosstalk determined to exist between ICD  16  and INS  26 . For example, if the visual alert includes displaying a colored display, the color of the display may change or change intensity as a function of the amount of crosstalk determined to exist between ICD  16  and INS  26 . As another example, if the interference indication comprises an audible alert, the tone, frequency, volume or another characteristic of the audible sound may change as a function of the amount of crosstalk determined to exist between ICD  16  and INS  26 . The amount of crosstalk determined to exist between ICD  16  and INS  26  may be based on a difference between the first and second values, where the first value is indicative of the difference between the characteristic of the baseline electrical signal and the sensing threshold of ICD  16  and the second value is indicative of the difference between the characteristic of the second electrical signal and the sensing threshold of ICD  16 . For example, processor  130  may determine that the greater the difference between the first and second values, the more crosstalk is present between ICD  16  and INS  26 . 
     The interference indication may also indicate that the delivery of neurostimulation by INS  26  was adjusted (e.g., suspended or the intensity of neurostimulation was reduced) and or that patient  12  should seek medical attention. As previously indicated, the tonal frequency of the audible alert or the pulse rate or intensity of the somatosensory alert may change as a function of the relative level of crosstalk between INS  26  and ICD  16 . For example, the intensity of the somatosensory alert or the pitch of the audible alert may change with the strength of the neurostimulation artifact present in the signal sensed by ICD  16 . 
     In some examples, processor  130  may transmit the interference indication to a remote site, such as a remote clinician&#39;s office, via a network, as described with respect to  FIG. 32 . In addition, in some examples, processor  130  may also store the interference indication in memory  132 . The interference indication may indicate, e.g., to a clinician, that the crosstalk between INS  26  and ICD  16  was not reducible by modifying one or more stimulation parameter of INS  26  or one or more sensing parameters of ICD  16 . After receiving the interference indication, the clinician may determine whether other measures may be taken in order to reduce the crosstalk between INS  26  and ICD  16 . For example, the clinician may determine whether repositioning lead  28  coupled to INS  26  within patient  12  may help reduce the crosstalk. 
     In other examples of the technique shown in  FIG. 21 , as well as  FIGS. 23 and 24 , processor  90  of ICD  16  or processor  110  of INS  26  may perform any part of the technique shown in  FIG. 21  in addition to or instead of processor  130  of programmer  24 . 
     In other examples of the technique shown in  FIG. 21 , processor  130  may evaluate the extent of crosstalk between ICD  16  and INS  26  based only on the second electrical signal, which is sensed by ICD  16  during delivery of neurostimulation by INS  26 . For example, rather than comparing the baseline and second electrical signals to determine whether the extent of crosstalk exceeds an acceptable level ( 302 ), processor  130  may determine that if the difference between an amplitude of the second electrical signal and a sensing threshold of sensing module  96  during a quiet segment of cardiac cycle of heart  14  ( FIG. 1 ) of patient  12  is greater than or equal to a stored value, the noise on the sensing channel of sensing module  96  is greater than an acceptable level. The amplitude may be a mean or median amplitude, a highest amplitude, a RMS amplitude, an amplitude that is equal to a certain percentage (e.g., about 95%) of the highest amplitude, and the like. The stored value may be a percentage of the sensing threshold of sensing module  96  of ICD  16 . For example, the stored value may be about 10% to about 50%, such as about 25% of the sensing threshold voltage. 
     The noise on the sensing channel of sensing module  96  may be at least partially attributable to the delivery of neurostimulation by INS  26 . In this way, a comparison of a stored value and the difference between an amplitude of the second electrical signal and a sensing threshold of sensing module  96  may indicate whether the extent of crosstalk between ICD  16  and INS  26  is undesirable. 
       FIG. 22  is a conceptual illustration of programmer  24 , which may display a status level of the INS  26  and ICD  16  interference. The interference status may be referred to as, for example, an electrical noise status or crosstalk status. In the example shown in  FIG. 22 , programmer  24  includes user input mechanisms  320 A- 320 G (collectively “user input mechanisms  320 ”) and display  322 . A user (e.g., patient  12  or a clinician) may interact with user input mechanisms  320  to input information into programmer  24 , and, in some cases, control aspects of therapy delivered by ICD  16  and/or INS  26  within the limits programmed by a clinician. User input mechanisms  320  include buttons  320 A and  320 B, which may be used to increase or decrease the therapy intensity delivered by INS  26 , if allowed, and may perform other functions. An intensity of therapy may be modified by, for example, modifying a therapy parameter value, such as the current or voltage amplitude of stimulation signals, the frequency of stimulation signals, the shape of a stimulation signal or the electrode combination used to deliver the stimulation signal. In some examples, user input mechanisms  320 C,  320 D may be used to decrease or increase the contrast of display  322 , and user input mechanism  320 E may be used to power programmer  24  on and off. 
     Multi-directional controller  320 F may allow a user to navigate through menus displayed by display  322 , and may include a button  320 G that is actuated when the center of multi-directional controller  320 F is pressed. Display  322  may comprise any suitable type of display, such as an LCD display, LED display or a touch screen display. Display  322  may present graphical user interface screens for presenting information to the user, such as information related to the sensed level of neurostimulation signal artifact on a selected sense channel of ICD  16 . In the example shown in  FIG. 21 , display  322  presents first screen  324  that indicates crosstalk status, a second screen  328  that illustrates a waveform of a baseline electrical signal that is sensed on the selected sense channel of ICD  16  when INS  26  is not delivering electrical stimulation to patient  12 , and a third screen  330  that illustrates a waveform of a second electrical signal that is sensed on the selected sense channel of ICD  16  when INS  26  is delivering electrical stimulation to patient  12 . 
     The user may review the different waveforms present in screens  328 ,  330  in order to visually ascertain the extent to which the neurostimulation artifact on the selected sensing channel of ICD  16  may be affecting the detection of true cardiac signals. Status screen  324  presents an indication of whether the neurostimulation signal artifact exceeds a threshold level or whether the stimulation signal artifact is sufficiently low, such that the neurostimulation signal delivered by INS  26  does not adversely affect the sensing of cardiac signals by ICD  16 . In the example shown in  FIG. 22 , status screen  324  provides an indication that ICD  16  may be oversensing cardiac signals, i.e., the neurostimulation artifact on the selected sensing channel of sensing module  96  of ICD  16  exceeds a threshold level. 
     In other examples, programmer  24  may present other types of displays to provide information to a user regarding the neurostimulation signal artifact on one or more sensing channels of ICD  16 . For example, in some examples, processor  130  of programmer  24  may categorize a neurostimulation signal artifact based on the probability that the artifact will affect the sensing of true cardiac signals by ICD  16 . The categorization of the neurostimulation signal artifact maybe useful for providing a relatively quick and easy way to ascertain the extent of crosstalk between INS  26  and ICD  16 . 
       FIG. 23  is a flow diagram illustrating an example technique for categorizing a neurostimulation signal artifact. Processor  130  of programmer  24  may measure the extent of the crosstalk between INS  26  and ICD  16  ( 300 ), e.g., by determining a difference between a characteristic of a baseline electrical signal and a respective characteristic of the second electrical signal sensed by ICD  16  while INS  26  is delivering stimulation, as described with respect to  FIG. 21 . Processor  130  may determine a difference between the characteristics of the baseline and second electrical signals using any suitable technique. In some examples, processor  130  determines a difference between the characteristics of the baseline and second electrical signals by determining a difference between a first value indicative of the difference between an amplitude of the baseline electrical signal and a sensing threshold value of sensing module  96  ( FIG. 6 ) of ICD  16  and a second value indicative of the difference between an amplitude of the second electrical signal and the sensing threshold value of sensing module  96 . 
     Processor  130  may determine whether the characteristics of baseline and second electrical signals differ by a first threshold value ( 340 ). As discussed with respect to  FIG. 21 , in some examples, the threshold value may be based on the sensing threshold of ICD  16 , e.g., may be less than the sensing threshold, such as about 1% to about 99% of the sensing threshold or about 25% to about 50% of the sensing threshold amplitude. In the example shown in  FIG. 23 , memory  132  of programmer  24  stores a plurality of threshold values (or threshold levels) that are each associated with a different neurostimulation signal artifact category. The different categories may represent the relative intensity of the neurostimulation artifact on a selected sense channel of ICD  16 . The threshold values may be adjustable. For example, a clinician may program the threshold values into programmer  24 , ICD  16 , INS  26  or another device. The threshold values may be specific to a particular patient. 
     If processor  130  determines that the characteristics of baseline and second electrical signals do not differ by at least the first threshold value, processor  130  may determine that the extent of the crosstalk between INS  26  and ICD  16  falls within a first category, and processor  130  may generate a category one indication ( 342 ). The first category of crosstalk may be associated with a crosstalk level in which crosstalk between INS  26  and ICD  16  is present, but the extent of the crosstalk is relatively low. Processor  130  may determine that modifications to one or more stimulation parameters of INS  26  or one or more sense parameters of ICD  16  are not necessary when a category one indication is generated. 
     If processor  130  determines that the characteristics of the baseline and second electrical signals differ by at least the first threshold value, processor  130  may determine whether the characteristics of the baseline and second electrical signals differ by a second threshold level that is different than the first threshold level ( 344 ). In some examples, the second threshold level may be associated with a greater artifact intensity than the first threshold level. For example, the first threshold value may include a first voltage amplitude value or a first percentage that indicates a percentages change of a voltage amplitude of the second electrical signal relative to a baseline electrical signal. The second threshold level may include a second voltage amplitude value or a second percentage, where the second voltage amplitude value or percentage are greater than the first voltage amplitude value or percentage, respectively. 
     If processor  130  determines that the characteristics of the baseline and second electrical signals do not differ by at least the second threshold value, processor  130  may determine that the extent of the crosstalk between INS  26  and ICD  16  is within a second category, and processor  130  may generate a category two indication ( 346 ). In some examples, the second category of crosstalk may be associated with a crosstalk level in which crosstalk between INS  26  and ICD  16  exceeds an acceptable level. Thus, as shown in  FIG. 23 , upon generating the category two indication, processor  130  may initiate the modification to one or more stimulation parameter values of INS  26  or one or more sensing parameters of ICD  16  ( 304 ). 
     If processor  130  determines that the characteristics of the baseline and second electrical signals differ by at least the second threshold level, processor  130  may determine whether the characteristics of the baseline and second electrical signals differ by a third threshold value that is different than the first and second threshold values ( 348 ). In some examples, the third threshold value may be associated with a greater artifact intensity than the first and second threshold levels. For example, the third threshold level may include a third voltage amplitude value or a third percentage, where the third voltage amplitude value or percentage are greater than the first and second voltage amplitude values or percentages, respectively. 
     If processor  130  determines that the characteristics of the baseline and second electrical signals do not differ by at least the third threshold value, processor  130  may determine that the extent of the crosstalk between INS  26  and ICD  16  is within the second category, and processor  130  may generate a category two indication ( 346 ). On the other hand, if processor  130  determines that the characteristics of the baseline and second electrical signals differ by at least the third threshold value ( 348 ), processor  130  may generate a category three indication ( 350 ). In some examples, the third category of crosstalk may be associated with a crosstalk level in which crosstalk between INS  26  and ICD  16  exceeds an acceptable level. Thus, as shown in  FIG. 23 , upon generating the category three indication, processor  130  may initiate the modification to one or more stimulation parameter values of INS  26  or one or more sensing parameters of ICD  16  ( 304 ). These modifications may be the same or different as the modifications made in response to the generation of a category two indication ( 346 ). In addition, in some examples, the modification to the one or more stimulation parameter values of INS  26  may result in the suspension of the delivery of neurostimulation by INS  26  upon generation of the category three indication. 
     In some examples, processor  130  of programmer  24  or a processor of another device may evaluate the extent of crosstalk between ICD  16  and INS  26  based on the difference between one or more characteristics of the baseline and second electrical signals during a quiet segment of a cardiac cycle of heart  14 . As previously indicated the second electrical signal may be the electrical signal sensed by ICD  16  on a particular sense channel while INS  26  delivers neurostimulation signals to patient  12 . The quiet segment of a cardiac cycle may be when the intrinsic electrical signal of heart  14  is least active, such as during the T-P segment of a sinus rhythm of heart  14 . Because the absolute value of a voltage amplitude of a true cardiac signal may be the lowest during the quiet segment, determining a voltage amplitude of a second electrical signal sensed by ICD  16  on a particular sensing channel during the quiet segment may provide a more useful indication of the artifact present on the sensing channel of ICD  16 . The difference in voltage amplitudes between a baseline signal and a second electrical signal during the quiet segment may be more pronounced and, therefore, more revealing of the crosstalk between ICD  16  and INS  26 . 
       FIG. 24  is a flow diagram illustrating an example technique for parsing data from a baseline electrical signal and the second electrical signal that is sensed by the selected sense channel of ICD  16  during active delivery of stimulation by INS  26 . The parsed data may indicate the voltage amplitude of the baseline signal or the second electrical signal sensed by ICD  16  during a quiet segment of a cardiac cycle of heart  14 . Processor  130  of programmer  24  may receive a cardiac signal that includes a plurality of cardiac cycles ( 360 ), such as about 10 cardiac cycles to about 20 cardiac cycles. A cardiac cycle may be defined by, for example, a sinus rhythm including a QRST segment. 
     Processor  130  may identify the portion of the received electrical cardiac signals that correspond to the quiet segment of each cardiac cycle ( 362 ). As previously indicated, in some examples, the quiet segment may include the T-P segment of a sinus rhythm. Processor  130  may digitize the portions of the cardiac signals corresponding to the quiet segments ( 364 ), e.g., defining each quiet segment as about six points, although any suitable number of digitized points may be used. 
     Processor  130  may convert the digitized quiet segment portions of the cardiac cycles into a waveform in order to determine the peak-to-peak voltage amplitude (V PK-PK ) ( 366 ). Processor  130  may filter the direct current (DC) component out of the waveform in order to remove low-frequency artifact prior to determining the root mean square (RMS) amplitude of the waveform indicative of the quiet segment ( 368 ). Processor  130  may determine the mean and median peak-to-peak voltage amplitudes (V PK-PK ) ( 370 ), and determine the mean and median root mean square amplitudes (V RMS ) of the waveform indicative of the quiet segment of the cardiac signal based on the mean and median peak-to-peak voltage amplitudes ( 372 ). For example, processor  130  may determine the mean root mean square amplitude by determining the square root of the square of the mean peak-to-peak voltage amplitudes. 
     In order to evaluate the extent of crosstalk between ICD  16  and INS  26 , processor  130  may compare the RMS voltage amplitudes of the baseline and second electrical signals and determine whether the RMS amplitudes differ by one or more threshold values, as generally described with respect to  FIG. 21 . 
     In some examples, the extent of the crosstalk between INS  26  and ICD  16  may be evaluated based on one or more characteristics of an electrical signal that is sensed by ICD  16  when INS  26  is delivering an electrical signal that does not provide any therapeutic benefits to patient  12 . For example, INS  26  may generate and deliver a test electrical signal that does not provide stimulation therapy to patient  12 , and ICD  16  may sense electrical cardiac signals while INS  26  is delivering the test signals. In some examples, patient  12  does not perceive the test electrical signal, due to, for example, the intensity of the test signal and/or the timing of delivery of the test signal. For example, test electrical signal may comprise a sub-threshold amplitude signal that does not capture or otherwise activate tissue (e.g., neurons within the tissue) of patient  12 . An intensity of stimulation may be modified by modifying the current or voltage amplitude of a stimulation signal, a frequency of the stimulation signal, and, if the signal comprises a pulse, a pulse width or pulse shape of the stimulation signal. 
       FIG. 25  is a flow diagram illustrating an example technique for determining an extent of crosstalk between INS  26  and ICD  16  with a test signal that does not provides little to no therapeutic benefits to patient  12 . In the example shown in  FIG. 25 , processor  110  of INS  26  may control signal generator  114  to generate and deliver a test signal to patient  12  ( 373 ). The test signal may be nontherapeutic, e.g., does not provide efficacious therapy to patient  12  or provides minimally efficacious therapy to patient  12 . In contrast, a therapeutic electrical stimulation signals delivered by INS  26  may have a greater voltage amplitude, current amplitude, frequency or a different burst pattern than the test signal delivered by INS  26 . Memory  112  of INS  26 , memory  132  of programmer  24  or a memory of another device may store a therapy program that defines the signal parameter values for the test signal. In addition, in some examples, the test signal may comprise an amplitude that is less than an activation threshold of tissue, such that the patient&#39;s tissue is not substantially affected by the delivery of the test signal. Furthermore, in some examples, the test signal may comprise an amplitude that is less than a perception threshold of patient  12 , such that patient  12  does not perceive the delivery of the test signal by INS  26 . 
     As INS  26  generates and delivers the test signal, sensing module  96  ( FIG. 6 ) of ICD  16  may sense an electrical signal via a selected sensing channel ( 374 ), although more than one sensing channel may also be used in other examples. Processor  90  of ICD  16  may determine whether a characteristic of the sensed electrical signal exceeds a threshold value ( 376 ). The threshold value may indicate an amplitude value at which the electrical signal sensed by the selected sensing channel of ICD  16  indicates that the extent of crosstalk between INS  26  and ICD  16  may exceed an acceptable level if INS  26  delivers neurostimulation signals in an ordinary course, e.g., according to a therapy program defining therapeutic neurostimulation signals. While the delivery of the test signal by INS  26  may not result in an unacceptable level of crosstalk between INS  26  and ICD  16 , one or more characteristics of the signal that is sensed by ICD  16  during the delivery of the test signal by INS  26  may be represent a neurostimulation artifact that may result if INS  26  delivers neurostimulation signals in an ordinary course. 
     A clinician may determine the threshold value using any suitable technique. In one example, the clinician may detect when there is an unacceptable level of crosstalk between INS  26  and ICD  16 , e.g., based on actual signals sensed by ICD  16  when INS  26  delivers therapeutic neurostimulation signals to patient  12 . Shortly thereafter, e.g., while leads  28 ,  29  are likely in the same position as when the unacceptable level of crosstalk was detected, the clinician may control INS  26  to deliver the test signal to patient  12 . The electrical signal that is sensed by ICD  16  while INS  26  delivers the test signal to patient  12  may be indicative of the unacceptable level of crosstalk between INS  26  and ICD  16 . Thus, one or more characteristics of the electrical signal that is sensed by ICD  16  while INS  26  delivers the test signal to patient  12  may be stored as a threshold value, e.g., in memory  92  of ICD  16  or memory  112  of INS  26 . 
     Determining the extent of potential crosstalk between INS  26  and ICD  16  prior to delivering therapeutic neurostimulation therapy to patient  12  may be useful for confirming that the extent of crosstalk between INS  26  and ICD  16  is within an acceptable range in advance of delivering the neurostimulation therapy. This may help mitigate the possibility that the delivery of neurostimulation by INS  26  interferes with the sensing of cardiac signals by ICD  16 . 
     If processor  90  of ICD  16  determines that one or more characteristics of the sensed electrical signal is greater than or equal to the threshold value ( 376 ), processor  90  may suspend the delivery of therapeutic electrical stimulation by INS  26  ( 378 ). If processor  90  of ICD  16  determines that the sensed electrical signal does not exceed the threshold value ( 376 ), processor  90  may determine that the relative level of crosstalk between INS  26  and ICD  16  is within an acceptable level. Processor  90  may then provide INS  26  with a controls signal that indicates that INS  26  may generate and deliver therapeutic electrical stimulation to patient  12  ( 380 ). 
     The technique shown in  FIG. 25  may be implemented to evaluate the extent of crosstalk between INS  26  and ICD  16  at any suitable evaluation frequency. In some examples, INS  26  may deliver the test signal to patient  12  ( 373 ) at a test frequency of about one to about ten times per minute, although more frequent (e.g., about 1 Hz to about 100 Hz) or less frequent testing frequencies are contemplated. INS  26  may notify ICD  16  prior to sending the test signal or ICD  16  and INS  26  may have synchronized clocks such that ICD  16  senses the electrical signal on a selected sensing channel ( 374 ) at substantially the same time that INS  26  delivers the test signal. 
     In some cases, the one or more characteristics of the electrical signal sensed by ICD  16  while INS  26  is delivering the non-therapeutic test signal may also indicate an intensity of stimulation signals that INS  26  may deliver without adversely affecting the sensing of cardiac signals by ICD  16 . For example, the one or more characteristics of the electrical signal (e.g., a voltage or current amplitude) may be associated with a specific therapy program or instructions for modifying a therapy program in memory  132  ( FIG. 8 ) of programmer  24 , memory  112  ( FIG. 7 ) of INS  26  or memory  92  ( FIG. 6 ) of ICD  16 . 
     Processor  130  of programmer  24  or another device may determine the one or more characteristics of the electrical signal sensed by ICD  16  while INS  26  is delivering the non-therapeutic test signal to patient  12 . If the one or more characteristics of the signal are less than the threshold value ( 376 ), thereby indicating that the crosstalk between ICD  16  and INS  26  is acceptable, processor  130  may determine an acceptable stimulation therapy program for INS  26 . For example, processor  130  may reference a data structure stored in memory  112  to determine the therapy program or instructions for modifying a therapy program. Processor  130  may then instruct processor  110  of INS  26  to deliver therapy to patient  12  in accordance with the therapy program associated with the one or more characteristics of the electrical signal or in accordance with therapy parameters modified based on the instructions associated with the one or more characteristics of the electrical signal. 
     The therapy programs or instructions for modifying a therapy program based on the one or more characteristics of the electrical signal sensed by ICD  16  while INS  26  is delivering the non-therapeutic test signal to patient  12  may be determined during a programming session with a clinician. The clinician may determine a characteristic of an electrical signal sensed by ICD  16  while INS  26  is delivering the non-therapeutic test signal to patient  12 , and determine the therapy parameter values that provide efficacious therapy to patient  12  without interfering with the sensing of cardiac signals by ICD  16 . These therapy parameter values may then be associated with the signal characteristic in memory  132  (or a memory of another device) as a therapy program or an instruction for modifying a baseline therapy program. 
     The delivery of electrical stimulation by INS  26  may change an amplitude of an electrical cardiac signal (e.g., an EGM) sensed by ICD  16 . Thus, in some examples, the crosstalk status of a therapy system including ICD  16  and INS  26  may be evaluated based on a change in amplitude of an electrical cardiac signal sensed while INS  26  is not actively delivering stimulation to patient  12  and an electrical cardiac signal sensed while INS  26  is delivering stimulation to patient  12 . 
       FIG. 26  illustrates a flow diagram of an example technique for determining a crosstalk status (or an electrical noise status) of therapy system  10 . In the technique shown in  FIG. 26  Processor  90  of ICD  16  may instruct processor  110  of INS  26  to suspend or otherwise adjust the delivery of neurostimulation ( 381 ). For example, processor  90  may transmit a control signal to processor  110  via the respective telemetry modules  98  ( FIG. 6 ),  118  ( FIG. 7 ). The control signal may not only indicate whether INS  26  should suspend or otherwise adjust the delivery of neurostimulation to patient  12 , but, in some examples, may indicate how long INS  26  should suspend neurostimulation or deliver therapy according to the adjusted parameters. In other examples, memory  112  ( FIG. 7 ) of INS  26  may store instructions for suspending or otherwise adjusting neurostimulation when processor  110  of INS  26  receives the control signal from processor  90  of ICD  16 . As another example, INS  26  may suspend or otherwise adjust delivery of stimulation without intervention from ICD  16 , e.g., according to schedule stored by memory  112 . 
     During the time in which neurostimulation is suspended or adjusted, sensing module  96  ( FIG. 6 ) of ICD  16  may sense a first electrical cardiac signal and processor  90  may determine a first characteristic of the first electrical cardiac signal ( 382 ). In some examples, the first characteristic may be a mean or median P-wave or R-wave amplitude over a predetermined period of time. Processor  90  of ICD  16  may then activate the delivery of stimulation by INS  26  ( 384 ). For example, processor  90  may generate a control signal that is transmitted to processor  110  of INS  26  via the respective telemetry modules  98  ( FIG. 6 ),  118  ( FIG. 7 ). Upon receiving the control signal, processor  110  of INS  26  may control stimulation generator  114  to begin generating and delivering neurostimulation therapy. In other examples, processor  110  of INS  26  may begin generating and delivering neurostimulation therapy based on a predetermined schedule that indicates the times at which processor  110  should suspend the delivery of neurostimulation and initiate the delivery of stimulation. 
     After INS  26  commences the delivery of neurostimulation to patient  12 , processor  90  may control sensing module  96  to sense a second electrical cardiac signal of heart  14  of patient  12 . Processor  90  may determine a second characteristic of the second electrical cardiac signal ( 386 ). In some examples, the first and second characteristics may be similar characteristics. For example, the first and second characteristics may be a mean or median P-wave or R-wave amplitude of the first and second electrical cardiac signals, respectively, over a predetermined period of time. 
     Processor  90  may determine whether the first and second characteristics are within a threshold range of each other ( 388 ). In general, if the first and second characteristics are similar, e.g., within a threshold range of each other, the crosstalk status of the therapy system including ICD  16  and INS  26  may be relatively low. The threshold range may be, for example, about 20% of the value of the first characteristic, such as about 5% to about 20%, about 10% to about 15%, or substantially equal. Thus, in some examples, if the difference between the first and second characteristics is less than about 20% of the value of the first characteristic, processor  90  may determine that the first and second characteristics are within a threshold range of each other. 
     First and second characteristics that are within a threshold range of each other may indicate that the delivery of neurostimulation by INS  26  has a minimal affect on the electrical cardiac signal sensed by ICD  16 , such that the possibility that ICD  16  may sense the neurostimulation signal and mischaracterize the signal as an electrical cardiac signal is relatively low. In such a situation, the crosstalk status may be acceptable. 
     If the first and second characteristics are within a threshold range of each other, processor  90  may continue comparing the first and second characteristics of subsequently sensed electrical cardiac signals in accordance with the technique shown in  FIG. 26 . On the other hand, if the first and second characteristics are not within a threshold range of each other, processor  90  may determine that the extent of crosstalk between ICD  16  and INS  26  is unacceptable, e.g., that the crosstalk status is unacceptable. Accordingly, processor  90  may generate a crosstalk indication ( 389 ) if the first and second characteristics are not within a threshold range of each other. The crosstalk indication may be a value, flag, or signal that is stored or transmitted to indicate the unacceptable crosstalk status. In some examples, processor  90  or  110  may transmit the crosstalk indication to programmer  24  or another external device, including remote devices, e.g., using a system described with respect to  FIG. 32 . In some examples, programmer  24  may present a notification to a user via user interface  134  ( FIG. 8 ) to indicate an unacceptable level of crosstalk was detected. 
     While  FIG. 26  is described with respect to processor  90  of ICD  16 , in other examples, processor  130  of programmer  24  or processor  110  of INS  26  or another device may perform any part of the technique shown in  FIG. 26 . For example, a clinician may evaluate the crosstalk status between ICD  16  and INS  26  with the aid of programmer  24 . Processor  130  of programmer  24  may perform any part of the technique shown in  FIG. 26 . For example, processor  130  may determine the first and second characteristics ( 382 ,  386 ) based on electrical cardiac signals sensed by ICD  16  and transmitted to programmer  24  by ICD  16 . 
     In some examples, ICD  16  and/or INS  26  may periodically check the impedance of one or more electrical paths, each path comprising two or more implanted electrodes on one or more implanted leads. For example, processor  90  of ICD  16  may initiate a check of the impedance of an electrical path comprising lead  18  ( FIG. 3 ) and electrodes  50 ,  52 ,  72 . ICD  16  and/or INS  26  may, for example, check the impedance of one or more electrical paths comprising an electrode prior to delivering electrical stimulation to patient  12  in order to confirm that electrical interference or lead-related conditions that may affect the efficacy of the delivery of stimulation to patient  12  are not present. 
     The impedance measurements may be used to detect lead-related conditions, such as short circuits, open circuits or significant changes in impedance that may adversely affect the performance of therapy delivery by ICD  16  or INS  26  or sensing by ICD  16  or INS  26 . Changes in impedance of an electrical path that is electrically connected to ICD  16  or INS  26  may increase the amount of crosstalk observed by ICD  16  by, for example, effectively widening a stimulation dipole of INS  26  or a sensing dipole of ICD  16  by creating a leakage path due to a lead-related condition, such as a lead fracture. A lead-related condition my often cause noise on a sensing channel of ICD  16 . Thus, the technique shown in  FIG. 25  may be useful for identifying a lead-related condition. 
     In some examples, lead integrity testing may also involve comparing the measured impedance to a threshold in order to determine whether the lead(s) have a lead-related condition. This integrity testing may be performed periodically, e.g., while patient  12  is sleeping or as patient  12  moves and subjects any of the leads  18 ,  20 ,  22 ,  28 ,  29  coupled to ICD  16  or INS  26  to mechanical stresses. 
     ICD  16  and INS  26  may measure impedance by determining an electrical parameter value indicative of the impedance. In some examples, ICD  16  or INS  26  may perform an impedance measurement by delivering, from the respective stimulation generator  94 ,  114 , an electrical signal having a constant voltage between at least two electrodes, and measuring a resulting current of the signal that is sensed by two or more electrodes. The respective processor  90 ,  110  may determine a resistance based upon the voltage amplitude of the electrical signal and the measured amplitude of the resulting current. The current of the sensed signal or the determined resistance may be electrical parameter values indicative of the impedance path comprising the electrodes. 
     In other examples, ICD  16  or INS  26  may perform impedance measurement by delivering, from the respective stimulation generator  94 ,  114 , a current pulse across at least two electrodes, and measuring a resulting voltage of a signal that is sensed by two or more electrodes. The respective processor  90 ,  110  may determine a resistance based upon the current amplitude of the pulse and the measured amplitude of the resulting voltage. The voltage of the sensed signal or the determined resistance may be electrical parameter values indicative of the impedance path comprising the electrodes. 
     Sensing module  96  of ICD  16  and a sensing module of INS  26  may include circuitry for measuring amplitudes of resulting currents or voltages, such as sample and hold circuitry. ICD  16  and INS  26  may use defined or predetermined pulse amplitudes, widths, frequencies, or electrode polarities for the pulses delivered for these various impedance measurements. In these examples, stimulation generators  94 ,  114  may deliver electrical signals that do not necessarily deliver stimulation therapy to patient  12 , due to, for example, the amplitudes of such signals and/or the timing of delivery of such signals. For example, these signals may comprise sub-threshold amplitude signals that may not stimulate tissue, e.g., below a threshold necessary to capture or otherwise activate tissue. In the case of ICD  16 , the electrical signals for measuring impedance of an electrical path may be delivered during a refractory period, in which case they also may not stimulate heart  14 . 
     In certain cases, ICD  16  and INS  26  may collect electrical parameter values that include both a resistive and a reactive (i.e., phase) component. In such cases, ICD  16  and INS  26  may measure impedance during delivery of a sinusoidal or other time varying signal by the respective stimulation generator  94 ,  114 . Thus, as used herein, the term “impedance” is used in a broad sense to indicate any collected, measured, and/or determined value that may include one or both of resistive and reactive components. Impedance data may include electrical parameter values that can be used to determine impedance (such as current and/or voltage values). 
     Crosstalk between INS  26  and ICD  16  may adversely affect the impedance measurements take by ICD  16  and INS  26 . For example, the electrical stimulation signals generated and delivered by INS  26  may be sensed by ICD  16  during a bipolar, tripolar or quadrapolar impedance measurement. Similarly, the electrical stimulation signals (e.g., pacing pulses or defibrillation pulses) generated and delivered by ICD  16  may be sensed by INS  26  during a bipolar, tripolar or quadrapolar impedance measurement. Inaccurate impedance measurements by either INS  26  or ICD  16  may adversely affect the system integrity checks performed by INS  26  or ICD  16 , such as by causing ICD  16  or INS  26  to over-sense or under-sense a system integrity issue. Oversensing a system integrity issue may be undesirable because of, for example, the time required for patient  12  to resolve a false-positive system integrity issue. Undersensing a system integrity issue may also be undesirable because a system integrity issue may affect the efficacy of therapy delivery by ICD  16  and INS  26 , and, therefore, it may be desirable for system integrity issues to be addressed by qualified individual as soon as possible. 
       FIG. 27  is a flow diagram of an example technique that may be implemented in order to determine whether the crosstalk between ICD  16  and INS  26  may be adversely affecting the impedance measurements taken by ICD  16 . Processor  90  of ICD  16  may control INS  26  to suspend or otherwise adjust (e.g., decrease the intensity) the delivery of neurostimulation ( 290 ), as described above with respect to  FIG. 20 . Processor  90  may determine a first electrical parameter value indicative of an impedance of an electrical path ( 390 ), e.g., by delivering a voltage pulse or a current pulse and determining a resulting current or voltage, respectively. Thereafter, processor  90  may activate the delivery of neurostimulation signals by INS  26 , e.g., as described above with respect to  FIG. 20  ( 294 ). 
     While INS  26  is delivering neurostimulation signals to patient  12 , processor  90  of ICD  16  may determine a second electrical parameter value indicative of the impedance of the electrical path ( 392 ). Processor  90  may compare the first and second electrical parameter values ( 394 ). If the first and second determined impedance values are within a threshold range, e.g., within about 20% or less of each other, such as about 5% to about 20%, about 10% to about 15%, or substantially equal, processor  90  may determine that the delivery of neurostimulation by INS  26  does not adversely affect the impedance measurement by ICD  16 . Processor  90  may periodically perform the technique shown in  FIG. 27 , such as at an impedance sampling frequency of about 1 Hz to about 100 Hz. Other frequencies are contemplated, such as a frequency of about one to about ten times per minute. In other examples, processor  90  may compare the first and second electrical parameter values indicative of impedance by, for example, comparing the difference between the first and second electrical parameter values to a threshold value. 
     If the difference exceeds a threshold value or falls outside of a threshold range of values, processor  90  may determine that the first and second electrical parameter values are not within the threshold range of each other. If the first and second determined impedance values are not within the threshold range of each other ( 394 ), processor  90  may generate an impedance measurement interference indication ( 396 ). The impedance measurement interference indication may be a value, flag, or signal that is stored in memory  92  of ICD  16  or transmitted to another device (e.g., programmer  24  or INS  26 ) to indicate that the delivery of neurostimulation by INS  26  may potentially be interfering with the accurate and precise impedance measurements of one or more electrical paths coupled to ICD  16 . In some cases, the change in impedance after INS  26  begins delivering stimulation to patient  12  may also indicate that a therapy system integrity issue is present, such as a lead-related condition (e.g., a lead fracture). The lead-related condition may be related to the integrity of one or more of the leads  18 ,  20 ,  22  ( FIG. 3 ) electrically connected to ICD  16  or one or more of the leads  28 ,  29  ( FIG. 5 ) electrically connected to ICD  16 . 
     In some examples, processor  90  may initiate the modification to one or more stimulation parameter values that define the neurostimulation delivered by INS  26  or suspend the delivery of neurostimulation by INS  26  if an impedance measurement interference indication determination is generated.  FIG. 28  is a flow diagram illustrating an example technique that may be implemented to modify the neurostimulation signal delivered by INS  26  in an attempt to mitigate the effect on impedance measurements of electrical paths taken by ICD  16 . The example technique shown in  FIG. 28  is substantially similar to the technique shown in  FIG. 27 . However, after generating the impedance measurement interference indication ( 396 ), processor  90  of ICD  16  may initiate the modification to one or more one or more neurostimulation parameter values ( 398 ). For example, processor  90  may instruct processor  110  of INS  26  to modify the one or more stimulation parameter values or switch therapy programs, or processor  90  of ICD  16  may transmit the modified stimulation parameter values to INS  26 . 
     After the one or more stimulation parameter values are modified, processor  90  may suspend or otherwise adjust the delivery of neurostimulation by INS  26  ( 290 ), determine a first electrical parameter value indicative of an impedance an electrical path ( 390 ), activate the delivery of neurostimulation by INS  26  ( 376 ), determine a second electrical parameter value indicative of the impedance of the electrical path ( 392 ), and determine whether the first and second electrical parameter values are within an threshold range of each other ( 394 ). Processor  90  of ICD  16  or processor  110  of INS  26  may continue modifying the INS  26  stimulation parameter values until processor  90  determines that the impedance measurement by ICD  16  is not substantially affected by the delivery of neurostimulation by INS  26  or until no further neurostimulation parameter values may be modified, i.e., all permissible neurostimulation modifications have been exhausted. The permissible neurostimulation modifications may set forth ranges for the different stimulation parameter values that provide efficacious therapy to patient  12 . Thus, modifying the neurostimulation parameters such that the values fall outside of the ranges may result in neurostimulation signals that do not provide efficacious therapy to patient  12 . 
     In some examples, the delivery of electrical stimulation, e.g., pacing pulses or defibrillation pulses, by ICD  16  may adversely affect impedance determinations by INS  26 .  FIG. 29  is a flow diagram illustrating an example technique for determining whether the delivery of electrical stimulation by ICD  16  adversely affects impedance determinations by INS  26 . The technique shown in  FIG. 29  is similar to the technique that may be implemented by ICD  16  and shown in  FIG. 27 . 
     Processor  110  of INS  26  may cause ICD  16  to suspend or otherwise adjust the delivery of stimulation ( 400 ), which may include, for example, a cardiac rhythm therapy. For example, processor  110  may transmit a control signal to processor  90  of ICD  16  via the respective telemetry modules  118  ( FIG. 7 ),  98  ( FIG. 6 ). The control signal may not only indicate whether ICD  16  should suspend the delivery of stimulation to patient  12 , but, in some examples, may indicate how long ICD  16  should suspend stimulation. In other examples, memory  92  of ICD  16  may store instructions for suspending stimulation when processor  90  receives the control signal from processor  110  of INS  26 . As another example, ICD  16  may suspend delivery of stimulation without intervention from INS  26 , e.g., according to schedule stored by memory  92 , where the schedule may indicate the times at which INS  26  takes impedance measurements. 
     Processor  110  may determine a first electrical parameter value indicative of an impedance of an electrical path ( 402 ), e.g., by generating and delivering a constant voltage signal or a constant current signal and measuring a resulting current or voltage, respectively, of a sensed signal, respectively. The electrical path may comprise, for example, a path between stimulation generator  114  and electrodes  124  ( FIG. 7 ) of lead  28 . Thereafter, processor  90  may activate the delivery of stimulation signals by ICD  16  ( 404 ). For example, processor  90  of ICD  16  may control stimulation generator  94  to generate and deliver stimulation upon the detection of an arrhythmia or at regular intervals, e.g., to pace heart  14 . 
     While ICD  16  is delivering stimulation signals to patient  12 , processor  110  of INS  26  may determine a second electrical parameter value indicative of the impedance of the electrical path ( 406 ). Processor  110  may compare the first and second determined impedance values ( 408 ). If the first and second determined impedance values are within a threshold range, e.g., within about 20% or less of each other, such as about 10% or substantially equal, processor  110  may determine that the delivery of stimulation by ICD  16  does not adversely affect the impedance determination by INS  26 . Processor  90  may periodically perform the technique shown in  FIG. 27 , such as at an impedance sampling frequency of about 1 Hz to about 100 Hz or about one to about ten times per minute. 
     On the other hand, if the first and second determined impedance values are not within the threshold range of each other ( 408 ), processor  110  may generate an impedance measurement interference indication ( 410 ). The impedance measurement interference indication may be a value, flag, or signal that is stored in memory  112  of INS  26  or transmitted to another device (e.g., programmer  24  or ICD  16 ) to indicate that the delivery of stimulation by ICD  16  may potentially be interfering with the accurate and precise impedance measurements of one or more electrical paths coupled to INS  26 . 
     In other examples, any part of the techniques shown in  FIGS. 27-29  may be performed by processor  130  of programmer  24  or another device. 
     In some cases, processor  90  of ICD  16 , processor  110  of INS  26  or another device may evaluate a change in the difference between the first and second electrical parameter values over time to evaluate the integrity of therapy system  10  ( FIG. 1 ). As indicated above, the first electrical parameter value may be indicative of an impedance of an electrical path electrically connected to ICD  16  or INS  26  while INS  26  or ICD  16 , respectively, is not actively delivering stimulation to patient  12 , and the second electrical parameter value may be indicative of the impedance of the electrical path while INS  26  or ICD  16 , respectively, is delivering stimulation to patient  12 . 
       FIG. 30  is a flow diagram illustrating an example technique for evaluating the integrity of therapy system  10  based on the difference between the first and second electrical parameter values over time. Processor  90  of ICD  16  or processor  110  of INS  26  may determine the difference between the first and second electrical parameter values over time ( 412 ). For example, for each impedance determination, e.g., as described above with respect to  FIG. 27 , processor  90  or processor  110  may determine the difference between the first and second electrical parameter values and store the value indicative of the difference in memory  92  ( FIG. 6 ). In other examples, processor  90  or processor  110  may determine the difference between the first and second electrical parameter values less frequently than the frequency with which the first and second electrical parameter values are determined. For example, processor  90  or processor  110  may determine the difference between the first and second electrical parameter values once for every two times the first and second electrical parameter values are determined. Other frequencies with which processor  90  or processor  110  the difference between the first and second electrical parameter values are contemplated. 
     Processor  90  or processor  110  may determine whether the difference between the first and second electrical parameter values is increasing over time ( 414 ). That is, processor  90  or processor  110  may determine a trend in a difference between the impedance of the electrical path electrically connected to ICD  16  that is determined while INS  26  is delivering stimulation begins to differ from the impedance that is determined while INS  26  is not actively delivering stimulation to patient  12 . This trend may indicate, for example, whether the crosstalk between ICD  16  and INS  26  is increasing over time. In addition, the trend may indicate whether another system integrity issue, such as a lead-related condition, may be present. 
     If the difference between first and second electrical parameter values remains substantially constant over time (e.g., stays within a particular range, such as less than about 25% of a mean or median difference value), processor  90  or processor  110  may determine that a system integrity issue is not present. Processor  90  or processor  110  may then continue monitoring the difference between the first and second electrical parameter values over time ( 412 ). 
     On the other hand, if the difference between first and second electrical parameter values increases over time, processor  90  or processor  110  may determine that a therapy system integrity issue is present. Accordingly, processor  90  or processor  110  may generate a system integrity indication ( 416 ). The system integrity indication may be a value, flag, or signal that is stored or transmitted to indicate that clinician attention is desirable. The clinician attention may be desirable to, for example, assess the integrity of leads  18 ,  20 ,  22 ,  28 ,  29  that may be implanted within patient  12 . In some examples, processor  90  or  110  may transmit the system integrity indication to programmer  24  or another external device, including remote devices, e.g., using a system described with respect to  FIG. 32 . 
     In some examples, processor  90  or processor  110  may generate the system integrity indication if the difference between the first and second electrical parameter values increases over time by a predetermined rate, which may be stored in memory  92  or  112  of ICD  16  or INS  26 , respectively. In other examples, processor  90  or processor  110  may generate the system integrity indication if the difference between the first and second electrical parameter values at a particular point in time exceeds the mean or median difference by a threshold value. The mean or median difference may be determined based on the mean or median value of the difference between the first and second electrical parameter values over a particular range of time preceding the current determination of the difference between the first and second electrical parameter values. 
     The techniques described herein, such as the techniques described with respect to  FIGS. 9-12B  for modifying one or more operating parameters of INS  26  in order to minimize crosstalk between INS  26  and ICD  16 , with respect to  FIGS. 16 ,  19 A, and  19 B for modifying one or more sensing parameters of ICD  16  in order to minimize crosstalk between INS  26  and ICD  16 , with respect to  FIGS. 20 ,  21 ,  23 - 30  for determining the extent of crosstalk between INS  26  and ICD  16 , may also be implemented for determining the extent of crosstalk in a device comprising the functionality of INS  26  and ICD  16  in a common housing. 
       FIG. 31  is a functional block diagram illustrating an example IMD  420  that includes a neurostimulation module  422  and a cardiac therapy module  424  in a common housing  426 . Neurostimulation therapy module  422  includes stimulation generator  114 , which is described above with respect to  FIG. 7 . Similarly, cardiac therapy module  424  includes stimulation generator  94  and sensing module  96 , which are described above with respect to  FIG. 6 . IMD  420  also includes processor  90 , memory  92 , telemetry module  98 , and power source  100 , which are described above with respect to  FIG. 6 . 
     Neurostimulation therapy module  422  may deliver electrical stimulation to a tissue site proximate to a nerve. As previously discussed with respect to INS  26 , the stimulation may be delivered to the nerve via an intravascular lead or an extravascular lead. In other examples, neurostimulation therapy module  422  may deliver electrical stimulation to a nonmyocardial tissue site that may or may not be proximate a nerve. Cardiac therapy module  424  may sense electrical cardiac signals of patient  12  and deliver cardiac rhythm management therapy to heart  14 , such as pacing, cardioversion or defibrillation therapy. 
     Processor  90  may control neurostimulation therapy module  422  and cardiac therapy module  424  according to any of the techniques described above to minimize the possibility that cardiac therapy module  424  delivers electrical stimulation to heart  14  in response to detecting electrical signals generated and delivered by neurostimulation therapy module  422  that resemble an arrhythmic cardiac signal. For example, with respect to the technique shown in  FIG. 9 , processor  90  may control neurostimulation therapy module  422  to deliver stimulation therapy to patient  12  ( 140 ). In addition, processor  90  may control sensing module  96  to sense electrical cardiac signals ( 142 ). 
     If processor  90  detects a potential arrhythmia based on the sensed electrical cardiac signals ( 144 ), processor may modify the stimulation signals delivered by neurostimulation therapy module  422  ( 146 ). For example, processor  90  may modify one or more therapy parameter values with which neurostimulation therapy module  422  generates electrical stimulation signals, e.g., using the techniques described with respect to  FIGS. 11A-11D . As another example, processor  90  may switch the therapy programs with which neurostimulation therapy module  422  generates the electrical stimulation signals, e.g., using the techniques described with respect to  FIGS. 12A and 12B . 
     Processor  90  of the IMD  420  including both neurostimulation therapy module  422  and cardiac therapy module  424  may also modify one or more sensing parameters of sensing module  96  if neurostimulation therapy module  422  is delivering electrical stimulation therapy to patient  12 , e.g., as described with respect to  FIGS. 16 ,  19 A, and  19 B. 
     Programmer  24  or another device may also evaluate the crosstalk between neurostimulation therapy module  422  and cardiac therapy module  424  using any of the techniques described herein, e.g., the techniques described with reference to  FIGS. 20 ,  21 , and  23 - 26 . However, instead of controlling ICD  16  and INS  26  or receiving information from separate devices  16 ,  26 , programmer  24  may control neurostimulation therapy module  422  and cardiac therapy module  424  of a common IMD  420 , and receive information from a single IMD  420 . In addition, the techniques shown in  FIGS. 27-30  may also be implemented by processor  90  in order to determine whether the delivery of electrical stimulation by neurostimulation therapy module  422  or cardiac therapy module  424  interferes with impedance measurements taken by processor  90 . 
       FIG. 32  is a block diagram illustrating a system  430  that includes an external device  432 , such as a server, and one or more computing devices  434 A- 434 N that are coupled to ICD  16 , INS  26 , and programmer  24  shown in  FIG. 1  via a network  436 , according to one example. In this example, ICD  16  and INS  26  uses their respective telemetry modules  98  ( FIG. 6) and 118  ( FIG. 7 ) to communicate with programmer  24  via a first wireless connection, and to communicate with an access point  438  via a second wireless connection. In the example of  FIG. 11 , access point  438 , programmer  24 , external device  432 , and computing devices  434 A- 434 N are interconnected, and able to communicate with each other, through network  436 . 
     In some cases, one or more of access point  438 , programmer  24 , external device  432 , and computing devices  434 A- 434 N may be coupled to network  436  through one or more wireless connections. ICD  16 , INS  26 , programmer  24 , external device  432 , and computing devices  434 A- 434 N may each comprise one or more processors, such as one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, that may perform various functions and operations, such as those described herein. 
     Access point  438  may comprise a device that connects to network  436  via any of a variety of connections, such as telephone dial-up, digital subscriber line (DSL), or cable modem connections. In other examples, access point  438  may be coupled to network  436  through different forms of connections, including wired or wireless connections. In some examples, access point  438  may communicate with programmer  24 , ICD  16 , and/or INS  26 . Access point  438  may be co-located with patient  12  (e.g., within the same room or within the same site as patient  12 ) or may be remotely located from patient  12 . For example, access point  438  may be a home monitor that is located in the patient&#39;s home or is portable for carrying with patient  12 . 
     During operation, ICD  16  and/or INS  26  may collect, measure, and store various forms of diagnostic data. For example, as described previously, ICD  16  or INS  26  may collect electrical parameter values indicative of an impedance of an electrical path. In certain cases, ICD  16  or INS  26  may directly analyze collected diagnostic data and generate any corresponding reports or alerts. In some cases, however, ICD  16  or INS  26  may send diagnostic data to programmer  24 , access point  438 , and/or external device  432 , either wirelessly or via access point  438  and network  436 , for remote processing and analysis. 
     For example, ICD  16  or INS  26  may send programmer  24  collected electrical parameter values indicative of the impedance of various electrical paths of therapy system  10  ( FIG. 1 ), arrhythmia indications that indicate an arrhythmia was detected (e.g., as discussed with respect to  FIG. 10 ), interference indications that indicate modification to the stimulation or sensing parameters of ICD  16  or INS  26  failed to reduce detected crosstalk between ICD  16  and INS  26  (e.g., as discussed with respect to  FIGS. 11A-11D ), interference indications that indicate the determined interference between ICD  16  and INS  26  or otherwise detected exceeds a certain level (e.g., as discussed with respect to  FIGS. 21 and 23 ), and impedance measurement interference indications that indicate that stimulation delivery by ICD  16  or INS  26  may be interfering with the measurement of the impedance of various electrical paths of therapy system  10  (e.g., as discussed with respect to  FIGS. 27-29 ). 
     Processor  24  may analyze the received electrical parameter values and/or indications. Programmer  24  may generate reports or alerts after analyzing the information from ICD  16  or INS  26  and determine whether the values and indications indicate that patient  12  requires medical attention, e.g., based on ICD  16  and INS  26  crosstalk that exceeds an acceptable level. In some cases, ICD  16 , INS  26 , and/or programmer  24  may combine all of the diagnostic data into a single displayable report, which may be displayed on programmer  24 . The report may contain information concerning the impedance measurements or indications, the time of day at which the measurements were taken or at which the indications were generated, and identify any patterns in the impedance measurements or arrhythmia or interference indications. 
     In another example, ICD  16  or INS  26  may provide external device  432  with collected impedance data via access point  438  and network  436 . External device  432  includes one or more processors  440 . In some cases, external device  432  may request collected impedance data and stored indications, and in some cases, ICD  16  or INS  26  may automatically or periodically provide such data to external device  432 . Upon receipt of the impedance data and indication data via input/output device  442 , external device  432  is capable of analyzing the data and generating reports or alerts upon determination that the impedance data indicates a lead integrity issue or upon determination that additional clinician assistance is necessary to decrease the crosstalk between ICD  16  and INS  26 . In some examples, ICD  16  or INS  26  may analyze the data and generate reports or alerts, which may be transmitted to external device  432  via network  436 . In addition, in some examples, a therapy system may not include programmer  24  to evaluate crosstalk, but, may instead rely on external device  432  or other devices to evaluate crosstalk between ICD  16  and INS  26 . 
     In one example, external device  432  may combine the diagnostic data into an report. One or more of computing devices  434 A- 434 N may access the report through network  436  and display the report to users of computing devices  434 A- 434 N. In some cases, external device  432  may automatically send the report via input/output device  442  to one or more of computing devices  434 A- 434 N as an alert, such as an audio or visual alert. In some cases, external device  432  may send the report to another device, such as programmer  24 , either automatically or upon request. In some cases, external device  432  may display the report to a user via input/output device  442 . 
     In one example, external device  432  may comprise a secure storage site for diagnostic information that has been collected from ICD  16 , INS  26 , and/or programmer  24 . In this example, network  436  may comprise an Internet network, and trained professionals, such as clinicians, may use computing devices  434 A- 434 N to securely access stored diagnostic data on external device  432 . For example, the trained professionals may need to enter usernames and passwords to access the stored information on external device  432 . In one example, external device  432  may be a CareLink server provided by Medtronic, Inc., of Minneapolis, Minn. 
     The examples therapy systems described herein include one ICD  16  and one INS  26 . In other examples, the techniques described herein may also apply to therapy systems that include more than one ICD  16  and/or more than one INS  26 . For example, the techniques shown in  FIGS. 9-11D  for modifying one or more electrical stimulation parameter values of an INS may be applicable to modifying one or more electrical stimulation parameter values for more than one INS. Some therapy systems may include more than one INS. For example, some therapy systems may include multiple microstimulators that each delivers electrical stimulation therapy to patient  12 . A microstimulator may include a substantially self-contained electrical stimulation device that includes electrodes on a housing of the microstimulator, rather than being coupled to electrodes via one or more leads that extend from the housing. However, the microstimulator may be coupled to electrodes of leads in some examples. The multiple implanted microstimulators or other INS&#39; may be distributed throughout the patient&#39;s body. In some examples, the microstimulators may communicate with each other to coordinate therapy delivery to patient  12 . In addition, in some examples, the microstimulators may communicate with a master microstimulator or ICD  16 , either of which may control the delivery of electrical stimulation by one or more of the other implanted microstimulators. Delivery of electrical stimulation signals by any one of the INS&#39; may generate crosstalk with ICD  16 . Thus, the techniques described herein may be used to minimize the crosstalk between one or more of the implanted INS&#39; and ICD  16 , evaluate the crosstalk between one or more of the implanted INS&#39; and ICD  16 , and the like. 
     The techniques described in this disclosure, including those attributed to ICD  16 , INS  26 , programmer  24 , or various constituent components, may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, stimulators, image processing devices or other devices. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. 
     Such hardware, software, firmware may be implemented within the same device or within separate devices to support the various operations and functions described in this disclosure. While the techniques described herein are primarily described as being performed by processor  90  of ICD  16 , processor  110  of INS  26 , and/or processor  130  of programmer  24 , any one or more parts of the techniques described herein may be implemented by a processor of one of the devices  16 ,  26 , programmer  24  or another computing device, alone or in combination with ICD  16 , INS  26  or programmer  24 . 
     In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     When implemented in software, the functionality ascribed to the systems, devices and techniques described in this disclosure may be embodied as instructions on a computer-readable medium such as RAM, ROM, NVRAM, EEPROM, FLASH memory, magnetic data storage media, optical data storage media, or the like. The instructions may be executed to support one or more aspects of the functionality described in this disclosure. 
     Various examples have been described in the disclosure. These and other examples are within the scope of the following example statements.