Patent Publication Number: US-2010114195-A1

Title: Implantable medical device including extravascular cardiac stimulation and neurostimulation capabilities

Description:
This application claims the benefit of U.S. Provisional Application No. 61/110,124, entitled, “IMPLANTABLE MEDICAL DEVICE INCLUDING EXTRAVASCULAR CARDIAC STIMULATION AND NEUROSTIMULATION CAPABILITIES,” and filed on Oct. 31, 2008, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to medical devices, and, more particularly, medical devices that deliver electrical stimulation therapy to a patient. 
     BACKGROUND 
     A wide variety of implantable medical devices for delivering a therapy or monitoring 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 may deliver electrical stimulation therapy and/or monitor physiological signals via one or more electrodes or sensor elements, 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. 
     For example, implantable cardiac therapy devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic 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, such an implantable medical device may sense intrinsic depolarizations of the heart, and control the delivery of such signals to the heart based on the sensing. When an abnormal cardiac rhythm 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 cardiac rhythm. For example, in some cases, an implantable cardiac therapy device may deliver pacing, cardioversion or defibrillation therapy 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. 
     SUMMARY 
     In general, the disclosure is directed an implantable medical device that includes a cardiac therapy module and a neurostimulation therapy module in a common housing. The cardiac therapy module may provide cardiac stimulation, e.g., pacing, cardioversion, and/or defibrillation to a patient via two or more extravascular electrodes (e.g., subcutaneous electrodes). The neurostimulation module may provide electrical stimulation therapy to a tissue site within a patient, such as a nonmyocardial tissue site (e.g., tissue proximate a nerve) or a nonvascular cardiac tissue site (e.g., a cardiac fat pad). In some examples, the neurostimulation module may provide stimulation to modulate an autonomic nervous system of the patient, which may provide cardiac benefits that complement the cardiac stimulation provided by the cardiac therapy module. 
     In one aspect, the disclosure is directed to a system comprising a housing, at least two extravascular electrodes implanted within a patient, a cardiac therapy module that delivers at least one of pacing, cardioversion, or defibrillation therapy to the patient via the at least two extravascular electrodes, a neurostimulation electrode, and a neurostimulation therapy module that delivers a neurostimulation signal to the patient via the neurostimulation electrode. The cardiac therapy module and the neurostimulation therapy module are enclosed in the housing. 
     In another aspect, the disclosure is directed to a method comprising generating at least one of pacing, cardioversion, or defibrillation therapy with a cardiac therapy module in a housing of a medical device, delivering the at least one of pacing, cardioversion, or defibrillation therapy to a patient via at least two extravascular electrodes implanted within a patient, generating a neurostimulation signal with a neurostimulation therapy module in the housing of the medical device, and delivering the neurostimulation signal to the patient via a neurostimulation electrode. 
     In another aspect, the disclosure is directed to a system comprising means for generating and delivering at least one of pacing, cardioversion, or defibrillation therapy via at least two extravascular electrodes implanted within a patient, means for generating and delivering a neurostimulation signal to the patient via a neurostimulation electrode, and a housing enclosing the means for generating and delivering the at least one of pacing, cardioversion, or defibrillation therapy and means for generating and delivering the neurostimulation signal. 
     In another aspect, the disclosure is directed to a computer-readable medium comprising instructions. The instructions cause a programmable processor to perform any part of the techniques described herein. 
     The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a conceptual diagram illustrating an example therapy system that delivers cardiac and neurostimulation therapy. 
         FIG. 1B  is a conceptual diagram illustrating another example therapy system that delivers cardiac and neurostimulation therapy. 
         FIG. 2A  is a conceptual diagram illustrating the therapy system of  FIG. 1B  in greater detail. 
         FIG. 2B  is a conceptual diagram illustrating another example therapy system. 
         FIG. 2C  is a conceptual diagram illustrating another example therapy system. 
         FIG. 3  is a functional block diagram of an example implantable medical device that includes cardiac and neurostimulation modules. 
         FIG. 4  is block diagram of an example medical device programmer. 
         FIG. 5  is a flow diagram illustrating an example technique for delivering both cardiac and neurostimulation therapy to a patient. 
     
    
    
     DETAILED DESCRIPTION 
     In general, the disclosure is directed toward therapy systems that include an implantable medical device that delivers pacing, cardioversion, and/or defibrillation stimulation to a heart of a patient via extravascular electrodes and delivers electrical stimulation to a tissue site, such as a nonmyocardial tissue site or a nonvascular cardiac tissue site. The nonmyocardial tissue site may be intravascular and/or extravascular. In some examples, the electrical stimulation may be provided to stimulate the autonomic nervous system of the patient. In examples described herein, an extravascular implantable cardioversion defibrillator (ICD) system includes a cardioversion defibrillation signal generator and a neurostimulation signal generator in a common housing. An extravascular electrode may include, for example, a subcutaneous electrode, a submuscular electrode, an epicardial electrode or an intramural electrode. In some examples, however, an extravascular electrode may not include an electrode that contacts the patient&#39;s heart. Thus, in some examples described herein, an extravascular electrode may not include epicardial or intramural electrodes located within the heart. 
       FIG. 1A  is a conceptual diagram illustrating an example therapy system  10 A that provides stimulation therapy to patient  12 . Patient  12  ordinarily, but not necessarily, will be a human. Therapy system  10 A includes an implantable medical device (IMD)  16 , which is coupled to lead  18 , and programmer  24 . As described in further detail with respect to  FIG. 3 , IMD  16  may include a cardiac therapy module and a neurostimulation module in a common housing. The cardiac therapy module may provide functionality similar to an implantable pacemaker, cardioverter, and/or defibrillator, and may generate and deliver electrical signals to heart  14  of patient  12  via extravascular electrodes (not shown in  FIG. 1A ) carried by lead  18 . 
     The neurostimulation module of IMD  16  may generate and deliver electrical stimulation to modulate the autonomic nervous system of patient  12 , e.g., induce a neurohormonal response and/or change in sympathetic and/or parasympathetic autonomic activity. Modulating may include both inhibiting and exciting the autonomic nervous system. For example, the neurostimulation module may generate and deliver stimulation to a nerve or other nonmyocardial tissue site of patient  12 , e.g., proximate a vagus nerve, a spinal cord or heart  14  of patient  12 , via intravascular and/or extravascular electrodes coupled to lead  18 . 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 heart  14 , or neural tissue. The nonmyocardial tissue site may include extravascular tissue sites or intravascular tissue sites. 
     In some examples, delivery of neurostimulation to a tissue site, such as a nonmyocardial tissue site or a nonvascular cardiac tissue site, may help modulate an autonomic nervous system of patient  12 . In some examples, the neurostimulation module of IMD  16  may deliver electrical stimulation therapy to a nerve of patient  12  via a lead implanted within vasculature (e.g., a blood vessel) of patient  12 . In some examples, the neurostimulation module may generate electrical stimulation that is delivered to peripheral nerves that innervate heart  14 , or fat pads on heart  14  that may contain nerve bundles. In the example shown in  FIG. 1A , electrodes of lead  18  are positioned outside the vasculature of patient  12  and positioned to deliver electrical stimulation to target tissue site  20  proximate a vagus nerve of patient  12 . Stimulation may be delivered to extravascular tissue sites, for example, when lead  18  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. 1A , the cardiac therapy module and the neurostimulation module are electrically coupled to a common medical lead  18  either directly or indirectly (e.g., via a lead extension). For example, as illustrated in  FIG. 1A , the cardiac therapy module may deliver stimulation via one or more electrodes on proximal portion  18 A of lead  18  positioned proximate to heart  14 , and the neurostimulation module may deliver stimulation via one or more electrodes on a distal portion  18 B of lead  18  positioned proximate to target stimulation site  20  near the vagus nerve of patient  12 . As described in further detail below, in some examples, the cardiac therapy module and the neurostimulation module may be electrically coupled to different electrodes carried by lead  18 . 
     In other example therapy systems, IMD  16  may be coupled to two or more leads, either directly or indirectly (e.g., via a lead extension and/or the leads may be coupled to IMD  16  in series). For example, the neurostimulation module of IMD  16  may be coupled to two leads in order to provide bilateral or multi-lateral stimulation. However, in some examples, due to the placement of electrodes of lead  18  within patient  12 , the neurostimulation module of IMD  16  may provide bilateral or multi-lateral stimulation via electrodes of a single lead  18 . As described with respect to  FIGS. 2B and 2C , in some examples, the cardiac therapy and neurostimulation modules of IMD  16  may deliver therapy to patient  12  with separate leads. That is, in some examples, the cardiac therapy module may deliver electrical stimulation therapy via extravascular electrodes coupled to one lead, and the neurostimulation therapy may deliver therapy via intravascular and/or extravascular electrodes coupled to a second lead that is separate from the first. 
     IMD  16  may sense electrical cardiac signals attendant to the depolarization and repolarization of heart  14  via extravascular electrodes carried by lead  18 . These signals may be referred to as electrocardiogram (ECG) signals or electrogram (EGM) signals. In some examples, the cardiac therapy module of IMD  16  may provide pacing pulses to heart  14  based on the electrical cardiac signals. Pacing therapies may include anti-tachyarrhythmia pacing and/or pacing therapies designed to prevent ventricular tachycardia, ventricular fibrillation, atrial tachycardia, and/or atrial fibrillation. The configurations of electrodes used by IMD  16  for sensing and pacing may be unipolar or bipolar. The cardiac therapy module of IMD  16  may also provide defibrillation therapy and/or cardioversion therapy via electrodes located on lead  18  and/or one or more electrodes on an outer housing of IMD  16 . IMD  16  may detect an arrhythmia of heart  14 , such as fibrillation of ventricles  26  and  28 , and the cardiac therapy module of IMD  16  may deliver defibrillation therapy to heart  14  in the form of electrical pulses based on the detection of the arrhythmia. In some examples, the cardiac therapy module of IMD  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. IMD  16  may detect fibrillation employing one or more fibrillation detection techniques known in the art. 
     The neurostimulation module of IMD  16  may provide a programmable stimulation signal, e.g., in the form of electrical pulses or a continuous signal, that is delivered to target stimulation site  20  by lead  18 , and more particularly, via one or more stimulation electrodes carried by lead  18  and/or the one or more electrodes on the outer housing of IMD  16 . In some examples, lead  18  may also carry sense electrodes to permit IMD  16  to sense electrical signals from target stimulation site  20 . Like the cardiac therapy module, the neurostimulation module may deliver stimulation based on the electrical cardiac signals. As one example, the neurostimulation module may deliver neurostimulation signals at specific timing related to cues of the EGM (or ECG) signal profile, such as during an arrhythmia. The neurostimulation signal may be modulated, such as rate or duration of pulses, in accordance with presence and/or type of arrhythmia. 
     In some examples, the neurostimulation module may deliver therapy in advance of any apparent need of arrhythmia correction, such as prior to confirmation of the existence of an arrhythmia. When an arrhythmia is detected, the neurostimulation module may deliver neurostimulation therapy to attempt to correct the arrhythmia, and the cardiac module may deliver a defibrillation shock if the neurostimulation therapy is unsuccessful in correcting the arrhythmia. Attempting to correct the arrhythmia with neurostimulation prior to delivering a defibrillation shock may help avoid delivering unnecessary defibrillation shocks to patient  12 . 
     In some examples, the neurostimulation module may deliver neurostimulation to reduce the severity of a sensed cardiac event, e.g., arrhythmia, fibrillation, electromechanical disassociation. The neurostimulation module may deliver therapy prior to, during, and/or after therapy delivery by the cardiac module of IMD  16 , e.g., delivery of a defibrillation shock. The neurostimulation module of IMD  16  may also deliver neurostimulation signals to reduce the risks associated with the cardiac therapy delivered by the cardiac module of IMD  16 . As one example, the neurostimulation module may reduce the risks associated with antitachycardia pacing delivered by the cardiac module by improving the heart&#39;s rhythm and reducing the chance of antitachycardia pacing inadvertently inducing a heart rhythm problem. In various examples, IMD  16  may deliver pacing that includes one or both of anti-tachycardia pacing (ATP) and cardiac resynchronization therapy (CRT). 
     In the example shown in  FIG. 1A , the neurostimulation module of IMD  16  provides electrical stimulation therapy of a parasympathetic nerve of the autonomic nervous system, 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 both facilitate antitachyarrhythmia therapy, e.g., antitachycardia pacing, cardioversion or defibrillation, delivered by the cardiac therapy module of IMD  16 . In other examples, the neurostimulation module of IMD  16  may deliver stimulation to increase the heart rate of heart  14 , e.g., by delivering stimulation signals to other locations of patient  12  and/or changing one or more parameters of the neurostimulation signal. In this way, neurostimulation by the neurostimulation module of IMD  16  may help control a heart rate of patient  12  and may provide therapy that complements the cardiac rhythm therapy delivered by the cardiac therapy module. In other examples, neurostimulation by the neurostimulation module of IMD  16  may not have an effect on the heart rate of patient  12 . For example, neurostimulation by the neurostimulation module of IMD  16  may affect vascular tone, which may improve a condition of patient  12 , such as heart failure status, and/or complement the cardiac rhythm therapy delivered by the cardiac therapy module. For example, the neurostimulation module of IMD  16  may deliver stimulation signals to regulate blood pressure, e.g., increase or decrease blood pressure. 
     In some examples, cardiac therapy by the cardiac module of IMD  16  may complement the neurostimulation by the neurostimulation module of IMD  16 . For example, the neurostimulation module may deliver therapy, and a patient response to the neurostimulation therapy may be monitored to determine if the cardiac therapy should deliver cardiac therapy. The patient response may include, for example, a cardiac response (e.g., that indicates cardiac capture), a neural response (e.g., a neural signal that indicates nerve capture), or any other suitable physiological response. The neurostimulation module may deliver neurostimulation therapy to, for example, modify a heart rate, improve a heart failure status, prevent an arrhythmia, terminate an arrhythmia, or modify a blood pressure of the patient. IMD  16  may sense the patient&#39;s response to the neurostimulation therapy delivered by the neurostimulation module, e.g., via signals indicative of cardiac electrical activity, blood pressure, or tissue perfusion, and the cardiac module of IMD  16  may apply cardiac therapy based on the sensed response to the neurostimulation therapy. 
     In other examples, electrodes of lead  18  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, the neurostimulation module of IMD  16  may deliver electrical stimulation to other sympathetic or parasympathetic nerves, peripheral nerves, cardiac fat pads, renal 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 the cardiac therapy module of IMD  16 . 
     Like the cardiac therapy module, the neurostimulation module of IMD  16  may also adjust and/or deliver therapy in response to a sensed cardiac event, e.g., depolarization or repolarization of heart  14 , or an arrhythmia. For example, if the neurostimulation module delivers therapy to control heart rate, the sensed depolarization and repolarization of heart  14  may provide feedback for therapy adjustment. As another example, the neurostimulation module may deliver therapy to control heart rate in response to a sensed arrhythmia, which may include abnormal heart rhythms such as bradycardia, tachycardia or fibrillation. A sensing module of IMD  16  may detect an arrhythmia using any suitable technique, such as techniques that rely on detected electrical cardiac signals generated by the depolarization and repolarization of heart  14 , heart rate, or hemodynamic parameters, such as blood oxygen levels, blood pressure, stroke volume, and the like. IMD  16  may also sense other parameters, such as tissue perfusion, transthoracic impedance, cardiac impedance, and/or acoustic cardiac sounds, and the cardiac and/or neurostimulation modules of IMD  16  may, additionally or alternatively, adjust and/or deliver therapy in response to these sensed parameters. 
     In some cases, the delivery of neurostimulation by the neurostimulation module of IMD  16  may help eliminate or reduce the demands of pacing or defibrillation provided by the cardiac therapy module. For example, in some examples, neurostimulation may decrease the threshold levels, e.g., voltage or current threshold levels, required to defibrillate heart  14  of patient  12  by, for example, reducing the threshold levels that are needed for the cardiac therapy module to terminate the fibrillation of heart  14 . This may help the cardiac therapy module terminate fibrillation at a threshold level less than the maximum output of the cardiac module, IMD  16  of extravascular therapy system  10 A may need to deliver higher amplitude therapy to defibrillate heart  14  with extravascular electrodes that are positioned outside of heart  14  compared to therapy systems that utilize intravascular electrodes. Thus, reducing the defibrillation threshold levels may be particularly useful in the therapy system  10 A, which utilizes extravascular electrodes to deliver defibrillation shocks. In addition to helping ensure the threshold level is within the stimulation output range of IMD  16 , reducing a defibrillation threshold level may reduce the patient that patient  12  experiences subsequent to receiving a defibrillation shock. 
     In other examples, the neurostimulation module of IMD  16  may deliver therapy to reduce the recovery time after defibrillation, for example, by improving cardiac output and/or stroke volume. For example, the neurostimulation module of IMD  16  may deliver stimulation signals subsequent to the delivery of a defibrillator shock delivered by the cardiac module of IMD  16 . The neurostimulation therapy may aid in returning heart  14  to a normal rhythm. Thus, the neurostimulation therapy may also reduce the need for post-shock pacing of heart  14  by the cardiac module of IMD  16 . Reducing the need for post-shock pacing of heart  14  may be particularly important in examples in which the cardiac module of IMD  16  does not deliver pacing therapy to heart  14 , e.g., cardiac modules that solely deliver cardioversion and/or defibrillation therapy. 
     In other examples, the delivery of neurostimulation by the neurostimulation module of IMD  16  may help prevent or reduce the tendency of heart  14  to beat irregularly, which may reduce the amount of energy consumed by the cardiac therapy module of IMD  16  to sustain a regular heart beat. Thus, in some examples, the neurostimulation module of IMD  16  may deliver therapy when the cardiac therapy module is not delivering therapy. This may help reduce the frequency with which the cardiac therapy module generates and delivers therapy to terminate an arrhythmia of heart. 
     In some examples, depending upon the neurostimulation target, the delivery of electrical stimulation by the neurostimulation module may also mitigate perceptible discomfort generated from the delivery of pacing pulses or cardioversion/defibrillation shocks by the cardiac therapy module. For example, if the neurostimulation module delivers electrical stimulation to a spinal cord of patient  12 , as shown and described with respect to  FIGS. 2B and 2C , the neurostimulation may produce paresthesia, which may help reduce the discomfort felt by patient  12  from the delivery of stimulation by the cardiac therapy module. 
     In general, the cardiac and neurostimulation modules of IMD  16  may deliver therapy at substantially the same time and/or at different times. For example, a processor within IMD  16  may control the cardiac and neurostimulation modules to deliver therapy substantially concurrently in response to detection of fibrillation. Additionally or alternatively, the neurostimulation module may provide neurostimulation prior to and/or subsequent to a defibrillation shock delivered by the cardiac therapy module. Other functions of therapy system  10 A, such as sensing electrical signals, may be performed coincident and/or in alternation with therapy delivery by the cardiac therapy module and/or neurostimulation module of IMD  16 . 
     In the example of  FIG. 1A , IMD  16  has been implanted in the chest cavity, e.g., in the intraclavicular, subclavicular, or mammary area, of patient  12 . Other implant locations are also contemplated, such as in the back or abdomen of patient  12 . IMD  16  may be subcutaneously or submuscularly implanted in the body of a patient  12  at any appropriate location. Upon implantation of IMD  16 , proximal end  18 C of lead  18  may be both electrically and mechanically coupled to connector  36  of IMD  16  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  18  to IMD  16 . 
     In some examples, lead  18  may be subcutaneously implanted in patient  12 . For example, lead may be tunneled between IMD  16 , heart  14 , and target stimulation site  20  near the vagus nerve of patient  12 . By tunneling lead  18  proximate to but outside of heart  14 , the cardiac therapy module of IMD  16  may deliver therapy to heart  14  without requiring lead  18  to be implanted in heart  14 . Proximal portion  18 A of lead  18  may include one or more extravascular electrodes that the cardiac therapy module may utilize to deliver therapy to heart  14 . For example, proximal portion  18 A of lead  18  may include extravascular electrodes that are positioned proximate to right atrium  30 , right ventricle  26 , left atrium  32 , and/or left ventricle  28 . Distal portion  18 B may include one or more electrodes with which the neurostimulation module of IMD  14  may deliver stimulation to target stimulation site  20  near the vagus nerve of patient  12 . Various electrode configurations of lead  18  are described in further detail with respect to  FIGS. 2A-2C . 
     In other examples, as shown in  FIG. 1B , lead  18  may be positioned to allow the neurostimulation module of IMD  12  to deliver electrical stimulation to spinal cord  38  of patient  12 . In some examples, lead  18  may be positioned within patient  12  such that at least some electrodes on distal portion  18 B of lead  18  are positioned in the intrathecal space or epidural space of spinal cord  38  near the spinal segments T1-T6, or, in some examples, adjacent nerves that branch off of spinal cord  38 . In one example technique for implanting lead  18  in patient  12 , lead  18  may be tunneled from IMD  16  to position electrodes proximate heart  14 , and then further tunneled in a ventral direction to the back of patient  12  in order to access spinal cord  38 . In some examples, electrodes of lead  18  may be positioned proximate to spinal cord  38  at approximately the same anteroposterior position as heart  14 . 
     Lead  18  may be introduced into spinal cord  38  in the thoracic region, as shown in  FIG. 1B . In other examples, lead  18  may be introduced into spinal cord  38  in the cervical or lumbar regions. Stimulation of spinal cord  38  or nerves branching therefrom by the neurostimulation module of IMD  16  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 the cardiac therapy module of IMD  16 . In this manner, the cardiac and neurostimulation modules of IMD  16  may operate in conjunction with each other to help prevent arrhythmias of heart  14  of patient  12 , as well as to terminate detected arrhythmias. 
     In some examples, programmer  24  may be a handheld computing device or a computer workstation. Programmer  24  may include a user interface that receives inputs 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 IMD  16 . For example, the user may interact with programmer  24  to retrieve physiological or diagnostic information from IMD  16 . A user may also interact with programmer  24  to program IMD  16 , e.g., select values for operational parameters of IMD  16 . 
     For example, the user may use programmer  24  to retrieve information from IMD  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 IMD  16  regarding other sensed physiological parameters of patient  12 , such as electrical depolarization/repolarization signals from the heart (e.g., EGM signals), intracardiac or intravascular pressure, activity, posture, respiration or thoracic impedance. 
     The user may use programmer  24  to program a therapy progression for IMD  16 . As one example, programmer  24  may select electrodes (i.e., an electrode combination) with which the cardiac therapy module of IMD  16  may deliver defibrillation pulses, select waveforms for the defibrillation pulse or select or configure a fibrillation detection algorithm for IMD  16 . The user may also use programmer  24  to program aspects of other therapies provided by the cardiac therapy module of IMD  16 , such as cardioversion or pacing therapies. 
     The user may also use programmer  24  to program aspects of the neurostimulation module. The therapy parameters for the neurostimulation module of IMD  16  may include an electrode combination for delivering neurostimulation signals, as well as an amplitude, which may be a current or voltage amplitude, and, if the neurostimulation module delivers electrical pulses, a pulse width, and a pulse rate for stimulation signals to be delivered to patient  12 . The electrode combination may include a selected subset of one or more electrodes located on implantable lead  18  coupled to IMD  16  and/or a housing of IMD  16 . 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 amplitude, pulse width, and pulse rate, the physician can attempt to generate an efficacious therapy for patient  12  that is delivered via the selected electrode subset. 
     As another example, the user may use programmer  24  to retrieve information from IMD  16  regarding the performance or integrity of IMD  16  or other components of the relevant therapy system  10 A or  10 B, such as lead  18  or a power source of IMD  16 . With the aid of programmer  24  or another computing device, a user may select values for therapy parameters for controlling therapy delivery by the cardiac and neurostimulation modules of IMD  16 . 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. 
     Programmer  24  may communicate with IMD  16  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 IMD  16  implant site in order to improve the quality or security of communication between IMD  16  and programmer  24 . 
       FIG. 2A  is a conceptual diagram illustrating IMD  16  and lead  18  of therapy system  10 B in greater detail. Lead  18  may be electrically coupled to the cardiac therapy module, the neurostimulation module, a sensing module, or other modules IMD  16  via connector block  36 . For example, the proximal end of lead  18  may include electrical contacts that electrically couple to electrical contacts within connector block  36 , which provides electrical connections to the modules within IMD  16 . In addition, in some examples, lead  18  may be mechanically coupled to connector block  36  with the aid of set screws, connection pins or another suitable mechanical coupling mechanism. In some examples, lead  18  may include an elongated insulative lead body, which may carry a number of conductors that are electrically coupled to a respective one of the electrodes carried by lead  18 . The conductors may be disposed in a common lead body in order to help define a lead  18  that is relatively easy to manipulate and implant within patient  12 . The conductors of lead  18  may be electrically coupled to circuitry within IMD  16  via the electrical connections provided by connector block  36 . In some examples, the conductors may be concentrically coiled and separated from one another by tubular insulative sheaths. Other lead configurations are also contemplated. 
     In the illustrated example, lead  18  includes electrodes  40 ,  42 ,  44 ,  46 ,  48 , and  50 . Each of the electrodes  40 ,  42 ,  44 ,  46 ,  48 , and  50  may be electrically coupled to a respective one of the conductors within the lead body of lead  18 , and thereby coupled to respective ones of the electrical contacts on proximal end  18 C of lead  18 . Electrodes  40 ,  42 ,  44 ,  46 ,  48 , and  50  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 stimulation electrodes. In the example illustrated in  FIG. 2A , lead  18  is positioned within patient  12  such that electrodes  40 ,  42 , and  44  are proximate to heart  14 . In this way, the cardiac therapy module of IMD  16  may deliver electrical stimulation therapy to heart  14  via at least electrodes  40 ,  42 ,  44  of lead  18 . Lead  18  is also implanted within patient  12  such that electrodes  46 ,  48 , and  50  are positioned proximate to spinal cord  38 . The neurostimulation module within IMD  16  may deliver electrical stimulation to spinal cord  38  via at least electrodes  46 ,  48 ,  50 . 
     Electrodes  40 ,  42 , and  44  are implanted proximate to, but outside of heart  14 . Therefore, electrodes  40 ,  42 , and  44  may be referred to as extravascular electrodes. An extravascular electrode may comprise an electrode that is not implanted within heart  14  or within an artery or other vasculature of the patient  12 . For example, electrodes  40 ,  42 , and  44  may comprise subcutaneous, submuscular, epicardial, and/or intramural electrodes. In some examples, electrodes  46 ,  48 , and  50  may also be extravascular electrodes. 
     In some examples, as illustrated in  FIG. 2A , IMD  16  may include one or more housing electrodes, such as housing electrode  52 , which may be formed integrally with an outer surface of hermetically-sealed housing  54  of IMD  16  or otherwise coupled to housing  54 . In some examples, housing electrode  52  is defined by an uninsulated portion of an outward facing portion of housing  54  of IMD  16 . In some examples, housing electrode  52  comprises substantially all of housing  54 . Other divisions between insulated and uninsulated portions of housing  54  may be employed to define two or more housing electrodes. For example, housing  54  may include three housing electrodes  52 . In examples in which IMD  16  includes two or more housing electrodes, the cardiac and/or neurostimulation module may deliver a stimulation signal between two housing electrodes. 
     One or more of housing electrodes, such as housing electrode  52 , may be embedded in a coating of housing  54 . IMD  16  may also include one or more electrodes separate from, but attached to housing  54 . For example, IMD  16  may include a modified housing electrode that is suspended from housing  52  via a connector element, such as an insulated electrical conductor. IMD  16  may also include one or more electrodes on an outer surface of connector block  36 . 
     A sensing module of IMD  16  may sense electrical physiological signals of patient  12  via two or more electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  52 . The electrical signals may be conducted to IMD  16  via lead  18 , or, in the case of housing electrode  52 , via a conductor within housing  54  and used by the cardiac and neurostimulation modules of IMD  16  to modulate therapy. As one example, electrodes  40 ,  42 , and  44  may sense signals attendant to the depolarization and repolarization of heart  14 . 
     The cardiac therapy module of IMD  16  may deliver pacing pulses via any combination of electrodes  40 ,  42 , and  44  and housing electrode  52 , e.g., any unipolar or bipolar electrode configuration, to cause depolarization of cardiac tissue of heart  14 . The cardiac therapy module of IMD  16  may alternatively or additionally deliver defibrillation and/or cardioversion pulses to heart  14  via electrodes  40 ,  42 ,  44 , and  52 . Electrodes  40 ,  42 , and  44  may comprise elongated electrodes that take the form of coil electrodes. Such coil electrodes may be useful in delivering high energy defibrillation pulses to heart  14 . 
     The neurostimulation module of IMD  16  may deliver neurostimulation via any combination of electrodes  46 ,  48 , and  50  and housing electrode  52 . In the example shown in  FIG. 2A , the neurostimulation module of IMD  16  delivers neurostimulation to spinal cord  38  of patient  12 . This may be referred to as spinal cord stimulation (SCS). In some examples, the neurostimulation module may deliver a stimulation signal between one of electrodes  46 ,  48 , or  50  and housing electrode  52 , i.e., in a unipolar configuration. As another example, the neurostimulation module may deliver a stimulation signal between a plurality of electrodes  46 ,  48 , and  50 , e.g., in a bipolar configuration. In some examples, electrodes  46 ,  48 , and  50  may take the form of ring, partial ring or segmented electrodes. Ring electrodes may extend substantially completely around the lead body of lead  18 , whereas partial ring and segmented electrodes may extend partially around the lead body of lead  18 . A plurality of segmented electrodes may be located at substantially the same axial position of lead  18 , e.g., to form a row of segmented electrodes. Ring electrodes may be relatively simple to program and are capable of delivering an electrical field to any tissue adjacent to the respective electrode  46 ,  48 , and  50 . In other examples, at least one of the electrodes  46 ,  48 , and  50  may have a different configuration. For examples, in some examples, at least one of the electrodes  46 ,  48 , and  50  may have a complex electrode array geometry that is capable of producing shaped electrical fields. The complex electrode array geometry may include multiple electrodes (e.g., partial ring or segmented electrodes) around the outer perimeter of each lead  20 , rather than one ring electrode. 
     In some examples, a first subset of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  52  that are used to deliver cardiac rhythm therapy to patient  12  and a second subset of  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  52  that are used to deliver neurostimulation to patient  12  may share a common return electrode. For example, housing electrode  52  may serve as a return electrode for both cardiac rhythm and neurostimulation therapies. As another example, cardiac rhythm and neurostimulation therapies may share one or more other electrodes, such as electrodes  40 ,  42 ,  44 ,  46 ,  48 , and/or  50 . Delivering cardiac rhythm and neurostimulation therapies with one or more of the same electrodes, e.g., one or more of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50  and  52 , may be particularly useful in examples in which IMD  16  delivers neurostimulation subcutaneously, transcutaneously, and/or in the form of peripheral nerve field stimulation (PNFS). In some examples, IMD  16  delivers different waveforms, e.g., sinusoidal, square, or pulse, to one or more of the same electrodes to elicit different responses from patient  12  ( FIG. 1 ) for the neurostimulation and the cardiac rhythm therapy. More specifically, IMD  16  may deliver cardiac pacing therapy using a first waveform and neuromodulation therapy using a second, different waveform. 
     In some examples, electrodes  46 ,  48 , and  50  may be electrically isolated from electrodes  40 ,  42 , and  44 . For example, lead  18  and/or IMD  16  may be configured to isolate any high energy shock that the cardiac therapy module delivers to electrodes  40 ,  42 ,  44 , and  52  from electrodes  46 ,  48 , and  50 . IMD  16  may include circuitry configured to achieve such isolation. 
     In some examples, the neurostimulation module of IMD  16  may utilize electrodes  40 ,  42 , and  44  to deliver neurostimulation. For example, the neurostimulation module of IMD  16  may deliver stimulation via any combination of electrodes  40 ,  42 ,  44 ,  46 ,  48 , and  50  and housing electrode  52 . The neurostimulation module of IMD  16  may utilize electrodes  40 ,  42 , and  44 , for example, to stimulate the cardiac fat pads on heart  14 . 
     The number, configuration, and type of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50  and  52  shown in  FIG. 2A  are merely exemplary. In other examples, lead  18  may include any number, configuration, and type of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 , and  52 . For example, lead  18  may include one or more additional electrodes proximate to heart  14 , e.g., proximate to left atrium  32  As another example, lead  18  may include a fewer or a greater number of electrodes proximate to spinal cord  38 . A greater number of electrodes may permit the neurostimulation module of IMD  16  to stimulate fewer or additional locations of spinal cord  38 . Also, electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  52  may be positioned at different locations on lead  18  or housing  54 , e.g., to permit stimulation at different locations of patient  12 . In the example illustrated in  FIG. 2A , therapy system  10 B includes a single lead  18 . In other examples, therapy system  10 B may also include two or more leads. 
       FIG. 2B  is a conceptual diagram illustrating another example of therapy system  10 C, which is similar to therapy systems  10 A and  10 B of  FIGS. 1A ,  1 B, and  2 A, but includes a neurostimulation extension lead  56  coupled to a cardiac lead  58  via a lead connector  60 . In the example illustrated in  FIG. 2B , cardiac lead  58  includes extravascular electrodes  62  and  64  positioned proximate to heart  14  and, more specifically, proximate to right ventricle  26  and left ventricle  28  of heart  14 , respectively. The cardiac therapy module of IMD  16  may deliver pacing, cardioversion, and/or defibrillation therapy, e.g., in the form electrical stimulation signals, to heart  14  using any combination of electrodes  62 ,  64 , and  52 . 
     A distal end  58 A of cardiac lead  58  may be mechanically and electrically coupled to a proximal end  56 A of neurostimulation lead  56  via lead connector  60 . Lead connector  60  may include electrical contacts similar to those of connector block  36 . For example, electrical contacts at distal end  58 A of cardiac lead  58  may electrically couple to electrical contacts within lead connector  60 . Electrical contacts at a proximal end  56 A of neurostimulation lead  56  may also couple to electrical contacts within lead connector  60  to electrically couple neurostimulation lead  56  to cardiac lead  58  and, ultimately, IMD  16  via conductors carried within cardiac lead  58 . 
     The electrical contacts at distal end  58 A of cardiac lead  58  may be electrically isolated from other conductors within cardiac lead  58  that electrically couple electrodes  62  and  64  to IMD  16 . For example, the conductors within cardiac lead  58  that electrically couple electrodes  62  and  64  to IMD  16  may be sufficiently insulated from the conductors that couple to neurostimulation lead  56  at lead connector  60 . In some examples, IMD  16  may include isolation circuitry to further aid in electrically isolating cardiac electrodes  62  and  64  from neurostimulation electrodes  66 ,  68 ,  70 ,  72 , and  74  of neurostimulation lead  56 , as described above. As one example, the isolation circuitry may create an electrical disconnect between neurostimulation lead  56  and IMD  16  when the cardiac therapy module delivers a high energy defibrillation pulse via a combination of electrodes  62 ,  64 , and/or  52 . 
     Electrodes  66 ,  68 ,  70 ,  72 , and  74  may be positioned proximate to spinal cord  38  to deliver spinal cord stimulation. As described with respect to  FIG. 2A , the neurostimulation module of IMD  16  may deliver a bipolar stimulation signal between two electrodes proximate to spinal cord  38 , e.g., between two of electrodes  66 ,  68 ,  70 ,  72 , and  74 . Alternatively, the neurostimulation module of IMD  16  may deliver unipolar stimulation between housing electrode  52  and an electrode proximate to spinal cord  38 , e.g., electrode  66 ,  68 ,  70 ,  72  or  74 . In some examples, the neurostimulation module of IMD  16  may utilize one or more of electrode  62  and  64  to deliver neurostimulation, e.g., to the cardiac fat pads, in a unipolar or multipolar configuration. The configuration of electrodes  62 ,  64 ,  66 ,  68 ,  70 ,  72 , and  74  illustrated in  FIG. 2B  is for purposes of example. In other examples, cardiac lead  58  and/or neurostimulation lead  56  may include any number, type or configuration of electrodes. 
     Therapy systems  10 C, as well as the other therapy systems described herein, may take advantage of the placement of cardiac lead  58  within patient  12  to provide the additional cardiac benefits that may result from the delivery of neurostimulation. In particular, systems that only include a cardiac therapy module may already include cardiac lead  58  that carries extravascular electrodes  62 ,  64  to provide pacing, cardioversion, and/or defibrillation pulses to heart  14 . Thus, therapy system  10 C leverages the existing location of a medical device and cardiac lead  58  to provide IMD  16  including both a cardiac therapy module and a neurostimulation module, as well as neurostimulation lead  56  as an extension on cardiac lead  58 . In this way, therapy system  10 C may provide further benefits to patient  12  in a minimally invasive manner compared to existing cardiac therapy systems that include extravascular cardiac therapy electrodes. 
     A neurostimulation lead  56  that is mechanically coupled to cardiac lead  58  via lead extension  60  may support a relatively flexible implantation process. For example, a clinician may implant leads  56  and  58  in any order. This flexibility may allow easier and/or more precise electrode placement. Moreover, the modular design of neurostimulation lead  56  also provides the flexibility of adding neurostimulation lead  56  to a therapy system at a later time. For example, IMD  16  and cardiac lead  62  may be implanted in a first implantation procedure and the therapy effectiveness may be evaluated. If needed or desired, neurostimulation lead  56  may be added in a second implantation procedure at a later time. Distal end  58 A of cardiac lead  58  and/or connector  60  may be sealed, e.g., by a cap, biocompatible material, or other mechanism, to avoid fluid ingress when neurostimulation lead  56  is not used, e.g., not connected to cardiac lead  58  via connector  60 . 
       FIG. 2C  is a conceptual diagram illustrating another example of therapy system  10 D, which is similar to therapy systems  10 A- 10 C of  FIGS. 1A ,  1 B,  2 A, and  2 B, but includes two separate, physically disconnected leads  76  and  78  that couple to IMD  16  in parallel. In some examples, leads  76 ,  78  may be attached at one or more points along the lengths of the leads  76 ,  78 , e.g., via a sheath, adhesive, and/or other attachment element. This may be useful for fixing the position of leads  76 ,  78  relative to each other. Lead  76  includes electrode  80  positioned proximate to heart  14 . The cardiac therapy module of IMD  16  may, for example, deliver a defibrillation pulse to heart  14  via electrodes  80  and  52 . Lead  78  includes electrodes  82 ,  84 ,  86 , and  88  positioned proximate to spinal cord  38 . The neurostimulation module of IMD  16  may, for example, deliver SCS via any combination of electrodes  82 ,  84 ,  86 ,  88 , and  52 . 
     In the example illustrated in  FIG. 2C , leads  76  and  78  may be utilized to deliver different therapies. For example, the cardiac therapy module of IMD  16  may deliver therapy directly to heart  14 , e.g., pacing, cardioversion, and/or defibrillation therapy, with electrode  80  of lead  76 , and the neurostimulation module may deliver stimulation signals to spinal cord  38  with any one or more of the electrodes  80 ,  82 ,  84 ,  86 , and  88  of lead  78  to deliver stimulation signals to spinal cord  38 . In other examples, one bifurcated lead may be provided instead of individual leads  76  and  78 . As another example, multiple leads may be utilized by the neurostimulation module of IMD  16 , e.g., to deliver bilateral stimulation. 
       FIG. 3  is a functional block diagram of an example configuration of IMD  16 , which includes processor  90 , memory  92 , therapy module  94 , sensing module  96 , telemetry module  98 , and power source  100 . The components shown in  FIG. 3  may be contained within a common, hermetically sealed housing  54  of IMD  16 . Therapy module  94  includes cardiac therapy module  102  and neurostimulation module  104 . In the example shown in  FIG. 3 , therapy module  94  is coupled to electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50  of lead  18  ( FIG. 2A ) and housing electrode  52 . In other examples, however, therapy module  94  may be coupled to electrodes  62 ,  64  of cardiac lead  58  ( FIG. 2B ) and electrodes  66 ,  68 ,  70 ,  72 ,  74  of neurostimulation lead  56  ( FIG. 2B ), electrode  80  of cardiac lead  76  ( FIG. 2C ) and electrodes  82 ,  84 ,  86 ,  88  of neurostimulation lead  78  ( FIG. 2C ), and/or electrodes of other leads. 
     Memory  92  includes computer-readable instructions that, when executed by processor  90 , cause IMD  16  and processor  90  to perform various functions attributed to IMD  16  and processor  90  herein. Memory  92  may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital media. 
     Processor  90  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. In some examples, processor  90  may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to processor  90  herein may be embodied as software, firmware, hardware or any combination thereof. Processor  90  controls therapy module  94  to deliver stimulation therapy according to a selected one or more of therapy programs, which may be stored in memory  92 . Specifically, processor  90  may control cardiac therapy module  102  and/or neurostimulation module  104  to deliver electrical signals with the amplitudes, frequency, electrode polarities, and, in the case of stimulation pulses, pulse widths specified by the selected one or more therapy programs. 
     Therapy module  94  is electrically coupled to electrodes  40 ,  42 ,  44 ,  46 ,  48 , and  50 , e.g., via conductors of lead  18 , or, in the case of housing electrode  52 , via an electrical conductor disposed within housing  54  of IMD  16 . Cardiac therapy module  102  and neurostimulation module  104  are disposed within a common housing  54  ( FIG. 2A ) of IMD  16 . Cardiac therapy module  102  and neurostimulation module  104  may each include a respective stimulation generator that generates the electrical stimulation signals for delivery to patient  12 . For example, cardiac therapy module  102  and neurostimulation module  104  may be physical separate components of IMD  16  disposed within housing  54 . In some examples, e.g., examples in which cardiac therapy module  102  and neurostimulation module  104  deliver stimulation solely in alternation, cardiac therapy module  102  and neurostimulation module  104  may share some or all stimulation generation circuitry. As one example, IMD  16  and/or lead  18  may include switching circuitry, such as one or more multipliers and/or transducers, to allow a single stimulation generator to deliver cardiac and neurostimulation therapy in alternation. 
     Cardiac therapy module  102  is configured to generate and deliver electrical stimulation therapy to heart  14  ( FIG. 1A ). For example, a stimulation generator of cardiac therapy module  102  may deliver cardioversion or defibrillation shocks and/or pacing pulses to heart  14  via electrodes  40 ,  42 , and  44  coupled to lead  18  and/or housing electrode  52 . In some examples, cardiac therapy module  102  delivers pacing, cardioversion, or defibrillation stimulation in the form of electrical pulses. In other examples, cardiac therapy module  102  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. 
     Neurostimulation module  104  is configured to generate and deliver electrical stimulation therapy to modulate the autonomic nervous system of patient  12 . Example stimulation sites for neurostimulation module  104  include, but are not limited to, tissue proximate a vagus nerve or spinal cord  38  of patient  12 . For example, a stimulation generator within neurostimulation module  104  may generate stimulation signals that are delivered to spinal cord  38  via electrodes  46 ,  48 , and  50  coupled to lead  18  and/or housing electrode  52 . Neurostimulation module  104  may include a single or multi-channel stimulation generator. In particular, the stimulation generator may be capable of delivering, a single stimulation pulse, multiple stimulation pulses, or a continuous signal at a given time via a single electrode combination or multiple stimulation pulses at a given time via multiple electrode combinations. In some examples, however, neurostimulation module  104  may be configured to deliver multiple channels on a time-interleaved basis. In this case, therapy module  94  may include a switching module that serves to time division multiplex the output of the stimulation generator across different electrode combinations at different times to deliver multiple programs or channels of stimulation energy to patient  12 . 
     Processor  90  may control cardiac therapy module  102  and neurostimulation therapy module  104  to coordinate the delivery of electrical stimulation to patient  12 . As previously described, in some examples, neurostimulation module  104  may deliver neurostimulation signals to patient  12  at substantially the same time as pacing, cardioversion, and/or defibrillation pulses delivered by cardiac therapy module  102 . In those examples, processor  90  may control cardiac therapy module  102  and neurostimulation module  104  to generate and deliver electrical stimulation at substantially the same time. While in some examples, the pulses or other signals delivered by cardiac therapy module  102  and neurostimulation therapy module  104  may not overlap in time, the general time frame that cardiac therapy module  102  and neurostimulation therapy module  104  actively generate and deliver electrical stimulation therapy to patient  12  may overlap. 
     In other examples, in addition to or instead of delivering neurostimulation signals at substantially the same time that cardiac therapy module  102  delivers stimulation, neurostimulation module  104  may deliver neurostimulation signals to patient  12  prior to or after cardiac therapy module  102  delivers stimulation. In some examples, processor  90  may control neurostimulation module  104  to deliver neurostimulation signals to patient  12  based on physiological parameter values sensed by sensing module  96 , which may indicate the presence of an arrhythmia. As one example, processor  90  may control neurostimulation module  104  to deliver neurostimulation signals, e.g., a predetermined number of pulses, when sensing module  96  detects an R-wave of an electrocardiogram (ECG) or electrogram (EGM) signal such that the neurostimulation signals are delivered during the refractory period of the ventricles of heart  14 . 
     In other examples, processor  90  may control neurostimulation module  104  to deliver neurostimulation signals to patient  12  according to a predetermined schedule that is independent of physiological parameter values sensed by sensing module  96 . The schedule may be determined by a clinician and stored in memory  92 . As previously indicated, the delivery of electrical stimulation by neurostimulation module  104  to a nonmyocardial tissue site to modulate an autonomic system of the patient&#39;s nervous system may help regulate the patient&#39;s heart rate. Thus, processor  90  may control neurostimulation module  104  to generate and deliver neurostimulation signals to patient  12  as a preventative measure, e.g., to reduce the occurrence of arrhythmias, and, therefore, reduce the frequency with which cardiac therapy module  102  generate and delivers stimulation. 
     If processor  90  detects an arrhythmia, e.g., based on cardiac signals sensed by sensing module  96 , processor  90  may initiate the delivery of stimulation by cardiac therapy module  102 . In addition, in some cases, upon the detection of an arrhythmia, processor  90  may control neurostimulation module  104  to generate neurostimulation signals based on a therapy program that is different than the therapy program used by neurostimulation module  104  to generate stimulation signals prior to the detection of the arrhythmia. The therapy programs may differ by at least one stimulation parameter value, such as a voltage or current amplitude, frequency, pulse rate, or pulse width. As another example, the electrode combinations defined by the therapy programs may differ. For example, neurostimulation module  104  may deliver electrical stimulation to patient  12  according to a first subset of electrodes  46 ,  48 ,  50 ,  52  prior to the detection of an arrhythmia, and according to a second subset of electrodes  46 ,  48 ,  50 ,  52  after the detection of an arrhythmia. Utilizing different subsets of electrodes to deliver the neurostimulation may permit neurostimulation module  104  to target different tissue sites, e.g., different parts of a nerve. 
     In general, if therapy module  94  includes a switch module, processor  90  may control the switch module to select, e.g., via a data/address bus, which of the available electrodes are used to deliver stimulation signals. 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, therapy module  94  may independently deliver stimulation to electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 , and  52  or selectively sense via one or more of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 , and  52  without a switch matrix. 
     Sensing module  96  monitors signals from at least two of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 , and  52  in order to monitor electrical activity of heart  14 , e.g., via electrocardiogram (ECG) signals. Sensing module  96  may also include a switch module to select the available electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 , and  52  that are used to sense the electrical activity of heart  14 . In some examples, processor  90  may select the electrodes that function as sense electrodes via the switch module within sensing module  96 , e.g., by providing signals via a data/address bus. In some examples, sensing module  96  includes one or more sensing channels, each of which may comprise an amplifier. In response to the signals from processor  90 , the switch module within sensing module  96  may couple the outputs from the selected electrodes to one of the sensing channels. 
     In some examples, one channel of sensing module  96  may include an R-wave amplifier that receives signals from electrode  42 , which may be used for pacing and sensing electrical cardiac activity from tissue proximate to right ventricle  26  ( FIG. 2A ) of heart  14 . Another channel may include another R-wave amplifier that receives signals from electrode  44 , which is used for pacing and sensing proximate to left ventricle  28  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 sensing module  96  may include a P-wave amplifier that receives signals from electrode  40 , which is used for pacing and sensing electrical cardiac activity in tissue proximate to right atrium  30  of heart  14 . As an alternative, the P-wave amplifier may receive signals from one or more of electrodes  46 ,  48 , and  50 , which may be positioned proximate to 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 sensing module  96  may be selectively coupled to housing electrode  52  or other sensing electrodes (not shown) with or instead of one or more of electrodes  40 ,  42 ,  44 ,  46 ,  48 , and  50 , e.g., for unipolar sensing of R-waves or P-waves in any of chambers  26 ,  28 ,  30 , and  32  of heart  14 . 
     In some examples, sensing module  96  includes a channel that comprises an amplifier with a relatively wider pass band than the R-wave or P-wave amplifiers. Signals from the selected sensing electrodes that are selected for coupling to this wide-band amplifier may be provided to a multiplexer, and thereafter converted to multi-bit digital signals by an analog-to-digital converter for storage in memory  92  as an electrogram (EGM). In some examples, the storage of such EGMs in memory  92  may be under the control of a direct memory access circuit. Processor  90  may employ digital signal analysis techniques to characterize the digitized signals stored in memory  92  to detect and classify the patient&#39;s heart rhythm from the electrical signals. Processor  90  may detect and classify the heart rhythm of patient  12  by employing any of the numerous signal processing methodologies known in the art. 
     If cardiac therapy module  102  of IMD  16  is configured to generate and deliver pacing pulses to heart  14 , processor  90  may include a pacer timing and control module, which may be embodied as hardware, firmware, software, or any combination thereof. The pacer timing and control module may comprise a dedicated hardware circuit, such as an ASIC, separate from other processor  90  components, such as a microprocessor, or a software module executed by a component of processor  90 , which may be a microprocessor or ASIC. The pacer timing and control module may include programmable counters which control the basic time intervals associated with DDD, VVI, DVI, VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and other modes of single and dual chamber pacing. In the aforementioned pacing modes, “D” may indicate dual chamber, “V” may indicate a ventricle, “I” may indicate inhibited pacing (e.g., no pacing), and “A” may indicate an atrium. The first letter in the pacing mode may indicate the chamber that is paced, the second letter may indicate the chamber in which an electrical signal is sensed, and the third letter may indicate the chamber in which the response to sensing is provided. When “D” is the third letter in the code, it indicates that the sensed signal is used for triggering a ventricular pace after a P-wave. 
     Intervals defined by the pacer timing and control module within processor  90  may include atrial and ventricular pacing escape intervals, refractory periods during which sensed P-waves and R-waves are ineffective to restart timing of the escape intervals, and the pulse widths of the pacing pulses. As another example, the pace timing and control module may define a blanking period, and provide signals from sensing module  96  to blank one or more channels, e.g., amplifiers, for a period during and after cardiac therapy module  102  delivers electrical stimulation to heart  14 . The durations of these intervals may be determined by processor  90  in response to stored data in memory  92 . The pacer timing and control module of processor  90  may also determine the amplitude of the cardiac pacing pulses. 
     During pacing, escape interval counters within the pacer timing/control module of processor  90  may be reset upon sensing of R-waves and P-waves. Cardiac therapy module  102  may include pacer output circuits that are coupled, e.g., selectively by a switching module, to any combination of electrodes  40 ,  42 ,  44 , and  52  appropriate for delivery of a bipolar or unipolar pacing pulse to one of the chambers of heart  14 . Processor  90  may reset the escape interval counters upon the generation of pacing pulses by cardiac therapy module  102 , and thereby control the basic timing of cardiac pacing functions, including anti-tachyarrhythmia pacing. 
     The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used by processor  90  to measure the durations of R-R intervals, P-P intervals, P-R intervals and R-P intervals, which are measurements that may be stored in memory  92 . Processor  90  may use the count in the interval counters to detect a tachyarrhythmia event, such as ventricular fibrillation event or ventricular tachycardia event. Upon detecting a threshold number of tachyarrhythmia events, processor  90  may identify the presence of a tachyarrhythmia episode, such as a ventricular fibrillation episode, a ventricular tachycardia episode, or a non-sustained tachycardia (NST) episode. Examples of tachyarrhythmia episodes that may qualify for delivery of responsive therapy, e.g., by cardiac therapy module  102  and/or neurostimulation module  104 , include a ventricular fibrillation episode or a ventricular tachyarrhythmia episode. In the case of a NST, however, processor  90  may not meet the requirements for triggering a therapeutic response, and, thus, processor  90  may continue normal operation. 
     In some examples, processor  90  may operate as an interrupt driven device, and is responsive to interrupts from pacer timing and control module, where the interrupts may correspond to the occurrences of sensed P-waves and R-waves and the generation of cardiac pacing pulses. Any necessary mathematical calculations to be performed by processor  90  and any updating of the values or intervals controlled by the pacer timing and control module of processor  90  may take place following such interrupts. A portion of memory  92  may be configured as a plurality of recirculating buffers, capable of holding series of measured intervals, which may be analyzed by processor  90  in response to the occurrence of a pace or sense interrupt to determine whether heart  14  of patient  12  is presently exhibiting atrial or ventricular tachyarrhythmia. 
     In some examples, an arrhythmia detection method may include any suitable tachyarrhythmia detection algorithms. In one example, processor  90  may utilize all or a subset of the rule-based detection methods described in U.S. Pat. No. 5,545,186 to Olson et al., entitled, “PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND GREATMENT OF ARRHYTHMIAS,” which issued on Aug. 13, 1996, or in U.S. Pat. No. 5,755,736 to Gillberg et al., entitled, “PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND TREATMENT OF ARRHYTHMIAS,” which issued on May 26, 1998. U.S. Pat. No. 5,545,186 to Olson et al. and U.S. Pat. No. 5,755,736 to Gillberg et al. are incorporated herein by reference in their entireties. However, other arrhythmia detection methodologies may also be employed by processor  90  in other examples. 
     In the examples described herein, processor  90  may identify the presence of an atrial or ventricular tachyarrhythmia episode by detecting a series of tachyarrhythmia events (e.g., R-R or P-P intervals having a duration less than or equal to a threshold) of an average rate indicative of tachyarrhythmia or an unbroken series of short R-R or P-P intervals. The thresholds for determining the R-R or P-P interval that indicates a tachyarrhythmia event may be stored within memory  92  of IMD  16 . In addition, the number of tachyarrhythmia events that are detected to confirm the presence of a tachyarrhythmia episode may be stored as a number of intervals to detect (NID) threshold value in memory  92 . In some examples, processor  90  may also identify the presence of the tachyarrhythmia episode by detecting a variable coupling interval between the R-waves of the heart signal. For example, if the differences between the coupling intervals are higher than a given threshold over a predetermined number of successive cycles, processor  90  may determine that the tachyarrhythmia is present. 
     If processor  90  detects an atrial or ventricular tachyarrhythmia based on signals from sensing module  96 , and an anti-tachyarrhythmia pacing regimen is desired, timing intervals for controlling the generation of anti-tachyarrhythmia pacing therapies by cardiac therapy module  102  may be loaded by processor  90  into the pacer timing and control module to control the operation of the escape interval counters therein and to define refractory periods during which detection of R-waves and P-waves is ineffective to restart the escape interval counters. 
     If IMD  16  is configured to generate and deliver defibrillation pulses to heart  14 , cardiac therapy module  102  may include a high voltage charge circuit and a high voltage output circuit. In the event that generation of a cardioversion or defibrillation pulse is required, processor  90  may employ the escape interval counter to control timing of such cardioversion and defibrillation pulses, as well as associated refractory periods. In response to the detection of atrial or ventricular fibrillation or tachyarrhythmia requiring a cardioversion pulse, processor  90  may activate a cardioversion/defibrillation control module, which may, like pacer timing and control module, be a hardware component of processor  90  and/or a firmware or software module executed by one or more hardware components of processor  90 . The cardioversion/defibrillation control module may initiate charging of the high voltage capacitors of the high voltage charge circuit of cardiac therapy module  102  under control of a high voltage charging control line. 
     Processor  90  may monitor the voltage on the high voltage capacitor, e.g., via a voltage charging and potential (VCAP) line. In response to the voltage on the high voltage capacitor reaching a predetermined value set by processor  90 , processor  90  may generate a logic signal that terminates charging. Thereafter, timing of the delivery of the defibrillation or cardioversion pulse by cardiac therapy module  102  is controlled by the cardioversion/defibrillation control module of processor  90 . Following delivery of the fibrillation or tachycardia therapy, processor  90  may return cardiac therapy module  102  to a cardiac pacing function and await the next successive interrupt due to pacing or the occurrence of a sensed atrial or ventricular depolarization. 
     Cardiac therapy module  102  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  52  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 therapy module  94 . 
     In some examples, sensing module  96  may sense non-cardiac electrical signals, e.g., via electrodes  46 ,  48 , and  50  proximate to spinal cord. The non-cardiac signals may be used to initiate and/or adjust therapy delivery from neurostimulation module  104 . Examples of non-cardiac electrical signals may include, for example, signals generated by an activity or motion sensor, such as an accelerometer, which may indicate the activity level or posture of patient  12 . As patient  12  moves and changes posture, the position of lead  18  ( FIG. 2A ) relative to spinal cord  38  may change, which may affect the efficacy of neurostimulation therapy. In some examples, processor  90  may detect the patient&#39;s posture or activity level via an electrical signal generated by sensing module  96  or a separate sensing module, which may or may not be mechanically coupled to IMD  16 , and select different therapy parameter values based on the patient&#39;s posture or activity level. Processor  90  may select different therapy parameter values by switching therapy programs (which may be stored in memory  92  of IMD  16 ) or by modifying one or more therapy parameter values of a stored therapy program. 
     As another example, sensing module  96  or a separate sensing module, which may or may not be mechanically coupled to IMD  16 , may sense neuroelectrogram signals indicative of neural activity, e.g., proximate to the vagus nerve and/or spinal cord  38  ( FIG. 1B ) of patient  12 . In some examples, processor  90  may select different therapy parameter values based on the level of neural activity indicated by the neuroelectrogram signal. Processor  90  may select different therapy parameter values by switching therapy programs (which may be stored in memory  92  of IMD  16 ) or by modifying one or more therapy parameter values of a stored therapy program. 
     Neurostimulation module  104  generates neurostimulation signals, which may be pulses as primarily described herein, or continuous time signals, such as sine waves, for delivery to patient  12  via selected combinations of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  52 . In some examples, cardiac signals sensed by sensing module  96  may be used to initiate and/or adjust therapy delivery from neurostimulation module  104 . 
     Telemetry module  98  includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as programmer  24  ( FIG. 1A ). Under the control of processor  90 , telemetry module  98  may receive downlink telemetry from and send uplink telemetry to programmer  24  with the aid of an antenna, which may be internal and/or external. Processor  90  may provide the data to be uplinked to programmer  24  and the control signals for the telemetry circuit within telemetry module  98 , e.g., via an address/data bus. In some examples, telemetry module  98  may provide received data to processor  90  via a multiplexer. 
     In some examples, processor  90  may transmit atrial and ventricular heart signals (e.g., ECG or EGM signals) produced sensing module  96  to programmer  24 . Programmer  24  may interrogate IMD  16  to receive the heart signals. Processor  90  may store heart signals within memory  92 , and retrieve stored heart signals from memory  92 . Processor  90  may also generate and store marker codes indicative of different cardiac episodes that sensing module  96  detects, and transmit the marker codes to programmer  24 . An example pacemaker with marker-channel capability is described in U.S. Pat. No. 4,374,382 to Markowitz, entitled, “MARKER CHANNEL TELEMETRY SYSTEM FOR A MEDICAL DEVICE,” which issued on Feb. 15, 1983 and is incorporated herein by reference in its entirety. 
     The various components of IMD  16  are coupled to power source  100 , which may include a rechargeable or non-rechargeable battery or a supercapacitor. 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 some examples, power source  100  may include two power sources such that cardiac module  102  and neurostimulation module  104  are powered by separate power sources. In examples in which cardiac module  102  and neurostimulation module  104  are powered by separate power sources, cardiac module  102  and neurostimulation module  104  may use the same power source when one power source is running low. In other examples, power source  100  may only have one power source shared by both cardiac module  102  and neurostimulation module  104 . Example implantable medical devices including more than one power source are described in U.S. Provisional Patent Application Ser. No. 61/110,393 to John Burnes et al. (attorney docket no. P0030860.00/1111-091USP1), which is entitled, “IMPLANTABLE MEDICAL DEVICE INCLUDING TWO POWER SOURCES” and was filed Oct. 31, 2008, the entire content of which is incorporated herein by reference. 
     In some examples, data generated sensing module  96  and stored in memory  92  may be uploaded to a remote server, from which a clinician or another user may access the data to determine whether a potential sensing integrity issue exists. An example of a remote server includes the CareLink Network, available from Medtronic, Inc. of Minneapolis, Minn. An example system may include an external device, such as a server, and one or more computing devices that are coupled to IMD  16  and programmer  24  via a network. 
       FIG. 4  is block diagram of an example programmer  24 . As shown in  FIG. 4 , programmer  24  includes processor  110 , memory  112 , user interface  114 , telemetry module  116 , and power source  118 . Programmer  24  may be a dedicated hardware device with dedicated software for programming of IMD  16 . Alternatively, programmer  24  may be an off-the-shelf computing device running an application that enables programmer  24  to program IMD  16 . 
     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 IMD  16  ( FIG. 1A ). The therapy programs may be for either or both cardiac therapy module  102  ( FIG. 3 ), neurostimulation module  104  ( FIG. 3 ). The clinician may interact with programmer  24  via user interface  114 , 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  110  can take the form one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, and the functions attributed to processor  110  herein may be embodied as hardware, firmware, software or any combination thereof. Memory  112  may store instructions that cause processor  110  to provide the functionality ascribed to programmer  24  herein, and information used by processor  110  to provide the functionality ascribed to programmer  24  herein. Memory  112  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  112  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  112  may also store information that controls therapy delivery by IMD  16 , such as stimulation parameter values. 
     Programmer  24  may communicate wirelessly with IMD  16 , such as using RF communication or proximal inductive interaction. This wireless communication is possible through the use of telemetry module  116 , 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 proximate to the patient&#39;s body near the IMD  16  implant site, as described above with reference to  FIG. 1A . Telemetry module  116  may be similar to telemetry module  98  of IMD  16  ( FIG. 3 ). As an alternative, IMD  16  ma communicate with programmer  24 , another computing device or another implanted medical device via transconductive communication (TCC) utilizing electrodes coupled to IMD  16 , e.g., the cardiac and/or neurostimulation electrodes. 
     Telemetry module  116  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  118  delivers operating power to the components of programmer  24 . Power source  118  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  118  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  118  may include circuitry to monitor power remaining within a battery. In this manner, user interface  114  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  118  may be capable of estimating the remaining time of operation using the current battery. 
       FIG. 5  is a flow diagram of an example technique for delivering both cardiac and neurostimulation therapy to patient  12  via IMD  16  including cardiac therapy module  102  and neurostimulation module  104  ( FIG. 3 ) in a common housing  54  ( FIG. 2A ). Sensing module  96  ( FIG. 3 ) of IMD  16  may sense an electrical cardiac signal from patient  12  ( 120 ). As described in further detail with respect to  FIG. 3 , sensing module  96  may sense electrical signals attendant to the depolarization and repolarization of heart  14  via electrodes coupled to lead  18 . Sensing module  96  may, additionally or alternatively, sense other cardiac or non-cardiac signals, such as signals that indicate patient posture, patient activity level, and/or neural activity. Cardiac therapy module  102  may deliver at least one pacing, cardioversion, or defibrillation therapy to heart  14  of patient  12  via an extravascular electrode, e.g., in response to electrical signals sensed by sensing module  96  ( 122 ). For example, cardiac therapy module  102  may deliver a defibrillation pulse to heart  14  in response to a detected fibrillation event sensed by sensing module  96 . 
     Neurostimulation module  104  may also deliver a neurostimulation signal to patient  12 , e.g., at a vagus nerve or spinal cord  38  of patient  12  ( 124 ). The neurostimulation signal may facilitate delivery of the pacing, cardioversion, or defibrillation therapy. For example, the neurostimulation signal may stimulate the autonomic nervous signal of patient  12  to produce a cardiac benefit, e.g., a change in heart rate, improvement in heart failure status, decrease in arrhythmia, and/or change in blood pressure. The neurostimulation therapy delivered by neurostimulation module  104  may aid in reducing the amount and/or intensity of the therapy cardiac therapy module  102  must deliver to support patient  12 . 
     Although many of the cardiovascular monitoring and analysis techniques described herein are performed by or otherwise controlled by processor  90  of IMD  16 , in other examples, another device may perform any part of the techniques described herein. 
     The techniques described in this disclosure, including those attributed to image IMD  16 , 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 have been described. These and other examples are within the scope of the following claims.