Patent Publication Number: US-2010114209-A1

Title: Communication between implantable medical devices

Description:
This application claims the benefit of U.S. Provisional Application No. 61/110,053, entitled, “COMMUNICATION BETWEEN IMPLANTABLE MEDICAL DEVICES,” 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 devices. 
     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, such as a nonmyocardial tissue site (e.g., tissue proximate a nerve) or a nonvascular cardiac tissue site (e.g., a cardiac fat pad), within a patient and cardiac rhythm management therapy to a heart of a patient. In some examples, the therapy system may include a first implantable medical device (IMD) that delivers electrical stimulation to the tissue site within a patient, such as a tissue site proximate a nerve (e.g., a vagus nerve or a spinal cord) or another tissue site, and a second 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 ICD may deliver any combination of pacing, cardioversion, and defibrillation pulses. The first and second implantable medical devices are not physically connected each other. 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). 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 that may be implemented to communicate information between the first and second IMDs are described herein. In some examples, the first and second IMDs may communicate with each other by encoding therapy information in a stimulation signal, which may be transmitted to the other device through tissue of the patient. The information may be encoded in a stimulation signal by, for example, varying one or more signal parameters, e.g., a slew rate, the frequency, phase, duty cycle, and, in the case of stimulation pulses, the pulse rate and pulse width. The encoded information may provide information regarding the therapy being delivered by the INS, such as the duration of the therapy and/or the type of therapy. The encoded information may also include information regarding the operation of the INS, such as information that indicates when the INS is being recharged. In some examples, the receiving device may modify its operation based on the received therapy information. 
     In addition, techniques for minimizing interference between the first and second IMDs or between the different therapy modules of a common medical device are described herein. In some examples, the first IMD may randomly or pseudo-randomly vary one or more signal parameters to generate a stimulation signal that has a spread spectrum energy distribution. Consequently, the signal artifact present on an electrical signal sensed by the second IMD may appear as wideband noise in the sensed signal. The second IMD may employ signal processing techniques well known in the art to suppress the wideband noise. These examples facilitate the removal of a signal artifact from the signal sensed by the ICD and, thus, may provide improved performance for therapy systems that include an INS and ICD. 
     In one aspect, the disclosure is directed to a method comprising generating an electrical stimulation signal with a first IMD implanted within a patient, encoding information in the electrical stimulation signal with the first IMD, and delivering the electrical stimulation signal to tissue within the patient, where delivering the electrical stimulation signal comprises transmitting the information to a second IMD implanted within the patient. 
     In another aspect, the disclosure is directed to a method comprising sensing an electrical stimulation signal with an electrode electrically connected to a first IMD implanted within a patient, where the electrical stimulation signal is generated by a second IMD implanted within the patient and encoded with information by the second IMD, and processing the electrical stimulation signal with the first IMD to retrieve the information. 
     In another aspect, the disclosure is directed to a method comprising generating an electrical stimulation signal with a first IMD, encoding information in the electrical stimulation signal with the first IMD, sensing the electrical stimulation signal with an electrode electrically connected to a second IMD, and processing the sensed electrical stimulation signal with the second IMD to retrieve the information. 
     In another aspect, the disclosure is directed to a system comprising a stimulation generator that generates an electrical stimulation signal, and a processor that controls the stimulation generator to encode information in the electrical stimulation signal and deliver the electrical stimulation signal to a patient. 
     In another aspect, the disclosure is directed to a system comprising an electrode electrically connected to a first IMD implanted within a patient, a sensing module that senses an electrical stimulation signal with the electrode, where the electrical stimulation signal is generated by a second IMD implanted within the patient and encoded with information, and a processor that processes the electrical stimulation signal to retrieve the information. 
     In another aspect, the disclosure is directed to a system comprising a first IMD that generates an electrical stimulation signal and encodes information in the electrical signal, and a second IMD that senses the electrical stimulation signal and processes the electrical stimulation signal to retrieve the information. The first and second IMD are implanted within a patient. 
     In another aspect, the disclosure is directed to a system comprising means for generating an electrical stimulation signal with a first IMD implanted within a patient, means for encoding information in the electrical stimulation signal with the first IMD, and means for delivering the electrical stimulation signal to tissue within a patient, where delivering the electrical stimulation signal comprises transmitting the information to a second IMD implanted within the patient. 
     In another aspect, the disclosure is directed to a system comprising means for sensing an electrical stimulation signal with an electrode electrically connected to a first IMD implanted within a patient, where the electrical stimulation signal is generated by a second IMD implanted within the patient and encoded with information by the second IMD, and means for processing the electrical stimulation signal with the first IMD to retrieve the information. 
     In another aspect, the disclosure is directed to a system comprising means for generating an electrical stimulation signal with a first IMD, means for encoding information in the electrical stimulation signal, means for sensing the electrical stimulation signal with an electrode electrically connected to a second IMD, and means for processing the sensed electrical stimulation signal to retrieve the information. 
     In another aspect, the disclosure is directed to a method comprising generating an electrical stimulation signal that has a predetermined signature with a first therapy module that delivers electrical stimulation therapy to a patient, where the predetermined signature is characterized by at least one of a duty cycle or a signal envelope of the electrical stimulation signal, delivering the electrical stimulation signal to tissue of the patient via a first set of electrodes electrically connected to the first therapy module, sensing electrical activity within the patient with a second set of electrodes electrically connected to a second therapy module, where the electrical activity includes a physiological signal of the patient and a signal artifact from the delivery of the electrical stimulation signal by the first therapy module, generating a sensed electrical signal based on the sensed electrical activity, and processing the sensed electrical signal to remove at least part of the signal artifact based on the predetermined signature. 
     In another aspect, the disclosure is directed to a method comprising sensing electrical activity within a patient with a second therapy module, where the electrical activity includes a physiological signal of the patient and a signal artifact from a delivery of an electrical stimulation signal to the patient by an IMD, the electrical stimulation signal comprising a predetermined signature that is characterized by at least one of a duty cycle or a signal envelope of the electrical stimulation signal, generating a sensed electrical signal based on the sensed electrical activity, and processing the sensed electrical signal to remove at least part of the signal artifact based on the predetermined signature of the electrical stimulation signal. 
     In another aspect, the disclosure is directed to a system comprising a first set of electrodes, a second set of electrodes, a first therapy module that generates and delivers an electrical stimulation signal having a predetermined signature to tissue of a patient via the first set of electrodes, where the predetermined signature is characterized by at least one of a duty cycle or a signal envelope of the electrical stimulation signal, a second therapy module that senses electrical activity within the patient via the second set of electrodes, where the electrical activity includes a physiological signal and a signal artifact from the delivery of the electrical stimulation signal by the first therapy module, and where the second therapy module generates a sensed electrical signal based on the electrical activity, and a processor that processes the sensed electrical signal to remove at least part of the signal artifact based on the predetermined signature. 
     In another aspect, the disclosure is directed to a system comprising means for generating an electrical stimulation signal that has a predetermined signature, where the predetermined signature is characterized by at least one of a duty cycle or a signal envelope of the electrical stimulation signal, means for delivering the electrical stimulation signal to tissue of the patient via a first set of electrodes electrically connected to the first therapy module, means for sensing electrical activity within the patient with a second set of electrodes electrically connected to a second therapy module, where the electrical activity includes a physiological signal of the patient and a signal artifact from the delivery of the electrical stimulation signal by the first therapy module, means for generating a sensed electrical signal based on the sensed electrical activity, and means for processing the sensed electrical signal to remove at least part of the signal artifact on the predetermined signature. 
     In another aspect, the disclosure is directed to a method comprising at least one of randomly or pseudo-randomly varying a value of at least one signal parameter to generate an electrical stimulation signal comprising a spread spectrum energy distribution, and delivering the electrical stimulation signal to tissue of a patient. 
     In another aspect, the disclosure is directed to a method comprising sensing electrical activity within a patient via a set of electrodes electrically connected to a first therapy module, where the electrical activity includes a physiological signal of the patient and a signal artifact from delivery of an electrical stimulation signal by a second therapy module, and where the electrical stimulation signal comprises a spread spectrum energy distribution, generating a sensed electrical signal based on the sensed electrical activity, and processing the sensed electrical signal to monitor cardiac activity of the patient. 
     In another aspect, the disclosure is directed to a system comprising a first set of electrodes, a second set of electrodes, a stimulation generator that generates and delivers an electrical stimulation signal comprising a spread spectrum energy distribution to a patient via the first set of electrodes, a sensing module that senses electrical activity of the patient via the second set of electrodes and generates an electrical signal based on the electrical activity, where the electrical activity includes a physiological signal of the patient and a signal artifact from delivery of the electrical stimulation signal by the stimulation generator, and a processor that processes the sensed electrical signal to monitor cardiac activity of the patient 
     In another aspect, the disclosure is directed to a system comprising means for generating an electrical stimulation signal, means for delivering the electrical stimulation signal to tissue of a patient, means for sensing electrical activity within the patient, where the electrical activity includes a physiological signal of the patient and a signal artifact from the delivery of the electrical stimulation signal, means for generating a sensed electrical signal based on the sensed electrical activity, and means for processing the sensed electrical signal to monitor cardiac activity of the patient. 
     In another aspect, the disclosure is directed to a computer-readable medium containing 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 claims provided below. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example therapy system that includes an implantable neurostimulator (INS) and an implantable cardiac device (ICD) in accordance with various examples described in this disclosure. 
         FIG. 2  is a conceptual diagram illustrating another example configuration of the therapy system of  FIG. 1 . 
         FIG. 3  is a conceptual diagram illustrating an example therapy system. 
         FIG. 4  is a conceptual diagram illustrating an example configuration of the ICD and leads attached to the ICD of  FIG. 1  in greater detail. 
         FIG. 5  is a conceptual diagram illustrating another example configuration of the ICD and attached leads in greater detail. 
         FIG. 6  is a functional block diagram of an example INS. 
         FIG. 7  is a functional block diagram of an example ICD. 
         FIG. 8  is a functional block diagram illustrating an example configuration of the artifact monitor of an ICD. 
         FIG. 9  is a functional block diagram illustrating another example configuration of the artifact monitor of an ICD. 
         FIG. 10  is a functional block diagram illustrating an example external programmer. 
         FIGS. 11A-11D  illustrate example stimulation waveforms for communication between the INS and the ICD. 
         FIGS. 12A ,  12 B,  13 A, and  13 B illustrate example stimulation waveforms that facilitate removal of the resulting signal artifact at the ICD. 
         FIG. 14A  illustrates an example EGM waveform generated by the ICD when the INS is not delivering therapy to the patient. 
         FIG. 14B  illustrates an example EGM waveform generated by the ICD when the INS is configured to deliver therapy by generating stimulation signals that have a spread spectrum energy distribution. 
         FIG. 15  is a flow diagram illustrating an example technique that either the INS or ICD may use to communicate with the ICD or INS, respectively. 
         FIGS. 16-18  are flow diagrams illustrating example techniques for reducing the effects of electrical crosstalk on sensing performed by the ICD. 
         FIG. 19  is a functional block diagram of an implantable medical device that includes an electrical stimulation module that generates and delivers electrical stimulation to a tissue site within a patient and a cardiac therapy module that generates and delivers cardiac rhythm management therapy to a heart of the patient. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure describes techniques that may be employed by a first implantable medical device (IMD) to communicate with a second IMD, where the first and second IMDs are implanted within a common patient. For example, the first IMD may encode information in a stimulation signal delivered to a patient, and the second IMD may sense the stimulation signal and decode the signal to receive the information. The second IMD may also implement the communication techniques described herein to communicate with the first IMD. 
     As described with respect to  FIG. 1 , the first IMD may comprise an implantable electrical stimulator that provides electrical stimulation therapy to a nonmyocardial tissue site (e.g., a tissue proximate a nerve of a patient or a tissue site outside of vasculature and not proximate a nerve), or a nonvascular cardiac tissue site (e.g., a cardiac fat pad). The second IMD may comprise a cardiac rhythm management device (i.e., an implantable cardiac device (ICD)) that senses electrical cardiac signals of a heart of the patient and, in some examples, provides at least one of pacing, cardioversion or defibrillation therapy to the heart of the patient. 
     Also described herein are techniques for reducing electrical crosstalk between first and second IMDs implanted within a patient, and, in some cases, between first and second therapy modules of a common medical device. The electrical crosstalk may be at least partially attributable to electrical stimulation signals generated and delivered by one IMD and sensed by the other IMD. The electrical crosstalk may be presented as an artifact present in an electrical signal sensed by a first IMD, where the artifact may be at least partially attributable to the stimulation signals generated by the second IMD. In this way, the artifact may be referred to as a “stimulation artifact” or a “signal artifact.” It may be desirable to minimize electrical crosstalk in order to minimize the possibility that interference from the electrical stimulation signals delivered by an electrical stimulator does not interfere with the proper detection of cardiac signals by an ICD. For example, if an ICD senses electrical stimulation signals generated and delivered by an electrical stimulator, such as an implantable neurostimulator (INS), and mischaracterizes the electrical stimulation signals as cardiac signals, the ICD may inappropriately detect an arrhythmia. This may result in the inappropriate delivery of pacing, cardioversion, and/or defibrillation therapy to the patient. 
       FIG. 1  is a conceptual diagram illustrating an example therapy system  10  provides therapy to patient  12 . Therapy system  10  includes ICD  16 , which is coupled to leads  18 ,  20 , and  22 , and programmer  24 . ICD  16  may be, for example, an IMD that sense electrical cardiac activity of heart  14  via electrodes coupled to one or more of leads  18 ,  20 , and  22 . In some examples, ICD  16  may also comprise at least one of an implantable pacemaker, cardioverter, and/or defibrillator that delivers cardiac rhythm management therapy to heart  14  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 therapy to heart  14 . In various examples, ICD  16  may deliver pacing that includes one or both of anti-tachycardia pacing (ATP) and cardiac resynchronization therapy (CRT). 
     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 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  delivers electrical stimulation therapy to a nerve of patient  12  via a lead implanted within vasculature (e.g., a blood vessel) of patient  12 . In addition, in some examples, INS  26  delivers 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. 19 , the functionality of ICD  16  and INS  26  may be performed by an implantable medical device (IMD) that includes both a cardiac therapy module that generates and delivers at least one of a pacing, cardioversion or defibrillation signal to patient  12  and an electrical stimulation therapy module that generates and delivers electrical stimulation to a target tissue site within patient  12 . 
     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. 3 , in other examples, ICD  16  may deliver stimulation therapy to heart  14  by delivering stimulation to an extravascular 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  detects 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 a 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 . 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 . Stimulation and sense electrodes carried by lead  28  are generally referred to as electrodes  46  throughout this disclosure. Furthermore, in some examples, INS  26  may be coupled to two or more leads, e.g., for bilateral or multi-lateral stimulation. 
     In the example illustrated in  FIG. 1 , lead  28  includes four electrodes  46 . In other examples, lead  28  may carry any suitable number of electrodes, such as fewer than four electrodes or greater than four electrodes (e.g., eight or sixteen electrodes). Electrodes  46  may comprise ring electrodes. In other examples, electrodes  46  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  46  illustrated in  FIG. 1  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  46 . 
     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 . 
     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 augment antitachycardia pacing by ICD  16  or provide back-up therapy to 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). 
     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  46  are implanted in a region where patient  12  experiences pain, but may not be proximate a nerve. 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 referred to herein, the disclosure is also applicable to examples in which INS  26  delivers electrical stimulation to other tissue sites, which may be intravascular or extravascular. 
     In the example shown in  FIG. 1 , lead  28  connected to INS  26  is positioned to provide 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 . 
     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  ( FIG. 2 ) 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 other examples, electrodes  46  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 program 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 facilitate the delivery of therapy by ICD  16 . Another example configuration of therapy system  10  is described below with respect to  FIG. 2 , in which INS  26  is positioned to deliver electrical stimulation to the spinal cord of patient  12 . 
     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) 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. 
     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  are described below with reference to  FIGS. 7-9  and  15 - 18 . 
     ICD  16  may sense electrical noise and interpret the electrical noise as electrical cardiac signals (e.g., an electrocardiogram (ECG) or electrogram (EGM) signal). This may cause ICD  16  to oversense the heart rhythms, and, in some cases, erroneously detect an arrhythmia based on the electrical noise. For example, a processor of ICD  16  may identify 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 of the 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. 
     Undersensing of the heart rhythms may also result in inappropriate delivery of pacing therapy to heart  14 . ICD  16  may undersense the heart rhythms when the electrical noise masks the actual electrical cardiac signals. For example, the electrical noise may cause a sense amplifier of ICD  16  that is used to sense electrical cardiac signals to be less sensitive. In this case, the electrical noise may have a sufficiently large amplitude, e.g., larger than the amplitude of the electrical cardiac signal, that ICD  16  calibrates its detection algorithm to detect signals having an amplitude larger than that of the electrical cardiac signal. As a result, ICD  16  may undersense the electrical cardiac signal and determine that the R-R intervals present a relatively slow rhythm. Consequently, undersensing may result in inappropriate delivery of electrical stimulation to heart  14 . 
     In another example, ICD  16  may undersense the electrical cardiac signals when heart  14  is not, in fact, in a normal sinus rhythm, and the electrical noise interferes with the electrical cardiac signals in a way that causes ICD  16  to interpret the combined signal of electrical noise and irregular electrical cardiac signal as a normal sinus rhythm. In this case, undersensing may result in inappropriate withholding of electrical stimulation to heart  14 . 
     Electrical noise that ICD  16  characterizes as heart rhythms may be attributable to different sources. The stimulation signal generated by INS  26  and delivered to the tissue site  40  of patient  12  may be coupled to ICD  16  through tissue of patient  12 . Thus, 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 . As previously described, ICD  16  senses electrical activity of patient  12  via the electrodes carried by leads  18 ,  20 , and  22 . The electrical activity includes an electrical cardiac signal that is produced by the electrical activity of heart  14  and an artifact resulting from the stimulation signal output by INS  26 . The artifact 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 . As another example, if the electrical noise causes ICD  16  to be less sensitive, ICD  16  may unnecessarily deliver electrical stimulation to heart  14 , e.g., when ICD  16  detects a heart rhythm slower than normal. 
     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 sensing parameters of ICD  16  may be modified in response to receiving input that indicates INS  26  is actively delivering electrical stimulation signals to patient  12 . For example, ICD  16  may implement a different filter to filter out the electrical stimulation signals delivered by INS  26  from the electrical signals sensed by ICD  16 . Filtering out the electrical stimulation signals from INS  26  based on the known characteristics of the electrical stimulation signals may help minimize a possibility that ICD  16  senses the electrical stimulation signals and mischaracterizes them as cardiac signals. 
     Filtering may be applied in response to receiving information from INS  26  that indicates INS  26  is delivering therapy may be useful because filtering the sensed electrical signals may affect normal cardiac sensing or EGM processing of ICD  16 . That is, filtering a sensed signal may inadvertently result in filtering of the cardiac signal component, such that the cardiac sensing or EGM processing is less effective. Thus, selectively applying the filter when the noise from delivery of stimulation by INS  26  is known to be occurring. 
     INS  26  and ICD  16  are configured to communicate with each other. Accordingly, ICD  16  may receive input from INS  26  that indicates INS  26  is actively delivering electrical stimulation signals to patient  12 . INS  26  may transmit information to ICD  16  indicating INS  26  is delivering therapy, and one or more therapy parameters, such as the duration of therapy or one or more electrical stimulation parameter values with which INS  26  generates the electrical stimulation signals. INS  26  may also transmit information to ICD  16  indicating INS  26  is being recharged because recharging INS  26  may also introduce electrical noise that may be mischaracterized by ICD  16 . In some examples, as described in further detail below with reference to  FIG. 15 , INS  26  may transmit the information by encoding the information in a stimulation signal that is delivered to tissue site  40  of patient  12 . Example stimulation waveforms that may be used for transmitting information to ICD  16  are shown in  FIGS. 10A-10D  and described below. 
     INS  26  may encode the therapy information by varying the value of one or more stimulation parameters, such as the slew rate, frequency (e.g., pulse rate), signal duration (e.g., pulse width), phase (e.g., positive and negative voltages), or duty cycle in a known manner, such that ICD  16  may sense the stimulation signals and extract information therefrom based on the known stimulation parameter values. In some examples, INS  26  varies the value of the one or more stimulation parameters within a predetermined range of values determined to provide efficacious therapy to patient  12 . 
     In some examples, INS  26  may encode information in a stimulation signal by varying signal parameters on a burst-by-burst basis. For example, INS  26  may generate the stimulation signal as a series of bursts of pulses or pulse trains and vary one or more signal parameters for each the bursts of pulses. In some examples, each pulse in particular burst of pulses may be generated using the same stimulation parameter values, but the pulses of subsequent bursts may be generated using different stimulation parameter values. Using this technique, INS  26  may encode information in the stimulation signal by associating particular burst shapes with information, where a burst shape may be defined by the stimulation parameter values (or “signal parameter values”) used to generate the pulses. For example, different burst shapes may be associated with specific instructions for ICD  16  or with different alphanumeric indicators, such as letters or numbers, and a plurality of burst shapes (symbols) may be arranged to form words or other indicators that are assigned a unique meaning or, more specifically, unique information relating to the stimulation therapy delivered by INS  26 . 
     In some examples, the alphanumeric indicator encoded in the stimulation signal from INS  26  may be associated with an instruction in the memory of ICD  16 . Thus, upon extracting the alphanumeric indicator from the sensed electrical stimulation signal from INS  26 , ICD  16  may reference a memory to determine what information was encoded in the stimulation signal. For example, ICD  16  may reference a memory to determine an operating modification associated with the alphanumeric indicator. As described in further detail below, the operating modification may include a modification to a sensing parameter of ICD  16 , such as a type of filter used to sense cardiac signals. 
     In another example, INS  26  may encode information in the stimulation signal by generating an electrical stimulation signal having one or more burst shapes that are associated with information, such as one or more alphanumeric indicators. A particular arrangement of multiple bursts of pulses may be associated with one or more alphanumeric indicators or with a specific instruction for ICD  16 . This may be referred to as burst pattern encoding because information is encoded using different “patterns” of burst shapes, where a burst pattern includes more than one burst of pulses. 
     As another example, INS  26  may encode information in the stimulation signal by varying one or more signal parameter values on a pulse-by-pulse basis. This technique may provide for a more robust stimulation technique compared to the burst pattern encoding technique because each pulse in the burst may be generated according to a different set of signal parameters. INS  26  may encode information in the stimulation signal by associating particular pulse shapes with an alphanumeric identifier or patterns in pulse shapes with alphanumeric identifiers, and may arrange the alphanumeric identifiers to form words or other indicators that have a unique predetermined meaning. ICD  16  may decode the stimulation signal using the same coding scheme with which INS  26  encoded the stimulation signal. In other examples, INS  26  may encode information in the stimulation signal by associating particular pulse shapes with respective instructions for ICD  16 , such as an instruction relating to a modification to a sensing parameter. In this manner, INS  26  may be configured to encode information in stimulation signals using well known techniques in the art of telecommunication. 
     Although INS  26  is primarily described herein as generating pulse waveforms, INS  26  may also generate continuous time signals, such as sine waves, and vary stimulation parameters including a slew rate, a signal amplitude, a signal frequency, and a signal phase in order to encode information in the signal. 
     ICD  16  and INS  26  may also communicate with each other via stimulation signals, but without encoding information in the stimulation signal. For example, ICD  16  may communicate with INS  26  by delivering a defibrillation pulse to heart  14 . In this example, the defibrillation pulse itself may be considered information that the INS  26  receives. INS  26  may suspend the delivery of neurostimulation upon receiving or sensing the defibrillation pulse, or may begin delivering neurostimulation that provides therapeutic benefits after a predetermined period of time has passed following the defibrillation pulse. 
     Examples of information that INS  26  may encode in a stimulation signal include, but are not limited to, therapy information, operational information, diagnostic information, and message information. Therapy information may include a duration and type of therapy, as well as signal parameter values of the neurostimulation signals. For example, therapy information may include a duration of a therapy session in which INS  26  will be actively delivering electrical stimulation signals and/or the type of therapy delivered by INS  26 . INS  26  may encode information indicating the duration of the therapy session by encoding information relating to a stop time of the therapy delivery, a start time and a stop time of the therapy delivery, a total duration of time of the therapy delivery or the time remaining in the current therapy session. As another example, INS  26  may encode the type of therapy by specifying the particular therapy program with which INS  26  is generating the stimulation signals (e.g., by identifying the therapy program by an alphanumeric identifier and transmitting the identifier to ICD  16 ), or by specifying the therapy program parameters. 
     In another example, INS  26  may encode operational information in a stimulation signal. Operational information may include information that specifies the operational mode of INS  26 . For example, the operational information may indicate that INS  26  is being recharged or is actively delivering stimulation signals to patient  14 . INS  26  may encode the type of operation by identifying the type of operation by an alphanumeric identifier and transmitting the identifier to ICD  16 . 
     In an additional example, INS  26  may encode diagnostic information in a stimulation signal. Diagnostic information may include information about the status of INS  26  or its leads. For example, the diagnostic information may include measured lead impedance values. The lead impedance values may be used for detecting lead-related conditions (e.g., a fractured conductor, a compromised electrical insulation, and the like). In some examples, INS  26  may transmit lead impedance values on a periodic basis, e.g., a daily basis, and ICD  16  may generate a combined INS  26 /ICD  16  lead impedance trend. This may allow a clinician to interrogate only one device, e.g., ICD  16 , to retrieve system specific diagnostic information. 
     In a further example, INS  26  may encode transmission information that includes message information and/and acknowledgement information. INS  26  may encode message information at one of or both of the beginning and end of a stimulation signal. The message information may include a header that indicates the beginning of the message and a footer than indicates the end of the message. In this way, ICD  16  may use the header to locate the beginning of the therapy, operational, and/or diagnostic information and the footer to confirm that the message has been transmitted properly. INS  26  may encode acknowledgement information in a stimulation signal in examples in which ICD  16  transmits information to INS  26 . The acknowledgement information may indicate to INS  26  that the information transmitted by INS  26  has been received by ICD  16 . Accordingly, ICD  16  may transmit acknowledgement information to INS  26  in response to receiving one or more of therapy information, operational information, and diagnostic information. 
     In examples in which ICD  16  transmits information to INS  26 , ICD  16  may encode therapy information in a stimulation signal. The therapy information may indicate to INS  26  that ICD  16  is delivering therapy and, may also indicate the type of therapy. For example, the therapy information may indicate prospective delivery a stimulation pulse by that ICD  16  to allow that INS  26  to suspend delivery neurostimulation or take another appropriate action to synchronize therapy delivery to the operation of ICD  16 . As another example, the therapy information may indicate the type and duration of therapy to be delivered by ICD  16 , so that INS  26  can begin delivering neurostimulation therapy that benefits the cardiac therapy after ICD  16  has finished delivering cardiac therapy. As an additional example, INS  26  may be configured to recognize a defibrillation pulse delivered by ICD  16  so that a defibrillation pulse itself indicates to INS  26  to stop delivering neurostimulation. In such examples, INS  26  may transmit acknowledgement information to ICD  16  upon receiving therapy information from ICD  16 , and ICD  16  may begin to deliver therapy after receiving acknowledgement information from INS  26 . 
     Again, techniques well known in the art of telecommunications may be used to encode this information, e.g., therapy information, operational information, diagnostic information, and message information in stimulation signals generated by INS  26 . Similarly, techniques well known in the art of telecommunications may be used to encode therapy information in stimulation signals generated by ICD  16 . 
     ICD  16  may be configured to sense the electrical stimulation signal generated by INS  26  and process the signal to retrieve the encoded information. For example, ICD  16  may include signal processing circuitry for detecting the signal artifact in the sensed signal and decoding the information. ICD  16  may then use the decoded information to modify its operation. For example, if the information encoded in the stimulation signal specifies the duration of a therapy session during which INS  26  will deliver electrical stimulation, ICD  16  may suspend the delivery of pacing, cardioversion or defibrillation signals to patient  12  in order to prevent delivering therapy in response to a cardiac arrhythmia that is detected based on the electrical stimulation signal delivered by INS  26 , rather than a true cardiac signal. In another example, ICD  16  may invoke additional signal processing methods while INS  26  delivers therapy, where the additional signal processing methods utilize more complex techniques for monitoring the cardiac signal so as not to deliver unnecessary stimulation therapy to heart  14 . The additional signal processing techniques may involve processing the sensed signal to remove the signal artifact resulting from the stimulation. In an example in which the therapy information specifies the type of therapy delivered by INS  26 , ICD  16  may modify its operation accordingly, for example by changing pacing and/or therapy parameters based on the received information. 
     Although interdevice communication has generally been described as one-way from INS to ICD  16 , ICD  16  and INS  26  may also be configured for two-way communication. ICD  16  may encode therapy information in a pacing, cardioversion or defibrillation signal that is coupled to INS  26  through tissue of patient  14  and INS  26  may be configured to retrieve the information from the sensed electrical activity. 
     Therapy system  10  may implement various techniques described herein to reduce the amount of crosstalk between INS  26  and ICD  16 . As one example, INS  26  may be configured to generate a stimulation signal characterized by a predetermined signature. INS  26  may vary one or more signal parameters, e.g., slew rate, frequency (e.g., pulse rate), signal duration (e.g., pulse width), phase, and duty cycle, to generate the stimulation signal with the signature. In examples in which INS  26  generates the stimulation signal as a plurality of bursts of pulses, the signature may comprise a plurality of bursts of pulses. In examples in which INS  26  generates the stimulation signal as a substantially continuous series of pulses or substantially continuous waveform, the signature may be characterized by a signal envelope that traces the outline of the amplitude of the stimulation signal for a given period of time. ICD  16  may be configured to process a sensed electrical signal to substantially remove the signal artifact attributable to the delivery of stimulation signals by INS  26 . For example, ICD  16  may include one or more filters designed to at least partially remove the signal artifact from the sensed signal. ICD  16  may analyze the processed signal, i.e., the signal with the reduced artifact, to monitor cardiac events and deliver cardiac rhythm management therapy. 
     As an additional example, INS  26  may be configured to generate a stimulation signal that has a narrow band energy spectrum centered at a predetermined frequency. The predetermined frequency may be selected as a frequency that does not generally interfere with the cardiac signal. Accordingly, ICD  16  may be configured to process the sensed signal to substantially remove the signal artifact from the sensed signal, for example, by applying a narrowband notch filter centered at the predetermined frequency to the sensed signal. 
     As another example, INS  26  may be configured to vary one or more signal parameters to mitigate the artifact present in electrical signals sensed by ICD  16 . In particular, INS  26  may randomly or pseudo-randomly vary one or more signal parameters, e.g., slew rate, frequency (pulse rate), pulse width, phase, and duty cycle, to generate a stimulation signal with a spread spectrum energy distribution. The spread spectrum energy distribution of the stimulation signal may cause the resulting signal artifact to appear as wideband noise in the sensed signal at ICD  16 . For example, the wideband noise may be spread over a frequency range of approximately 2.5 Hz to approximately 100 Hz, although other frequency ranges are contemplated. ICD  16  may employ signal processing techniques known in the art to substantially remove or suppress wideband noise. For example, Wiener filtering or adaptive noise cancellation schemes (e.g., a least means square approach) may be used to filter the wideband noise from a sensed signal. A Wiener filter may reduce the amount of noise present in a sensed signal by comparison with an estimation of the desired noiseless signal. 
     Alternatively, the resulting “wideband noise” may be such that ICD  16  may employ well known signal processing techniques for monitoring cardiac activity. In other words, ICD  16  may not need to be configured to include additional processing features for removing the resulting “wideband noise.” 
     Programmer  24  of therapy system  10  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 EGM), intracardiac or intravascular pressure, activity, posture, respiration, 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 therapy 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. 
     As another example, the user may use programmer  24  to retrieve information from INS  26  regarding the performance or integrity of INS  26  or lead  28 , or a power source of INS  26 . 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 . The therapy parameters may also include phase of the signal or a duty cycle of the signal. The therapy parameters may also be modulated to vary the rise and fall time of a soft start/stop signal. A soft/start stop signal is a signal in which the amplitude is gradually increased at the onset of therapy from a low value to a maximum value and subsequently gradually decreased back to the low value. Thus, INS  26  may encode information in a stimulation signal by varying one or more of the low amplitude value, the maximum amplitude value, and the rise and fall times between the low and maximum value. An electrode combination may include a selected subset of one or more electrodes  46  located on implantable lead  28  coupled to INS  26 . 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 addition, by selecting values for signal parameters such as, amplitude, pulse width, pulse rate, phase, and duty cycle, 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 radiofrequency (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. 2  is a conceptual diagram illustrating another example of therapy system  11 . As shown in  FIG. 2 , INS  26  and lead  28  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 reduce the level of aggressiveness of the cardiac therapy, such as pacing, cardioversion or defibrillation, 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 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. 
       FIG. 3  is a conceptual diagram illustrating another example therapy system  500  that includes two medical devices to provide therapy to patient  12 . In addition to INS  26 , therapy system  500  includes ICD  502 , which delivers electrical stimulation to heart  14  via extravascular leads  503 ,  504 . Extravascular leads  503 ,  504  each include at least one electrode  505 ,  506 , respectively. Electrodes  505 ,  506  may be subcutaneous coil electrodes, which may be positioned within a subcutaneous tissue layer of patient  12 . In other examples, electrodes  505 ,  506  may comprise any other suitable type of extravascular electrode. For example, electrodes  505 ,  506  may include any other type of subcutaneous electrode, such as subcutaneous ring electrodes, subcutaneous plate electrodes, subcutaneous patch or pad electrodes, or an extrathoracic electrode, a submuscular electrode, an epicardial electrode or an intramural electrode. 
     Electrode  505  may be located within the right ventricular cavity of the patient&#39;s chest, on the patient&#39;s side or back, or any other portion of the body appropriate for providing electrical stimulation to heart  14 . Electrode  506  may be located within the left ventricular cavity of the patient&#39;s chest, 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  503 ,  504  may be electrically coupled to a stimulation module, and, in some cases, a sensing module that are enclosed within housing  507  of ICD  502 . Housing  507  may comprise a hermetic housing that substantially encloses the components of ICD  502 , such as a sensing module, stimulation module, processor, memory, telemetry module, power source, and the like. Components of an example ICD  16  and ICD  502  are described with respect to  FIG. 7 . ICD  502  may deliver electrical stimulation (e.g., pacing, cardioversion or defibrillation pulses) to heart  14  between electrodes  505 ,  506 , e.g., in a bipolar configuration. In other examples, ICD  502  may deliver electrical stimulation to heart  14  between electrodes  505  and housing  507 , or between electrode  506  and housing  507 , e.g., in a unipolar configuration. 
     Just as with ICD  16  ( FIGS. 1 and 2 ) that delivers stimulation to heart  14  via intravascular electrodes, the delivery of electrical stimulation by INS  26  may interfere with the ability of ICD  502  to sense cardiac signals and deliver appropriate therapy upon the detection of an arrhythmia. ICD  502  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  502  to deliver inappropriate therapy to heart  14  of patient  12 . 
     While the disclosure primarily refers to therapy system  10  including ICD  16  ( FIGS. 1 and 2 ) and INS  26 , the description of the techniques, systems, and devices herein are also applicable to therapy system  500  including ICD  502  and INS  26 . 
       FIG. 4  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 of 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  50  and  51  are located proximate to a distal end of lead  18 . In addition, bipolar electrodes  52  and  53  are located proximate to a distal end of lead  20  and bipolar electrodes  54  and  55  are located proximate to a distal end of lead  22 . 
     Electrodes  50 ,  52 , and  54  may take the form of ring electrodes, and electrodes  51 ,  53 , and  55  may take the form of extendable helix tip electrodes retractably mounted within insulative electrode heads  62 ,  64 , and  66 , respectively. Each of the electrodes  50 - 55  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 - 55  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 - 55  to cause depolarization of cardiac tissue of heart  14 . In some examples, as illustrated in  FIG. 3 , ICD  16  may include 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 . Other division 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 - 55  may be used for unipolar sensing or pacing in combination with housing electrode  68 . As described in further detail with reference to  FIG. 7 , 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 electrical cardiac signals of heart  14 . 
     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. 
       FIG. 5  is a conceptual diagram illustrating another example configuration of ICD  16  for use in therapy system  10 . As shown in  FIG. 5 , ICD  16  may be configured to include two leads  18  and  22  in some examples, rather than three leads as illustrated in  FIG. 4 . In the two lead configuration shown in  FIG. 5 , leads  18 ,  22  are implanted within right ventricle  32  and right atrium  30 , respectively, and may be useful for providing cardioversion, defibrillation, and pacing pulses to heart  14 . Therapy system  10  may include ICD  16  as shown in  FIG. 5  and INS  26  which is configured to deliver electrical stimulation therapy to a nonmyocardial or nonvascular cardiac tissue site within patient  14  in order to help prevent or mitigate an arrhythmia of patient  16 , to treat heart failure or to provide other cardiac benefits to patient  12 . 
     The configuration of therapy system  10  and ICD  16  illustrated in  FIGS. 1 ,  2 ,  4 , AND  5  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 electrodes. In examples in which INS  26  is not implanted in patient  12 , INS  26  may deliver electrical stimulation to target tissue sites or sense stimulation delivered by ICD  16  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 . For example, other examples of therapy systems may include three transvenous leads located as illustrated in  FIGS. 1 and 2 , and an additional lead located within or proximate to left atrium  38 . As another example, 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 . In addition, in other examples, a therapy system may include extravascular electrodes for providing pacing, cardioversion or defibrillation pulses to heart  14 , as described with respect to  FIG. 3 . 
       FIG. 6  is a functional block diagram of an example INS  26 . INS  26  includes processor  80 , memory  82 , power source  84 , telemetry module  86 , and stimulation generator  88 . In the example shown in  FIG. 6 , processor  80 , memory  82 , power source  84 , telemetry module  86 , and stimulation generator  88  are enclosed within the housing of INS  26  which may be, for example, a hermetic housing. As shown in  FIG. 6 , stimulation generator  88  is coupled to electrodes  46  carried by lead  28  either directly or indirectly (e.g., via a lead extension). In other examples, such as in the example shown in  FIG. 2 , stimulation generator  88  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 . 
     Processor  80  may include any one or more microprocessors, controllers, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated digital or analog logic circuitry. The functions attributed to processor  80  herein may be embodied as software, firmware, hardware or any combination thereof. Processor  80  controls stimulation generator  88  to generate and deliver electrical stimulation signals to patient  12 . Processor  80  may set and adjust stimulation parameter values with which stimulation generator  88  generates electrical stimulation signals, e.g., based on stored therapy programs  100  and other instructions stored in memory  82 , as described in further detail below. In examples in which stimulation generator  88  generates electrical stimulation pulses, the stimulation parameters may include, for example, a slew rate, a pulse amplitude, pulse rate (frequency), pulse width (duration), phase, and duty cycle. In other examples, stimulation generator  88  may generate continuous electrical signals, e.g., a sine wave, in which case the stimulation parameters may include a signal amplitude, signal width, and signal frequency. 
     Memory  82  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. Memory  82  stores computer-readable instructions that, when executed by processor  80 , cause INS  26  to perform various functions. For example, memory  82  may stores instructions for execution by processor  80 , including operational commands and programmable parameter settings. Example storage areas of memory  82  may include instructions associated with therapy programs  100 , interdevice communication features  102 , and crosstalk mitigation features  104 . 
     Therapy programs  100  may be stored as individual therapy programs and/or organized into therapy program groups that include one or more therapy programs. The therapy programs may define a particular program of therapy in terms of respective values for electrical stimulation parameters. A therapy 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  80 . 
     Interdevice communication features  102  may include instructions for encoding information in a stimulation signal generated by stimulation generator  88  and decoding information received from ICD  16 . The instructions may include instructions for selecting the information that is encoded and signal parameter variation instructions for encoding the selected information in the stimulation signal. As previously, information that processor  80  may encode in a stimulation signal include, but is not limited to, therapy information, such as the start and stop times for a therapy session in which stimulation generator  88  delivers stimulation therapy to patient  12 , the duration of the therapy session, the time remaining in a current therapy session, the type of stimulation, and the stimulation parameter values of stimulation delivered by INS  26  in a particular therapy session, operational information, diagnostic information, and message information. 
     Crosstalk mitigation features  104  may include instructions that processor  80  may execute to control stimulation generator  88  to generate a stimulation signal that may minimize the neurostimulation artifact present in an electrical signal sensed by ICD  16 . In one example, crosstalk mitigation features  104  include instructions for generating a stimulation signal with a predetermined signature. In an additional example, crosstalk mitigation features  104  include signal parameter instructions for generating a stimulation signal with a narrowband energy spectrum centered at a predetermined frequency. In another example, crosstalk mitigation features  104  may store instructions for generating a stimulation signal with a spread spectrum energy distribution. 
     It should be understood that although INS  26  is described as implementing the interdevice communication and crosstalk mitigation functions for reducing the neurostimulation signal artifact present in an electrical signal sensed by ICD  16 , INS  26  may be configured to implement only one of these functions. Accordingly, memory  82  may include only one of interdevice communication  102  and crosstalk mitigation features  104 . 
     Stimulation generator  88  generates stimulation signals, which may be pulses as primarily described herein, or continuous time signals, such as sine waves, for delivery to patient  12  via selected subset of electrodes  46 . In particular, processor  80  may control stimulation generator  88  according to stored therapy programs  100 , interdevice communication features  102 , and/or crosstalk mitigation features  104  loaded from memory  82  to produce an electrical stimulation signal with particular stimulation parameter values, such as amplitude, frequency, phase, and duty cycle, and, in the case of stimulation pulses, pulse width and pulse rate. As shown in  FIG. 6 , stimulation generator  88  may include a charging circuit  92 , a DC to DC converter  94 , and a stimulation interface  96 . 
     DC to DC converter  94  is primarily described as a capacitor module, but this disclosure is not limited to examples in which DC to DC converter  94  is a capacitor module. In other examples, DC to DC converter  94  may comprise, for example, an inductor-based charge pump, a capacitor-based charge pump, and/or any other type of DC to DC converter. 
     Charging circuit  92  selectively, e.g., based on signals from processor  80 , applies energy from power source  84  to DC to DC converter  94  to charge the capacitor module for delivery of a stimulation signal, e.g., pulse. For delivery of pulses, charging circuit  92  may control the pulse rate by controlling the rate at which DC to DC converter  94  is recharged. Similarly, charging circuit  92  may also control the duty cycle. In addition to capacitors, DC to DC converter  94  may include switches. In this manner, capacitor module  94  may be configurable, e.g., based on signals from stimulation control module  90 , to store a desired voltage for delivery of stimulation at a voltage or current amplitude specified by a program. For delivery of stimulation pulses, switches within capacitor module  94  may control the width of the pulses based on signals from processor  80 . 
     Stimulation interface  96  conditions charge from capacitor module  94  to produce an electrical stimulation signal, e.g., a pulse, under control of processor  80  for application to a subset of electrodes  46  carried by lead  28 . Stimulation interface  96  may control the voltage or current amplitude, or shape of the signal based on signals from stimulation control module  90 . Stimulation generator  88  is coupled to electrodes  46  via stimulation interface  96  and conductors within leads  28 . Stimulation interface  96  may control the subset of electrodes  46  that are selected to deliver the stimulation signal to patient  12  and the polarities of the selected electrodes based on signals from processor  80 . For example, processor  80  may control stimulation interface  96  to apply the stimulation signals to selected combinations of electrodes  46 . In particular, stimulation interface  96  may couple stimulation signals to selected conductors within lead  28 , which may deliver the stimulation signals across the selected electrodes  46 . Stimulation interface  96  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 some examples, INS  26  does not include stimulation interface  96 . 
     Stimulation generator  88  may be a single or multi-channel stimulation generator. In particular, stimulation generator  88  may be capable of delivering a single stimulation pulse, multiple stimulation pulses (as a series of pulses or as a burst/train of 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  88  and stimulation interface  96  may be configured to deliver stimulation signals to one or more channels on a time-interleaved basis. In this case, stimulation interface  96  serves to time division multiplex the stimulation signal across different electrode combinations at different times to deliver multiple programs or channels of stimulation energy to patient  12 . 
     In one example, processor  80  may control operation of stimulation generator  88  to encode information in a stimulation signal that provides therapeutic benefits to patient  12 . In another example, processor  80  may control operation of stimulation generator  88  to reduce crosstalk between INS  26  and ICD  16 , e.g., by producing a stimulation signal having predetermined characteristics in order to facilitate removal of the resulting signal artifact by ICD  16 , or by generating a stimulation signal comprising one or more characteristics that reduces the possibility that ICD  16  senses the electrical stimulation signal generated by INS  26  and mischaracterizes the signal as a cardiac signal. These examples are described in greater detail below. 
     In examples in which processor  80  controls stimulation generator  88  to encode information in a stimulation signal that provides therapy to patient  12 , stimulation generator  88  may encode information in the stimulation signal by varying one or more stimulation signal parameters, such as a slew rate, pulse amplitude, pulse rate (frequency), pulse width (duration), phase, and duty cycle. One or more stimulation parameters may be varied at any given time. Example modulation schemes include amplitude modulation, frequency modulation, and on/off keying (OOK), which may be viewed as a form of amplitude modulation. Other modulation schemes are also contemplated, such as modulating a minimum value, a maximum value, or the rise/fall time of a soft start/stop signal. As previously described, the information to be encoded in the stimulation signal may be stored as interdevice communication features  102  in memory  82  and may include therapy information, such as the start and stop times for stimulation therapy, the duration of a particular therapy session, the time remaining for a current therapy session, and the type of stimulation or therapy programs delivered, operational information, diagnostic information, and message information. 
     In some examples, stimulation generator  88  generates stimulation signals that comprise bursts of pulses, where each burst includes a plurality of pulses. A burst of pulses may also be referred to as a pulse train. Stimulation generator  88  may encode information in a stimulation signal comprising a plurality of bursts by varying signal parameters on a burst-by-burst basis or on a pulse-by-pulse basis. Stimulation generator  88 , and INS  26  in general, may be configured to encode information in a stimulation signal in this manner using well known techniques in the art of telecommunication. That is, stimulation generator  88  may employ various known encoding techniques to encode information in a stimulation signal for transmission to ICD  16 . The encoding schemes may include, for example, amplitude modulation and/or frequency modulation. 
     For example, when encoding information on a burst-by-burst basis, stimulation generator  88  may generate each pulse in a burst of pulses using the same signal parameter values. Information may be encoded by associating a particular burst shape with an alphanumeric indicator. The multiple burst shapes may be configured such that the associated alphanumeric indicators form code words for transmitting the desired information or are otherwise associated with desired information. The code words may be assigned a unique predefined meaning or may be arranged to form a message that has a unique predefined message. ICD  16  may sense the stimulation signal and process the sensed electrical signal to identify the burst shapes, and, therefore, extract the encoded alphanumeric indicators from the sensed electrical signal. In other examples, burst shapes may be directly associated with a respective therapy modification instruction or other information that is stored in a memory of ICD  16 . 
     Similarly, in some examples, stimulation generator  88  may encode information in a stimulation signal by associating a plurality of bursts of pulses, referred to herein as a burst pattern, with an alphanumeric indicator. The multiple burst patterns may be arranged to define code words or other alphanumeric codes in order to transmit the desired information to ICD  16 . When using burst pattern encoding, stimulation generator  88  may encode information by varying the duty cycle for a burst pattern, or by varying the duty cycle between burst patterns. In other examples, burst pattern may be directly associated with a respective therapy modification instruction or other information that is stored in a memory of ICD  16 . Again, ICD  16  may sense the stimulation signal and process the sensed electrical signal to identify the burst pattern, and, therefore, extract the encoded alphanumeric indicators from the sensed electrical signal. In other examples, burst patterns may be directly associated with a respective therapy modification instruction or other information that is stored in a memory of ICD  16 . 
     When encoding information on a pulse-by-pulse basis, stimulation generator  88  may vary one or more signal parameter values for each pulse. That is, stimulation generator  88  may generate each pulse according to a different set of signal parameter values. This encoding technique may provide a greater information rate, i.e., may be used to transmit more information for a given period of time, but may require greater resolution at ICD  16 . 
     Stimulation generator  88  may also encode information by varying one or more signal parameter values in a particular pattern, where the values are varied within an acceptable range of stimulation parameter values that provide efficacious therapy to patient  12 . For example, stimulation generator  88  may encode therapy information by varying one or more signal parameters, such as a slew rate, pulse amplitude, pulse rate (frequency) and pulse width (rate), and may encode duration information by varying one or more other signal parameters, such as duty cycle. Varying specific types of signal parameters may permit stimulation generator  88  to encode different types of therapy information. ICD  16  may sense the stimulation signal and process the sensed electrical signal to identify the one or more signal parameter variation patterns, and, therefore, extract the encoded information from the sensed electrical signal. The signal parameter variation patterns may be associated with alphanumeric indicators or directly associated with a respective therapy modification instruction or other information that is stored in a memory of ICD  16 . 
     In order to vary the signal parameters, processor  80  may load stimulation parameter values according to therapy programs  100  stored in memory  82 . Therapy programs  100  may each define initial values for generating a stimulation signal. Processor  80  may also load stimulation variation parameter values stored in memory  82  as interdevice communication features  102 . The stimulation variation parameter values may provide a range of values over which signal parameters may be varied and instructions for varying the signal parameters to encode the desired information. These stimulation variation parameter values may provide a range of values over which the signal parameter values may be modified without adversely affecting the efficacy of stimulation therapy delivered by INS  16 . 
     In some examples, interdevice communication features  102  may store instructions that permit processor  80  to modify a pulse rate (frequency) for an electrical stimulation signal between approximately 10 Hertz (Hz) and approximately 100 Hz. The interdevice communication features  102  may also store instructions that permit processor  80  to modify a pulse width (duration) of an electrical stimulation signal between approximately 30 microseconds (μs) and approximately 480 μs. The interdevice communication features  102  may also store instructions that permit processor  80  to modify a duty cycle of an electrical stimulation signal between parameter values that indicate stimulation is delivered in a ratio of approximately 20% ON and 80% OFF to approximately 80% ON and 20% OFF. 
     Stimulation generator  88  may encode information in a stimulation signal in a predetermined order or sequence. Additionally, because INS  26  and ICD  16  may not necessarily be synchronized with each other and communication may be one-way, from INS  26  to ICD  16 , the information may be encoded repeatedly in the stimulation signal. Repeating the encoded information may allow ICD  16  to reliably retrieve the encoded information by providing ICD  16  multiple opportunities to sense the stimulation signal and extract the therapy information therefrom. 
     In some examples, the information may be encoded in the stimulation signal in a sequence that includes a header marking the beginning of the sequence and a predefined sequence of information useful to INS  26 , such as therapy information, operational information, and diagnostic information. The stimulation signal may also include a footer marking the end of the information sequence. Again, the header and footer may be referred to as message information. As an example, the predefined sequence may include in order, a header, one or more of therapy information, operational information, and diagnostic information, and a footer. Other orders of encoded information are contemplated, but the header and footer typically remain at the beginning and end, respectively, of the encoded information. The information located between the header and footer may require a variable number of “bits” or “bytes” to transmit the information. The bits or bytes refer to the number of pulses required to transmit the required information. However, in examples in which the number of bits or bytes is fixed for transmitting this information, a footer may not be needed. In other examples, the types of therapy information may be encoded in the stimulation signal in any particular order. In examples in which therapy information is transmitted, processor  80  may access a clock or other timing device within INS  26  to determine pertinent times. 
     ICD  16  may use the encoded information to modify its operation. For example, as described in further detail below, ICD  16  may blank its sensing circuitry while INS  26  delivers stimulation or may invoke additional signal processing to suppress crosstalk resulting from the stimulation signal output by INS  26 . 
     The example modulation techniques described in this disclosure are not limiting of the scope of the systems, devices, and methods described herein. The purpose of the examples described herein is to provide functional examples and a framework for which more complex systems, that are contemplated within the score of this disclosure, Accordingly, the scope of this disclosure encompasses more any suitable encoding, decoding, and transmission techniques that are well known in the art of telecommunications. 
     In another example, processor  80  may control operation of stimulation generator  88  to reduce the possibility that ICD  16  may sense stimulation signals generated by INS  26  and mischaracterize the stimulation signals as cardiac signals. For example, processor  80  may control stimulation generator  88  to generate a stimulation signal in a way that facilitates filtering of the electrical stimulation signal generated by INS  26  from a signal sensed by ICD  16 , or by producing stimulation signal in a way that reduces the impact of the resulting signal artifact at ICD  16 . 
     Processor  80  may control operation of stimulation generator  88  based on information and/or instructions stored as crosstalk mitigation features  104  loaded from memory  82 . As one example, processor  80  may control stimulation generator  88  to generate a stimulation signal with a predetermined signature, which may be stored as crosstalk mitigation features  104  in memory  82 . Stimulation generator  88  may vary one or more signal parameters, e.g., a slew rate, pulse rate (frequency), pulse width (rate), phase, and duty cycle, to generate the stimulation signal with the predetermined signature. In particular, stimulation generator may vary one or more signal parameter values from the initial value specified in therapy programs  100  to generate the stimulation signal with the predetermined signature. When stimulation generator  88  generates the stimulation signal as a plurality of pulses, the signature may comprise a plurality of bursts of pulses. When stimulation generator  88  generates the stimulation signal as a continuous waveform, the signature may be characterized by a signal envelope that traces the outline of the stimulation signal for a given period of time. Stimulation generator  88  may generate the stimulation signal so that the predetermined signature repeats throughout the signal. 
     As an additional example, stimulation generator  88  may generate a stimulation signal that has a narrowband energy spectrum centered at a predetermined frequency. In such an example, crosstalk mitigation features  104  stored in memory  82  may specify signal parameter values that define a stimulation signal having an energy focused within the predetermined frequency band. The predetermined frequency may be selected by a clinician to be frequency that does not generally interfere with a cardiac signal. That is, the predetermined frequency may be selected as a frequency that contains little information for cardiac events. 
     As will be described in greater detail with respect to  FIGS. 6 and 8 , in examples in which INS  26  generates and delivers a stimulation signal includes a predetermined signature or a narrow band stimulation signal, ICD  16  may be configured to substantially remove the artifact present in a sensed electrical signal, where the artifact is attributable to a stimulation signal generated by INS  26 . The known signature of the stimulation signal and narrowband energy spectrum may allow ICD  16  to relatively easily filter the artifact from a sensed electrical signal. For example, ICD  16  may sense an electrical signal with select electrodes of leads  18 ,  20 ,  22  or housing  70  ( FIG. 4 ), and process the sensed electrical signal with a notch filter at the predetermined frequency band in order to filter the stimulation signals out of the sensed electrical signal. 
     In other examples, processor  80  of INS  26  may control stimulation generator  88  to generate a stimulation signal that has a spread spectrum energy distribution. In particular, stimulation generator  88  may randomly or pseudo-randomly vary one or more signal parameters, e.g., a slew rate, pulse rate (frequency), pulse width (rate), phase, and duty cycle, under the control of processor  80 . When stimulation generator  88  outputs a pulse waveform, stimulation generator  88  may vary the one or more signal parameters for each burst, i.e., on a burst-by-burst basis, or for each pulses, i.e., on a pulse-by-pulse basis. Processor  80  may load instructions and/or signal parameters values from crosstalk mitigation features  104  stored in memory  82 . The spread spectrum energy distribution of the stimulation signal may cause the signal artifact coupled to ICD  16  to appear as wideband noise in the sensed signal, as described in further detail below with reference to  FIGS. 12A and 12B . In addition, as described below with respect to  FIGS. 6 and 8 , ICD  16  may be configured to suppress the resulting wideband noise or may be configured to remove the wideband noise via processing techniques. 
     Telemetry module  86  supports wireless communication between INS  26  and an external programmer  24  ( FIG. 1 ) or another computing device under the control of processor  80 . Processor  80  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  86 . The updates to the therapy programs may be stored within memory  82 . Telemetry module  86  may include an antenna  87 , which may take on a variety of forms. Antenna  87  may comprise an internal antenna or an external antenna. For example, antenna  87  may be formed by a conductive coil or wire embedded in a housing associated with INS  26 . Alternatively, antenna  87  may be mounted on a circuit board carrying other components of INS  26  or take the form of a circuit trace on the circuit board. In addition, in some examples, telemetry module  86  may support communication between INS  26  and another device (e.g., programmer  24 ) with the aid of more than one antenna, such as an external antenna and an internal antenna. 
     The various components of INS  26  are coupled to power supply  84 , 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 supply  84  may be powered by proximal inductive interaction with an external power supply carried by patient  12 . 
       FIG. 7  is a functional block diagram of an example configuration of ICD  16 , which includes processor  110 , memory  112 , stimulation generator  114 , sensing module  116 , telemetry module  118 , and power source  120 . In general, ICD  16  may monitor electrical activity of heart  14  and deliver cardiac rhythm therapy to heart  14  in the form of pacing, cardioversion, and/or defibrillation pulses. Electrical signals sensed by ICD  16  may include a signal artifact attributable to stimulation delivery by INS  26 . As previously described, the signal artifact may be used for communication purposes, i.e., INS  26  may encode information in a stimulation signal, in one example. In other examples, INS  26  may generate a stimulation signal that reduces the signal artifact at ICD  16 . 
     In examples in which INS  26  encodes information in a stimulation signal, ICD  16  may be configured to analyze the sensed electrical signal to retrieve the encoded information. ICD  16  may then modify its operation based on the retrieved information. In examples in which INS  26  generates a stimulation signal to reduce the signal artifact, ICD  16  may be configured to process the sensed signal to substantially remove the signal artifact. 
     As shown in  FIG. 7 , sensing module  116  of ICD  16  includes artifact monitor  122  and cardiac sensing module  124 . Cardiac sensing module  124  is configured to monitor electrical activity of heart  14  using techniques known in the art of cardiac therapy. Artifact monitor  122  may provide interdevice communication features and crosstalk mitigation features described herein. In particular, communication monitor  150  may be configured to provide interdevice communication features and artifact removal module  160  may be configured to provide crosstalk mitigation features.  FIG. 8  provides a more detailed description of communication module  150  and artifact removal module  160 . The following paragraphs provide a general description for the operation of ICD  16  with respect to the block diagram illustrated in  FIG. 7 . 
     Memory  112  includes computer-readable instructions that, when executed by processor  110 , cause ICD  16  and to perform various functions attributed to ICD  16  herein. 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. In the example illustrated in  FIG. 7 , memory  112  includes therapy programs  126 , interdevice communication features  127 , and crosstalk mitigation features  128 . Therapy programs  126  may be stored as individual therapy programs or as therapy program groups. 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 therapy delivery by ICD  16  under control of processor  110 . 
     Interdevice communication features  127  may include instructions for analyzing a sensed electrical signal to extract information encoded in the sensed signal. As previously indicated, INS  26  may encode a stimulation signal with one or more alphanumeric identifiers or other therapy information indicators. Interdevice communication features  127  may provide instructions executable by processor  110  to decode information encoded in a sensed signal. In this way, the stimulation signal generated and delivered by INS  26  may also be used to support wireless communication between ICD  16  and INS  26 . Processor  110  may also load instructions from interdevice communication features  127  to modify its operation based on the retrieved information. For example, interdevice communication features  127  may provide instructions that, when executed by processor  110 , cause processor  110  to time the blanking of sensing circuitry of cardiac sensing module  124  or selectively apply modified signal processing techniques. An example modified signal processing technique may involve applying a matched filter to the sensed signal. In this example, the matched filter may be matched to the stimulation signal or, more particularly, the signal artifact in the sensed signal. ICD  16  may then disregard portions of the sensed signal that are detected by the matched filter. These instructions may be, for example, associated with the alphanumeric identifiers encoded in the sensed stimulation signal or one or more signal characteristics of the sensed stimulation signal. 
     In some examples, processor  110  may load instructions from crosstalk mitigation features  128  in order to minimize the crosstalk resulting from stimulation therapy delivered by INS  26 . In particular, crosstalk mitigation features  128  may provide instructions that guide processor  110  to implement signal processing techniques for filtering out at least some of the neurostimulation artifact from a sensed electrical signal. As an example, the stored instructions may cause processor  110  to load addresses of registers that store the one or more signal parameter values that characterize a predefined electrical stimulation signal signature. As an additional example, instructions stored within crosstalk mitigation features  128  may include instructions for loading addresses of registers that store values for digital filter used for filtering the received signal at a predetermined frequency. 
     As previously described, ICD  16  may generally be configured to analyze a sensed electrical signal in order to retrieve information encoded in the signal artifact. In addition or instead of decoding a sensed electrical signal to retrieve information communicated by INS  26 , ICD  16  may be configured to at least partially remove the signal artifact from a sensed electrical signal. Thus, it should be understood that in some examples, memory  112  may include only one of interdevice communication features  127  or crosstalk mitigation features  128 . 
     Processor  110  may include any one or more of a microprocessor, a controller, a DSP, an ASIC, an FPGA, or equivalent discrete or integrated logic circuitry. In some examples, processor  110  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  110  herein may be embodied as software, firmware, hardware or any combination thereof. Processor  110  controls stimulation generator  114  to deliver stimulation therapy to heart  14  according to a selected one or more of therapy programs  126 , which may be stored in memory  112 . Specifically, processor  110  may control stimulation generator  114  to deliver therapy to heart  14  in the form of electrical pulses with the amplitudes, pulse widths, frequency, or electrode polarities specified by the selected one or more therapy programs stored as therapy programs  126 . 
     Stimulation generator  114  is electrically coupled to electrodes  50 - 55 ,  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  114  is configured to generate and deliver electrical stimulation therapy to heart  14 . For example, stimulation generator  114  may deliver defibrillation shocks to heart  14  via at least two electrodes  68 ,  72 ,  74 ,  76 . Stimulation generator  114  may deliver pacing pulses via ring electrodes  50 ,  52 ,  54  coupled to leads  18 ,  20 , and  22 , respectively, and/or helical electrodes  51 ,  53 ,  55  of leads  18 ,  20 , and  22 , respectively. In some examples, stimulation generator  114  delivers pacing, cardioversion, or defibrillation stimulation in the form of electrical pulses. In other examples, stimulation generator  114  may deliver one or more of these types of stimulation in the form of other signals, such as sine waves, square waves, or other substantially continuous time signals. 
     Stimulation generator  114  may include a switch module and processor  110  may use the switch module to select, e.g., via a data/address bus, which of the available electrodes are used to deliver the stimulation 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  114  may independently deliver stimulation to electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76 . 
     As shown in  FIG. 7 , sensing module  116  includes artifact monitor  122  and cardiac sensing module  124 . Generally, sensing module  116  monitors electrical signals from at least two electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76  in order to monitor electrical activity of heart  14 , e.g., via EGM signals. In one example, artifact monitor  122  and, more particularly, communication module  150 , may process a voltage signal between two or more of electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76  to retrieve information encoded in a stimulation signal output by INS  26 . In such an example, artifact monitor  122  (communication module  150 ) may provide the retrieved information to processor  110  which may then modify operation of ICD  16  based on the retrieved information. For example, processor  110  may reference memory  112  to determine the instruction that is associated with the retrieved information, which may be in the form of, for example, an alphanumeric indicator or another symbolic indicator. Operation of artifact monitor  122  (communicate module  150 ) and ICD  16  in accordance with such an example is described in greater detail with respect to  FIG. 8 . 
     In another example, artifact monitor  122  and, more particularly, artifact removal module  160 , may be configured to substantially remove the signal artifact caused by the delivery of stimulation by INS  26  from the sensed electrical signal. In such an example, artifact monitor  122  (artifact removal module  160 ) may process a voltage signal between two or more of electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76  by applying a filter. The filter may be designed based on a predetermined signature used by INS  26  to generate a stimulation signal. Alternatively, the filter may be designed with a predetermined center frequency that is used by INS  26  to generate a stimulation signal. A more detailed description of artifact monitor  122  (artifact removal module  160 ) configured for substantially removing crosstalk is provided with respect to  FIG. 9 . 
     Artifact monitor  122  may preprocess the voltage signal and output the processed signal to cardiac sensing module  124 . In this way, cardiac sensing module  124  may apply signal processing techniques known in the art to monitor the activity of heart  14 , such as the amplification techniques described below for sensing R-waves and P-waves of electrical cardiac signals. Sensing module  116  may include a switch module to select which of the available electrodes are used to sense the electrical cardiac activity. In some examples, processor  110  may select the electrodes that function as sense electrodes via the switch module within sensing module  116 , e.g., by providing signals via a data/address bus. In some examples, sensing module  116  may include one or more sensing channels, each of which may comprise an amplifier. In response to the signals from processor  110 , the switch module within sensing module  116  may couple the outputs from the selected electrodes to one of the sensing channels. 
     In some examples, one channel of cardiac sensing module  124  may include an R-wave amplifier that receives signals from electrodes  50  and  51 , 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  52  and  53 , which are used for pacing and sensing proximate to left ventricle  36  of heart  14 . In some examples, 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 cardiac sensing module  124  may include a P-wave amplifier that receives signals from electrodes  54  and  55 , which are used for pacing and sensing in right atrium  30  of heart  14 . In some examples, 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 cardiac sensing module  124  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 - 55 , 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, cardiac sensing module  124  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  112  as an electrogram (EGM). In some examples, the storage of such EGMs in memory  112  may be under the control of a direct memory access circuit. Processor  110  may employ digital signal analysis techniques to characterize the digitized signals stored in memory  112  to detect and classify the patient&#39;s heart rhythm from the electrical signals. Processor  110  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  110  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  110  components, such as a microprocessor, or a software module executed by a component of processor  110 , 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 “D” is used with the third letter in the code, it may indicate that the signal is used for tracking purposes. 
     Intervals defined by the pacer timing and control module within processor  110  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 to cardiac sensing module  124  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  110  in response to stored data in memory  112 . The pacer timing and control module of processor  110  may also determine the amplitude of the cardiac pacing pulses. 
     In an example in which ICD  16  is configured to retrieve information that has been encoded in a stimulation signal output (e.g., delivered to tissue) by INS  26 , ICD  16  may modify its operation based on the retrieved information. For example, artifact monitor  122  may decode a sensed electrical signal in order to determine whether the blanking period of sensing module  116  should be modified. Thus, information that may be extracted from the sensed stimulation signal may include information that specifies the timing of therapy delivered by INS  26 . Processor  110  may use the retrieved information to define a blanking period and provide signals to cardiac sensing module  124  to blank one or more channels during stimulation delivered by INS  26  and for a period following the therapy. 
     During pacing, escape interval counters within the pacer timing/control module of processor  110  may be reset upon sensing of R-waves and P-waves. Stimulation generator  114  may include pacer output circuits that are coupled, e.g., selectively by a switching module, to any combination of electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76  appropriate for delivery of a bipolar or unipolar pacing pulse to one of the chambers of heart  14 . Processor  110  may reset the escape interval counters upon the generation of pacing pulses by stimulation generator  114 , 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  110  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  112 . Processor  110  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  110  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, processor  110  may not meet the requirements for triggering a therapeutic response, and, thus, processor  110  may continue normal operation. 
     In some examples, processor  110  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  110  and any updating of the values or intervals controlled by the pacer timing and control module of processor  110  may take place following such interrupts. A portion of memory  112  may be configured as a plurality of recirculating buffers, capable of holding series of measured intervals, which may be analyzed by processor  110  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  110  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  110  in other examples. 
     In the examples described herein, processor  110  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  112  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  112 . In some examples, processor  110  may also identify the presence of the tachyarrhythmia episode by detecting a variable coupling interval between the R-waves of the heart signal. 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  110  may determine that the tachyarrhythmia is present. 
     If processor  110  detects an atrial or ventricular tachyarrhythmia based on signals from sensing module  116 , and an anti-tachyarrhythmia pacing regimen is desired, timing intervals for controlling the generation of anti-tachyarrhythmia pacing therapies by stimulation generator  114  may be loaded by processor  110  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  114  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  110  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  110  may activate a cardioversion/defibrillation control module, which may, like pacer timing and control module, be a hardware component of processor  110  and/or a firmware or software module executed by one or more hardware components of processor  110 . 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  110  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  110 , processor  110  may generate a logic signal that terminates charging. Thereafter, timing of the delivery of the defibrillation or cardioversion pulse by stimulation generator  114  is controlled by the cardioversion/defibrillation control module of processor  110 . Following delivery of the fibrillation or tachycardia therapy, processor  110  may return stimulation generator  114  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  114  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  114 . 
     Telemetry module  118  includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as programmer  24  ( FIG. 1 ). Under the control of processor  110 , telemetry module  118  may receive downlink telemetry from and send uplink telemetry to programmer  24  with the aid of antenna  119 . Antenna  119  may be similar to antenna  87  coupled to telemetry module  86  ( FIG. 6 ). For example, antenna  119  may be internal or external to the housing of ICD  16 . Processor  110  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  118  may provide received data to processor  110  via a multiplexer. 
     In some examples, processor  110  may transmit atrial and ventricular heart signals (e.g., EGM signals) produced by atrial and ventricular sense amp circuits within cardiac sensing module  124  to programmer  24 . Programmer  24  may interrogate ICD  16  to receive the heart signals. Processor  110  may store heart signals within memory  112 , and retrieve stored heart signals from memory  112 . Processor  110  may also generate and store marker codes indicative of different cardiac episodes that cardiac sensing module  124  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  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. 
     The configuration of the functional blocks in  FIG. 7  is merely one example. Other configurations are contemplated. For example, although  FIG. 7  illustrates artifact monitor  122  and cardiac sensing module  124  are being part of sensing module  116 , in other examples, processor  110  may include artifact monitor  122  and cardiac sensing module  124 . 
       FIG. 8  is a functional block diagram of an example configuration of artifact monitor  122  and illustrates communication module  150  in greater detail. As shown in the illustrated example of  FIG. 8 , communicate module  150  may include filter  130 , amplifier  132 , analog to digital converter (A/D)  134 , DSP  136 , and decoder  138 . As previously described, INS  26  may encode information in a stimulation signal that provides therapeutic effects. The stimulation signal or, more specifically, a signal artifact of the stimulation signal, may be transmitted to ICD  16  by electrical conduction through tissue of patient  12 . 
     In the illustrated example of  FIG. 8 , communication module  150  senses electrical activity within patient  12  by sensing a voltage  140 , i.e., a voltage difference, across two or more of electrodes  50 - 55 ,  68 ,  70 ,  72 , and  76  ( FIG. 3 ) coupled to sensing module  116  of ICD  16 . In particular, voltage  140  is sensed across inputs to filter  130 . Voltage  140  may comprise signal components from one or more of a signal artifact resulting from a stimulation signal output by INS  26  and electrical activity of heart  14 . Filter  130  may remove high frequency signals from the sensed wideband signal and output a narrowband filtered signal, i.e., signal  141 . As an example, filter  130  may have a pass band of approximately 2.5 Hz to approximately 100 Hz. 
     Amplifier  132  amplifies filtered signal  141  to produce amplified signal  142  as an input to A/D  134 . Amplified signal  142  may also be supplied to cardiac sensing module  124 , which may process amplified signal  142  in accordance with the description provided in  FIG. 7 . For example, cardiac sensing module  124  may amplify signal  142 , which may help processor  110  identify R-waves and P-waves of electrical cardiac signals. 
     A/D  134  may convert amplified signal  142  to a digital signal  144  for processing by DSP  136 . 
     DSP  136  may be configured to employ various techniques for retrieving the information encoded in digital signal  144 . In particular, DSP  136  may include hardware and/or software for measuring the pulse rate (frequency), pulse width (duration), phase, and duty cycle of digital signal  144 . DSP  136  may output a signal  146  based on the measurement. For example, DSP  136  may employ peak detection techniques to determine a pattern in the pulse rate, pulse width or duty cycle of the stimulation signal generated by INS  26  in order to retrieve the encoded information. Additionally or alternatively, DSP  136  may be configured to operate as a matched filter. When INS  26  encodes information in the stimulation signal, INS  26  may prepend the information with a predetermined header. DSP  136  may be configured as a correlator for locating the header in digital signal  144 . After locating the header, DSP  136  may apply additional processing, such as peak detection or other signal processing techniques, to retrieve the encoded information that may follow the header. 
     The output of DSP  136  is a processed digital signal  146  that represents the encoded information. Decoder  138  may analyze the series of digital values that form digital signal  146  in order to retrieve the encoded information. For example, each digital value may correspond to a particular signal characteristic, such as an amplitude value or a frequency value. Decoder  138  may output electrical signal  148  based on the retrieved information. Electrical signal  148  may be a control signal that conveys the encoded information to processor  110 . For example, decoder  138  may be capable of outputting a number of predefined electrical signals that are associated with corresponding information in memory  112  ( FIG. 7 ) of ICD  16 . The predefined electrical signals may correspond to alphanumeric identifier or another type of symbolic identifier. Decoder  138  may match the digital values provided by DSP  136  to one or more corresponding identifiers and output electrical signal  148  accordingly. 
     Processor  110  may modify operation of ICD  16  based on electrical signal  148 . For example, processor  110  may reference stored instructions within memory  112  to determine the instructions that are associated with electrical signal  148  from decoder  138 . As previously indicated, memory  112  of ICD  16  may store a plurality of alphanumeric identifiers and associated instructions. The instructions may, for example, cause processor  110  to blank sensing circuitry, e.g., sensing module  116 , at specified times. In this way, the specified times may be ultimately conveyed from INS  26  to ICD  16  by electrical signal  140 . 
     As shown in  FIG. 8 , processor  110  may also provide control signals to communication module  150 . For example, processor  110  may activate and deactivate communication module  150 . Processor  110  may deactivate communication module  150  when INS  26  is not actively delivering therapy to patient  12  (e.g., when the delivery of stimulation by INS  26  is suspended). Processor  110  may determine when INS  26  delivers therapy based on information received from communication module  150 . If communication module  150  is deactivated, processor  110  may periodically and temporarily reactivate communication module  150  to determine whether INS  26  has started to deliver therapy again. If communication module  150  determines that INS  26  is not delivering therapy, then processor  110  may deactivate communication module  150  for a given period of time. In this way, one-way communication between INS  26  and ICD  16  may be performed in an energy efficient manner. 
     Other configurations of communication module  150  are contemplated. For example, in other examples, communication module  150  may not include at least one of the components  130 ,  132 ,  134 ,  136 ,  138  or a single component may provide the functions attributed to the separate components  130 ,  132 ,  134 ,  136 ,  138  shown in  FIG. 8 . It should be understood that the modules shown illustrated in  FIG. 8  illustrate logical functions and, thus, certain features of the modules may be provided by shared or common circuitry. For example, decoder  138  may share at least some circuitry with processor  110 . Moreover, the order of the components  130 ,  132 ,  134 ,  136 ,  138  of communication module  150  shown in  FIG. 8  is merely one example. For example, in other examples, amplifier  132  may amplify a signal prior to filtering by filter  130 . 
       FIG. 9  is a functional block diagram of an example configuration of artifact monitor  122  that shows artifact removal module  160  in greater detail. As shown in  FIG. 9 , artifact removal module  160  may include signature module  162  and programmable filter  164  for removing the signal artifact from a sensed electrical signal. In particular, signature module  162  may detect and remove a signal artifact having a predetermined signature from the sensed signal, and programmable filter  154  may remove a signal artifact with a narrowband energy spectrum centered at a predetermined frequency from the sensed signal. Although artifact removal module  160  is shown in  FIG. 9  as including both signature module  162  and programmable filter  164 , artifact removal module  160  may generally include one or both of signature module  162  and programmable filter  164 . Artifact removal module  160  may include signature module  162  in examples in which INS  26  is configured to generate a stimulation signal with a predetermined signature. Artifact removal module  160  may include programmable filter  164  in examples in which INS  26  is configured to generate a stimulation signal with a narrowband energy spectrum centered at a predetermined frequency. 
     Just as with communication module  150  in  FIG. 8 , artifact removal module  160  may sense a voltage  151 , i.e., a voltage difference across two or more of electrodes  50 - 55 ,  68 ,  70 ,  72 , and  76  ( FIG. 4 ). Again, voltage  151  may include a signal artifact of the stimulation signal output by INS  26  and electrical cardiac signals. Voltage  151  is sensed across inputs to filter  161 , which may remove high frequency signals. Thus, filter  161  may filter the electrical signal generated by voltage  151  at its inputs to output a narrowband signal  152  to amplifier  163 . In some examples, filter  161  may have a passband of approximately 2.5 Hz to approximately 100 Hz, although other frequency ranges are contemplated. Amplifier  163  amplifies narrowband signal  152  to produce amplified signal  154 . Filter  161  and amplifier  163  in  FIG. 9  may be substantially similar to filter  130  and amplifier  132  in  FIG. 8 . Filters  161 ,  130  and amplifiers  163 ,  132  may generally used to condition a sensed signal for processing by additional circuitry. Accordingly, amplified signal  154  is provided as an input to both signature module  162  and programmable filter  164 . 
     In  FIG. 9 , signature module  162  includes matched filter  166  and filter  168 . In general, matched filter  166  may be used to detect a predetermined signature in amplified signal  154  and filter  168  may be used to remove the predetermined signature from amplified signal  154 . As shown in  FIG. 9 , matched filter  166  outputs control signal  156  to processor  110 . Control signal  156  may be a logic signal that is high when the predetermined signature is detected and is low when the predetermined signature is not detected. Matched filter  166  also outputs amplified signal  154  to filter  168  and cardiac sensing module  124 . In particular, matched filter may output amplified signal  154  to filter  168  when the predetermined signature is detected and output amplified signal  154  to cardiac sensing module  124  when the predetermined signature is not detected. When the predetermined signature is detected, filter  168  filters amplified signal  154  to substantially remove the predetermined signature, i.e., the signal artifact, from the signal. The filtered signal  158  is output to cardiac sensing module  124  to monitor the heart rhythm of patient  12 . However, when matched filter  166  does not detect the predetermined signature, amplified signal  154  is output directly to cardiac sensing module  124  because a signal artifact is determined not to be present in the sensed signal. 
     Matched filter  166  may be implemented as an analog matched filter or a digital matched filter. When implemented as a digital matched filter, signature module  162  may also include an A/D to convert analog signal  154  to a digital signal suitable for input to the digital matched filter. In other examples, signature module  162  may be implemented using other components for analyzing amplified signal  154  for a predetermined signature. 
     In an example in which INS  26  is configured to generate a stimulation signal with a narrowband energy spectrum, amplifier  163  applies amplified signal  154  to programmable filter  164 . Programmable filter  164  may be an analog or digital filter that passes frequencies outside of a stop band centered at a center frequency, e.g., a notch filter. Thus, programmable filter  164  may substantially remove a signal artifact with a narrowband energy spectrum centered at the predetermined frequency from amplified signal  154 . Thus, programmable filter  164  outputs signal  155  to cardiac sensing module  124 , and cardiac sensing module  124  may reliably process filtered signal  155  to monitor the heart of patient  12  while INS  26  delivers neurostimulation to patient  12 . 
     In particular, programmable filter  164  may have a variable center frequency that is controlled by processor  110  via control signal  157 . Processor  110  may select the center frequency of programmable filter  164  based on pre-programmed information or based on information received from INS  26 . INS  26  may transmit the information to ICD  16  via RF communication or via stimulation signals output by INS  26  in as described in this disclosure. When the center frequency is a pre-selected parameter, programmable filter  164  need not be programmable and, thus, may be implemented as a filter with a fixed center frequency. In a similar manner, processor  110  may also use control signal  157  to control the bandwidth of programmable filter  164 , i.e., the range of frequency for the stop band. 
       FIG. 10  is block diagram of an example programmer  24 . As shown in  FIG. 10 , programmer  24  includes processor  170 , memory  172 , user interface  174 , telemetry module  176 , and power source  178 . 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  174 , 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  170  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  172  may store instructions that cause processor  170  to provide the functionality ascribed to programmer  24  herein, and information used by processor  170  to provide the functionality ascribed to programmer  24  herein. Memory  172  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  172  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  172  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  176 , 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  176  may be similar to telemetry module  98  of ICD  16  ( FIG. 6 ) or telemetry module  118  of INS  26  ( FIG. 7 ). 
     Telemetry module  176  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  178  delivers operating power to the components of programmer  24 . Power source  178  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  178  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  178  may include circuitry to monitor power remaining within a battery. In this manner, user interface  174  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  178  may be capable of estimating the remaining time of operation using the current battery. 
       FIGS. 11A-11D  illustrate example waveforms that stimulation generator  88  of INS  26  ( FIG. 6 ) may generate and deliver to a tissue site within patient  12  in order to provide therapeutic benefits to patient  12 . The example waveforms shown in  FIGS. 11A-11D  are encoded with information. In particular,  FIGS. 11A-11D  illustrate example pulse waveforms that may be generated by INS  26  and encoded with information by varying one or more signal parameters, e.g., pulse rate (frequency), pulse width (duration), phase, and duty cycle. It should be understood that the pulse waveforms illustrated in  FIGS. 11A-11D  are merely exemplary and should not be considered limiting of the disclosure. Rather, the purpose of pulse waveforms in  FIGS. 11A-11D  is to show various examples for encoding information by varying one or more signal parameter values. 
     As shown in  FIGS. 11A-11D , INS  26  may generate a stimulation signal as a series of bursts of pulses, where each burst of pulses includes a plurality of pulses. The number of pulses for each burst of pulses may be a predefined parameter for a selected therapy program, which may be stored in memory  82  of INS  26  ( FIG. 6 ). As previously described, INS  26  may generate the waveforms shown in  FIGS. 11A-11D  in accordance with one or more selected therapy programs and may encode information in the stimulation signal by varying the values one or more of the signal parameters, e.g., pulse rate (frequency), pulse width (duration), phase, and duty cycle, in a predetermined manner. ICD  16  extract the information from the stimulation signals shown in  FIGS. 11A-11D  based on the pattern with which the stimulation parameters are varied. For example, INS  26  and ICD  16  may share a set of instructions that associate different patterns in stimulation signal parameter values with certain types of information (e.g., instructions relating to the modification to the sensing parameters of ICD  16 ). 
     Information is encoded in the waveforms in  FIGS. 11A-11C  by varying one signal parameter on a burst-by-burst basis. In other examples, information may be encoded in the stimulation signals by varying more than one signal parameter at any given time. Each pulse for a particular burst in the waveforms in  FIGS. 11A-11C  is generated with the same signal parameter values, but the signal parameter values may vary between bursts. In this way, each burst or group of bursts may be associated with an alphanumeric identifier or another type of identifier. In examples in which the bursts or group of bursts are associated with alphanumeric identifiers, the burst or group of bursts may be may be arranged such that the associated alphanumeric identifiers to form alphanumeric code words that can be used to transmit messages to the receiving device, e.g., ICD  16 . These messages may contain information regarding the therapy delivered by INS  26 , such as the type of therapy and duration of a current therapy session. The current therapy session may be, for example, the therapy session in which INS  26  is delivering therapy to patient  12  when ICD  16  senses the stimulation signal artifact. The type of therapy may be encoded by specifying the selected therapy program(s) or by specifying the therapy parameter values. In addition, as previously indicated, the duration of therapy may be encoded by specifying a stop time, by specifying a start and a stop time, by specifying the total duration of time or by specifying the time remaining. 
     Information is encoded in the waveform in  FIG. 11D  by varying one or more signal parameters on a pulse-by-pulse basis. Thus, at least two pulses in a particular burst of pulses may be generated with different signal parameter values. This technique may also be used to encode information by associating each burst of pulses or group of bursts with a symbol from a predefined alphanumeric code. Alternatively, this method may also be used to encode information by associating each pulse with an alphanumeric identifier in order to provide a greater information transmission rate. 
     In general, the purpose of the waveforms shown in  FIGS. 11A-11D  is not to illustrate a stimulation waveform encoded with particular information. Instead, the purpose of the waveforms is to provide an example that shows how information may be encoded in a stimulation waveform by varying one or more signal parameters, such as duty cycle (FIG  11 A, frequency ( FIG. 11B ), pulse width ( FIG. 11C ), and frequency and pulse width (FIG  11 D). 
     With respect to  FIG. 11A , stimulation generator  88  of INS  26  varies the duty cycle of pulse waveform  200  in order to transmit information to ICD  16 . Waveform  200  includes bursts of pulses  202 A- 202 C (collectively bursts of pulses “ 202 ” or “bursts  202 ”) and  204 A- 204 D (collectively “bursts of pulses  204 ” or “bursts  204 ”). Bursts of pulses  202  are delivered in accordance with a first duty cycle and bursts of pulses  204  are delivered in accordance with a second duty cycle that is different than the first duty cycle. 
     In  FIG. 11A , T ON  is the duration of time during which stimulation generator  88  delivers pulses of waveform  200 , and T OFF1  and T OFF2  are the durations of times during which stimulation generator  88  is not delivering pulses. The duty cycle of bursts of pulses  202  and  204  may be the ratio of T ON  to a total cycle time including T ON  and T OFF1  or T OFF2 , respectively. In the following description, T ON  is a constant value so the total cycle time for is different for the first and second duty cycles, and the values for T OFF1  and T OFF2  are selected to encode information. However, in other examples, that first and second duty cycles may be selected by using a constant value for T OFF  and using different values for T ON . Additionally, it is contemplated that the total cycle time be a constant value and different T ON  and T OFF  values be selected for each of the different duty cycles used for encoding information. 
     In any case, with respect to  FIG. 11A , the duty cycle of bursts  202  and  204  are generally selected to be different from each other and may be selected anywhere between 0% and 100% ON. In a 0% ON, T ON  is substantially equal to zero, such that stimulation generator  88  does not deliver any pulses in the burst having the duty cycle of approximately 0% ON. In a 100% ON duty cycle, T OFF  is substantially equal to zero, such that the burst of pulses is substantially one continuous pulse. In some examples, a duty cycle may typically be selected to be less than 50% ON and more than 50% OFF, such that the duration of times T OFF1  or T OFF2  between bursts  202 ,  204 , respectively, is greater than the duration of each burst  202 ,  204 . Accordingly, the duty cycle for bursts  202  and  204  may each be selected to be less than 50% ON, but with different values, such as 25% ON for bursts  202  and 50% ON for bursts  204 . In a duty cycle of approximately 25% ON, T ON  may be approximately one third the value of T OFF1  or T OFF2 . In a duty cycle of approximately 50% ON, T ON  may be approximately equal to the value of T OFF1  or T OFF2 . As another example, the duty cycle for one of bursts  202  and  204  maybe selected to be more than 50% ON and the duty cycle for the other may be selected to be less than 50% ON. 
     Waveform  200  in  FIG. 11A  includes bursts  202  and bursts  204 . More specifically, bursts  202  and  204  comprise bursts of pulses that each include the same number of pulses and in which the pulses are generated with substantially equal pulse rates (frequency) and pulse width (rate). The number, frequency, and pulse width for the pulses is merely exemplary and may be selected according to a particular therapy program, which may depend on the patient&#39;s condition or the desired therapeutic results. The duty cycle for bursts  202  and  204 , however, is varied to encode information in waveform  200 . In particular, bursts  202  are generated using a duty cycle defined as the ratio of T ON  to T OFF1 , and bursts  204  are generated using a duty cycle defined as the ratio of T ON  to T OFF2 . 
     In general, stimulation generator  88  encodes information in pulse waveform  200  by selectively generating the bursts of pulses using the different duty cycle values. For example, stimulation generator  88  may encode binary information, i.e., a sequence of “0” and “1” values that correspond to predefined therapy information, by selectively generating bursts of pulses using the different duty cycle values. In this example, the first duty cycle value may correspond to a “0” value and the second duty cycle value may correspond to a “1” value. In this way, each burst of pulses may be viewed as a digital bit and groups of bursts may be arranged to form digital words that correspond to predefined messages. Encoding information on a burst-by-burst basis may be referred to as “burst encoding.” 
     In another example, stimulation generator may encode binary information by selectively generating groups of bursts using the different duty cycle values or combination of the duty cycle values. In this example, each burst of the group may be generated using the same duty cycle value, or bursts in the group may be generated using more than duty cycle value. If each group is generated using the same duty cycle value, the information rate decreases, but may allow for increased system performance, i.e., increased reliability because there may be a higher probability that ICD  16  will properly sense the stimulation artifact and decode the information encoded therein. Communication module  150  ( FIG. 8 ) of ICD  16  may extract the pattern in the duty cycles from a sensed electrical signal and retrieve the information contained in an artifact signal. 
       FIG. 11B  illustrates an example stimulation waveform  210  in which stimulation generator  88  may vary the frequency of pulses to encode information in a stimulation signal. In  FIG. 11B , stimulation waveform  210  includes bursts of pulses  212 A and  212 B (collectively “bursts  212 ”) and bursts of pulses  214 A and  214 B (collectively “bursts of pulses  214 ”). Stimulation generator  88  generates the pulses in bursts  212  with a frequency f 1  and the pulses in bursts  214  with frequency f 2 . Generally, frequencies f 1  and f 2  may be predetermined values and selected to be different than one another. The value for frequency f 1  is selected to be smaller than the value for frequency f 2  in  FIG. 11B . The predetermined frequencies may be selected within a range of approximately 10 Hz to approximately 100 Hz. 
     Stimulation generator  88  may encode information in pulse waveform  210  by selectively generating the bursts of pulses using the different frequencies. For example, stimulation generator  88  may encode binary information, i.e., a sequence of “0” and “1” values that correspond to predefined therapy information, by selectively generating bursts of pulses having different pulse frequencies. In this example, a burst having a first pulse frequency value may correspond to a “0” value and a burst having a second pulse frequency value may correspond to a “1” value. Communication module  150  ( FIG. 8 ) of ICD  16  may extract the different pulse frequency values from a sensed electrical signal and retrieve the information contained in an artifact signal. 
     It may be important to select the pulse frequency for the bursts of pulses to minimize the possibility of different bursts of pulses from being confused with each other. For example, in some cases, it may be undesirable to select the frequency, f 1 , for bursts  212  to be about 50 Hz and select the frequency, f 2  for bursts  214  to be about 100 Hz because undersensing every other pulse for bursts  214  may result in bursts  214  and burst  212  appearing the same. For this reason, it may be beneficial to select, for example, f 1  and f 2  as 50 Hz and 55 Hz, respectively. 
       FIG. 11C  illustrates an example stimulation waveform  230  in which stimulation generator  88  may vary the pulse width to encode information in a stimulation signal. In  FIG. 11C , stimulation waveform  230  includes bursts of pulses  232 A- 232 C (collectively “bursts of pulses  232 ”) and bursts of pulses  234 A and  234 B (collectively “bursts of pulses  234 ”). Stimulation generator  88  generates the pulses in bursts  232 A- 232 C with a width W 1 , and the pulses in bursts  234  with a width W 2 . The pulse width or pulse duration may be selected within a range of approximately 30 μs to approximately 480 μs, although other pulse widths are contemplated and may depend upon the pulse widths that provide therapeutic stimulation therapy to patient  12 . In the example shown in  FIG. 11C , width W 1  is selected to be smaller than width W 2 . 
     Just as with the other stimulation parameter value modulations, stimulation generator  88  may encode information in pulse waveform  230  by selectively generating the bursts of pulses having the different pulse widths. For example, stimulation generator  88  may encode binary information, i.e., a sequence of “0” and “1” values that correspond to predefined therapy information, by selectively generating bursts of pulses having different pulse widths. In this example, a burst having a first pulse width value may correspond to a “0” value and a burst having a second pulse width value may correspond to a “1” value. Communication module  150  ( FIG. 8 ) of ICD  16  may extract the different pulse frequency values from a sensed electrical signal and retrieve the information contained in an artifact signal. 
       FIG. 11D  illustrates an example pulse waveform  240  in which stimulation generator  88  may vary a signal parameter on a pulses-to-pulse basis to encode information in a stimulation signal. Moreover, stimulation generator  88  may encode information in pulse waveform  240  by varying the value of more than one type of signal parameter on a pulse-to-pulse basis. In  FIG. 11D , stimulation waveform  240  includes bursts of pulses  242 ,  244 , and  246 . Burst  242  includes pulses  242 A- 242 E. Burst  244  includes pulses  244 A- 244 E. Burst  246  includes pulses  246 A- 246 E. For each of bursts  242 ,  244 , and  246 , the value of at least one stimulation parameter is varied to encode information. 
     With respect to burst  242 , pulses  242 A- 242 E have substantially the same pulse width W 1 , but are generated with varying frequency to encode information. In particular, pulses  242 B, and  242 D are generated with a frequency f 1  relative to respective pulses  242 A and  242 C, and pulses  242 C and  242 E are generated with a frequency f 2  relative to respective pulses  242 B and  242 D. With respect to burst  244 , pulses  244 B- 244 E have substantially the same frequency, f 1 , but are generated to have different pulse widths. The pattern in the pulse widths may be modified in order to encode information in the stimulation waveform  240 . In particular, pulses  243 A and  243 D have a pulse width W 1 , and pulses  243 B,  243 C, and  243 E have a pulse width W 2 . 
     With respect to burst  246 , the value of more than one type of signal parameter is varied to encode information in pulses  246 A- 246 E. Pulse  246 A and pulse  246 D have a pulse width W 1  with pulse  246 D having a frequency f 1 . Pulses  246 B and  246 C have a frequency f 1  relative to respective pulses  246 A and  246 B, and a pulse width W 2 , which is different than pulse width W 1 . Pulse  246 E has a frequency f 2  relative to pulse  246 D, and pulse width W 2 . 
       FIGS. 12A and 12B  and  FIGS. 13A and 13B  illustrate example stimulation waveforms with a predetermined signature that may be generated by stimulation generator  88  ( FIG. 6 ) of INS  26 . Generally, the predetermined signature may be uniquely characterized by one or more of frequency content, duty cycle, and/or signal envelope. Because the signature is known by both INS  26  and ICD  16 , ICD  16  may be configured to include signal processing components specifically configured to remove the stimulation signals generated by INS  26  from a sensed electrical signal. The processing components may include, for example, such as envelope detectors, correlators, and filters to substantially remove the corresponding signal artifact (crosstalk) from the sensed signal. 
       FIG. 12A  illustrates an example stimulation waveform  300  that stimulation generator  88  may generate and deliver in order to reduce interference with the sensing of cardiac signals by ICD  16 . Stimulation generator  88  may generate example stimulation waveform  300  including a plurality of pulses that follow a predetermined signal envelope  302 . Signal envelope  302  traces the outline of example stimulation waveform  300  and is characterized by three substantially equal amplitudes and three substantially equal duration peaks. Stimulation generator  88  may generate stimulation waveform  300  by outputting bursts of pulses and alternating the amplitude of the pulses in successive bursts of pulses between two different values. With respect to  FIG. 12A , each pulses in burst of pulses  304 A,  304 C, and  304 E have a first amplitude value, and each pulse in bursts of pulses  304 B,  304 D, and  304 F have a second amplitude value that is greater than the first amplitude value. As a result, signal envelope  302  may appear similar to a square wave. 
       FIG. 12B  illustrates another example stimulation waveform  310  that stimulation generator  88  may generate and deliver to patient  12  in order to reduce interference with the sensing of cardiac signals by ICD  16 . As shown in  FIG. 12B , stimulation waveform  310  includes bursts of pulses  314 A- 314 F (collectively “bursts of pulses  314 ”). Each of bursts of pulses  314  includes pulses that increase in amplitude and then decrease in amplitude over time to define a rising edge, falling edge, and peak. With respect to  FIG. 12B , bursts  314 A and  314 B,  314 C and  314 D, and  314 E and  314 F form closely spaced pairs of amplitude peaks. Consequently, signal envelope  312  is characterized by closely spaced pairs of amplitude peaks where each pair of amplitude peaks are spaced from each other. 
       FIGS. 13A and 13B  illustrate example stimulation waveforms that may be generated by INS  26  with a predetermined signature by varying values of one or more signal parameters. In particular,  FIG. 13A  illustrates an example stimulation waveform  320  with a predetermined signature for facilitating the removal of resulting crosstalk at ICD  16 . Stimulation waveform  320  includes bursts of pulses  322 A- 322 F (collectively “bursts of pulses  322 ”) that each includes seven pulses of substantially equal amplitude and duration. The signature of waveform  320  is characterized by generating bursts  322  with progressively decreasing duty cycle values that reach a minimum value, and then repeat the progression from the initial maximum duty cycle value to the minimum duty cycle value. 
     In  FIG. 13A , stimulation generator  88  ( FIG. 6 ) of INS  26  generates waveform  320  with the predetermined signature by generating bursts  322  using three different duty cycle values, i.e., first, second, and third duty cycle values. The first, second, and third duty cycle values are defined as the ratio of T ON  to T OFF1 , T OFF2 , and T OFF3 , respectively. The first, second, and third duty cycle values decrease in value over time. More specifically, INS  26  generates stimulation waveform  320  by generating burst  322 A using the first duty cycle value, burst  322 B using the second duty cycle value, and burst  322 C using the third duty cycle value, where the first duty cycle value is larger than the second and third duty cycle values, and the second duty cycle value is larger than the third duty cycle value. INS  26  then repeats the pattern by generating bursts  322 D- 322 F using the first, second, and third duty cycle values. In this way, INS  26  may generates stimulation waveform  320  with a signature characterized by a progressively descending duty cycle that reaches a minimum value and then repeats the descent from the initial maximum value to the minimum value. 
     The minimum duty cycle value may indicate, for example, the minimum duty cycle value that provides therapeutic efficacy to patient  12 . Thus, although the duty cycles vary in stimulation waveform  320 , the delivery of waveform  320  to tissue of patient  12  may provide efficacious therapy to patient  12 , e.g., to provide cardiac benefits. 
       FIG. 13B  illustrates another example stimulation waveform  330  that may be generated by INS  26  with a predetermined signature. In  FIG. 13B , stimulation waveform  330  includes bursts of pulses  332 A- 323 E (collectively “bursts of pulses  332 ”) that each include five pulses. The signature of waveform  330  is characterized by bursts  332  that each include pulses having progressively increasing pulse width values followed by bursts including pulses having progressively decreasing pulse width values. The pulse width values may be varied between a maximum value and a minimum value, where the minimum and maximum pulse width values indicate the range of pulse width values that provide efficacious therapy to patient  12 . 
     As shown in  FIG. 13B , INS  26  generates each of bursts  332  using one of three different pulse width values, i.e., W 1 , W 2 , and W 3 , in accordance with the predetermined signature or pattern. In particular, stimulation generator  88  may generate waveform  320  by generating burst  332 A including pulses having a first pulse width value W 1 , followed by burst  332 B including pulses having a second pulse width value W 2 , and followed by burst  332 C including pulses having a third pulse width value W 3 . Thereafter, stimulation generator  88  may generate bursts  332 D and  332 E using pulses having pulse width values W 2  and W 1 , respectively. 
     In general, INS  26  may deliver therapy to patient  12  by continuously repeating waveforms  300 ,  310 ,  320 , and  330  in  FIGS. 12A ,  12 B,  13 A, and  13 B, respectively. By repeating the predetermined signature in this way, ICD  16  may not be required to be synchronized with INS  26 . Rather, ICD  16  may continuously analyze a sensed signal and remove the signal artifact when the predetermined signature is detected. 
     In general, the waveforms shown in  FIGS. 12A ,  12 B,  13 A, and  13 B are merely examples and should not be considered limiting of the disclosure as described herein. Rather, the purpose of  FIGS. 12A ,  12 B,  13 A, and  13 B are to provide examples for using a predetermined signature to facilitate removal of a signal artifact by ICD  16 . Other signatures that may be characterized by varying the values of one or more signal parameters are contemplated and, thus, within the scope of this description. It is recognized that the complexity of the signature may be restricted by the processing capabilities of ICD  16 , and that a system designer may be responsible for balancing the tradeoff between performance and complexity. However, as processors utilized by ICD  16  improve, increasingly complex signal processing techniques may be used. 
       FIGS. 14A and 14B  illustrate example EGM waveforms that represent an electrical signal sensed by sensing module  116  ( FIG. 7 ) of ICD  16  when INS  26  is configured to generate stimulation signals with a spread spectrum energy distribution. In particular, EGM waveform  340  in  FIG. 14A  may be generated by ICD  16  when INS  26  is not delivering therapy. Accordingly, EGM waveform  340  is substantially void of an artifact attributable to the stimulation delivery by INS  26 . Waveform  340  is a relatively smooth signal. 
       FIG. 14B  illustrates EGM waveform  342  indicative of an electrical signal sensed by sensing module  116  of by ICD  16  while INS  26  is delivering therapy to patient  12 . Consequently, a stimulation artifact is present in EGM waveform  342 . In the example shown in  FIG. 14B , stimulation generator  88  ( FIG. 6 ) of INS  26  is generating and delivering a stimulation signal having one or more randomly or pseudo-randomly varied signal parameters while sensing module  116  senses an electrical signal. The signal parameters may include, for example, a current amplitude, a voltage amplitude, a pulse width, duty cycle and/or a pulse rate. 
     In  FIG. 14B , the stimulation artifact appears as wideband noise in ECG waveform  342  because the random or pseudo-random variation of the values of one or more signal parameters of the stimulation signal by INS  26  produces a spread spectrum energy distribution. Because the energy of the stimulation signal is spread substantially across the frequency spectrum, rather than concentrated in a relatively narrow frequency band, crosstalk between INS  26  and ICD  16  may be mitigated. In other words, the energy of the stimulation signal is spread out in such a way that the crosstalk does not adversely interfere with the electrical signal sensed by ICD  16  or the ability of ICD  16  to monitor cardiac events using the EGM waveform generated via the sensed electrical signal. 
     When INS  26  generates a stimulation signal with a spread spectrum energy distribution, ICD  16  may utilize techniques well known in the art for analyzing the ECG waveform and may not require additional processing to suppress the wideband noise. Alternatively, ICD  16  may use wideband filters or filtering techniques to substantially remove or mitigate the wideband noise. In some examples, a filter may be applied to a time domain signal. In other examples, ICD  16  may convert the received analog signal to a digital signal and use digital signal processing techniques, such as performing a frequency analysis and apply digital filters to the digital signal. 
       FIG. 15  is a flow diagram of an example technique INS  26  and ICD  16  may implement in order to communicate. In the example method of  FIG. 15 , the communication is one-way from INS  26  to ICD  16 . Stimulation generator  88  ( FIG. 6 ) of INS  26  generates an electrical stimulation signal that provides therapeutic benefits to patient  12 , and varies the value of one or more signal parameters of the stimulation signal in order to encode information in the stimulation signal ( 400 ). Processor  80  ( FIG. 6 ) may load an initial set of signal parameter values (or stimulation parameter values) for generating the stimulation signal according to one or more selected therapy programs and encode information by varying one or more of the signal parameters defined by the one or more selected therapy programs. That is, the therapy programs may provide initial values for generating the stimulation signal, and processor  80  may control stimulation generator  88  to vary one or more of the signal parameters from the initial value to encode the information. The variation may be restricted to a predefined range from the initial value. The signal parameters may include, for example, a slew rate, pulse rate (frequency), pulse width (rate), voltage/current amplitude, phase, and duty cycle. As previously described, the encoded information may include therapy information, operational information, diagnostic information, and message information. For example, the encoded therapy information may include information regarding the type and duration of therapy by specifying one or more of the selected therapy program(s), the therapy parameters, a stop time for the therapy, a start time for the therapy, the duration of the therapy, or the remaining time that therapy will be delivered. 
     INS  26  outputs the stimulation signal ( 402 ) which may, at least partially, be coupled to ICD  16  by electrical conduction through tissue of patient  12 . As previously described, the stimulation signal output by INS  26  may result in a signal artifact, i.e., crosstalk, in the electrical signal sensed by ICD  16 . 
     ICD  16  senses electrical activity of patient  12  that includes the signal artifact of the stimulation signal ( 404 ) and generates an electrical signal based on the electrical activity ( 406 ). This electrical signal may be referred to as a sensed electrical signal because it is generated based on electrical activity sensed by ICD  16 . As previously described with respect to  FIGS. 7 and 8 , ICD  16  may be configured to generate the sensed signal by sensing a voltage difference between two or more of electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76  ( FIG. 4 ). ICD  16  is also configured to process the sensed signal to retrieve the encoded information ( 408 ). Sensing module  116  and/or processor  110  ( FIG. 7 ) of ICD  16  may utilize various signal processing techniques well known in the art of telecommunications to retrieve the encoded information. For example, as described above with respect to  FIGS. 7-9 , ICD  16  may be configured to process the sensed signal with peak detectors, correlators, comparators, frequency analysis components, decoders, and other signal processing techniques. 
     Finally, ICD  16  may modify its operation based on the retrieved information ( 410 ). For example, if the retrieved information specifies the duration of the therapy, ICD  16  may suspend the delivery of cardiac rhythm therapy in order to prevent delivering unnecessary stimulation therapy to heart  14 . In other examples, ICD  16  may blank sensing channels while INS  26  delivers therapy. This may effectively also prevent ICD  16  from delivering therapy to patient  12 . In another example, ICD  16  may apply additional signal processing to remove the signal artifact from the sensed signal. In this way, ICD  16  may pre-process the sensed signal to retrieve information and substantially remove the signal artifact from the sensed signal before processing the sensed signal (that now has the signal artifact substantially removed) to monitor cardiac activity and detect an arrhythmia. 
       FIG. 16  is a flow diagram of an example technique that may be implemented in order to substantially remove at least some of the stimulation artifact present in an electrical signal sensed by ICD  16 . Stimulation generator  88  of INS  26  may generate a stimulation signal having a predetermined signature ( 420 ). The predetermined signature may be uniquely characterized by one or more of the frequency (pulse rate), duty cycle, or signal envelope, as described with respect to  FIGS. 12A and 12B  and  13 A and  13 B. Moreover, the predetermined signature may be designed to facilitate removal of the resulting signal artifact at ICD  16 , such as the use of different notch filters or bandpass filters specifically designed to remove the signal artifact having the predetermined signature. Example stimulation waveforms having predetermined signatures are illustrated and described in greater detail in  FIGS. 12A ,  12 B,  13 A, and  13 B. 
     In some examples, INS  26  may generate the stimulation signal with the predetermined signature by initially loading one or more therapy programs from memory  82  ( FIG. 6 ). Processor  80  may control stimulation generator  88  to vary the initial values for one or more the signal parameters, e.g., amplitude, frequency, pulse width, duty cycle, to generate the stimulation signal with the predetermined signature. The values of one or more stimulation parameters may be varied based on a set of rules stored in memory  82 , which may, for example, indicate minimum and maximum values for the stimulation parameter values. Stimulation generator  88  may output (i.e., deliver) the stimulation signal ( 422 ) in order to provide therapy to patient  12 . 
     Similar to the method in  FIG. 15 , sensing module  116  ( FIG. 7 ) of ICD  16  may sense electrical activity of patient  12  that includes the signal artifact of the stimulation signal ( 424 ). ICD  16  may generates an electrical (sensed) signal based on the electrical activity ( 426 ), e.g., by sensing a voltage difference between two or more of electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76 . ICD  16  may, for example, generate the sensed signal as an ECG signal or an EGM signal. In such an example, the ECG or EGM signal includes the signal artifact. Sensing module  116  and/or processor  110  of ICD  16  may process the sensed electrical signal to substantially remove the signal artifact ( 428 ). 
     As an example, ICD  16  may be configured to process the sensed signal using a matched filter, comparators, and other signal process components to identify the signal artifact in the sensed signal, and to filter the signal to substantially remove the signal artifact, as described above with respect to  FIGS. 7-9 . Finally, ICD  16  may analyze the processed signal, i.e., the sensed signal with the signal artifact substantially removed, to monitor the cardiac activity of patient  12  ( 430 ). That is, in some examples ICD  16  may analyze an ECG or EGM signal that has been processed to remove the signal artifact to monitor cardiac activity of patient  12 . In this way, ICD  16  may reliably analyze an ECG or EGM signal while INS  26  delivers neurostimulation to patient  12 . Processor  110  may detect an arrhythmia based on the processed signal (ECG/EGM signal with signal artifact substantially removed), e.g., based on R-R or P-P intervals of the processed signal or the morphology of the ECG/EGM signal. Consequently, the processed (ECG/EGM) signal may be used to control the delivery of cardiac rhythm management therapy to patient  12  while INS  26  delivers neurostimulation to patient  12 . 
       FIG. 17  is a flow diagram of another example technique that may be implemented in order to substantially remove at least some of the stimulation artifact present in an electrical signal sensed by ICD  16 . In the method illustrated in  FIG. 17 , INS  26  generates a stimulation signal with a narrowband energy spectrum ( 440 ). In order to generate the stimulation signal with a narrowband energy spectrum, processor  80  ( FIG. 6 ) of INS  26  may load one or more therapy programs from memory  82  ( FIG. 6 ). The therapy programs may define a set of stimulation parameter values for generating the stimulation signal. INS  26  may selectively vary the values of one or more signal parameters from the respective initial value to produce the stimulation signal with the narrowband energy spectrum. For example, INS  26  may change the initial value for one or more of the frequency, pulse width, phase, and duty cycle to produce the stimulation signal with a narrowband energy spectrum. 
     Generating the stimulation signal with a narrowband energy spectrum may help focus the energy of the stimulation signal at a particular frequency or a particular frequency band. The frequency may be predetermined so that it is known by INS  26  and ICD  16 , and selected as a frequency that may not substantially interfere with a cardiac signal. INS  26  outputs the stimulation signal ( 442 ) to provide therapy to patient  12 . 
     Sensing module  116  ( FIG. 7 ) of ICD  16  may sense electrical activity of patient  12  that includes the signal artifact from the stimulation signal output by INS  26  ( 444 ). ICD  16  then generates an electrical (sensed) signal based on the electrical activity ( 446 ), e.g., by sensing a voltage difference between two or more of electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76  ( FIG. 4 ). Processor  110  may process the sensed electrical signal to substantially remove the signal artifact ( 448 ). Because the stimulation signal generated and delivered by INS  26  has a narrowband energy spectrum, the energy of the signal artifact is also focused in the same band of the frequency spectrum. Accordingly, ICD  16  may apply a notch filter centered at the predetermined frequency to substantially remove the signal artifact from the sensed signal. Processor  110  may analyze the processed signal, i.e., the sensed signal with the signal artifact substantially removed, to monitor the cardiac activity of patient  12  ( 450 ), e.g., to control the delivery of cardiac rhythm management therapy to patient  12 . 
       FIG. 18  is a flow diagram of an example technique that may be implemented in order to reduce the impact of crosstalk between INS  26  and ICD  16  on the sensing of cardiac signals by ICD  16 . Stimulation generator  88  ( FIG. 7 ) of INS  26  may generate a stimulation signal having a spread spectrum energy distribution ( 460 ). In order to generate the stimulation signal with a spread spectrum energy distribution, processor  80  ( FIG. 6 ) of INS  26  may load one or more therapy programs from memory  82  ( FIG. 6 ). INS  26  may then randomly or pseudo-randomly vary the values of one or more signal parameters defined by the therapy programs. The signal parameters may include, for example, one or more of frequency, pulse width, phase, and duty cycle. By randomly or pseudo-randomly varying one or more of the signal parameters, the energy of the stimulation signal may be spread over a wide frequency spectrum. Because the energy of the stimulation signal is spread substantially over a wide frequency spectrum, the resulting signal artifact may not substantially interfere with the cardiac signal sensed by ICD  16  and, more particular, may not adversely interfere with the cardiac signal at frequencies that contain critical cardiac information for detecting an arrhythmia. 
     INS  26  outputs the stimulation signal ( 462 ) to provide therapy to patient  12 . ICD  16  may senses electrical activity of patient  12 , and inadvertently sense an artifact of the stimulation signal output by INS  26  ( 464 ). ICD  16  may generate an electrical signal based on the electrical activity ( 466 ), e.g., by sensing a voltage difference between two or more of electrodes  50 - 55 ,  68 ,  72 ,  74 , and  76 . The signal artifact may appear as wideband noise in the sensed electrical signal because it has a spread spectrum energy distribution. 
     In some examples, processor  110  ( FIG. 7 ) of ICD  16  may process the sensed electrical signal to substantially remove the signal artifact ( 468 ). For example, processor  110  may filter the sensed electrical signal, e.g., by applying a wideband filter or otherwise process the signal to remove wideband noise. In other examples, however, the signal artifact may not substantially interfere with the cardiac signal because it may appear as a relatively low level wideband noise. Thus, ICD  16  may not apply additional processing to the sensed signal. In either case, processor  110  of ICD  16  may analyze the sensed signal, which may or may not be processed to remove wideband noise, to monitor the cardiac activity of patient  12  ( 470 ). 
     The techniques described in this disclosure are described with reference to therapy systems  10 ,  11 ,  500  ( FIGS. 1-3 ) including physically separate devices  16 ,  26 . In some examples, the techniques described herein may also be applicable to a single medical device including an electrical stimulation module that generates and delivers electrical stimulation to one or more tissue sites, e.g., proximate a nerve and/or an extravascular tissue site (which may or may not be proximate a nerve), and a cardiac therapy module that senses electrical cardiac activity of patient  12  and delivers cardiac rhythm management therapy to heart  14  of patient  12 . 
       FIG. 19  is a functional block diagram illustrating an example IMD  472  that includes an electrical stimulation module  474  and a cardiac therapy module  476  in a common housing  478 . Electrical stimulation therapy module  474  includes stimulation generator  88 , which is described above with respect to  FIG. 6 . Similarly, cardiac therapy module  476  includes stimulation generator  114  and sensing module  116 , which are described above with respect to  FIG. 7 . IMD  472  also includes processor  110 , memory  112 , telemetry module  118 , and power source  120 , which are described above with respect to  FIG. 7 . 
     Electrical stimulation therapy module  474  may deliver electrical stimulation to a nonmyocardial tissue site or a nonvascular cardiac tissue site. 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, electrical stimulation therapy module  474  may deliver electrical stimulation to an extravascular tissue site that may or may not be proximate a nerve. Cardiac therapy module  476  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  110  may control electrical stimulation therapy module  474  and cardiac therapy module  476  according to any of the techniques described above to minimize the possibility that cardiac therapy module  476  delivers electrical stimulation to heart  14  in response to detecting electrical signals generated and delivered by electrical stimulation therapy module  474  that resemble an arrhythmic cardiac signal. For example, processor  110  may implement any of the techniques described with respect to  FIGS. 16-18  in order to control stimulation generator  88  of electrical stimulation therapy module  474  to generate an electrical stimulation signal that is either easily filtered by sensing module  116  or processor  110 , or that has a spread spectrum energy distribution that minimizes the interference with sensing of true cardiac signals by sensing module  116 . 
     For example, with respect to the technique shown in  FIG. 16 , processor  110  may control electrical stimulation therapy module  474  to generate and deliver a stimulation signal having a predetermined signature to patient  12  ( 420 ,  422 ). As discussed above, the predetermined signature may be designed to facilitate removal of the resulting signal artifact by sensing module  116  or processor  110 . 
     Sensing module  116  of IMD  472  may sense electrical activity of patient  12  that includes the signal artifact of the stimulation signal and generate an electrical signal based on the electrical activity ( 424 ,  426 ). Sensing module  116  and/or processor  110  of IMD  472  may process the sensed electrical signal to substantially remove the signal artifact ( 428 ). Processor  110  may analyze the processed signal, i.e., the sensed signal with the signal artifact substantially removed, to monitor the cardiac activity of patient  12  ( 430 ). 
     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. 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 of the disclosure have been described. These and other examples are within the scope of the following example statements.