Patent Publication Number: US-10765870-B2

Title: Method and apparatus for detection of intrinsic depolarization following high energy cardiac electrical stimulation

Description:
This application is a continuation of U.S. patent application Ser. No. 15/450,182, filed Mar. 6, 2017, entitled “METHOD AND APPARATUS FOR DETECTION OF INTRINSIC DEPOLARIZATION FOLLOWING HIGH ENERGY CARDIAC ELECTRICAL STIMULATION,” which claims the benefit of U.S. patent application Ser. No. 14/693,933, now U.S. Pat. No. 9,586,051, filed Apr. 23, 2015, entitled “METHOD AND APPARATUS FOR DETECTION OF INTRINSIC DEPOLARIZATION FOLLOWING HIGH ENERGY CARDIAC ELECTRICAL STIMULATION,” the content of both of which is incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to implantable medical devices and, in particular, to a method and apparatus for sensing intrinsic cardiac electrical signals during post-stimulation polarization signals following high-energy cardiac electrical stimulation pulses. 
     BACKGROUND 
     A variety of implantable medical devices (IMDs) for delivering a therapy, monitoring a physiological condition of a patient or a combination thereof have been clinically implanted or proposed for clinical implantation in patients. Some IMDs may employ one or more elongated electrical leads carrying stimulation electrodes, sense electrodes, and/or other sensors. IMDs may deliver therapy to or monitor conditions of a variety of organs, nerves, muscle or tissue, such as the heart, brain, stomach, spinal cord, pelvic floor, or the like. Implantable medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of electrical stimulation or sensing of physiological conditions. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to an implantable medical device housing, which may contain circuitry such as signal generation circuitry and/or sensing circuitry. 
     Some IMDs, such as cardiac pacemakers or implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to or monitor the heart of the patient via electrodes carried by one or more implantable leads. The leads may be transvenous, e.g., implanted in the heart through one or more veins, to position intracardiac electrodes. Other leads may be non-transvenous leads implanted outside the heart, e.g., implanted epicardially, pericardially, or subcutaneously. In either case, the electrical stimulation provided by the IMD may include signals such as pacing pulses, cardioversion shocks or defibrillation shocks to address abnormal cardiac rhythms such as bradycardia, tachycardia or fibrillation. 
     In some cases, the IMD senses signals representative of intrinsic depolarizations of the heart and analyzes the sensed signals to identify normal or abnormal cardiac rhythms. Upon detection of an abnormal rhythm, the device may deliver an appropriate electrical stimulation pulse or pulses to restore or maintain a more normal rhythm. For example, an IMD may deliver pacing pulses to the heart upon detecting asystole, tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation. 
     SUMMARY 
     In general, the disclosure is directed to techniques for detecting intrinsic cardiac electrical signals after delivery of a high energy, therapeutic electrical stimulation pulse, such as a cardioversion/defibrillation (CV/DF) shock or a high energy transthoracic pacing pulse delivered to a patient&#39;s heart. An IMD operating in accordance with the techniques of this disclosure detects cardiac electrical signals during a post-stimulation polarization signal caused by the therapeutic electrical stimulation pulse. The detected cardiac electrical signal is identified as an intrinsic cardiac electrical signal based on criteria that discriminates the intrinsic cardiac signal from a cardiac evoked response signal. 
     In one example, the disclosure provides a method performed by a medical device comprising delivering a high-energy electrical stimulation pulse to a patient that produces a post-stimulation polarization signal, detecting a cardiac electrical signal superimposed on the post-stimulation polarization signal, determining at least one feature of the detected cardiac electrical signal, comparing the feature to criteria that differentiate an intrinsic cardiac event during the post-stimulation polarization signal from an evoked response signal, and identifying the detected cardiac electrical signal as the intrinsic cardiac event if the feature meets the criteria. 
     In another example, the disclosure provides a medical device comprising a therapy delivery module configured to deliver a high-energy electrical stimulation pulse to a patient that produces a post-stimulation polarization signal and a cardiac signal analyzer configured to receive an electrical signal developed across a pair of electrodes coupled to the medical device. The cardiac signal analyzer is configured to detect, from the received electrical signal, a cardiac electrical signal superimposed on the post-stimulation polarization signal, determine at least one feature of the detected cardiac electrical signal, compare the feature to criteria that differentiate an intrinsic cardiac event during the post-stimulation polarization signal from an evoked response signal, and identify the detected cardiac electrical signal as the intrinsic cardiac event if the feature meets the criteria. 
     In another example, the disclosure provides a non-transitory, computer-readable storage medium comprising instructions that, when executed by a processor of a medical device, cause the medical device to deliver a high-energy electrical stimulation pulse to a patient that produces a post-stimulation polarization signal, detect a cardiac electrical signal superimposed on the post-stimulation polarization signal, determine a feature of the detected cardiac electrical signal, compare the feature to criteria that differentiates an intrinsic cardiac event during the post-stimulation polarization signal from an evoked response signal, and identify the detected cardiac electrical signal as the intrinsic cardiac event if the feature meets the criteria. 
     This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the apparatus and methods described in detail within the accompanying drawings and description below. Further details of one or more examples are set forth in the accompanying drawings and the description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram of a patient implanted with an example IMD system that includes an ICD coupled to a defibrillation lead. 
         FIG. 2  is a transverse view of a patient implanted with an IMD system according to another example. 
         FIG. 3  is a schematic diagram of the ICD shown in  FIG. 1  according to one example. 
         FIG. 4  is an illustration of a pace polarization signal during which an evoked R-wave and intrinsic R-wave occur. 
         FIG. 5  is a flow chart of a method for detecting intrinsic cardiac signals during a post-stimulation polarization signal. 
         FIG. 6  is a flow chart of a method for sensing intrinsic cardiac activity during a post-stimulation polarization signal according to another example. 
     
    
    
     DETAILED DESCRIPTION 
     In general, this disclosure describes techniques for sensing intrinsic cardiac electrical events that may occur during the polarization signal that is produced by therapeutic electrical stimulation pulses delivered to a patient. Immediately following delivery of an electrical stimulation pulse, such as a CV/DF shock pulse or an extracardiac pacing pulse, a residual post-stimulation polarization signal is generated by the charge induced in the patient&#39;s tissue by delivery of the stimulation pulse. If the stimulation pulse causes an evoked response in cardiac tissue, then an evoked response signal is superimposed on the large amplitude polarization signal. 
     Relatively high energy stimulation pulses are required to achieve therapeutic benefit when extracardiac electrodes, e.g., subcutaneous or substernal electrodes that are not in direct physical contact with the myocardium, are used to deliver the stimulation pulses compared to when stimulation pulses are delivered using electrodes that are in direct contact with the myocardium, e.g., intracardiac, endocardial, or epicardial electrodes. As used herein, a “high energy stimulation pulse” refers to an electrical stimulation pulse having a pulse energy that is on the order of milliJoules or higher. For example, extracardiac pacing pulses may be on the order of approximately 20 mJ to 140 mJ when biphasic, 200 mA extracardiac pacing pulses are delivered across an impedance load between 25 and 200 Ohms (resulting in a pulse amplitude range of 5 V to 40 V). 
     In contrast, pacing pulses delivered using endocardial or epicardial electrodes may be on the order of microJoules. Typical pacing pulses delivered using endocardial electrodes might be 2 V in amplitude with 0.5 ms pulse width across a load of between 400 and 1200 ohms, resulting in a typical pulse energy ranging from approximately 2 μJ to 60 μJ. Even at a maximum programmable voltage amplitude and pulse width, e.g., 8 V and pulse width of 1.5 ms (which would rarely be used), the pulse energy may be as high as approximately 240 μJ, but still well below the mJ range. As used herein, the term “approximately” when referring to a stated numerical value refers to a value within ±10% of the stated value 
     The techniques disclosed herein for sensing intrinsic cardiac events during post-stimulation polarization signals may also be used following high energy electrical stimulation pulses delivered as CV/DF shocks. CV/DF shocks are even higher energy pulses than extracardiac pacing pulses, e.g., on the order of at least 10 Joules. 
     The relatively higher energy of extracardiac pacing pulses and of CV/DF shock pulses cause even higher amplitude polarization signals that decay over an even longer period of time than the polarization signals caused by lower energy pulses delivered via endocardial or epicardial electrodes. If an intrinsic cardiac event such as an R-wave occurs during the large polarization signal, the intrinsic cardiac event signal is superimposed on the polarization signal, and may occur during or after the evoked response signal. The large polarization signal interferes with sensing of intrinsic cardiac electrical events, such as R-waves, and therefore may interfere with the ability to sense the intrinsic heart rhythm by an ICD coupled to extracardiac electrodes. Without being able to reliably sense intrinsic cardiac events, the ICD or pacemaker may deliver unnecessary stimulation pulses, such as pacing pulses, in the presence of an adequate intrinsic heart rate. 
     When relatively lower energy pulses are delivered via endocardial or epicardial electrodes, the polarization signal is smaller and decays more quickly such that normal sensing of intrinsic cardiac signals is restored relatively soon after the stimulation pulse. A low polarization coating can be applied to the electrode surfaces to reduce the polarization signal making cardiac event sensing even more reliable in the presence of low energy stimulation pulses. Such coatings can be costly and may not be effective enough to reduce the larger polarization signals caused by higher energy stimulation pulses. The techniques described herein, however, may be used alone or in combination with low polarization coatings to enable an ICD to detect the intrinsic cardiac events superimposed on or occurring during the polarization signal. 
     Sensing of intrinsic cardiac events during delivery of high energy stimulation pulses is important because a pacing pulse delivered asynchronously with an underlying intrinsic rhythm could be delivered during the vulnerable period associated with myocardial repoloarization. Pacing during the vulnerable period can be pro-arrhythmic. Sensing of intrinsic R-waves that are attendant to the depolarization of the ventricular myocardium enables post-shock pacing or other electrical stimulation therapies to be properly timed or withheld in the presence of intrinsic cardiac events. 
       FIG. 1  is a conceptual diagram of a patient  12  implanted with an example IMD system  10  that includes an ICD  14  coupled to a defibrillation lead  16 . Defibrillation lead  16  includes a proximal end that is connected to ICD  14  and a distal portion that includes one or more electrodes. Defibrillation lead  16  is illustrated in  FIG. 1  as being implanted subcutaneously, e.g., in tissue and/or muscle between the skin and the ribcage  32  and/or sternum  22 . In the example shown, defibrillation lead  16  extends subcutaneously from ICD  14  toward xiphoid process  20 . At a location near xiphoid process  20 , defibrillation lead  16  bends or turns and extends subcutaneously superior, substantially along sternum  22  or offset from sternum  22 . 
     In other instances, lead  16  may be implanted at other extravascular or extracardiac locations. As shown in a transverse view of patient  12  in  FIG. 2 , lead  16  may be implanted at least partially in a substernal location, e.g., between the ribcage  32  and/or sternum  22  and heart  26 . In one such configuration, a proximal portion of lead  16  extends subcutaneously from ICD  14  toward sternum  22  (not seen in the transverse view of  FIG. 2 ) and the distal portion of lead  16  extends superior under or below the sternum  22  in the anterior mediastinum  36 . Anterior mediastinum  36  is bounded laterally by pleurae  39 , posteriorly by pericardium  38 , and anteriorly by sternum  22 . Lead  16  may be at least partially implanted in other intrathoracic locations, e.g., other non-vascular, extra-pericardial locations but not attached to the pericardium or other portion of heart  26 . 
     Referring again to  FIG. 1 , lead  16  includes an elongated lead body  18  carrying electrodes  24 ,  28  and  30  located along the distal portion of the length of the lead body  18 . Lead body  18  insulates one or more elongated electrical conductors (not illustrated) that each extend from a respective electrode  24 ,  28  and  30  through the lead body  18  to a proximal connector (not shown) that is coupled to ICD  14 . Lead body  18  may be formed from a non-conductive material, such as silicone, polyurethane, fluoropolymers, or mixtures thereof or other appropriate materials, and may define one or more lumens within which the one or more conductors extend. 
     ICD  14  includes connector assembly  17  (sometimes referred to as a connector block or header) that includes a connector bore for receiving the proximal connector of lead  16  and electrical feedthroughs through which electrical connections are made between electrical conductors within lead  16  and electronic components included within the housing  15 . The lead conductors are electrically coupled to ICD circuitry, such as a therapy delivery module and a sensing module, via connections in the ICD connector assembly  17  and associated electrical feedthroughs crossing ICD housing  15  as necessary. The electrical conductors transmit electrical stimulation pulses from a therapy delivery module within ICD  14  to one or more of electrodes  24 ,  28 , and  30 , and transmit cardiac electrical signals developed across a pair of electrodes  24 ,  28 , and  30  to a sensing module within ICD  14 . 
     Defibrillation lead  16  is shown in  FIG. 1  to include a defibrillation electrode  24 , which may be an elongated coil electrode, along the distal portion of defibrillation lead  16 . Defibrillation electrode  24  is located on lead  16  such that when ICD system  10  is implanted a therapy vector between defibrillation electrode  24  and housing  15  of ICD  14  is substantially through or across the ventricle(s) of heart  26 . 
     Defibrillation lead  16  also includes one or more sensing electrodes  28  and  30 , located toward the distal portion of defibrillation lead  16 . Electrodes  28  and  30  are referred to herein as “sensing electrodes,” however it is recognized that electrodes  28  and  30  may be used, together or in any combination with defibrillation electrode  24  or housing  50 , to deliver pacing pulses. Likewise, while electrode  24  is referred to herein as a “defibrillation electrode,” electrode  24  may be used as a pacing electrode, e.g., with housing  15  for delivering high energy pacing pulses such as post shock pacing pulses. In the example illustrated in  FIG. 1 , sensing electrodes  28  and  30  are separated from one another by defibrillation electrode  24 . In other words, sensing electrode  28  is located distal to defibrillation electrode  24  and sensing electrode  30  is proximal to defibrillation electrode  24 . In other embodiments, however, electrodes  28  and  30  may be located on the same side of defibrillation electrode  24  either distal or proximal to electrode  24 . ICD system  10  may sense electrical activity of heart  26  via one or more sensing vectors that include combinations of electrodes  28  and  30  and the housing  15 , sometimes referred to as a “can electrode,” of ICD  14 . For example, ICD  14  may receive a subcutaneous electrocardiogram (ECG) signal across a sensing vector between electrodes  28  and  30 , a sensing vector between electrode  28  and the housing  15 , a sensing vector between electrode  30  and housing  15 , or any combination of electrodes  28  and  30  and housing  15 . In some instances, ICD  14  may even sense cardiac electrical signals using a sensing vector that includes defibrillation electrode  24  and any one or more of electrodes  28  or  30  and housing  15 . 
     ICD  14  analyzes the electrical signals received from one or more of the sensing vectors described above to detect and treat shockable tachyarrhythmias, such as VT or VF. ICD  14  may deliver one or more cardioversion or defibrillation shocks via defibrillation electrode  24  in response to detecting VT or VF. ICD  14  may also provide pacing therapy, such as anti-tachycardia pacing (ATP) in response to detecting a tachyarrhythmia and/or post-shock pacing after a cardioversion or defibrillation shock to treat post-shock asystole or bradycardia. After delivering a shock pulse or a high voltage pacing pulse using defibrillation electrode  24  and housing  15 , a high amplitude polarization signal will occur and decay over time. As described herein, ICD  14  analyzes electrical signals received from one or more of the sensing vectors described above after delivery of a high energy therapeutic electrical stimulation pulse to identify intrinsic cardiac signals that are attendant to intrinsic cardiac events occurring during the post-stimulation polarization and are therefore superimposed on the polarization signal. 
     ICD  14  includes a housing  15  which forms a hermetic seal that protects internal electronic components of ICD  14 . The housing  15  may be formed of a conductive material, such as titanium, titanium alloy, or other conductive material to serve as an electrode. Housing  15  may function as a “can electrode” since the conductive housing or a portion thereof may be coupled to internal circuitry to be used as an indifferent or ground electrode during sensing, pacing or cardioversion/defibrillation shock delivery. As will be described in further detail herein, housing  15  may enclose one or more processors, memory devices, transmitters, receivers, sensors, sensing circuitry, therapy circuitry and other appropriate components. 
     The example illustrated in  FIG. 1  is illustrative in nature and should not be considered limiting of the techniques described in this disclosure. In other examples, ICD  14  and one or more associated leads may be implanted at other locations. For example, ICD  14  may be implanted in a subcutaneous pocket in the right chest. In this case, defibrillation lead  16  may extend subcutaneously from the device toward the manubrium of the sternum  22  and bend or turn and extend subcutaneously or substernally inferiorly from the manubrium of the sternum, substantially parallel with the sternum. 
     In another example, ICD  14  may be implanted subcutaneously outside the ribcage  32  in an anterior medial location. Lead  16  may be tunneled subcutaneously into a location adjacent to a portion of the latissimus dorsi muscle of patient  12 , from a medial implant pocket of ICD  14  laterally and posterially to the patient&#39;s back to a location opposite heart  26  such that the heart  26  is generally disposed between the ICD  14  and distal defibrillation electrode  24  and distal sensing electrode  28 . 
     The techniques disclosed herein may be implemented in numerous ICD or pacemaker and electrode configurations that include one or more housing-based electrodes and/or one or more lead-based electrodes for enabling sensing of a cardiac electrical signal developed across one or more sensing vectors and for delivering electrical stimulation therapies to heart  26  including shock pulses and/or pacing pulses. The IMD system  10  is an extravascular and extracardiac IMD system because lead  16  is positioned in an extravascular and extracardiac location outside the blood vessels, heart  26  and pericardium  38 . It is understood that while ICD  14  and lead  16  may be positioned between the skin and a muscle layer of the patient  12 , ICD  14  and any associated leads could be positioned in any extravascular and extracardiac location of the patient, such as below a muscle layer or even within the thoracic cavity but without direct contact with heart  26 , e.g., in a substernal location. Furthermore, the techniques disclosed herein may be implemented in an automatic external defibrillator (AED) that employs surface electrodes sensing cardiac signals and delivering therapy transcutaneously or in an ICD system that employs transvenous electrodes to deliver high energy CV/DF shock pulses and sensing of intrinsic cardiac events during the post-shock polarization signal is desired. 
     An external device  40  is shown in telemetric communication with ICD  14  by a communication link  42 . Communication link  42  may be established between ICD  14  and external device  40  using a radio frequency (RF) link such as Bluetooth, Wi-Fi, or Medical Implant Communication Service (MICS) or other RF bandwidth. External device  40  may include a processor, display, user interface, and external telemetry unit and may be a programmer used in a hospital, clinic or physician&#39;s office to retrieve data from ICD  14  and to program operating parameters and algorithms in ICD  14  for controlling ICD  14  functions. External device  40  may alternatively be embodied as a home monitor or hand held device. 
     External device  40  may be used to program cardiac event sensing parameters such as parameters used to control sensing intrinsic R-waves during a post-shock or post-pace polarization signal. External device  40  may also be used to program ICD tachyarrhythmia detection parameters and therapy control parameters, including post-shock pacing control parameters and shock therapy control parameters. 
       FIG. 3  is a schematic diagram of ICD  14  according to one example. The electronic circuitry enclosed within the housing of ICD  14  includes software, firmware and hardware that cooperatively monitor one or more ECG signals, determine when a CV/DF shock or pacing therapy is necessary, and deliver prescribed CV/DF and pacing therapies. In some examples, ICD  14  may be coupled to a lead, such as lead  16 , carrying electrodes, such as electrodes  24 ,  28  and  30 , positioned in operative relation to the patient&#39;s heart for delivering cardiac pacing pulses, including post-shock pacing, in addition to shock therapies. CV/DF shocks may be delivered using defibrillation electrode  24  and housing  15 , represented in  FIG. 3  as a return electrode. CV/DF shocks delivered transthoracically via defibrillation electrode  24  and housing  15  may typically be in the range of at least 10 Joules and up to 80 Joules. Extracardiac pacing pulses may be delivered, for example, using defibrillation electrode  24  and housing  15  having a pulse energy of at least approximately 1 mJ, e.g., between 20 mJ and 140 mJ. In contrast, typical cardiac pacing pulses delivered using endocardial electrodes may be on the order of 20 to 240 μJ. 
     ICD  14  and the associated techniques for sensing intrinsic cardiac events are described herein in conjunction with extracardiac electrodes  24 ,  28 ,  30  and housing  15  used to deliver high-energy stimulation pulses and sensing cardiac electrical signals. It is contemplated, however, that the disclosed techniques for sensing intrinsic events may be implemented in other ICD configurations that include transvenous leads carrying electrodes used to deliver high energy CV/DF shock pulses. 
     ICD  14  includes control module  80 , memory  82 , therapy delivery module  84 , electrical sensing module  86 , telemetry module  88 , and cardiac signal analyzer  90 . A power source  98  provides power to the circuitry of ICD  14 , including each of the modules  80 ,  82 ,  84 ,  86 ,  88 , and  90  as needed. Power source  98  may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. 
     The functional blocks shown in  FIG. 3  represent functionality that may be included in ICD  14  and implemented in any discrete and/or integrated electronic circuit components capable of producing the functions attributed to ICD  14  herein. As used herein, the term “module” refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine, or other suitable components that provide the described functionality. The particular form of software, hardware and/or firmware employed to implement the functionality disclosed herein will be determined primarily by the particular system architecture employed in the device and by the particular detection and therapy delivery methodologies employed by the device. Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modern ICD, given the disclosure herein, is within the abilities of one of skill in the art. 
     The functions attributed to the modules herein may be embodied as one or more processors, hardware, firmware, software, or any combination thereof. Depiction of different features as modules is intended to highlight different functional aspects and does not necessarily imply that such modules must be realized by separate hardware or software components. Rather, functionality associated with one or more modules may be performed by separate hardware or software components, or integrated within common hardware or software components. 
     Memory  82  may include any volatile, non-volatile, magnetic, or electrical non-transitory computer readable storage 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 memory device. Furthermore, memory  82  may include non-transitory computer readable media storing instructions that, when executed by a processor included in control module  80  or another module included in ICD  14 , cause ICD  14  to perform various functions described herein. The non-transitory computer readable media storing the instructions may include any of the media listed above, with the sole exception being a transitory propagating signal. 
     Control module  80  communicates with therapy delivery module  84 , cardiac signal analyzer  90  and electrical sensing module  86  for detecting cardiac rhythms and generating cardiac therapies as needed in response to sensed cardiac signals. Therapy delivery module  84  and electrical sensing module  86  are electrically coupled to electrodes  24 ,  28 , and  30  carried by lead  16  (shown in  FIG. 1 ) and housing  15 , which may serve as a common or ground electrode. 
     Electrical sensing module  86  may be coupled to electrodes  28 ,  30  and housing  15  in order to monitor electrical activity of the patient&#39;s heart via one or more ECG sensing vectors. Electrical sensing module  86  may additionally be selectively coupled to electrode  24 . For example, sensing module  86  may include switching circuitry for selecting which of electrodes  24 ,  28 ,  30  and housing  15  are coupled to sense amplifiers included in sensing module  86  used to sense cardiac electrical signals, such as R-waves for detecting arrhythmias. Switching circuitry may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple sense amplifiers to selected electrodes. 
     In some examples, electrical sensing module  86  includes multiple sensing channels for sensing multiple ECG sensing vectors selected from electrodes  24 ,  28 ,  30  and housing  15 . Sensing module  86  is shown to include two sensing channels  83  and  85  in the example of  FIG. 3 . Each sensing channel  83  and  85  may be configured to amplify and filter the ECG signal received from selected electrodes coupled to the respective sensing channel to improve the signal quality for sensing cardiac events, e.g., R-waves. 
     In one example, a first sensing channel  83  (ECG 1 ) may be selectably configured to sense an ECG signal between sensing electrode  28  and ICD housing  15  and a second sensing channel  85  (ECG 2 ) may be selectably configured to sense an ECG signal between sensing electrode  30  and ICD housing  15 . In another example, one sensing channel  83  or  85  may receive an ECG signal using electrodes  28  and  30  and the other sensing channel  83  or  85  may receive an ECG signal using one of electrodes  28  and  30  paired with the housing  15 . 
     Each sensing channel  83  and  85  may include cardiac event detection circuitry for sensing cardiac event signals from the received ECG signal developed across the selected electrodes  24 ,  28 ,  30  or  15 . Cardiac event sensing thresholds used by each sensing channel  83  and  85  may be automatically adjusted according to sensing control parameters, which may be stored in memory  82 . Control of the automatically-adjusted cardiac event sensing threshold for each sensing channel  83  and  85  may be implemented in control module  80 , sensing module  86  or a combination of both. A given sensing channel  83  and  85  senses a cardiac event when the respective received ECG signal crosses auto-adjusting cardiac event sensing threshold outside a blanking interval. A sensed event signal, e.g., an R-wave sensed event signal, is produced when the ECG signal crosses the threshold. The R-wave sensed event signal is passed to control module  80  for use in controlling the timing of electrical stimulation pulses delivered by therapy delivery module  84 . 
     Sensing module  86  may include an analog-to-digital converter for providing a digital ECG signal from one or both ECG sensing channels  83  and  85  to cardiac signal analyzer  90 . For example each ECG signal received by channels  83  and  85  may be converted to a multi-bit digital signal by sensing module  86  and provided to cardiac signal analyzer  90  for signal analysis. Cardiac signal analyzer  90  may include a tachyarrhythmia detector  94  for detecting and discriminating shockable and non-shockable rhythms by analysis of the ECG signal(s). Results of analysis performed by tachyarrhythmia detector  94  may be used by control module  80  in combination with R-wave sense event signals received from electrical sensing module  86  for detecting tachyarrhythmia and controlling therapy delivery module  84 . 
     Examples of algorithms that may be performed by ICD  14  for detecting, discriminating and treating shockable rhythms are generally disclosed in U.S. Pat. No. 5,354,316 (Keimel); U.S. Pat. No. 5,545,186 (Olson, et al.); U.S. Pat. No. 6,393,316 (Gillberg et al.); U.S. Pat. No. 7,031,771 (Brown, et al.); U.S. Pat. No. 8,160,684 (Ghanem, et al.), U.S. Pat. No. 8,301,233 (Zhang et al.), and U.S. Pat. No. 8,437,842 (Zhang, et al.), all of which patents are incorporated herein by reference in their entirety. The detection algorithms are highly sensitive and specific for the presence or absence of life threatening, shockable VT and VF. It should be noted that implemented arrhythmia detection algorithms may utilize not only ECG signal analysis methods but may also utilize supplemental sensors  96 , such as blood pressure, tissue oxygenation, respiration, patient activity, heart sounds, and the like, for contributing to a decision by processing and control module  80  to apply or withhold a therapy. 
     In response to detecting a shockable tachyarrhythmia, control module  80  controls therapy delivery module  84  to deliver a CV/DF shock pulse. Therapy delivery module  84  includes a high voltage (HV) therapy delivery module including one or more HV output capacitors for delivering HV CV/DF shock pulses. When a malignant tachycardia is detected, the HV capacitors are charged to a pre-programmed voltage level by a HV charging circuit. HV CV/DF shock pulses may be delivered with a pulse energy of approximately 10 J to 80 J in some examples. 
     Control module  80  applies a signal to trigger discharge of the HV capacitors upon detecting a feedback signal from therapy delivery module  84  that the HV capacitors have reached the voltage required to deliver a programmed shock energy. In this way, control module  80  controls operation of the high voltage output circuit of therapy delivery module  84  to deliver high energy CV/DF shocks using defibrillation electrode  24  and housing  15 . 
     After a CV/DF shock, post-shock pacing pulses may be required to treat asystole or bradycardia during shock recovery using extracardiac electrodes coupled to ICD  14 . Therapy delivery module  84  may also be configured for generating and delivering cardiac pacing pulses, e.g., transthoracic pacing pulses delivered using extracardiac electrodes carried by lead  16  for treating post-shock asystole, and/or for delivering tachyarrhythmia induction pulses delivered to induce VT or VF during ICD testing. High energy extracardiac pacing pulses may be delivered having a pulse energy ranging from approximately 20 mJ to 140 mJ. Depending on the location of the extracardiac electrodes, e.g., intrathoracic vs. extrathoracic, the pulse energy of extracardiac pacing pulses may be higher or lower than this approximate range but are expected to be on the order of mJ, e.g., at least 1 mJ or higher. 
     Cardiac signal analyzer  90  further includes a post-stimulation R-wave detector  92 . ECG signal analysis is performed by post-stimulation R-wave detector  92  to detect intrinsic activity during the polarization signal that follows a CV/DF shock or a pacing pulse so that control module  80  is not blinded to intrinsic cardiac signals during the polarization signal. Control module  80  includes a timing circuit comprising various timers and/or counters for measuring time intervals, such as RR intervals between sensed R-waves and for setting pacing escape intervals for controlling pacing pulses delivered by therapy delivery module  84  when an R-wave is not sensed prior to expiration of the pacing escape interval. 
     Post-stimulation R-wave detector  92  analyzes electrical signals sensed using extracardiac electrodes to detect R-waves superimposed on or occurring during the post-stimulation polarization caused by delivering high-energy stimulation pulses, CV/DF shocks and/or pacing pulses. If an intrinsic R-wave signal is detected during the polarization signal by post-stimulation R-wave detector  92 , cardiac signal analyzer  90  may pass a sensed event signal to control module  80 . A sensed event signal received during a pacing escape interval set by control module  80  may be used to reset the escape interval without delivering a scheduled pacing pulse. In this way, intrinsic activity sensed during a prolonged high-amplitude polarization signal can be used to inhibit a pacing pulse to allow intrinsic heart activity to control the heart rhythm, e.g., after delivery of a shock pulse. 
     Therapy delivery module  84  may be configured to deliver relatively high-energy pacing pulses, e.g., up to 40 volts or higher in voltage amplitude or up to 200 milliamps or higher, to provide subcutaneously delivered transthoracic pacing pulses. Therapy delivery module  84  may programmed to deliver extracardiac pacing pulses having a pulse width between 5 ms and 10 ms. For example, with no imitation intended extracardiac post-shock pacing pulses may be delivered as biphasic, 200 mA pulses having a pulse voltage amplitude between approximately 5 and 40 Volts and a pulse width between 5 and 10 ms for treating post-shock asystole. User-programmable therapy control parameters may be programmed into memory  82  via telemetry module  88 . 
     Telemetry module  88  includes a transceiver and antenna for communicating with external device  40  (shown in  FIG. 1 ) using RF communication. Under the control of control module  80 , telemetry module  88  may receive downlink telemetry from and send uplink telemetry to external device  40 . ECG episode data related to the detection of a shockable rhythm and the delivery of a CV/DF shock may be stored in memory  82  and transmitted by telemetry module  88  to external device  40  upon receipt of an interrogation command. Clinician review of episode data facilitates diagnosis and prognosis of the patient&#39;s cardiac state and therapy management decisions, including selecting programmable control parameters used for detecting shockable rhythms, sensing cardiac signals, and delivering therapy. 
       FIG. 4  is a depiction of a post-stimulation polarization signal  102  during which an evoked response R-wave  104  and an intrinsic R-wave  106  occur. The polarization signal  102  includes a sharply rising portion  102   a  upon delivery of an electrical stimulation pulse  108  followed by a decaying portion  102   b . Stimulation pulse  108  is a high energy pulse, e.g., a transthoracic shock or pacing pulse delivered by ICD  14  using extracardiac electrodes carried by lead  16 . In the case of stimulation pulse  108  being a CV/DF shock, sensing of intrinsic cardiac events may be performed during polarization signal  102  for verifying success of the shock in cardioverting or defibrillating the heart, for early redetection of the tachyarrhythmia if the shock did not succeed, and for determining a need for post-shock pacing. In the example shown, stimulation pulse  108  is a pacing pulse, followed by an evoked response signal  104 . In the case of stimulation pulse  108  being a pacing pulse, a sensing of intrinsic R-wave  106  is used to inhibit the next pacing pulse. 
     A pacing escape interval  110  may be set upon delivery of the stimulation pulse  108  to control post-shock pacing. The polarization signal  102  is large in amplitude and the decaying portion  102   b  may not return to baseline for a relatively long period of time, e.g., for more than 500 ms, which may be a majority of or more than one pacing escape interval  110 . In the example shown, the polarization signal  102  extends for more than one pacing escape interval  110 . 
     Sensing of intrinsic R-waves by a threshold-based cardiac event detector included in electrical sensing module  86  during escape interval  110  is impaired due to the high amplitude of polarization signal  102 . The amplitude of signal  102  may be much greater than the amplitude of intrinsic R-waves and greater than an R-wave sensing threshold used by sensing module  86  for detecting R-waves from the ECG signal when no polarization artifact is present. As a result, sensing module  86  may conceivably sense an ECG signal amplitude greater than the R-wave sensing threshold throughout a pacing escape interval  110 , masking the presence of an intrinsic R-wave  106  that is superimposed on the polarization signal  102 . 
     In some examples, cardiac signal analyzer  90  may monitor for an evoked response signal  104  following stimulation pulse  108  to verify that the pulse  108  has captured the heart. A method and apparatus for detecting an evoked response following a pacing pulse during a pace polarization signal is generally disclosed in U.S. Pat. No. 6,134,473 (Hemming, et al.), incorporated herein by reference in its entirety. The polarization signal following a relatively low energy pacing pulse, e.g., 5 Volts or less, delivered using endocardial electrodes decays relatively quickly. The evoked response signal following the low energy pacing pulse may occur during the polarization signal, but interference of the post-pace polarization signal with intrinsic R-wave sensing disappears quickly following low energy pacing pulses and can be further minimized using low polarization electrodes. Sensing intrinsic R-waves following low energy pacing pulses, therefore, typically does not require special techniques since reliable sensing by a sense amplifier using an auto-adjusted sensing threshold is restored relatively soon after the low energy pacing pulse. In contrast, the large amplitude long duration polarization signal  102  caused by the high energy stimulation pulse  108  will impair sensing of intrinsic R-wave  106  based on an R-wave sensing threshold by electrical sensing module  86  for a relatively long period of time. The techniques disclosed herein enable sensing of the intrinsic R-wave  106  during the polarization signal  102  and discrimination of the intrinsic R-wave  106  from the evoked response signal  104  (when it occurs). 
     As described below, cardiac signal analyzer  90  of ICD  14  is configured to sense the intrinsic R-wave  106  during the polarization signal  102 , when the sensing function of electrical sensing module  86  may be impaired by the polarization signal  102 . Post-stimulation R-wave detector  92  may detect cardiac electrical signals during post-stimulation polarization signal  102  based on an accelerated slope  103  of the decaying portion  102   b  or a reversed polarity slope  107  of decaying portion  102   b . As described below in conjunction with  FIGS. 5 and 6 , post-stimulation R-wave detector  92  identifies intrinsic R-wave  106  based on features that differentiate it from evoked response signal  104 . 
     In some examples, a sensing delay interval  126  is applied after the stimulation pulse  108  to allow time for the polarization signal  102  to decay. If stimulation pulse  108  is a shock pulse, polarization signal  102  may have a very large amplitude that is out of range of the post-stimulation R-wave detector  92 . A sensing delay interval  126  may be used to allow polarization signal  102  to come back into a sensing range. 
     Upon identifying intrinsic R-wave  106 , cardiac signal analyzer  90  produces an R-wave sense event signal  112  (labeled “VS” in  FIG. 4 ), which causes control module  80  to terminate ventricular pacing escape interval  110 , which was started upon delivery of stimulation pulse  108 . A pacing pulse  114  scheduled for delivery upon expiration of pacing escape interval  110  is withheld. Control module  80  restarts a new ventricular pacing escape interval  116  upon the VS event signal  112 . If a VS signal is not received by control module  80  during escape interval  116 , a pacing pulse  118  is delivered upon expiration of escape interval  116 , resulting in another polarization signal. Cardiac signal analyzer  90  may continue to detect intrinsic R-waves during the next polarization signal. 
     Characteristics of R-wave  106  that differentiate it from the pacing evoked response signal  104  are used to positively identify intrinsic R-wave  106  during polarization signal  102 . For example, since the myocardial depolarization wavefront associated with intrinsic R-wave  106  travels toward a sensing electrode (e.g., electrode  28  or  30 ) then away from the sensing electrode, R-wave  106  is typically a biphasic signal having a positive-going peak  120  followed by a negative-going peak  122  (or a negative-going peak followed by a positive-going peak). The evoked depolarization caused by stimulation pulse  108  is traveling through the myocardial tissue away from the pacing electrode. Evoked response signal  104  will therefore typically be monophasic, having a single, negative-going, peak as illustrated in  FIG. 4 . The biphasic intrinsic R-wave  106  will typically have a narrower signal width  124  than the signal width  105  of the monophasic evoked response signal  104  due to the specialized conduction pathways that conduct the intrinsic depolarization. 
     Since the intrinsic R-wave  106  will typically occur later in time after a stimulation pulse  108  than the evoked response signal  104 , timing relative to a stimulation pulse  108  may be used to differentiate the evoked response signal  104  from an intrinsic R-wave  106  in some cases. For instance, if a cardiac electrical signal is detected on the polarization signal  102  later than a time interval threshold after the stimulation pulse  108 , for example later than approximately 200 ms after the stimulation pulse  108 , the detected cardiac signal may be presumed to be an intrinsic R-wave  106  and not an evoked response signal  104 . 
     However, the intrinsic R-wave  106  may not always arrive at a distinctly different time than the evoked response  104 . Fusion beats are beats in which the timing of the stimulation pulse  108  results in fusion of the evoked response  104  and the intrinsic R-wave  106 . It may be desirable to detect a fusion beat as intrinsic activity during a pacing escape interval to inhibit pacing pulses, e.g., during post-shock pacing to enable recovery of the intrinsic heart rhythm post-shock. Since the evoked response signal and the intrinsic R-wave signal are merged, the fusion beat signal will include characteristics of the R-wave  106 , such as two signal peaks having differing polarities, which differentiate the fusion beat from a pure evoked response. For the purposes of sensing intrinsic activity to withhold pacing, the fusion beat may be detected as an intrinsic R-wave, and an R-wave sense event signal may be produced by cardiac signal analyzer  90  to cause control module  80  to reset the currently running pacing escape interval. 
       FIG. 5  is a flow chart  200  of a method for detecting intrinsic cardiac signals during a post-stimulation polarization signal. At block  202 , the cardiac signal analyzer  90  waits for an electrical stimulation pulse delivery, which may be a CV/DF shock or a pacing pulse. Upon stimulation pulse delivery, the electrical sensing module  86  may be enabled by control module  80  to pass an ECG signal from one or both sensing channels  83  and  85  to cardiac signal analyzer  90  for detecting intrinsic cardiac signals during the post-stimulation polarization signal. One or both ECG signals may be analyzed by post-stimulation R-wave detector  92  for detecting an R-wave during the polarization signal. 
     In system  10  shown in  FIGS. 1 and 3 , ECG 1   83  may be received across electrode  28  and housing  15 , and ECG 2   85  may be received across electrode  30  and housing  15 . In other examples, ECG 1   83  may be received between the defibrillation electrode  24  and the housing  15 , and ECG 2   85  may be received between one of sensing electrodes  28  and  30 . The vertical sensing vector between electrodes  28  and  30 , however, may be inferior for sensing R-waves due to lower ECG R-wave amplitude compared to a relatively more transverse vector between one of electrodes  24 ,  28 ,  30  and housing  15 . Using the techniques disclosed herein, a vector that includes housing  15  and/or defibrillation electrode  24  may be used for sensing intrinsic events after the stimulation pulse, even when the stimulation pulse is delivered using housing  15  and defibrillation electrode  24 . Switching to a different sensing vector that excludes both electrodes  15  and  24  used to deliver a stimulation pulse in order to avoid polarization signal interference may be unnecessary using the presently disclosed techniques. Sensing an ECG signal using a relatively more transverse vector that has a relatively higher R-wave amplitude, e.g., a vector that employs housing  15  rather than the vertical vector between electrodes  18  and  30 , may be particularly useful during post-stimulation sensing when detecting of intrinsic activity is important for appropriately controlling ICD functions. 
     For each ECG signal received, post-stimulation R-wave detector  92  tracks the post-stimulation polarization signal, such as signal  102  of  FIG. 4 , to detect a cardiac signal superimposed on the polarization signal at block  204 . In one example, the post-stimulation R-wave detector  92  includes a peak tracking circuit that detects a change in polarity of the polarization signal or detects a change in slope of the polarization signal, e.g., an acceleration of a negative slope of the polarization signal  102  or a reversal of the polarity of the slope. The slope of the decaying portion of the polarization signal  102  is not expected to suddenly increase or change direction. Either of these slope changes can be used at block  204  to detect a cardiac electrical signal, evoked or intrinsic, superimposed on the polarization signal  102 . 
     These slope changes of the decaying portion  102   b  of the polarization signal  102  that are evidence of a cardiac electrical signal superimposed on the polarization signal  102  can be observed in  FIG. 4 . For example, the negative-going peak of evoked response  104  is preceded by an accelerated negative slope  103  along the decaying portion  102   b  of the polarization signal  102 . In the case of the intrinsic R-wave  106 , the slope of the decaying portion  102   b  switches in polarity from a negative slope to a positive slope  107  that rises to positive-going peak  120 . If the intrinsic R-wave  106  is characterized by a biphasic signal having a negative-going peak first followed by a positive-going peak, the negative slope of the decaying portion  102   b  of polarization signal  102  will accelerate to the leading negative peak, similar to the situation of evoked response  104 , then be followed by the change in slope polarity to a positive slope leading to a positive-going peak. 
     Thus an acceleration of the negative slope of the decaying portion  102   b  of polarization signal  102  and/or a change in the polarity of the slope can be used to detect the presence of a cardiac electrical signal superimposed on polarization signal  102 . Circuitry included in post-stimulation R-wave detector  92  for detecting a cardiac electrical signal superimposed on the polarization signal  102  may correspond generally to circuitry disclosed in the above-incorporated &#39;473 patent (Hemming, et al.) for detecting an evoked response during a post-pace polarization signal. 
     If a cardiac electrical signal is not detected during the polarization signal, the post-stimulation R-wave detector  92  returns to block  202  to wait for the next electrical stimulation pulse delivery. Post-stimulation R-wave detector  92  may be enabled to continue searching for a cardiac signal by monitoring for a change in the slope of decaying portion  102   b  until the next stimulation pulse or until an intrinsic R-wave is sensed. In practice, a back-up pacing pulse may be scheduled when an evoked response signal is not detected within a predetermined time interval, e.g., 50 ms, following a pacing pulse. If a cardiac signal is not detected at block  204  within the predetermined time interval, a back-up pulse may be delivered immediately in which case the next stimulation pulse may come relatively soon at block  202 , sometimes with a higher energy (higher amplitude and/or pulse width). 
     If a cardiac electrical signal is detected at block  204 , additional signal features may be determined by post-stimulation R-wave detector  92  in order to discriminate intrinsic R-waves from evoked R-waves. For example, after detecting a cardiac electrical signal based on a slope change of the decaying portion  102   b , the peak tracking circuit of post-stimulation R-wave detector  92  may determine how many peaks the detected superimposed cardiac signal has and the polarity of each detected peak relative to the polarity signal  102  at block  206 . Peaks may be identified based on a change in sign of the amplitude difference determined between signal sample points that occur a predetermined time interval apart, e.g., 20 ms apart or less. Alternatively, peaks could be identified by setting a threshold greater than the polarization signal and a threshold less than the polarization signal at a given point in time such that if a threshold is crossed a peak is counted. Once one threshold is crossed resulting in the first peak being counted, e.g., a positive going peak, the next peak is counted only after the opposite threshold is crossed, e.g., a negative-going peak. The peak tracking circuit of post-stimulation R-wave detector  92  may be configured to detect peaks within a time window of detecting the cardiac electrical signal, e.g., 100 ms or less. Alternatively the peak tracking circuit may detect a return to the expected slope of the decaying portion  102   b  of polarization signal  102  and stop searching for peaks of the detected cardiac signal. 
     At block  208 , the post-stimulation R-wave detector  92  may determine a width of the detected cardiac signal. For example, the signal width may be determined as the time from detection of the cardiac electrical signal (based on the change in the polarity of the slope or the steepness of the slope of the decaying portion  102   b ) until a return to the expected slope of the decaying portion  102   b . An expected slope of the decaying portion  102   b  may be based on a predefined slope threshold that is expected to always be less than the slope  103  of an evoked response signal  102  and the slope  107  of an R-wave  106 . Alternatively, the slope of the polarization signal may be measured when a cardiac signal is not being detected and used to establish an expected slope or slope range of the decaying portion  120   b . The signal width  105  or  124  may be determined as the time from the onset of detecting the cardiac signal (evoked response signal  104  or intrinsic R-wave  106 ) until the expected slope is detected again. 
     Determination of the expected slope of polarization signal  102  in the absence of a cardiac signal could be performed by regional modelling of the exponential polarization decay. Absence of a cardiac signal may be inferred from a lack of slope disturbance, i.e., no change in polarity of the slope or acceleration of the slope. 
     In another example, the signal onset is determined as a time point that the slope of the decaying portion  102   b  changes from a slow slope to a fast slope. The slope may be determined between two sample points at predetermined time intervals apart, e.g., 20 ms apart. The difference between two consecutively determined slopes may be compared to a difference threshold. If the difference exceeds the threshold, a large change in the slope is detected as the onset of the signal. In a similar manner, the end of the signal may be detected as a time point that the slope changes back from a fast slope to a slow slope, e.g., by determining that the difference between two consecutively determined slopes is less than a difference threshold. The signal width is the time interval from the detected onset to the detected end point. 
     In yet another example, the signal width may be determined at a predetermined fraction of the peak amplitude. For example, the time interval between a signal sample point at 50% of the peak amplitude prior to the peak and a signal sample point at 50% of the peak amplitude after the peak may be determined as the signal width. 
     Instead of determining signal width at block  208 , or in addition to, the signal area may be determined at block  208 . The signal width and consequently signal area of the intrinsic R-wave is expected to be less than the signal width and signal area of the evoked response signal. Signal area may be determined as the sum of the absolute values of the samples points during the detected signal (based on determining an onset and end as described above). Signal area may be determined as the signed area where sample points greater than a mean amplitude of the detected signal are positive terms and sample points less than the mean amplitude of the detected signal are negative terms. The negative summation of the negative terms is subtracted from the positive summation of the positive terms yielding a signed signal area. 
     At blocks  210  through  214 , one or more features of the detected cardiac signal may be compared to criteria that differentiate the intrinsic R-wave  106  from the evoked response signal  104 . For example, the number of signal peaks determined at block  206  may be used to determine whether the detected cardiac signal is an evoked response or an intrinsic R-wave. More than one signal peak supports the decision that the detected cardiac signal is an intrinsic R-wave at block  216 . If only a single peak is detected as determined at block  210 , the detected cardiac signal is determined to be the evoked response signal  104  at block  224 . The process may return to block  204  to detect another cardiac electrical signal during the polarization signal  102 . If no additional superimposed cardiac electrical signal is detected during the decaying portion  102   b , the ECG signal(s) received after the next electrical stimulation pulse is delivered at block  202  is(are) analyzed at block  204 . 
     If more than one signal peak is detected at block  210 , the post-stimulation R-wave detector  92  may determine whether the detected signal is at least biphasic based on at least one signal peak being negative-going relative to the polarization signal  102  and at least one signal peak being positive going relative to the polarization signal  102 . Both signal peaks, e.g., peaks  120  and  122 , may have a positive amplitude, but one peak  120  follows a positive-going slope while the other peak  124  follows a negative-going slope. Two peaks or more detected within the detected cardiac signal that follow slopes of the same polarity may not be detected as an intrinsic cardiac signal since the signal may be a monophasic signal relative to the polarization signal  102  having multiple peaks of the same polarity. An evoked response signal  104  could have a double peak in the same direction in some instances, e.g., a negative-going slope reaching a first negative peak, a brief flat or positive-going slope, then another negative-going slope followed by a second negative peak. In order to verify that the detected cardiac signal is an intrinsic R-wave, the detected cardiac signal must be at least biphasic having at least one peak following a positive-going slope and one peak following a negative-going slope need to be detected in some embodiments. If the signal is monophasic, as determined at block  212 , the detected signal may be identified as evoked response signal  104  at block  224 . 
     If the detected cardiac signal is not monophasic (e.g., biphasic, triphasic, etc.) at block  212 , the signal width and/or area determined at block  208  may be compared to a respective signal width threshold or area threshold at block  214 . In various examples, the signal width threshold is a predetermined threshold or a threshold set based on a previous signal width measurement of a known evoked response signal or a known intrinsic signal. The signal width threshold may be set to a percentage of the known evoked response signal width  105  (shown in  FIG. 4 ), e.g., 70% of signal width  105 . Similarly, a signal area threshold may be set based on the area determined for a previously detected known evoked response signal or intrinsic signal, 
     In other examples, if the evoked response signal  104  ( FIG. 4 ) has been identified after the most recent electrical stimulation pulse  108  and prior to the currently detected but still unknown cardiac electrical signal  106 , the signal width  124  of the currently unknown detected cardiac signal  106  may be compared to the signal width  105  of the identified evoked response signal  104 . If the signal width  124  is less than a predetermined signal width threshold, or less a preceding detected evoked response signal width  105 , the unknown signal  106  is identified to be an intrinsic R-wave  106  at block  216 . 
     In  FIG. 5 , all of decision blocks  210 ,  212  and  214  are performed in order to identify a detected cardiac signal as an intrinsic cardiac signal at block  106  or an evoked response  104  at block  224 . In various examples, however, all of blocks  210 ,  212  and  214  may not be required. One or more of the criteria shown in the decision blocks  210 ,  212  and  214  may be used for identifying a detected cardiac signal superimposed on the polarization signal  102  as an evoked response  104  or an intrinsic cardiac signal  106 . If a detected cardiac signal is identified as an evoked response signal at block  224  and another cardiac signal is detected during the same polarization signal after the evoked response signal, the next cardiac signal may be identified as an intrinsic R-wave based on the knowledge that an evoked response signal has already been detected. Alternatively, less stringent criteria may be applied to identify an intrinsic R-wave if an evoked response signal has already been identified, e.g., only verifying at least two peaks of opposite polarity relative to the polarization signal  102  without determining signal width. 
     The criteria applied to discriminate the intrinsic R-wave  106  from the evoked response signal  104  and the polarization signal  102  may be established for a particular ICD  14  (or other device employing the techniques disclosed herein) taking into account the behavior of the polarization signal  102 , evoked response signal  104 , and intrinsic R-wave  106  when filtered by the particular input filter properties of the ICD  14 . Criteria relating to the number of signal peaks, the signal width and the signal area may differ between different hardware applications depending in part on the filtering of the ECG signal. 
     If an intrinsic R-wave is detected at block  216 , the pacing escape interval started upon delivery of the pacing pulse delivered at block  202  is reset at block  218  by control module  80  such that the next scheduled pacing pulse is inhibited. The process returns to block  202  to wait for the next electrical stimulation pulse. It is recognized that in some cases, a second intrinsic event may occur during the same polarization signal if the post-stimulation polarization signal is relatively long, e.g., greater than 500 ms and/or the intrinsic heart rhythm is fast due to sinus or non-sinus tachycardia. As such, after resetting the escape interval, the ongoing decaying polarization signal  102  may be analyzed for detecting another intrinsic cardiac signal. Additional intrinsic cardiac signals identified during the polarization signal  102  may be used by control module  80  for resetting the pacing escape interval as well as for tachyarrhythmia detection by tachyarrhythmia detector  94 . 
       FIG. 6  is a flow chart  300  of a method for sensing intrinsic cardiac activity during a post-stimulation polarization signal according to another example. At block  302 , an intrinsic cardiac signal waveform template is established. This template may be based on a waveform transformation of the ECG signal during a polarization signal or when a known intrinsic event occurs outside of the polarization signal (e.g., before a stimulation pulse or after full decay of the post-stimulation polarization signal). A wavelet transformation of the ECG signal may be performed to generate a template of the intrinsic R-wave waveform shape in some examples. The template may be generated from an R-wave signal or an average of multiple R-wave signals. 
     At block  304 , the cardiac signal analyzer  90  waits for an electrical stimulation pulse delivered by therapy delivery module  84 . A pacing escape interval may be started by control module  80  upon delivery of the stimulation pulse. After the stimulation pulse, the control module  80  may disable sensing of intrinsic events by electrical sensing module  86  during the post-stimulation polarization signal at block  306 . The sensing module  86  may be disabled by setting a blanking period during which sense amplifiers are disabled in electrical sensing module  86  that become saturated by the polarization signal and are not the amplifiers used to provide an ECG signal to post-stimulation R-wave detector  92 . In some examples, one or more sensing channels may be included in electrical sensing module  86  that pass ECG signals to post-stimulation R-wave detector  92 . These sensing channels may remain enabled during the polarization signal, whereas the one or more sensing channels of sensing module  86  that are used for sensing R-waves based on an auto-adjusted sensing threshold amplitude when a polarization signal is not present may be blanked during the post-stimulation polarization signal. For instance, the sense amplifiers for those channels may be powered down while continuing to provide power to the sensing channel(s) that provide an ECG signal to post-stimulation R-wave detector  92 . 
     Alternatively, cardiac event sensing by sensing module  86  is disabled during a blanking period by ignoring sense event signals produced by sensing module  86  during the polarization signal. The blanking period applied to sensing module  86  at block  306  may be a predetermined time interval expected to encompass a portion of the post-stimulation polarization signal  102  after which the sensing module  86  is expected to reliably sense cardiac signals, e.g., based on an auto-adjusted sensing threshold that decays after each sensed event. 
     In other examples, the blanking period is terminated by the control module  80  after the polarization signal  102  has reached a predetermined threshold. The predetermined threshold may be a programmed nominal threshold or a threshold set by control module  80 . Electrical sensing module  86  may set an auto-adjusted R-wave sensing threshold to a percentage of the peak amplitude of the most recently sensed R-wave, e.g., 60% of the previous R-wave peak amplitude. The period of time that electrical sensing module  86  is blanked or disabled at block  306  may be until the polarization signal amplitude falls below the current auto-adjusted R-wave sensing threshold set by electrical sensing module  86 . 
     At block  308 , the post-stimulation R-wave detector  92  may be configured to wait for an evoked response blanking interval (which may be set by control module  80 ). An evoked response blanking interval may optionally be applied for a fixed interval of time after the stimulation pulse  108  is delivered to avoid detection of the evoked response signal  104  during the polarization signal decaying portion  102   b . A peak tracking circuit of post-stimulation R-wave detector  92  may not be enabled to detect a cardiac signal at block  310  until after the evoked response blanking interval has expired. In this way, the evoked response signal  104  and the intrinsic R-wave  106  can be discriminated based on time of occurrence after the stimulation pulse  108 . Fusion beats that occur during the evoked response blanking interval, however, may be missed. 
     After the evoked response blanking interval, a cardiac signal superimposed on the polarization signal  102  is detected by post-stimulation R-wave detector  92  as described previously in conjunction with  FIG. 5 . Upon detecting a superimposed cardiac signal, the morphology of the detected cardiac signal waveform is determined at block  312 , using the same transformation technique used to generate an intrinsic waveform signal template. At block  314 , the morphology of the detected signal is compared to the previously established template to determine a correlation metric. If the detected signal waveform matches the template e.g., based on the correlation metric exceeding a correlation threshold, the detected cardiac signal is identified as an R-wave at block  316 . 
     A pacing escape interval may be restarted at block  318  in response to detecting the intrinsic R-wave at block  316 . A pacing escape interval that was set previously by control module  206  at block  304  upon delivery of the stimulation pulse may be restarted without delivery of a pacing pulse by therapy delivery module  84 . 
     Depending on the intrinsic cardiac rate and the duration of the polarization signal, additional intrinsic R-waves may be detected during the polarization signal. At block  320 , the control module  80  or cardiac signal analyzer  90  may determine whether the polarization signal  102  has fallen below a threshold below which cardiac event sensing by sensing module  86  is deemed reliable. If not, post-stimulation R-wave detector  92  remains enabled and continues searching for another cardiac signal superimposed on the polarization signal  102  at block  310 . If so, post-stimulation R-wave detector  92  may be disabled at block  322  until the next stimulation pulse is delivered. The electrical sensing module  86  may be enabled at block  324  to sense intrinsic events after the polarization signal has substantially decayed, enabling reliable amplitude-based cardiac event sensing using an auto-adjusted sensing threshold. As described above, electrical sensing module  86  may be enabled at block  324  when the polarization signal falls below the current R-wave sensing threshold automatically adjusted by electrical sensing module  86  based on a preceding R-wave peak amplitude. 
     Alternatively, the post-stimulation R-wave detector  92  is disabled at block  322  after a predetermined time interval after the most recent electrical stimulation pulse and the sensing module  86  is enabled at block  324  for sensing intrinsic R-waves until the next stimulation pulse is delivered at block  304 . For example, searching for intrinsic events during the polarization signal  102  by post-stimulation R-wave detector using the methods shown by flow chart  300  or flow chart  200  may be enabled for up to 500 ms after a stimulation pulse, after which post-stimulation R-wave detector  92  is disabled and sensing module  86  is enabled for R-wave sensing. 
     Thus, a method and apparatus for sensing intrinsic cardiac electrical signals during a post-stimulation polarization signal following high-energy stimulation pulses have been presented in the foregoing description with reference to specific embodiments. In other examples, various methods described herein may include steps performed in a different order or combination than the illustrative examples shown and described herein. It is recognized, for example, that various aspects of a method for detecting intrinsic cardiac signals during post-stimulation polarization may include a different order or a different combination of steps than the order and combinations shown by the illustrative flow charts of  FIGS. 5 and 6 . Aspects of the two flow charts  200  and  300  may be implemented in any combination. It is appreciated that various modifications to the referenced embodiments may be made without departing from the scope of the disclosure and the following claims.