Patent Publication Number: US-2021170170-A1

Title: Method and apparatus for detecting cardiac event oversensing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/945,430 filed Dec. 9, 2019, the entire disclosure of which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to a medical device system and method for detecting oversensing of cardiac events in a cardiac electrical signal. 
     BACKGROUND 
     Medical devices, such as cardiac pacemakers and implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to a heart of a patient via electrodes carried by one or more medical electrical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include signals such as pacing pulses or cardioversion or defibrillation shocks. In some cases, a medical device may sense cardiac electrical signals attendant to the intrinsic or pacing-evoked depolarizations of the heart and control delivery of stimulation signals to the heart based on sensed cardiac electrical signals. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm of the heart. For example, an ICD may deliver pacing pulses to the heart of the patient upon detecting bradycardia or tachycardia or deliver cardioversion or defibrillation (CV/DF) shocks to the heart upon detecting tachycardia or fibrillation. 
     The ICD may sense the cardiac electrical signals in a heart chamber and deliver electrical stimulation therapies to the heart chamber using electrodes carried by transvenous medical electrical leads. Cardiac signals sensed within a heart chamber generally have a high signal strength and quality for reliably sensing near-field cardiac electrical events, such as ventricular R-waves sensed from within a ventricle, without oversensing of far-field events arising from other heart chambers, such as P-waves. In some proposed or available ICD systems, a non-transvenous lead may be coupled to the ICD, in which case cardiac signal sensing presents new challenges in accurately sensing cardiac electrical events from outside a heart chamber. 
     SUMMARY 
     In general, the disclosure is directed to techniques for detecting oversensing of cardiac events from a cardiac electrical signal and rejecting an arrhythmia detection in response to detecting the cardiac event oversensing. In some instances, the oversensed cardiac events may be P-waves that are falsely sensed as R-waves from a cardiac electrical signal. Oversensed P-waves may be counted toward the detection of a ventricular tachyarrhythmia leading to anti-tachyarrhythmia pacing or a CV/DF shock. A medical device, such as a pacemaker or ICD, operating according to the techniques disclosed herein may detect oversensing of P-waves and does not detect a ventricular tachyarrhythmia based on a count of tachyarrhythmia intervals reaching a detection threshold criteria in response to detecting the oversensed P-waves. In this way, a tachyarrhythmia therapy, such as anti-tachycardia pacing and/or CV/DV shock(s), may be withheld when oversensing is detected. 
     In some examples, the cardiac electrical signal is sensed via an extracardiovascular lead used for sensing the cardiac events, e.g., R-waves, and delivering cardiac electrical stimulation therapies via extracardiovascular electrodes based on sensed cardiac events. A medical device operating according to the techniques disclosed herein may sense cardiac events from a first cardiac electrical signal and detect oversensing of cardiac events from the first cardiac electrical signal by determining features of consecutive time segments of a second cardiac electrical signal. The device may detect cardiac event oversensing based on identifying an alternating pattern of the second cardiac electrical signal features. Cardiac event oversensing detection may include verifying that the consecutive second cardiac electrical signal segments presenting the alternating pattern of signal features do not include a tachyarrhythmia morphology. 
     In one example, the disclosure provides a medical device including a cardiac electrical signal sensing circuit and a control circuit. The cardiac electrical signal sensing circuit is configured to sense at least one cardiac electrical signal and detect cardiac events from the at least one cardiac electrical signal. The control circuit is configured to determine a signal feature from a segment of the at least one cardiac electrical signal in response to each one of a plurality of cardiac events detected from the at least one cardiac electrical signal, detect an alternating pattern of the signal feature determined from the consecutive segments of the at least one cardiac electrical signal, determine a gross morphology metric from each of at least one segment of the consecutive segments, and detect cardiac event oversensing in response to the gross morphology metric not meeting a tachyarrhythmia morphology criteria and detecting the alternating pattern. The control circuit withholds detection of an arrhythmia in response to detecting the cardiac event oversensing. 
     In another example, the disclosure provides a method including sensing at least one cardiac electrical signal and detecting cardiac events from the at least one cardiac electrical signal. The method includes determining a signal feature from a segment of the at least one cardiac electrical signal in response to each one of a plurality of cardiac events detected from the at least one cardiac electrical signal, detecting an alternating pattern of the signal feature determined from consecutive segments of the at least one cardiac electrical signal and determining a gross morphology metric from each of at least one segment of the consecutive segments. The method further includes determining that the gross morphology metric does not meet a tachyarrhythmia morphology criteria, detecting cardiac event oversensing in response to the gross morphology metric not meeting the tachyarrhythmia morphology criteria and detecting the alternating pattern, and withholding detection of an arrhythmia in response to detecting the cardiac event oversensing evidence. 
     In another example, the disclosure provides a non-transitory, computer-readable storage medium storing a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to sense at least one cardiac electrical signal, detect cardiac events from the at least one cardiac electrical signal, and determine a signal feature from a segment of the at least one cardiac electrical signal in response to each one of a plurality of cardiac events detected from the at least one cardiac electrical signal. The medical device is further caused to detect an alternating pattern of the signal features determined from consecutive segments of the at least one cardiac electrical signal, determine a gross morphology metric from each of at least one segment of the consecutive segments, determine that the gross morphology metric does not meet a tachyarrhythmia morphology criteria, and detect cardiac event oversensing in response to the gross morphology metric not meeting the tachyarrhythmia morphology criteria and detecting the alternating pattern. The medical device is caused to withhold detection of an arrhythmia in response to detecting the cardiac event oversensing. 
     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 DRAWINGS 
         FIGS. 1A and 1B  are conceptual diagrams of an extra-cardiovascular ICD system configured to sense cardiac electrical events and deliver cardiac electrical stimulation therapies according to one example. 
         FIGS. 2A-2C  are conceptual diagrams of a patient implanted with an extra-cardiovascular ICD system in a different implant configuration than the arrangement shown in  FIGS. 1A-1B . 
         FIG. 3  is a schematic diagram of an ICD according to one example. 
         FIG. 4  is a diagram of circuitry included in a sensing circuit of the ICD of  FIG. 3 . 
         FIG. 5  is a flow chart of a method for detecting cardiac event oversensing according to one example. 
         FIG. 6A  is a conceptual diagram of a method for detecting an alternating pattern of cardiac signal features according to one example. 
         FIG. 6B  is an illustration of a cardiac electrical signal segment stored over a time interval in response to an R-wave sensed event signal. 
         FIG. 7  is a flow chart of a method for determining a gross morphology metric for verifying a suspected true sensed cardiac event in an alternating signal feature pattern of cardiac electrical signal segments according to one example. 
         FIG. 8  is a flow chart of a method for verifying a suspected true sensed cardiac event in a cardiac electrical signal segment based on a gross morphology metric according to another example. 
         FIG. 9  is a flow chart of a method for detecting P-wave oversensing and rejecting a ventricular tachyarrhythmia detection in response to detecting P-wave oversensing according to one example. 
     
    
    
     DETAILED DESCRIPTION 
     In general, this disclosure describes techniques for detecting cardiac event oversensing by a medical device or system. In some examples, a medical device may be configured to sense R-waves attendant to ventricular depolarizations for use in controlling ventricular pacing and detecting ventricular tachyarrhythmias. A ventricular tachyarrhythmia may be detected in response to sensing a threshold number of R-waves occurring at time intervals that are less than a tachyarrhythmia detection interval. In some instances, atrial P-waves attendant to atrial depolarizations may be oversensed as R-waves. An oversensed P-wave may cause the medical device to count a ventricular tachyarrhythmia interval when a P-wave is falsely sensed as an R-wave within the tachyarrhythmia interval of a true sensed R-wave, potentially leading to a false ventricular tachyarrhythmia detection and inappropriate CV/DF shock or other therapy delivered by the cardiac medical device, such as anti-tachyarrhythmia pacing (ATP). By identifying oversensing of P-waves, a ventricular tachyarrhythmia detection due to the P-wave oversensing (PWOS) may be rejected. A medical device performing the techniques disclosed herein may detect cardiac event oversensing for the purposes of controlling a tachyarrhythmia therapy. 
     In some examples, the medical device system performing the techniques disclosed herein may be an extra-cardiovascular ICD system. As used herein, the term “extra-cardiovascular” refers to a position outside the blood vessels, heart, and pericardium surrounding the heart of a patient. Implantable electrodes carried by extra-cardiovascular leads may be positioned extra-thoracically (outside the ribcage and sternum) or intra-thoracically (beneath the ribcage or sternum) but generally not in intimate contact with myocardial tissue. The techniques disclosed herein for detecting P-wave oversensing may be applied to a cardiac electrical signal sensed using extra-cardiovascular electrodes. 
     The cardiac event oversensing detection techniques are described herein in conjunction with an ICD and an implantable extra-cardiovascular medical lead carrying sensing and therapy delivery electrodes, but aspects disclosed herein may be utilized in conjunction with other cardiac medical devices or systems. For example, the techniques for detecting P-wave oversensing as described in conjunction with the accompanying drawings may be implemented in any implantable or external medical device enabled for sensing intrinsic cardiac electrical events from cardiac signals received from a patient&#39;s heart via sensing electrodes, including implantable pacemakers, ICDs or cardiac monitors coupled to transvenous, pericardial or epicardial leads carrying sensing and therapy delivery electrodes; leadless pacemakers, ICDs or cardiac monitors having housing-based sensing electrodes; and external or wearable pacemakers, defibrillators, or cardiac monitors coupled to external, surface or skin electrodes. 
     Furthermore, while the techniques disclosed herein are described for the detection of oversensing of P-waves as R-waves, the disclosed techniques may be used for detecting oversensing of other cardiac events, e.g., oversensing of R-waves as P-waves potentially leading to false atrial tachyarrhythmia detection. Atrial P-waves may be sensed from a cardiac electrical signal, e.g., received by lead-based electrodes or a leadless pacemaker in the atrium, and oversensing of R-waves as P-waves may be detected. 
       FIGS. 1A and 1B  are conceptual diagrams of an extra-cardiovascular ICD system  10  configured to sense cardiac electrical events and deliver cardiac electrical stimulation therapies according to one example.  FIG. 1A  is a front view of ICD system  10  implanted within patient  12 .  FIG. 1B  is a side view of ICD system  10  implanted within patient  12 . ICD system  10  includes an ICD  14  connected to an extra-cardiovascular electrical stimulation and sensing lead  16 .  FIGS. 1A and 1B  are described in the context of an ICD system  10  capable of providing high voltage CV/DF shocks, and in some examples cardiac pacing pulses, in response to detecting a cardiac tachyarrhythmia. However, the techniques disclosed herein for detecting cardiac event oversensing may be implemented in other cardiac devices configured for sensing cardiac events and, for example, determining a cardiac event interval or rate, for use in determining the cardiac rate or rhythm and controlling a cardiac electrical stimulation therapy. 
     ICD  14  includes a housing  15  that forms a hermetic seal that protects internal components of ICD  14 . The housing  15  of ICD  14  may be formed of a conductive material, such as titanium or titanium alloy. The housing  15  may function as an electrode (sometimes referred to as a “can” electrode). Housing  15  may be used as an active can electrode for use in delivering CV/DF shocks or other high voltage pulses delivered using a high voltage therapy circuit. In other examples, housing  15  may be available for use in delivering unipolar, low voltage cardiac pacing pulses and/or for sensing cardiac electrical signals in combination with electrodes carried by lead  16 . In other instances, the housing  15  of ICD  14  may include a plurality of electrodes on an outer portion of the housing. The outer portion(s) of the housing  15  functioning as an electrode(s) may be coated with a material, such as titanium nitride, e.g., for reducing post-stimulation polarization artifact. 
     ICD  14  includes a connector assembly  17  (also referred to as a connector block or header) that includes electrical feedthroughs crossing housing  15  to provide electrical connections between conductors extending within the lead body  18  of lead  16  and electronic components included within the housing  15  of ICD  14 . As will be described in further detail herein, housing  15  may house one or more processors, memories, transceivers, cardiac electrical signal sensing circuitry, therapy delivery circuitry, power sources and other components for sensing cardiac electrical signals, detecting a heart rhythm, and controlling and delivering electrical stimulation pulses to treat an abnormal heart rhythm. 
     Elongated lead body  18  has a proximal end  27  that includes a lead connector (not shown) configured to be connected to ICD connector assembly  17  and a distal portion  25  that includes one or more electrodes. In the example illustrated in  FIGS. 1A and 1B , the distal portion  25  of lead body  18  includes defibrillation electrodes  24  and  26  and pace/sense electrodes  28  and  30 . In some cases, defibrillation electrodes  24  and  26  may together form a defibrillation electrode in that they may be configured to be activated concurrently. Alternatively, defibrillation electrodes  24  and  26  may form separate defibrillation electrodes in which case each of the electrodes  24  and  26  may be activated independently. 
     Electrodes  24  and  26  (and in some examples housing  15 ) are referred to herein as defibrillation electrodes because they are utilized, individually or collectively, for delivering high voltage stimulation therapy (e.g., cardioversion or defibrillation shocks). Electrodes  24  and  26  may be elongated coil electrodes and generally have a relatively high surface area for delivering high voltage electrical stimulation pulses compared to pacing and sensing electrodes  28  and  30 . However, electrodes  24  and  26  and housing  15  may also be utilized to provide pacing functionality, sensing functionality or both pacing and sensing functionality in addition to or instead of high voltage stimulation therapy. In this sense, the use of the term “defibrillation electrode” herein should not be considered as limiting the electrodes  24  and  26  for use in only high voltage cardioversion/defibrillation shock therapy applications. For example, either of electrodes  24  and  26  may be used as a sensing electrode in a sensing vector for sensing cardiac electrical signals and determining a need for an electrical stimulation therapy. 
     Electrodes  28  and  30  are relatively smaller surface area electrodes which are available for use in sensing electrode vectors for sensing cardiac electrical signals and may be used for delivering relatively low voltage pacing pulses in some configurations. Electrodes  28  and  30  are referred to as pace/sense electrodes because they are generally configured for use in low voltage applications, e.g., used as either a cathode or anode for delivery of pacing pulses and/or sensing of cardiac electrical signals, as opposed to delivering high voltage CV/DF shocks. In some instances, electrodes  28  and  30  may provide only pacing functionality, only sensing functionality or both. 
     ICD  14  may obtain cardiac electrical signals corresponding to electrical activity of heart  8  via a combination of sensing electrode vectors that include combinations of electrodes  24 ,  26 ,  28  and/or  30 . In some examples, housing  15  of ICD  14  is used in combination with one or more of electrodes  24 ,  26 ,  28  and/or  30  in a sensing electrode vector. Various sensing electrode vectors utilizing combinations of electrodes  24 ,  26 ,  28 , and  30  and housing  15  are described below for acquiring first and second cardiac electrical signals using respective first and second sensing electrode vectors that may be selected by sensing circuitry included in ICD  14 . 
     In the example illustrated in  FIGS. 1A and 1B , electrode  28  is located proximal to defibrillation electrode  24 , and electrode  30  is located between defibrillation electrodes  24  and  26 . One, two or more pace/sense electrodes may be carried by lead body  18 . For instance, a third pace/sense electrode may be located distal to defibrillation electrode  26  in some examples. Electrodes  28  and  30  are illustrated as ring electrodes; however, electrodes  28  and  30  may comprise any of a number of different types of electrodes, including ring electrodes, short coil electrodes, hemispherical electrodes, directional electrodes, segmented electrodes, or the like. Electrodes  28  and  30  may be positioned at other locations along lead body  18  and are not limited to the positions shown. In other examples, lead  16  may include fewer or more pace/sense electrodes and/or defibrillation electrodes than the example shown here. 
     In the example shown, lead  16  extends subcutaneously or submuscularly over the ribcage  32  medially from the connector assembly  27  of ICD  14  toward a center of the torso of patient  12 , e.g., toward xiphoid process  20  of patient  12 . At a location near xiphoid process  20 , lead  16  bends or turns and extends superiorly, subcutaneously or submuscularly, over the ribcage and/or sternum, substantially parallel to sternum  22 . Although illustrated in  FIG. 1A  as being offset laterally from and extending substantially parallel to sternum  22 , the distal portion  25  of lead  16  may be implanted at other locations, such as over sternum  22 , offset to the right or left of sternum  22 , angled laterally from sternum  22  toward the left or the right, or the like. Alternatively, lead  16  may be placed along other subcutaneous or submuscular paths. The path of extra-cardiovascular lead  16  may depend on the location of ICD  14 , the arrangement and position of electrodes carried by the lead body  18 , and/or other factors. 
     Electrical conductors (not illustrated) extend through one or more lumens of the elongated lead body  18  of lead  16  from the lead connector at the proximal lead end  27  to electrodes  24 ,  26 ,  28 , and  30  located along the distal portion  25  of the lead body  18 . The elongated electrical conductors contained within the lead body  18 , which may be separate respective insulated conductors within the lead body  18 , are each electrically coupled with respective defibrillation electrodes  24  and  26  and pace/sense electrodes  28  and  30 . The respective conductors electrically couple the electrodes  24 ,  26 ,  28 , and  30  to circuitry, such as a therapy delivery circuit and/or a sensing circuit, of ICD  14  via connections in the connector assembly  17 , including associated electrical feedthroughs crossing housing  15 . The electrical conductors transmit therapy from a therapy delivery circuit within ICD  14  to one or more of defibrillation electrodes  24  and  26  and/or pace/sense electrodes  28  and  30  and transmit sensed electrical signals produced by the patient&#39;s heart  8  from one or more of defibrillation electrodes  24  and  26  and/or pace/sense electrodes  28  and  30  to the sensing circuit within ICD  14 . 
     The lead body  18  of lead  16  may be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and/or other appropriate materials, and shaped to form one or more lumens within which the one or more conductors extend. Lead body  18  may be tubular or cylindrical in shape. In other examples, the distal portion  25  (or all of) the elongated lead body  18  may have a flat, ribbon or paddle shape. Lead body  18  may be formed having a preformed distal portion  25  that is generally straight, curving, bending, serpentine, undulating or zig-zagging. 
     In the example shown, lead body  18  includes a curving distal portion  25  having two “C” shaped curves, which together may resemble the Greek letter epsilon, “c.” Defibrillation electrodes  24  and  26  are each carried by one of the two respective C-shaped portions of the lead body distal portion  25 . The two C-shaped curves are seen to extend or curve in the same direction away from a central axis of lead body  18 , along which pace/sense electrodes  28  and  30  are positioned. Pace/sense electrodes  28  and  30  may, in some instances, be approximately aligned with the central axis of the straight, proximal portion of lead body  18  such that mid-points of defibrillation electrodes  24  and  26  are laterally offset from pace/sense electrodes  28  and  30 . 
     Other examples of extra-cardiovascular leads including one or more defibrillation electrodes and one or more pacing and sensing electrodes carried by curving, serpentine, undulating or zig-zagging distal portion of the lead body  18  that may be implemented with the techniques described herein are generally disclosed in pending U.S. Pat. Publication No. 2016/0158567 (Marshall, et al.), incorporated herein by reference in its entirety. The techniques disclosed herein are not limited to any particular lead body design, however. In other examples, lead body  18  is a flexible elongated lead body without any pre-formed shape, bends or curves. 
     ICD  14  analyzes the cardiac electrical signals received from one or more sensing electrode vectors to monitor for abnormal rhythms, such as bradycardia, ventricular tachycardia (VT) or ventricular fibrillation (VF). ICD  14  may analyze the heart rate and morphology of the cardiac electrical signals to monitor for tachyarrhythmia in accordance with any of a number of tachyarrhythmia detection techniques. 
     ICD  14  generates and delivers electrical stimulation therapy in response to detecting a tachyarrhythmia (e.g., VT or VF) using a therapy delivery electrode vector which may be selected from any of the available electrodes  24 ,  26 ,  28   30  and/or housing  15 . ICD  14  may deliver anti-tachycardia pacing (ATP) in response to VT detection and in some cases may deliver ATP prior to a CV/DF shock or during high voltage capacitor charging in an attempt to avert the need for delivering a CV/DF shock. If ATP does not successfully terminate VT or when VF is detected, ICD  14  may deliver one or more CV/DF shocks via one or both of defibrillation electrodes  24  and  26  and/or housing  15 . ICD  14  may deliver the CV/DF shocks using electrodes  24  and  26  individually or together as a cathode (or anode) and with the housing  15  as an anode (or cathode). ICD  14  may generate and deliver other types of electrical stimulation pulses such as post-shock pacing pulses or bradycardia pacing pulses using a pacing electrode vector that includes one or more of the electrodes  24 ,  26 ,  28 , and  30  and the housing  15  of ICD  14 . 
     ICD  14  is shown implanted subcutaneously on the left side of patient  12  along the ribcage  32 . ICD  14  may, in some instances, be implanted between the left posterior axillary line and the left anterior axillary line of patient  12 . ICD  14  may, however, be implanted at other subcutaneous or submuscular locations in patient  12 . For example, ICD  14  may be implanted in a subcutaneous pocket in the pectoral region. In this case, lead  16  may extend subcutaneously or submuscularly from ICD  14  toward the manubrium of sternum  22  and bend or turn and extend inferiorly from the manubrium to the desired location subcutaneously or submuscularly. In yet another example, ICD  14  may be placed abdominally. Lead  16  may be implanted in other extra-cardiovascular locations as well. For instance, as described with respect to  FIGS. 2A-2C , the distal portion  25  of lead  16  may be implanted underneath the sternum/ribcage in the substernal space.  FIGS. 1A and 1B  are illustrative in nature and should not be considered limiting of the practice of the techniques disclosed herein. 
     An external device  40  is shown in telemetric communication with ICD  14  by a communication link  42 . External device  40  may include a processor  52 , memory  53 , display  54 , user interface  56  and telemetry unit  58 . Processor  52  controls external device operations and processes data and signals received from ICD  14 . Display  54 , which may include a graphical user interface, displays data and other information to a user for reviewing ICD operation and programmed parameters as well as cardiac electrical signals retrieved from ICD  14 . 
     User interface  56  may include a mouse, touch screen, key pad or the like to enable a user to interact with external device  40  to initiate a telemetry session with ICD  14  for retrieving data from and/or transmitting data to ICD  14 , including programmable parameters for controlling cardiac event sensing and therapy delivery. Telemetry unit  58  includes a transceiver and antenna configured for bidirectional communication with a telemetry circuit included in ICD  14  and is configured to operate in conjunction with processor  52  for sending and receiving data relating to ICD functions via 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 or communication frequency bandwidth or communication protocols. Data stored or acquired by ICD  14 , including physiological signals or associated data derived therefrom, results of device diagnostics, and histories of detected rhythm episodes and delivered therapies, may be retrieved from ICD  14  by external device  40  following an interrogation command. 
     External device  40  may be embodied as 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 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 signal sensing parameters, cardiac rhythm detection parameters and therapy control parameters used by ICD  14 . At least some control parameters used in detecting cardiac event oversensing according to techniques disclosed herein may be programmed into ICD  14  using external device  40  in some examples. 
       FIGS. 2A-2C  are conceptual diagrams of patient  12  implanted with extra-cardiovascular ICD system  10  in a different implant configuration than the arrangement shown in  FIGS. 1A-1B .  FIG. 2A  is a front view of patient  12  implanted with ICD system  10 .  FIG. 2B  is a side view of patient  12  implanted with ICD system  10 .  FIG. 2C  is a transverse view of patient  12  implanted with ICD system  10 . In this arrangement, extra-cardiovascular lead  16  of system  10  is implanted at least partially underneath sternum  22  of patient  12 . Lead  16  extends subcutaneously or submuscularly from ICD  14  toward xiphoid process  20  and at a location near xiphoid process  20  bends or turns and extends superiorly within anterior mediastinum  36  in a substernal position. 
     Anterior mediastinum  36  may be viewed as being bounded laterally by pleurae  39 , posteriorly by pericardium  38 , and anteriorly by sternum  22  (see  FIG. 2C ). The distal portion  25  of lead  16  may extend along the posterior side of sternum  22  substantially within the loose connective tissue and/or substernal musculature of anterior mediastinum  36 . A lead implanted such that the distal portion  25  is substantially within anterior mediastinum  36 , may be referred to as a “substernal lead.” 
     In the example illustrated in  FIGS. 2A-2C , lead  16  is located substantially centered under sternum  22 . In other instances, however, lead  16  may be implanted such that it is offset laterally from the center of sternum  22 . In some instances, lead  16  may extend laterally such that distal portion  25  of lead  16  is underneath/below the ribcage  32  in addition to or instead of sternum  22 . In other examples, the distal portion  25  of lead  16  may be implanted in other extra-cardiovascular, intra-thoracic locations, including the pleural cavity or around the perimeter of and adjacent to the pericardium  38  of heart  8 . 
       FIG. 3  is a schematic diagram of ICD  14  according to one example. The electronic circuitry enclosed within housing  15  (shown schematically as an electrode in  FIG. 3 ) includes software, firmware and hardware that cooperatively monitor cardiac electrical signals, determine when an electrical stimulation therapy is necessary, and deliver therapy as needed according to programmed therapy delivery algorithms and control parameters. ICD  14  may be coupled to an extra-cardiovascular lead, such as lead  16  carrying extra-cardiovascular electrodes  24 ,  26 ,  28 , and  30 , for delivering electrical stimulation pulses to the patient&#39;s heart and for sensing cardiac electrical signals. 
     ICD  14  includes a control circuit  80 , memory  82 , therapy delivery circuit  84 , cardiac electrical signal sensing circuit  86 , and telemetry circuit  88 . A power source  98  provides power to the circuitry of ICD  14 , including each of the components  80 ,  82 ,  84 ,  86 , and  88  as needed. Power source  98  may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source  98  and each of the other components  80 ,  82 ,  84 ,  86  and  88  are to be understood from the general block diagram of  FIG. 3 , but are not shown for the sake of clarity. For example, power source  98  may be coupled to one or more charging circuits included in therapy delivery circuit  84  for charging holding capacitors included in therapy delivery circuit  84  that are discharged at appropriate times under the control of control circuit  80  for producing electrical pulses according to a therapy protocol. Power source  98  is also coupled to components of cardiac electrical signal sensing circuit  86 , such as sense amplifiers, analog-to-digital converters, switching circuitry, etc. as needed. 
     The circuits shown in  FIG. 3  represent functionality included in ICD  14  and may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to ICD  14  herein. Functionality associated with one or more circuits may be performed by separate hardware, firmware or software components, or integrated within common hardware, firmware or software components. For example, cardiac event sensing and detection of oversensing cardiac events may be performed cooperatively by sensing circuit  86  and control circuit  80  and may include operations implemented in a processor or other signal processing circuitry included in control circuit  80  executing instructions stored in memory  82  and control signals such as blanking and timing intervals and sensing threshold amplitude signals sent from control circuit  80  to sensing circuit  86 . 
     The various circuits of ICD  14  may include 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 or combinations of 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 ICD and by the particular detection and therapy delivery methodologies employed by the ICD. Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modern implantable cardiac device system, given the disclosure herein, is within the abilities of one of skill in the art. 
     Memory  82  may include any volatile, non-volatile, magnetic, or electrical non-transitory computer readable storage media, such as 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 one or more processing circuits, cause control circuit  80  and/or other ICD components to perform various functions attributed to ICD  14  or those ICD components. The non-transitory computer-readable media storing the instructions may include any of the media listed above. 
     Control circuit  80  communicates, e.g., via a data bus, with therapy delivery circuit  84  and sensing circuit  86  for sensing cardiac electrical activity, detecting cardiac rhythms, and controlling delivery of cardiac electrical stimulation therapies in response to sensed cardiac signals. Therapy delivery circuit  84  and sensing circuit  86  are electrically coupled to electrodes  24 ,  26 ,  28 ,  30  carried by lead  16  and the housing  15 , which may function as a common or ground electrode or as an active can electrode for delivering CV/DF shock pulses or cardiac pacing pulses. 
     Sensing circuit  86  may be selectively coupled to electrodes  28 ,  30  and/or housing  15  in order to monitor electrical activity of the patient&#39;s heart. Sensing circuit  86  may additionally be selectively coupled to defibrillation electrodes  24  and/or  26  for use in a sensing electrode vector together or in combination with one or more of electrodes  28 ,  30  and/or housing  15 . Sensing circuit  86  may be enabled to selectively receive cardiac electrical signals from at least two sensing electrode vectors from the available electrodes  24 ,  26 ,  28 ,  30 , and housing  15 . At least two cardiac electrical signals from two different sensing electrode vectors may be received simultaneously by sensing circuit  86  in some examples. Sensing circuit  86  may monitor one or both of the cardiac electrical signals simultaneously for sensing cardiac electrical events and/or producing digitized cardiac signal waveforms for analysis by control circuit  80 . For example, sensing circuit  86  may include switching circuitry for selecting which of electrodes  24 ,  26 ,  28 ,  30 , and housing  15  are coupled to a first sensing channel  83  and which electrodes are coupled to a second sensing channel  85  of sensing circuit  86 . 
     Each sensing channel  83  and  85  may be configured to amplify, filter and digitize the cardiac electrical signal received from selected electrodes coupled to the respective sensing channel to improve the signal quality for detecting cardiac electrical events, such as R-waves or performing other signal analysis. The cardiac event detection circuitry within sensing circuit  86  may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), timers or other analog or digital components as described further in conjunction with  FIG. 4 . A cardiac event sensing threshold may be automatically adjusted by sensing circuit  86  under the control of control circuit  80 , based on timing intervals and sensing threshold values determined by control circuit  80 , stored in memory  82 , and/or controlled by hardware, firmware and/or software of control circuit  80  and/or sensing circuit  86 . 
     Upon detecting a cardiac event based on a sensing threshold crossing, first sensing channel  83  may produce a sensed event signal, such as an R-wave sensed event signal, that is passed to control circuit  80 . The sensed event signal is used by control circuit  80  to trigger storage of a time segment of a cardiac electrical signal for post-processing and analysis for detecting cardiac event oversensing as described below, e.g., in conjunction with  FIGS. 5 through 9 . In some examples, sensing circuit  86  senses at least one cardiac electrical signal received by a sensing electrode vector selected from the available electrodes, e.g., electrodes  24 ,  26 ,  28 ,  30  and housing  15 , for detecting R-waves and buffering multiple cardiac electrical signal segments, where each cardiac electrical signal segment corresponds to a detected R-wave, for processing and analysis for detecting P-wave oversensing. A single cardiac electrical signal sensed by first sensing channel  83  may be used for both R-wave detection and analysis of cardiac electrical signal segments for PWOS detection. In other examples, R-waves are detected from the first cardiac electrical signal sensed by the first sensing channel  83  and segments of a second cardiac electrical signal sensed by the second sensing channel  85  may be buffered, with each segment corresponding to an R-wave sensed from the first cardiac electrical signal. PWOS detection may be based on the analysis of the second cardiac electrical signal segments. The second cardiac electrical signal may be received via a different sensing electrode pair coupled to the second sensing channel  85  than the sensing electrode pair coupled to the first sensing channel  83  for sensing the first cardiac electrical signal, and/or the second cardiac electrical signal may be received by the same sensing electrode pair but processed differently, e.g., filtered differently, by the second sensing channel  85  to produce a second cardiac electrical signal sensed by sensing circuit  86  different than the first cardiac electrical signal. 
     Memory  82  may be configured to store a predetermined number of cardiac electrical signal segments in a circulating buffer under the control of control circuit  80 , e. g., at least one, two, three or other number of cardiac electrical signal segments. Each segment may be written to memory  82  over a time interval extending before and after an R-wave sensed event signal produced by the first sensing channel  83 . Control circuit  80  may access stored cardiac electrical signal segments when confirmation of R-waves sensed by the first sensing channel  83  is required based on the detection of a predetermined number of tachyarrhythmia intervals, which may precede tachyarrhythmia detection. 
     The R-wave sensed event signals are also used by control circuit  80  for determining RR intervals (RRIs) for detecting tachyarrhythmia and determining a need for therapy. An RRI is the time interval between consecutively sensed R-waves and may be determined between consecutive R-wave sensed event signals received from sensing circuit  86 . For example, control circuit  80  may include a timing circuit  90  for determining RRIs between consecutive R-wave sensed event signals received from sensing circuit  86  and for controlling various timers and/or counters used to control the timing of therapy delivery by therapy delivery circuit  84 . Timing circuit  90  may additionally set time windows such as morphology template windows, morphology analysis windows or perform other timing related functions of ICD  14  including synchronizing cardioversion shocks or other therapies delivered by therapy delivery circuit  84  with sensed cardiac events. 
     Control circuit  80  is also shown to include a tachyarrhythmia detector  92  configured to analyze signals received from sensing circuit  86  for detecting tachyarrhythmia. Tachyarrhythmia detector  92  may detect tachyarrhythmia based on cardiac events detected from a sensed cardiac electrical signal meeting tachyarrhythmia criteria, such as a threshold number of detected cardiac events occurring at a tachyarrhythmia interval. Tachyarrhythmia detector  92  may be implemented in control circuit  80  as hardware, software and/or firmware that processes and analyzes signals received from sensing circuit  86  for detecting VT and/or VF. In some examples, the timing of R-wave sense event signals received from sensing circuit  86  is used by timing circuit  90  to determine RRIs between sensed event signals. Tachyarrhythmia detector  92  may include comparators and counters for counting RRIs determined by timing circuit  90  that fall into various rate detection zones for determining a ventricular rate or performing other rate- or interval-based assessment of R-wave sensed event signals for detecting and discriminating VT and VF. 
     For example, tachyarrhythmia detector  92  may compare the RRIs determined by timing circuit  90  to one or more tachyarrhythmia detection interval zones, such as a tachycardia detection interval zone and a fibrillation detection interval zone. RRIs falling into a detection interval zone are counted by a respective VT interval counter or VF interval counter and in some cases in a combined VT/VF interval counter included in tachyarrhythmia detector  92 . The VF detection interval threshold may be set to 300 to 350 milliseconds (ms), as examples. For instance, if the VF detection interval is set to 320 ms, RRIs that are less than 320 ms are counted by the VF interval counter. When VT detection is enabled, the VT detection interval may be programmed to be in the range of 350 to 420 ms, or 400 ms as an example. In order to detect VT or VF, the respective VT or VF interval counter is required to reach a threshold number of intervals to detect (NID) the tachyarrhythmia. 
     As an example, the NID to detect VT may require that the VT interval counter reaches 32 VT intervals counted out of the most recent 32 consecutive RRIs. The NID to detect VF may be programmed to 18 VF intervals out of the most recent 24 consecutive RRIs or 30 VF intervals out 40 consecutive RRIs, as examples. When a VT or VF interval counter reaches a detection threshold, a ventricular tachyarrhythmia may be detected by tachyarrhythmia detector  92 . The NID may be programmable and range from as low as 12 to as high as 40, with no limitation intended. VT or VF intervals may be detected consecutively or non-consecutively out of the specified number of most recent RRIs. In some cases, a combined VT/VF interval counter may count both VT and VF intervals and detect a tachyarrhythmia episode based on the fastest intervals detected when a specified NID is reached. 
     Tachyarrhythmia detector  92  may be configured to perform other signal analysis for determining if other detection criteria are satisfied before detecting VT or VF, such as R-wave morphology criteria, onset criteria, and noise and oversensing rejection criteria. Examples of parameters that may be determined from cardiac electrical signals received from sensing circuit  86  for detecting cardiac event oversensing that may cause withholding of a VT or VF detection are described in conjunction with  FIGS. 5-9 . 
     To support these additional analyses, sensing circuit  86  may pass a digitized electrocardiogram (ECG) signal to control circuit  80  for morphology analysis performed by tachyarrhythmia detector  92  for detecting and discriminating heart rhythms. A cardiac electrical signal from the selected sensing vector, e.g., from first sensing channel  83  and/or the second sensing channel  85 , may be passed through a filter and amplifier, provided to a multiplexer and thereafter converted to a multi-bit digital signal by an analog-to-digital converter, all included in sensing circuit  86 , for storage in memory  82 . Memory  82  may include one or more circulating buffers to temporarily store digital cardiac electrical signal segments for analysis performed by control circuit  80 . Control circuit  80  may be a microprocessor-based controller that employs digital signal analysis techniques to characterize the digitized signals stored in memory  82  to recognize and classify the patient&#39;s heart rhythm employing any of numerous signal processing methodologies for analyzing cardiac signals and cardiac event waveforms, e.g., R-waves. As described below, processing and analysis of digitized signals may include determining signal features for detecting patterns of oversensing and verifying that a tachyarrhythmia morphology is not present in cardiac electrical signal segments presenting a detected pattern of oversensing. When a tachyarrhythmia morphology is not present, and an alternating pattern of signal features is detected, a tachyarrhythmia detection based on RRIs may be withheld to inhibit a tachyarrhythmia therapy. 
     Therapy delivery circuit  84  includes charging circuitry, one or more charge storage devices such as one or more high voltage capacitors and/or low voltage capacitors, and switching circuitry that controls when the capacitor(s) are discharged across a selected pacing electrode vector or CV/DF shock vector. Charging of capacitors to a programmed pulse amplitude and discharging of the capacitors for a programmed pulse width may be performed by therapy delivery circuit  84  according to control signals received from control circuit  80 . Control circuit  80  may include various timers or counters that control when cardiac pacing pulses are delivered. For example, timing circuit  90  may include programmable digital counters set by a microprocessor of the control circuit  80  for controlling the basic pacing time intervals associated with various pacing modes or ATP sequences delivered by ICD  14 . The microprocessor of control circuit  80  may also set the amplitude, pulse width, polarity or other characteristics of the cardiac pacing pulses, which may be based on programmed values stored in memory  82 . 
     In response to detecting VT or VF, control circuit  80  may schedule a therapy and control therapy delivery circuit  84  to generate and deliver the therapy, such as ATP and/or CV/DF therapy. Therapy can be generated by initiating charging of high voltage capacitors via a charging circuit, both included in therapy delivery circuit  84 . Charging is controlled by control circuit  80  which monitors the voltage on the high voltage capacitors, which is passed to control circuit  80  via a charging control line. When the voltage reaches a predetermined value set by control circuit  80 , a logic signal is generated on a capacitor full line and passed to therapy delivery circuit  84 , terminating charging. A CV/DF pulse is delivered to the heart under the control of the timing circuit  90  by an output circuit of therapy delivery circuit  84  via a control bus. The output circuit may include an output capacitor through which the charged high voltage capacitor is discharged via switching circuitry, e. g., an H-bridge, which determines the electrodes used for delivering the cardioversion or defibrillation pulse and the pulse wave shape. 
     In some examples, the high voltage therapy circuit configured to deliver CV/DF shock pulses can be controlled by control circuit  80  to deliver pacing pulses, e.g., for delivering ATP, post shock pacing pulses or ventricular pacing pulses during atrio-ventricular conduction block or bradycardia. In other examples, therapy delivery circuit  84  may include a low voltage therapy circuit for generating and delivering pacing pulses for a variety of pacing needs. 
     It is recognized that the methods disclosed herein for detecting cardiac event oversensing may be implemented in a medical device that is used for monitoring cardiac electrical signals by sensing circuit  86  and control circuit  80  without having therapy delivery capabilities or in a pacemaker that monitors cardiac electrical signals and delivers cardiac pacing therapies by therapy delivery circuit  84 , without high voltage therapy capabilities, such as cardioversion/defibrillation shock capabilities. 
     Control parameters utilized by control circuit  80  for sensing cardiac events and controlling therapy delivery may be programmed into memory  82  via telemetry circuit  88 . Telemetry circuit  88  includes a transceiver and antenna for communicating with external device  40  (shown in  FIG. 1A ) using RF communication or other communication protocols as described above. Under the control of control circuit  80 , telemetry circuit  88  may receive downlink telemetry from and send uplink telemetry to external device  40 . 
       FIG. 4  is a diagram of circuitry included in sensing circuit  86  having first sensing channel  83  and second sensing channel  85  according to one example. First sensing channel  83  may be selectively coupled via switching circuitry included in sensing circuit  86  to a first sensing electrode vector including at least one electrode carried by extra-cardiovascular lead  16  for receiving a first cardiac electrical signal. In some examples, first sensing channel  83  may be coupled to a sensing electrode vector that is a short bipole, having a relatively shorter inter-electrode distance or spacing than the second electrode vector coupled to second sensing channel  85 . First sensing channel  83  may be coupled to a sensing electrode vector that is approximately vertical (when the patient is in an upright position) or approximately aligned with the cardiac axis to increase the likelihood of a relatively high R-wave signal amplitude relative to P-wave signal amplitude. In one example, the first sensing electrode vector may include pace/sense electrodes  28  and  30 . In other examples, the first sensing electrode vector coupled to sensing channel  83  may include a defibrillation electrode  24  and/or  26 , e.g., a sensing electrode vector between pace/sense electrode  28  and defibrillation electrode  24  or between pace/sense electrode  30  and either of defibrillation electrodes  24  or  26 . In still other examples, the first sensing electrode vector may be between defibrillation electrodes  24  and  26 . 
     Sensing circuit  86  includes second sensing channel  85  for sensing a second cardiac electrical signal in some examples. For instance, second sensing channel  85  may receive a raw cardiac electrical signal from a second sensing electrode vector, for example from a vector that includes one electrode  24 ,  26 ,  28  or  30  carried by lead  16  paired with housing  15 . Second sensing channel  85  may be selectively coupled to other sensing electrode vectors, which may form a relatively longer bipole having an inter-electrode distance or spacing that is greater than the sensing electrode vector coupled to first sensing channel  83  in some examples. The second sensing electrode vector may be, but not necessarily, approximately orthogonal to the first channel sensing electrode vector in some cases. For instance, defibrillation electrode  26  and housing  15  may be coupled to second sensing channel  85  to provide the second cardiac electrical signal. As described below, the second cardiac electrical signal received by second sensing channel  85  via a long bipole may be used by control circuit  80  for analysis and detection of P-wave oversensing (when atrial P-waves are falsely sensed as R-waves by the first sensing channel  83 ). The long bipole coupled to second sensing channel  85  may provide a relatively far-field or more global cardiac signal compared to the relatively shorter bipole coupled to the first sensing channel. In the relatively more global signal, the amplitude of P-waves may be relatively higher or lower (depending on the location of the electrodes relative to atrium) than in the more near-field signal received by the first sensing channel  83  and used for sensing R-waves for determining RRIs. In other examples, any vector selected from the available electrodes, e.g., electrodes  24 ,  26 ,  28 ,  30  and/or housing  15 , may be included in a sensing electrode vector coupled to second sensing channel  85 . The sensing electrode vectors coupled to first sensing channel  83  and second sensing channel  85  may be different sensing electrode vectors, which may have no common electrodes or only one common electrode but not both. 
     In other examples, however, the sensing electrode vectors coupled to the first sensing channel  83  and the second sensing channel  85  may be the same sensing electrode vectors. The two sensing channels  83  and  85  may include different filters or other signal processing circuitry such that two different signals are sensed by the respective sensing channels and different analyses may be performed on the two signals. For example, the first sensing channel  83  may sense a first cardiac electrical signal by filtering and processing the received cardiac electrical signal for detecting R-waves in response to an R-wave sensing threshold crossing for determining RRIs. The second sensing channel  85  may sense a second cardiac electrical signal different than the first by filtering and processing the received cardiac electrical signal for passing signal segments to control circuit  80  for determination and analysis of signal waveform morphology and specific morphological features for detecting alternating signal features and detecting segments having a tachyarrhythmia morphology. The first sensing channel  83  may apply relatively narrower band pass filtering, and the second sensing channel  85  may apply relatively wider band pass filtering and notch filtering to provide two different sensed cardiac electrical signals. 
     In the illustrative example shown in  FIG. 4 , the electrical signals developed across the first sensing electrode vector, e.g., electrodes  28  and  30 , are received by first sensing channel  83  and electrical signals developed across the second sensing electrode vector, e.g., electrodes  26  and housing  15 , are received by second sensing channel  85 . The cardiac electrical signals are provided as differential input signals to the pre-filter and pre-amplifier  62  or  72 , respectively, of first sensing channel  83  and second sensing channel  85 . Non-physiological high frequency and DC signals may be filtered by a low pass or bandpass filter included in each of pre-filter and pre-amplifiers  62  and  72 , and high voltage signals may be removed by protection diodes included in pre-filter and pre-amplifiers  62  and  72 . Pre-filter and pre-amplifiers  62  and  72  may amplify the pre-filtered signal by a gain of between 10 and 100, and in one example a gain of 17.5, and may convert the differential signal to a single-ended output signal passed to analog-to-digital converter (ADC)  63  in first sensing channel  83  and to ADC  73  in second sensing channel  85 . Pre-filters and amplifiers  62  and  72  may provide anti-alias filtering and noise reduction prior to digitization. 
     ADC  63  and ADC  73 , respectively, convert the first cardiac electrical signal from an analog signal to a first digital bit stream and the second cardiac electrical signal to a second digital bit stream. In one example, ADC  63  and ADC  73  may be sigma-delta converters (SDC), but other types of ADCs may be used. In some examples, the outputs of ADC  63  and ADC  73  may be provided to decimators (not shown), which function as digital low-pass filters that increase the resolution and reduce the sampling rate of the respective first and second cardiac electrical signals. 
     The digital outputs of ADC  63  and ADC  73  are each passed to respective filters  64  and  74 , which may be digital bandpass filters. The bandpass filters  64  and  74  may have the same or different bandpass frequencies. For example, filter  64  may have a bandpass of approximately 13 Hz to 39 Hz for passing cardiac electrical signals such as R-waves typically occurring in this frequency range. Filter  74  of the second sensing channel  85  may have a bandpass of approximately 2.5 to 100 Hz. In some examples, second sensing channel  85  may further include a notch filter  76  to filter 60 Hz or 50 Hz noise signals. 
     The bandpass filtered signal in first sensing channel  83  is passed from filter  64  to rectifier  65  to produce a filtered, rectified signal. First sensing channel  83  includes an R-wave detector  66  for sensing cardiac events in response to the first cardiac electrical signal crossing an R-wave sensing threshold. R-wave detector  66  may include an auto-adjusting sense amplifier, comparator and/or other detection circuitry that compares the filtered and rectified cardiac electrical signal to an R-wave sensing threshold in real time and produces an R-wave sensed event signal  68  when the cardiac electrical signal crosses the R-wave sensing threshold outside of a post-sense blanking interval. The R-wave sensing threshold may be a multi-level sensing threshold as disclosed in commonly assigned U.S. Pat. No. 10,252,071 (Cao, et al.), incorporated herein by reference in its entirety. Briefly, the multi-level sensing threshold may have a starting sensing threshold value held for a time interval, which may be equal to a tachycardia detection interval or expected R-wave to T-wave interval, then drops to a second sensing threshold value held until a drop time interval expires, which may be 1 to 2 seconds long. The sensing threshold drops to a minimum sensing threshold, which may correspond to a programmed sensitivity sometimes referred to as the “sensing floor,” after the drop time interval. In other examples, the R-wave sensing threshold used by R-wave detector  66  may be set to a starting value based on the peak amplitude determined during the most recent post-sense blanking interval and decay linearly or exponentially over time until reaching a minimum sensing threshold. The techniques described herein are not limited to a specific behavior of the sensing threshold or specific R-wave sensing techniques. Instead, other decaying, step-wise adjusted or other automatically adjusted sensing thresholds may be utilized. 
     The notch-filtered, digital cardiac electrical signal  78  from second sensing channel  85  may be passed to memory  82  for buffering a segment of the second cardiac electrical signal  78  in response to an R-wave sensed event signals  68  produced by the first sensing channel  83 . In some examples, the buffered segment of the second cardiac electrical signal  78  is rectified by rectifier  75  before being stored in memory  82 . In some cases, both the filtered, non-rectified signal  78  and the rectified signal  79  are passed to control circuit  80  and/or memory  82  for use in determining features of multiple segments of the second cardiac electrical signal, where each segment extends over a time interval that encompasses the time point of an R-wave sensed event signal produced by the first sensing channel  83 . 
     Control circuit  80  is configured to detect tachyarrhythmia based on cardiac events detected from at least one cardiac electrical signal sensed by sensing circuit  86 . For example, control circuit  80  may be configured to detect tachyarrhythmia when a threshold number of detected cardiac events each occur at a tachyarrhythmia interval. Control circuit  80  may buffer segments of a sensed cardiac electrical signal in memory  82  and retrieve stored signal segments from memory  82  for analysis when a lower threshold number of tachyarrhythmia intervals have been detected, before the detection threshold is reached. In some examples, the RRIs for detecting tachyarrhythmia intervals are determined from the first cardiac electrical signal sensed by first sensing channel  83 , and cardiac electrical signal segments are buffered from the second cardiac electrical signal received by control circuit  80  from second sensing channel  85  for P-wave oversensing analysis when the lower threshold number of tachyarrhythmia intervals is detected. Analysis of the second cardiac electrical signal segments may be performed for use in detecting P-wave oversensing as described below in conjunction with  FIGS. 5-9 . In other examples, a single cardiac electrical signal sensed by sensing circuit  86  is used to both determine RRIs for detecting tachyarrhythmia intervals and buffer cardiac electrical signal segments. The buffered cardiac electrical signal segments are analyzed for detecting evidence of cardiac event oversensing. 
     For instance, control circuit  80  may be configured to determine a signal feature from each of multiple, consecutive second cardiac electrical signal segments for detecting an alternating pattern of the signal feature as evidence of PWOS. When evidence of PWOS is detected based on the alternating pattern of the determined signal feature, additional analysis may be performed on at least one of the second cardiac electrical signal segments of the alternating pattern to detect a tachyarrhythmia morphology present in the segment. Time segments of the notch-filtered, rectified signal  79  received from second sensing channel  85  may be used to detect a tachyarrhythmia morphology. In some examples, as described below, at least one segment of the second cardiac electrical signal that corresponds to a suspected true R-wave, based on the detected alternating pattern of the signal feature, is identified. When a tachyarrhythmia morphology is not detected in second cardiac electrical signal segment(s) identified as a suspected true R-wave in an alternating pattern, evidence of PWOS is detected. A threshold number of PWOS evidence detections may cause control circuit  80  to withhold a tachyarrhythmia detection and therapy delivery circuit  84  to withhold a tachyarrhythmia therapy. 
     The configuration of sensing channels  83  and  85  as shown in  FIG. 4  is illustrative in nature and should not be considered limiting of the techniques described herein. The sensing channels  83  and  85  of sensing circuit  86  may include more or fewer components than illustrated and described in  FIG. 4  and some components may be shared between sensing channels  83  and  85 . For example, one or more of pre-filter and pre-amplifiers  62 / 72 , ADC  63 / 73 , and/or filters  64 / 74  may be shared components between sensing channels  83  and  85  with a single, sensed signal output split to two sensing channels for subsequent processing and analysis. Sensing circuit  86  and control circuit  80  include circuitry configured to perform the functionality attributed to ICD  14  in detecting cardiac event oversensing and rejecting or withholding tachyarrhythmia detection as disclosed herein. 
       FIG. 5  is a flow chart  100  of a method for detecting cardiac event oversensing according to one example. In various examples presented herein, the cardiac event oversensing being detected is PWOS, when R-waves are being intentionally sensed by the first sensing channel  83  based on an R-wave sensing threshold crossing. PWOS occurs when a P-wave of the cardiac electrical signal crosses the R-wave sensing threshold, causing the first sensing channel  83  to produce a false R-wave sensed event signal that is passed to control circuit  80 . It is to be understood, however, that the techniques for detecting cardiac event oversensing may be applied for detecting R-wave oversensing when P-waves are being intentionally sensed by a sensing circuit of a medical device. In this case, R-wave oversensing occurs when an R-wave of the cardiac electrical signal crosses the P-wave sensing threshold, causing the sensing circuit to produce a false P-wave sensed event signal. 
     At block  102 , sensing circuit  86  senses cardiac events based on cardiac event sensing threshold crossings by a first cardiac electrical signal. The cardiac electrical signal may be a relatively near field signal for increasing the likelihood of sensing cardiac events in a desired heart chamber, e.g., ventricular or atrial, without oversensing a cardiac event in the adjacent heart chamber, e.g., atrial or ventricular. In one example, cardiac events sensed at block  102  are intended to be R-waves sensed based on an R-wave sensing threshold crossing detected by the first sensing channel  83  of sensing circuit  86  as described above. 
     At block  104 , control circuit  80  may determine one or more signal features from a cardiac electrical signal segment corresponding in time to a sensed event signal produced by the first sensing channel  83 . The signal features may be determined from the second cardiac electrical signal sensed via a sensing electrode vector that is different than the sensing electrode vector used to sense the first cardiac electrical signal, from which the cardiac events are being sensed. The second cardiac electrical signal may be a relatively far field or more global cardiac signal compared to the first cardiac electrical signal and/or filtered or processed differently to enhance the specificity and sensitivity of the morphological analysis for detecting alternating signal features and tachyarrhythmia morphology signal segments. For example, the second cardiac electrical signal may be sensed using a sensing electrode vector having an interelectrode distance that is greater than the interelectrode distance of the first sensing electrode vector. Additionally or alternatively, the second cardiac electrical signal segment may be filtered by a relatively wider band pass to retain features of the cardiac electrical signal waveforms and, in some examples, filtered by a notch filter for attenuating 50-60 Hz noise. In other examples, the first cardiac electrical signal and the second cardiac electrical signal are received by the same sensing electrode vector but processed differently, e.g., filtered differently, to produce a first cardiac electrical signal that is different than the second cardiac electrical signal. 
     At block  104 , a segment of the second cardiac electrical signal over a predetermined time interval may be buffered in memory  82 . The predetermined time interval encompasses a time point at which a cardiac event was sensed from the first cardiac electrical signal. For example, in response to each R-wave sensed event signal  68  received from the first sensing channel  83  (see  FIG. 4 ), control circuit  80  may buffer a time segment of the second cardiac electrical signal  78  (and the rectified signal  79  in some examples) from the second sensing channel  85  in memory  82 . The time segment may extend from a time point earlier than the time of the R-wave sensing threshold crossing to a time point later than the R-wave sensing threshold crossing that caused the first sensing channel  83  to generate an R-wave sensed event signal  68 . The time segment may be 300 to 500 ms in duration, e.g., 360 ms in duration, including sample points preceding and following the R-wave sensed event signal. For instance, as described in conjunction with  FIG. 6B , a 360 ms segment may include 92 sample points when the sampling rate is 256 Hz with  24  of the sample points occurring after the R-wave sensed event signal that triggered the storage of the signal segment and 68 sample points extending from the R-wave sensed event signal earlier in time from the R-wave sensed event signal. 
     One or more signal features may be determined from each second cardiac electrical signal segment by control circuit  80 . In one example, at least a maximum peak-to-peak amplitude occurring during the time segment of the second cardiac electrical signal is determined. The maximum peak-to-peak amplitude may be determined over the time segment of the second cardiac electrical signal as the absolute difference between a minimum sample point amplitude and a maximum sample point amplitude. As such, control circuit  80  may include a peak detector that holds the values of a minimum peak and a maximum peak detected in a comparative analysis of sample point amplitudes of the digitized, notch-filtered cardiac signal. In this example of determining a maximum peak-to-peak amplitude, the cardiac electrical signal segment is a non-rectified signal. In other examples, a rectified signal may be used and a maximum peak amplitude may be determined. A series of maximum peak-to-peak amplitudes, e.g., at least three to twelve maximum peak-to-peak amplitudes determined from a corresponding number of buffered second cardiac electrical signal segments may be stored in memory  82  so that they are available for analysis for cardiac event oversensing detection. 
     In other examples, the polarity (positive or negative) of the maximum absolute peak amplitude may be determined among the one or more signal features determined from each second cardiac electrical signal segment at block  104 . In still other examples, a signal feature determined at block  104  may be a maximum slope, total area (e.g., integral or summation of the sample point amplitudes), or other feature of the second cardiac electrical signal that is expected to alternate when the sensed event signals produced by the first sensing channel correspond to an alternating pattern of R-waves and P-waves (e.g., P-R-P or R-P-R). Two or more signal features may be determined from each segment to detect an alternating pattern of the combination of two or more signal features in some examples. 
     At block  106 , control circuit  80  compares consecutively determined signal features and/or event intervals to each other to criteria for detecting an alternating pattern of signal features. For example, an alternating pattern of relatively high and relatively low maximum peak-to-peak amplitudes of the second cardiac electrical signal segments may be detected at block  106 . In other examples, an alternating pattern of event signal polarity may be detected at block  106 . In still other examples, event time intervals may be determined between consecutively sensed event signals produced by the first sensing channel to detect an alternating pattern of long and short event intervals. The alternating pattern detected at block  106  may be one pair of high and low maximum peak-to-peak amplitudes (e.g., high-low or low-high), one pair of positive and negative event polarities (e.g., positive-negative or negative-positive), and/or one pair of long and short event intervals (e.g., long-short or short-long), occurring in either order. In other examples, an alternating pattern of at least three consecutive event signal features is required to detect the alternating pattern at block  106 . Using the example of maximum peak-to-peak amplitude, a relative peak-to-peak amplitude pattern of high-low-high or low-high-low may be detected as an alternating pattern at block  106 . Methods and criteria for detecting alternating patterns of relatively high and low maximum peak-to-peak amplitude differences and relatively long and short intervals are described below in conjunction with  FIG. 6A . 
     When an alternating pattern is not detected (“no” branch of block  106 ), no further signal analysis may be performed by control circuit  80  for detecting cardiac event oversensing. The process may return to block  102  to continue sensing cardiac events. In response to detecting an alternating pattern of at least one signal feature (“yes” branch of block  106 ), e.g., alternating maximum peak-to-peak amplitude, from consecutive second cardiac electrical signal segments, control circuit  80  may determine a gross morphology feature at block  108  from one or more of the second cardiac electrical signal segments corresponding to the alternating signal feature pattern. 
     In some examples, when the alternating signal feature pattern suggests that the middle cardiac electrical signal segment out of three consecutive segments is an oversensed event, e.g., based on a high-low-high maximum peak-to-peak amplitude, control circuit  80  may identify the first and third segments of the cardiac electrical signal as corresponding to suspected true sensed cardiac events. The gross morphology features may be determined from the first and third segments identified as suspected true sensed cardiac events (e.g., true sensed R-waves). When the alternating pattern suggests that the first and third segments of the second cardiac electrical signal are oversensed events, e.g., based on a low-high-low maximum peak-to-peak amplitude, the gross morphology feature may be determined from the middle (second) segment of the three consecutive segments. In this case, the middle segment is identified as a second cardiac electrical signal segment corresponding to a suspected true sensed event. The gross morphology feature(s) for only the signal segments suspected to be true sensed R-waves may be compared to tachyarrhythmia morphology criteria at block  110  in some examples. In other examples, the gross morphology feature(s) may be determined from all of the segments analyzed at block  106  determined to present the alternating feature pattern and compared to tachyarrhythmia morphology criteria at block  110 . 
     Each gross morphology feature is determined from an analysis of the sample points spanning the second cardiac electrical signal segment. The gross morphology features determined at block  108  may include an amplitude morphology metric and/or a signal width morphology metric, as examples. Gross morphology features that may be used for detecting a tachyarrhythmia morphology may include a maximum slope, a number of peaks, a total signal area, a pulse count, a low slope content, a normalized mean rectified amplitude or other morphology features. Examples of gross morphology features may include any signal feature or metric correlated to a tachyarrhythmia waveform. Example methods for determining the gross morphology features at block  108  are described below in conjunction with  FIGS. 7 and 8 . 
     At block  110 , the gross morphology feature(s) may be compared to tachyarrhythmia morphology criteria. When a gross morphology feature meets tachyarrhythmia morphology criteria, cardiac event oversensing is not detected. As further described below, the tachyarrhythmia morphology criteria may require that at least one of the amplitude morphology metric or the signal width morphology metric is greater than a respective threshold as evidence of the morphology of a tachyarrhythmia waveform. When the gross morphology features do not meet tachyarrhythmia morphology criteria at block  110  (“no” branch), cardiac event oversensing evidence is detected at block  112 . This detection at block  112  indicates that at least one of the second cardiac electrical signal segments corresponds to an oversensed cardiac event. When the gross morphology features do not meet criteria for detecting a tachyarrhythmia morphology at block  110 , e.g., for at least one cardiac electrical signal segment identified as a suspected true sensed cardiac event, the suspected true sensed cardiac event is verified. This verification of the true sensed cardiac event in the absence of evidence of a tachyarrhythmia morphology (“no” branch of block  110 ), and the detection of an alternating signal feature pattern, results in an oversensing evidence detection at block  112 . 
     Conversely, when the gross morphology feature of at least one second cardiac electrical signal segment, which may be identified as a suspected true sensed cardiac event in the alternating signal feature pattern, is determined to meet tachyarrhythmia morphology criteria, the suspected true sensed cardiac event is not verified and oversensing evidence is not detected (“yes” branch of block  110 ). As described below in conjunction with  FIGS. 7 and 8 , the gross morphology features determined at block  108  may be compared to criteria at block  110  that distinguish between a likely tachyarrhythmia morphology and a likely true cardiac event signal morphology such as a true R-wave. The process returns to block  102  to continue sensing cardiac events from the first cardiac electrical signal and monitoring for cardiac event oversensing as needed. 
     Control circuit  80  may include a counter or buffer that tracks the number of times evidence of oversensing is detected at block  112  out of a specified number of recent sensed cardiac events. At block  114 , a counter may be increased or a flag may be set indicating the oversensing evidence detection made at block  112 . For instance, a first-in-first-out buffer may store a specified number of flags. When an alternating pattern is detected and tachyarrhythmia morphology criteria are not met, a flag may be set high (e.g., to 1) to indicate oversensing evidence is detected. When an alternating pattern is not detected, or tachyarrhythmia morphology criteria are met, the flag may be set low (e/g/. to 0) to indicate oversensing evidence is not detected. The first-in-first-out buffer may update on a beat-by beat basis in some examples, as oversensing evidence is detected (or not) with the oldest flag value being discarded. The buffer may store a predetermined number of flags, e.g., up to 32 (or more) corresponding to up to 32 (or more) consecutively sensed cardiac events. The oversensing evidence detection made at block  112  indicates that at least one of the second cardiac electrical signal segments of the alternating feature pattern corresponds to a suspected oversensed event, e.g., an oversensed P-wave falsely sensed as an R-wave. 
     At block  116 , the number of oversensing evidence detections counted (or number of oversensing evidence flags set in a buffer storing a specified number of flags in a first-in-first out basis) may be compared to an oversensing detection threshold. For example, if oversensing evidence is detected X times during a series of Y sensed cardiac event signals sensed from the first cardiac electrical signal, cardiac event oversensing is detected at block  120 . In one example, if more than a threshold percentage (e.g., 20% 25%, 30%, 35%, 40% or other percentage) of sensed event signals received from the first sensing channel  83  result in an oversensing evidence detection based on the analysis of multiple, consecutive second cardiac electrical signal segments, oversensing is detected at block  120 . For instance, if 3 oversensing evidence detections occur over 12 consecutive R-wave sensed event signals, or if 4 oversensing evidence detections occur over 16 consecutive R-wave sensed event signals or 8 oversensing evidence detections occur over 32 R-wave sensed event signals, cardiac event oversensing is detected at block  120 . In these examples, each oversensing evidence detection may be based on an analysis of three consecutive segments of the second cardiac electrical signal. 
     When the threshold for detecting cardiac event oversensing is not reached at block  116 , the process may return to block  102  without any therapy delivery adjustments being made by the therapy delivery circuit at block  118 . When cardiac event oversensing is detected at block  120 , control circuit  80  may withhold an arrhythmia detection at block  122 . Control circuit  80  may, in some instances, cause therapy delivery circuit  80  to withhold an arrhythmia therapy at block  122  in response to the cardiac event oversensing detection. For example, scheduling of a cardiac electrical stimulation therapy may be withheld in response to detecting the cardiac event oversensing. As described below in conjunction with  FIG. 9 , when other tachyarrhythmia detection criteria are met at block  121 , e.g., based on the sensed cardiac event rate or a threshold number of RRIs being less than a tachyarrhythmia detection interval at block  121 , the tachyarrhythmia detection may be withheld at block  122  in response to detecting cardiac event oversensing. 
     In other examples, consecutively determined RRIs may be buffered in memory  82  on a first-in-first-out basis so that a specified number of RRIs are stored. When all slots of the buffer are filled with an RRI value, each RRI that is less than a tachyarrhythmia detection interval may be counted as a tachyarrhythmia interval. Control circuit  80  may determine that tachyarrhythmia rate criteria are met at block  121  when a threshold number of tachyarrhythmia intervals are counted in the buffer. However, an RRI that is less than a tachyarrhythmia detection interval corresponds in time to a flag set in response to detecting cardiac event oversensing, the RRI may be ignored and not counted toward reaching the threshold number of tachyarrhythmia detection intervals. In this way, arrhythmia detection may be withheld by ignoring RRIs that coincide with cardiac oversensing detections. 
     An anti-tachyarrhythmia therapy, e.g., ATP and/or CV/DF shock(s) may be withheld by rejecting or withholding the tachyarrhythmia detection at block  122  to avoid delivering an inappropriate therapy due to cardiac event oversensing. In this way, the performance of the medical device for detecting and treating tachyarrhythmias is improved by detecting cardiac event oversensing and avoiding an inappropriate or unnecessary therapeutic response of the medical device due to cardiac event oversensing. 
       FIG. 6A  is a conceptual diagram  150  of a method for detecting an alternating pattern of cardiac signal features according to one example. Two examples  151   a  and  151   b  are depicted in which an alternating high-low-high maximum peak-to-peak amplitude pattern is detected in example  151   a , and an alternating low-high-low maximum peak-to-peak amplitude pattern is detected in example  151   b . In these examples  151   a  and  151   b , the first sensing channel  83  is configured to sense R-waves based on R-wave sensing threshold crossings by the first cardiac electrical signal. In response to each R-wave sensing threshold crossing, an R-wave sensed event signal  190 ,  191 ,  192 ,  194 ,  195 , and  196  is produced by the sensing circuit  86 . In the separate examples  151   a  and  151   b , R-wave sensed event signals  190 ,  191  and  192  are three consecutive R-wave sensed event signals, and R-wave sensed event signals  194 ,  195  and  196  are three consecutive R-wave sensed event signals. Each R-wave sensed event signal  190 - 196 , triggers the storage of a segment of the second cardiac electrical signal  78  (see  FIG. 4 ) produced by the second sensing channel  85  over a respective time interval  153 ,  155 ,  157 ,  163 ,  165 , and  167 , with each individual time interval encompassing a time point at which the triggering R-wave sensed event signal ( 190 ,  191 ,  192 ,  194 ,  195 , and  196 ) occurred. The second cardiac electrical signal may be buffered in memory  82  as it is sensed such that a signal segment may be stored over each time interval  153 ,  155 ,  157 ,  163 ,  165  and  167  that starts earlier than the respective triggering R-wave sensed event signal and ends after the triggering R-wave sensed event signal. 
       FIG. 6B  is an illustration  220  of the second cardiac electrical signal segment  221  stored over time interval  155  in response to the R-wave sensed event signal  191  (corresponding to identically number time interval  155  and R-wave sensed event signal  191  shown in  FIG. 6A ). Time segment  155  begins after the preceding R-wave sensed event signal  190  and ends prior to the next R-wave sensed event signal  192 . The second cardiac electrical signal (from sensing channel  85 ) may be buffered in memory  82  at a desired sampling rate, e.g., 128 Hz or 256 Hz, until an R-wave sensed event signal  191  is received at which point the desired number of sample points preceding the R-wave sensed event signal  191 , e.g., 68 sample points when the sampling rate is 256 Hz, and the desired number of sample points following the R-wave sensed event signal  191 , e.g.,  24  of the sample points occurring after the R-wave sensed event signal  191 , are stored in memory  82 , in a designated buffer, as a second cardiac electrical signal segment  221 . In other examples, a higher or lower sampling rate may be used, e.g., a sampling rate of 512 Hz or 128 Hz. A correspondingly higher or lower number of sample points may be used to analyze the cardiac signal segment over the same or similar time interval extending before and after the time point that the R-wave was sensed. Time interval  155  over which second cardiac electrical signal segment  221  is stored for analysis may be between 100 ms and 500 ms in length in various examples and is 360 ms in one example. Sample points of the second cardiac electrical signal that are buffered before the start of the time segment  155  and after preceding time segment  153 , may be discarded. 
     In some examples, the second cardiac electrical signal segment  221  is a non-rectified signal such that a maximum peak-to-peak amplitude  154  may be determined as the absolute difference between the maximum sample point amplitude  222  and the minimum sample point amplitude  224  detected over the signal segment  155 . In other examples, a maximum peak, minimum peak or a maximum peak of the rectified signal may be determined. In the example shown, control circuit  80  determines a maximum peak-to-peak amplitude of the second cardiac electrical signal during each of the time segments  153 ,  155 ,  157 ,  163 ,  165  and  167 . For example, the maximum peak-to-peak amplitude A 2   154  is determined from the second cardiac electrical signal segment  221  stored over time segment  155  as the absolute difference between the maximum peak sample point amplitude  222  and the minimum peak sample point amplitude  224 . Referring again to  FIG. 6A , the maximum peak amplitude A 1   152  is determined from the second cardiac electrical signal segment stored over time segment  153 ; the maximum peak-to-peak amplitude A 3   156  is determined from the second cardiac electrical signal segment stored over time segment  157 , and so on. The maximum and minimum sample point amplitudes used to determine the peak-to-peak amplitudes  152 ,  154 ,  156 ,  162 ,  164  and  166  may occur at any time during the respective time segment  153 ,  155 ,  157 ,  163 ,  165 , and  167  and are not necessarily the amplitude at the time of the R-wave sensed event signals  190 ,  191 ,  192 ,  194 ,  195  and  196 . 
     Once three consecutive maximum peak-to-peak amplitudes are determined from three consecutively stored second cardiac electrical signal segments, the differences between consecutive maximum peak-to-peak amplitudes are determined. In example  151   a , the first amplitude difference D 1   158  is determined between amplitude A 1   152  and amplitude A 2   154 . A second amplitude difference D 2   160  is determined between the second amplitude A 2   154  and the third amplitude A 3   156 . The first and second amplitude differences  158  and  160  between the three consecutively determined maximum peak-to-peak amplitudes  152 ,  154  and  156  may be compared to difference thresholds to detect an alternating pattern of maximum peak-to-peak amplitude in the second cardiac electrical signal segments. 
     The difference threshold may be a fixed value or set based on at least one of the determined maximum peak-to-peak amplitudes of the three consecutively determined peak-to-peak amplitudes  152 ,  154  and  156 . In some examples, the larger peak-to-peak amplitude of the two amplitudes being compared is identified, and the difference threshold is set to a percentage of the larger of the two amplitudes. For instance, in the first example  151   a , the absolute value of the first difference D 1   158  is compared to a percentage of the first, larger maximum peak-to-peak amplitude A 1   152 . The second difference D 2   160  is compared to a percentage of the third (larger) maximum peak-to-peak amplitude A 3   156 . The percentage of the maximum peak-to-peak amplitude used for setting a difference threshold may be 15 to 30%, as examples, and is 22% in one example. When the absolute value of difference D 1   158  is greater than 22% of peak-to-peak amplitude A 1   152  and difference D 2   160  is greater than 22% of peak-to-peak amplitude A 3   156 , an alternating pattern of high-low-high is detected by control circuit  80 . The R-wave sensed event signal  191  corresponding to the low maximum peak-to-peak amplitude A 2   154  in the detected high-low-high pattern is a suspected oversensed P-wave. The R-wave sensed event signals  190  and  192 , corresponding to high maximum peak-to-peak amplitudes  152  and  156  in the second cardiac electrical signal segments stored over time intervals  153  and  157 , respectively, may be identified as suspected true sensed R-waves. Control circuit  80  may increase an oversensed cardiac event evidence counter or set an oversensed cardiac evidence flag to track the number of times a pattern of cardiac event oversensing is identified over three consecutive R-wave sensed event signals. 
     In addition to detecting the high-low-high peak-to-peak amplitude pattern, control circuit  80  may verify stability in the alternating high peak-to-peak amplitudes A 1   152  and A 3   156 . These first and third amplitudes  152  and  156 , which may correspond to true sensed R-waves, may be required to be within a stability threshold of each other. In one example, the difference D 3   161  between peak-to-peak amplitude A 1   152  and peak-to-peak amplitude A 3   156  may be required to be less than a threshold percentage, e.g., less than 50%, of the highest maximum peak-to-peak amplitude (A 1   152  in this example) out of the three signal segments. The requirement of D 3  being less than a stability threshold, which may be set based on the highest maximum peak-to-peak amplitude, requires the maximum peak-to-peak amplitudes A 1   152  and A 3   156  of the signal segments corresponding to the two suspected true R-wave sensed event signals  190  and  192  to be stable. 
     The comparisons of signal amplitude differences for detecting high-low-high or low-high-low patterns may be performed using the maximum peak amplitude of R-waves sensed from the first sensing channel in some examples. The first and second differences between three consecutively sensed R-waves of the first cardiac electrical signal may be compared to respective thresholds, which may be different than the thresholds applied to the maximum peak-to-peak amplitude differences determined from the second cardiac electrical signal segments. In one example, the difference between first and second sensed R-wave amplitudes is required to be greater than 20% (or other percentage) of the largest one of the first and second sensed R-wave amplitudes. Additionally or alternatively, the difference between first and second maximum peak amplitudes determined from the second cardiac electrical signal is required to be greater than 20% of the highest one of the first and second maximum peak amplitudes or multiple thereof, e.g., 20% of 1.5 times the highest maximum peak amplitude. 
     In other examples, additional criteria may be applied before counting the detected alternating pattern of signal features as a detection of PWOS evidence. For example, a polarity (positive or negative) of the maximum peak amplitude of the non-rectified signal may be determined for each of the second cardiac electrical signal segments or at least the segment(s) suspected of being an oversensed event. In other examples, the RR intervals  180  and  182  between the consecutive R-wave sensed event signals  190  and  191  and the consecutive R-wave sensed event signals  191  and  192 , respectively, may be compared to alternating pattern detection criteria at block  106  of  FIG. 5 . 
     For instance, the sum of RR interval  180  and RR interval  182  may be required to fall within a threshold range, e.g., greater than 380 ms or other fast interval threshold and less than 1200 ms or other slow interval threshold. The threshold range may correspond to an expected range of true, non-tachyarrhythmia RR intervals. When at least one of the three consecutive R-wave sensed event signals  190 ,  191  or  192  is an oversensed P-wave, the sum of the two RR intervals  180  and  182  may represent a true RR interval. Thus their sum should fall within an expected RR interval range. 
     The RR intervals  180  and  182  may additionally or alternatively be compared to each other or their difference may be compared to a difference threshold to verify that a likely long-short pattern exists corresponding to an R-P-R pattern of the R-wave sensed event signals  190 ,  191  and  192 . In some examples, when one RR interval  180  or  182  is shorter than a threshold percentage (e.g., less than 60%, 50%, 40% or other percentage) than the other RR interval  182  or  180  a short-long or long-short pattern is detected. Additionally or alternatively, at least one of the two consecutive RR intervals may be required to be less than a threshold interval, e.g., less than a threshold interval of 360 to 400 ms. In some instances, the threshold interval is set to a tachyarrhythmia detection interval plus an offset, e.g., 40 ms longer than a VT or VF detection interval. This short interval in combination with an alternating amplitude pattern may be evidence of PWOS. 
     These examples of comparative analysis of RR intervals may be performed on RR intervals determined between consecutive R-wave sensed event signals produced by the first sensing channel  83 . Additionally or alternatively, any or all of these examples of comparative analysis of RR intervals may be performed by determining RR intervals between maximum peak amplitudes or R-wave sensing threshold amplitude crossings of the second cardiac electrical signal. The determination of a long-short RR interval pattern in combination with a high-low-high amplitude pattern may result in detection of PWOS evidence by control circuit  80 . As described below, tachyarrhythmia morphology criteria may be applied to one or more of the second cardiac electrical signal segments presenting the alternating signal feature pattern before detecting the alternating pattern as evidence of PWOS. 
     In the second example  151   b , control circuit  80  is configured to detect the low-high-low pattern of the maximum peak-to-peak amplitudes of the second cardiac electrical signal buffered over time segments  163 ,  165  and  167 , triggered by respective R-wave sensed event signals  194 ,  195  and  196 . In this case, the first amplitude difference D 1   168  is determined between the first maximum peak-to-peak amplitude A 1   162  and the second maximum peak-to-peak amplitude A 2   164  (e.g., A 1  minus A 2 ). The second amplitude difference D 2   170  is determined between the second maximum peak-to-peak amplitude A 2   164  and the third maximum peak-to-peak amplitude A 3   166  (e.g., A 2  minus A 3 ). The absolute value of D 1   168  may be compared to a difference threshold set based on one of the amplitudes A 1   162 , A 2   164 , or A 3   166 , e.g., to a percentage of the higher, second maximum peak-to-peak amplitude A 2   164 . In one example, the difference threshold is set to 22% of A 2   164  (the greater of A 1  and A 2 ). The second amplitude difference D 2   170  may also be compared to a difference threshold set to a percentage of the higher second maximum peak-to-peak amplitude A 2   164 . When the first amplitude difference D 1   168  and the second amplitude difference D 2   170  are both greater than a percentage of the second maximum peak amplitude A 2   164 , the low-high-low maximum peak-to-peak amplitude pattern is detected by control circuit  80 . 
     In addition to detecting the low-high-low peak-to-peak amplitude pattern, control circuit  80  may verify stability in the alternating low peak-to-peak amplitudes A 1   162  and A 3   166 . These first and third amplitudes  162  and  166 , which may correspond to oversensed P-waves, may be required to be within a stability threshold of each other. In one example, the difference D 3   171  between A 1   162  and A 3   166  may be required to be less than a threshold percentage, e.g., less than 50%, of the maximum of these two low peak-to-peak amplitudes (A 1   162  in this example). The requirement of D 3  being less than a stability threshold, e.g., set based on the higher peak-to-peak amplitude of the alternating low peak-to-peak amplitudes A 1   162  and A 3   166 , requires the peak-to-peak amplitudes of the second cardiac electrical signal segments corresponding to the two suspected oversensed P-waves to be stable. 
     Accordingly, once a low-high-low or a high-low-high peak-to-peak amplitude pattern is detected based on consecutive peak-to-peak amplitude differences being greater than a difference threshold, the difference between the first and third peak-to-peak amplitudes may be required to be less than a stability threshold for the three consecutive signal segments to be detected as an alternating pattern of signal features. The difference threshold may be set to a percentage of the highest maximum peak-to-peak amplitude out of all three consecutive signal segments or out of the two signal segments being compared. The stability threshold may be set to a percentage of the higher of the first and third peak-to-peak amplitudes. It is to be understood that, in other examples, the difference and stability thresholds may be defined differently than the specific examples give here. For example, the difference and stability thresholds for detecting an alternating signal feature pattern may be based on a percentage of a lower one of the maximum peak-to-peak amplitudes instead of the highest maximum peak-to-peak amplitude or based on a mean or median peak-to-peak amplitude of all segments or only segments correspond to suspected true R-waves, to a predetermined threshold value, etc. 
     As described above, additional criteria, such as the polarity of the maximum peak amplitude(s) of first and third time segments  163  and  165  and/or second time segment  165  may be compared to polarity criteria for detecting PWOS evidence. In other examples, the consecutive RR intervals  184  and  186  may be compared to each other or their difference may be compared to an RR interval difference threshold to verify that a short-long pattern of RRIs is detected to support the detection of the low-high-low maximum peak-to-peak amplitude pattern that suggests the first and third R-wave sensed event signals  194  and  196  are suspected oversensed P-waves and the second, middle R-wave sensed event signal  195  is a suspected true R-wave. In response to detecting the low-high-low pattern, an oversensing evidence counter may be increased or an oversensing evidence flag may be set to track the number of times an alternating pattern of consecutively determined second cardiac electrical signal features is detected. 
     Even though two likely P-waves may be oversensed in the series of the three R-wave sensed event signals  194 ,  195  and  196 , the oversensing evidence counter may be increased by one or a single flag may be set. In some examples, overlapping, moving sets of three second cardiac electrical signal segments may be evaluated for detecting alternating signal feature patterns. For instance, the third R-wave sensed event signal  196  may become the second R-wave sensed event signal of the next set of three consecutive R-wave sensed event signals, which may also meet the oversensing evidence criteria (in a high-low-high pattern). When a moving set of three (or other selected number of) consecutively determined maximum peak-to-peak amplitudes (or other signal feature) is used to detect the alternating signal feature pattern, the oversensing evidence counter may be increased once (or a single oversensing evidence flag set) in response to each detection of an alternating signal feature pattern. An oversensing detection threshold set to X detections of oversensing evidence out of Y consecutively sensed events may take into account the possible double counting of some suspected oversensed events in the detected alternating patterns. 
     In other examples, consecutive sets of second cardiac electrical signal segments may be non-overlapping. For instance, three signal segments corresponding to three consecutive R-wave sensed event signals may be analyzed then control circuit  80  may wait for the next three R-wave sensed event signals to analyze the next three, non-overlapping segments of the second cardiac electrical signal. An oversensing evidence counter may be increased by one for each set of signal segments detected as an alternating signal feature pattern. In other examples, the oversensing evidence counter (or number of flags set) may be increased by the number of suspected oversensed cardiac events in each detected alternating signal feature pattern, e.g., by one in example  151   a  and by two in example of  151   b.    
       FIG. 7  is a flow chart  200  of a method for determining a gross morphology metric for verifying a suspected true sensed cardiac event in an alternating signal feature pattern of cardiac electrical signal segments according to one example. Each cardiac electrical signal segment analyzed for determining a gross morphology metric may be buffered as generally described in conjunction with  FIG. 6B . The method of flow chart  200  may generally be performed at blocks  108  and  110  of  FIG. 5  to analyze at least one second cardiac electrical signal segment out of a consecutive series of segments presenting the alternating signal feature pattern for detecting a tachyarrhythmia morphology. For example, when three segments of the second cardiac electrical signal present an alternating pattern, the first and third segments may be analyzed to determine gross morphology metrics when the alternating pattern suggest that the second, middle segment corresponds to an oversensed P-wave and the first and third segments correspond to suspected true R-waves. When the alternating pattern suggests that the second middle segment corresponds to a suspected true sensed cardiac event and the first and third segments correspond to suspected oversensed events, the second, middle segment may be analyzed to determine gross morphology metrics. In other examples, all of the segments included in a detected alternating pattern sequence may be analyzed for determining gross morphology metrics. 
     The method of  FIG. 7  corresponds to the analysis of one second cardiac electrical signal segment for verifying a suspected true cardiac event in an alternating signal feature pattern based on the absence of evidence of a tachyarrhythmia morphology. At block  202 , the second cardiac electrical signal segments stored on a triggered basis in response to R-wave sensed event signals may be rectified. In some examples, a 360 ms segment of the notch-filtered second cardiac electrical signal may be rectified by rectifier  75  included second sensing channel  85 . At block  202 , the buffered, rectified signal segment may be retrieved by control circuit  80  from memory  82 . In other examples, a notch-filtered signal segment may be buffered in memory  82  and control circuit  80  may perform the rectification of the stored signal segment at block  202 . The rectified signal segment obtained at block  202  may correspond to a signal segment identified as a suspected true sensed cardiac event, e.g., a true sensed R-wave, based on the detected high-low-high or low-high-low alternating signal feature pattern as described above in conjunction with  FIG. 6A . 
     Control circuit  80  determines the maximum absolute amplitude of the rectified, notch-filtered signal segment at block  204 . The maximum absolute amplitude may be determined from among all sample points spanning the selected signal segment. As described above, a 360 ms segment of the second cardiac electrical signal may include 92 sample points when the sampling rate is 256 Hz, with  24  of the sample points occurring after the R-wave sensed event signal that triggered the storage of the signal segment and 68 sample points extending from the R-wave sensed event signal earlier in time from the R-wave sensed event signal. The sample point having the maximum amplitude in the rectified signal is determined at block  204 . 
     At block  206 , the amplitudes of all sample points of the rectified signal segment are summed. At block  208 , the gross morphology metric of the signal segment is determined as a normalized rectified amplitude (NRA) based on the maximum absolute amplitude determined at block  204  and the summed sample point amplitudes determined at block  206 . In one example, the NRA is determined as a predetermined multiple or weighting of the summation of all sample point amplitudes of the notch-filtered and rectified signal segment normalized by the maximum amplitude. For instance, the NRA may be determined as four times the summed amplitudes divided by the maximum absolute amplitude, which may be truncated to an integer value. This NRA may be determined as a gross morphology amplitude metric at block  108  of  FIG. 5 . 
     The gross morphology amplitude metric may be inversely correlated to the probability of the signal segment sample points being at a baseline amplitude during the time interval of the signal segment. The higher the gross morphology amplitude metric is, the lower the probability that the signal is at a baseline amplitude at any given time point during the time interval of the signal segment. A relatively low probability that the signal is at baseline during the time interval may be correlated to a tachyarrhythmia morphology, e.g., a ventricular fibrillation morphology, which may resemble a sinusoidal signal. When the gross morphology amplitude metric exceeds a threshold value for verifying a suspected true sensed R-wave, the more likely the second cardiac electrical signal segment has a tachyarrhythmia morphology. When the gross morphology amplitude metric is less than the threshold value, the higher the probability that the signal is at a baseline amplitude at a given time point during the time interval of the signal segment. A relatively higher probability of a signal sample point being at baseline during the time interval of the signal segment may be correlated to a true, relatively narrow R-wave signal occurring during the signal segment, with baseline amplitude portions of the signal segment occurring before and after the true R-wave. 
     As such, when the gross morphology amplitude metric is higher than the true sensed event threshold, evidence of a true sensed R-wave is not confirmed. In this case, the suspected true sensed event is not verified, precluding detection of oversensing evidence. At block  210 , the NRA is compared to a true sensed event threshold. The true sensed event threshold for verifying a suspected true cardiac event may be set between 100 and 150, and is set to 125 in some examples, such as when 92 sample points are summed and multiplied by a weighting factor of four and normalized by the maximum absolute amplitude. The threshold applied at block  210  to discriminate between a true sensed R-wave and a tachyarrhythmia morphology in the second cardiac electrical signal segment will depend on various factors such as the amplification and number of sample points summed, the multiplication or weighting factor of the summed sample points, etc. 
     Control circuit  80  may verify a suspected true cardiac event of an alternating signal feature pattern at block  212  in response to the NRA being less than the true sensed event threshold. When the NRA is greater than (or equal to) the true event threshold at block  210 , the suspected true sensed event is not verified at block  214 . Tachyarrhythmia morphology evidence is detected at block  216 . With reference to  FIG. 5 , evidence of oversensing is not detected (“yes” branch of block  110 ) in response to evidence of a tachyarrhythmia morphology based on the NRA being greater than the true sensed event threshold even though an alternating signal feature pattern may be detected. 
     When the gross morphology amplitude metric determined at block  108  of  FIG. 5  (according to the method of  FIG. 7 ) is greater than the true sensed event threshold at block  210 , evidence of a tachyarrhythmia morphology is detected (block  110  of  FIG. 5 ), precluding detection of oversensing evidence. When at least one (or all) second cardiac electrical signal segment(s) corresponding to a suspected true R-wave signal in the alternating pattern of signal segments is/are verified based on the gross morphology amplitude metric being less than or equal to a true sensed event threshold, control circuit  80  does not detect tachyarrhythmia morphology evidence and may detect oversensing evidence in response to the verification of the suspected true sensed event(s) and the alternating pattern of the second cardiac electrical signal segment features. In this case, tachyarrhythmia morphology evidence is not detected at block  110  of  FIG. 5 , leading to the detection of oversensing evidence at block  112 . 
       FIG. 8  is a flow chart  250  of a method for verifying a suspected true sensed cardiac event in a cardiac electrical signal segment based on a gross morphology metric according to another example. The process of flow chart  250  may be performed by ICD  14  for determining a gross morphology signal width metric at block  108  of  FIG. 5 . Blocks  202  and  204  correspond to identically-numbered blocks described above in conjunction with  FIG. 7 . The notch-filtered, rectified signal segment determined at block  202 , which may correspond to a suspected true sensed cardiac event based on a detected alternating signal feature pattern. The notch-filtered, rectified signal segment may be used to determine a maximum absolute amplitude of the signal segment at block  204 . 
     Control circuit  80  determines a pulse amplitude threshold at block  252  based on the maximum absolute amplitude determined at block  204 . This pulse amplitude threshold may be used for identifying a signal pulse having a maximum signal width out of all signal pulses occurring during the time interval of the second cardiac electrical signal segment. For example, the pulse amplitude threshold used for determining the gross morphology signal width metric may be set to half the maximum absolute amplitude of the rectified, notch-filtered signal segment. 
     At block  254 , control circuit  80  determines the signal width for all signal pulses of the second cardiac electrical signal segment. Each signal pulse in the signal segment may be identified by identifying two consecutive zero amplitude or baseline amplitude sample points of the rectified signal segment (or two consecutive zero crossings of a non-rectified signal segment). All signal pulses between two consecutive baseline amplitude sample points are identified at block  254 . The signal pulses may be identified from the non-rectified signal segment to enable signal pulses to be identified between zero-crossings, in some examples. The signal width of each identified signal pulse is determined as the number of sample points (or corresponding time interval) between the pair of consecutive baseline amplitude sample points (or zero crossings). The absolute maximum amplitude of each rectified signal pulse is determined at block  256 . All signal pulses having an absolute maximum amplitude that is greater than or equal to the pulse amplitude threshold determined at block  252  are identified at block  258 . For example, all signal pulses having a maximum amplitude that is at least half the maximum absolute amplitude determined at block  204  are identified at block  258 . The maximum signal pulse width is determined at block  260  by comparing the signal pulse widths of all signal pulses identified at block  258  as having a maximum amplitude that is at least the pulse amplitude threshold. The maximum pulse width identified at block  260 , out of all signal pulses identified at block  258 , may be determined as the gross morphology signal width metric at block  108  of  FIG. 5 . 
     The gross morphology signal width metric may be correlated to the probability of the signal segment having a tachyarrhythmia morphology. For example, a relatively high gross morphology signal width metric may be evidence of a tachyarrhythmia morphology, such as relatively wide ventricular fibrillation waves. Conversely, a relatively low gross morphology signal width metric may be evidence of a relatively narrow, true R-wave occurring during the time interval of the second cardiac electrical signal segment. A suspected true sensed R-wave may be verified by a relatively low gross morphology signal width metric, which supports detection of oversensing evidence based on a detected alternating signal feature pattern. 
     Control circuit  80  compares the maximum pulse width identified at block  260  to a true sensed cardiac event width threshold at block  262 . In one example, the true sensed cardiac event width threshold is set to 20 sample points when the sampling rate is 256 Hz. When the maximum signal pulse width is less than the width threshold, control circuit  80  verifies the suspected true cardiac event at block  264 . A maximum signal pulse width that is less than or equal to the width threshold may correspond to a true, relatively narrow R-wave, e.g., during a sinus rhythm. Verification of the suspected true sensed cardiac event may support a cardiac event oversensing detection when an alternating pattern of signal features is detected. Oversensing evidence may be detected at block  112  of  FIG. 5  in response to detecting the alternating pattern at block  106  and verifying at least one suspected true cardiac sensed event based on at least the gross morphology signal width metric being less than the true sensed event width threshold. 
     When the maximum pulse width is greater than the width threshold at block  262 , the suspected true sensed cardiac event is not verified at block  266 . Instead, the relatively wide maximum signal pulse width may be detected at block  268  as evidence of a tachyarrhythmia morphology present in the segment of the second cardiac electrical signal being analyzed. Evidence of the tachyarrhythmia morphology precludes verification of a suspected true sensed cardiac event and detection of oversensing evidence at block  112  of  FIG. 5 , even when an alternating pattern of second cardiac electrical signal segment features has been detected. 
     In various examples, both the gross morphology amplitude metric and the gross morphology signal width metric may be determined (at block  110  of  FIG. 5  according to the techniques of  FIG. 7  and  FIG. 8 ) and compared to true cardiac event criteria or thresholds at block  110  of  FIG. 5 . In some examples, both of the gross morphology amplitude metric and the gross morphology signal width metric may be required to meet the true cardiac sensed event criteria, e.g., both may be required to be less than or equal to a respective threshold value, for suspected true cardiac sensed event to be verified and allow oversensing evidence to be detected. When only one of the gross morphology amplitude or the gross morphology signal width is greater than the respective true cardiac sensed event threshold value, evidence of a tachyarrhythmia morphology is detected in the second cardiac electrical signal segment at block  110  of  FIG. 5 , precluding detection of oversensing evidence. In other examples, at least one of the gross morphology amplitude and signal width metrics of the suspected true sensed cardiac event signal segment(s) may be required to be less than or equal to the respective true cardiac event threshold value in order to detect oversensing evidence based on the alternating signal feature pattern. 
     The gross morphology amplitude metric determined by the method of  FIG. 7  and the gross morphology signal width metric determined by the method of  FIG. 8  may be used in combination to verify suspected true R-waves in an alternating signal feature pattern. Evidence of a tachyarrhythmia morphology based on the gross morphology metrics prevents verification of the suspected true R-waves and precludes detection of oversensing evidence, in some examples. A segment of the second cardiac electrical signal that has a relatively high gross morphology amplitude metric and/or relatively high gross morphology signal width metric is evidence of a tachyarrhythmia morphology. 
       FIG. 9  is a flow chart  300  of a method performed by ICD  14  for detecting P-wave oversensing and rejecting a ventricular tachyarrhythmia detection in response to detecting P-wave oversensing according to one example. At blocks  302  and  304 , two different sensing electrode vectors may be selected by sensing circuit  86  for receiving a first cardiac electrical signal by a first sensing channel  83  and a second cardiac electrical signal by a second sensing channel  85 , respectively. The two sensing electrode vectors may be selected by switching circuitry included in sensing circuit  86  under the control of control circuit  80 . In some examples, the two sensing electrode vectors are programmed by a user and retrieved from memory  82  by control circuit  80  and passed to sensing circuit  86  as vector selection control signals. 
     The first sensing electrode vector selected at block  302  for obtaining a first cardiac electrical signal may be a relatively short bipole, e.g., between electrodes  28  and  30  or between electrodes  28  and  24  of lead  16  or other electrode combinations as described above. The relatively short bipole may include electrodes that are in relative close proximity to each other and to the ventricular heart chambers compared to the second sensing vector selected at block  304 , to provide sensing of a relatively “near-field” ventricular signal for sensing R-waves. The first sensing vector may be a vertical sensing vector (with respect to an upright or standing position of the patient) or approximately aligned with the cardiac axis for maximizing the amplitude of R-waves in the first cardiac electrical signal for reliable R-wave sensing. The first sensing electrode vector, however, is not limited to any particular interelectrode spacing or orientation and may be selected as any available electrode pair. 
     The second sensing electrode vector used to obtain a second cardiac electrical signal at block  304  may be a relatively long bipole having an inter-electrode distance that is greater than the first sensing electrode vector. For example, the second sensing electrode vector may be selected as the vector between one of the pace sense electrodes  28  or  30  and ICD housing  15 , one of defibrillation electrodes  24  or  26  and housing  15  or other combinations of one electrode along the distal portion of the lead  16  and the housing  15 . This second sensing vector may be orthogonal or almost orthogonal to the first sensing vector in some examples, but the first and second sensing vectors are not required to be orthogonal vectors. The second sensing electrode vector may receive a relatively more global or far-field cardiac electrical signal compared to the first sensing electrode vector. The second cardiac electrical signal received by the second sensing vector selected at block  304  may be analyzed by control circuit  80  for detecting P-wave oversensing. In other examples, the sensing vector  1  and sensing vector  2  selected for sensing the first and second cardiac electrical signals at blocks  302  and  304  may be the same sensing electrode vector, such that single cardiac electrical signal is received by the sensing circuit  86 , but the raw, received signal is processed by two different sensing channels  83  and  85  of sensing circuit  86  having different filtering and/or other signal processing features to sense two different cardiac electrical signals, one used by the first sensing channel  83  for detecting R-waves and one sensed by the second sensing channel for performing signal feature and morphological analysis. 
     Sensing circuit  86  may produce an R-wave sensed event signal at block  306  in response to the first sensing channel  83  detecting an R-wave sensing threshold crossing by the first cardiac electrical signal. The R-wave sensed event signal may be passed to control circuit  80 . In response to the R-wave sensed event signal, down-going “yes” branch of block  306 , control circuit  80  is triggered at block  308  to store a segment of the second cardiac electrical signal received from the second sensing channel  85  over a predetermined time interval. Segments of the second cardiac electrical signal may be stored in a circulating buffer of memory  82  configured to store multiple sequential segments, where storage of each segment is triggered by an R-wave sensed event signal produced by the first sensing channel  83 . A digitized segment of the second cardiac electrical signal may be 100 to 500 ms long, for example, including sample points before and after the time of the R-wave sensed event signal. The segment of the second cardiac electrical signal may or may not be centered in time on the R-wave sensed event signal received from sensing circuit  86 . For instance, the segment may extend 100 ms after the R-wave sensed event signal and be 200 to 500 ms in duration such that the segment extends from about 100 to 400 ms before the R-wave sensed event signal to 100 ms after. In other examples, the segment may be centered on the R-wave sensed event signal or extend a greater number of sample points after the R-wave sensed event signal than before. In one example, the buffered segment of the cardiac electrical signal is at least 50 sample points obtained at a sampling rate of 256 Hz, or about 200 ms. In another example, the buffered segment is at least 92 sample points, or approximately 360 ms, sampled at 256 Hz and is available for analysis for detecting P-wave oversensing. 
     Memory  82  may be configured to store a predetermined number of second cardiac electrical segments, e.g., at least 1 and in some cases two or more cardiac electrical signal segments, in circulating buffers such that the oldest segment is overwritten by the newest segment. However, previously stored segments may never be analyzed for P-wave oversensing detection and be overwritten if an R-sense confirmation threshold is not reached at block  314  as described below. In some examples, at least one segment of the second cardiac electrical signal may be stored and if not needed for detecting P-wave oversensing, the segment is overwritten by the next segment corresponding to the next R-wave sensed event signal. 
     In addition to buffering a segment of the second cardiac electrical signal, control circuit  80  responds to the R-wave sensed event signal produced at block  306  by determining an RRI at block  310  ending with the current R-wave sensed event signal and beginning with the most recent preceding R-wave sensed event signal. The timing circuit  90  of control circuit  80  may pass the RRI timing information to the tachyarrhythmia detection circuit  92  which adjusts tachyarrhythmia interval counters at block  312 . If the RRI is longer than a tachycardia detection interval (TDI), the tachyarrhythmia interval counters remain unchanged. If the RRI is shorter than the TDI but longer than a fibrillation detection interval (FDI), e.g., if the RRI is in a tachycardia detection interval zone, a VT interval counter is increased at block  312 . If the RRI is shorter than or equal to the FDI, a VF interval counter is increased at block  312 . In some examples, a combined VT/VF interval counter is increased if the RRI is less than the TDI. 
     After updating the tachyarrhythmia interval counters at block  312 , tachyarrhythmia detector  92  compares the counter values to an R-sense confirmation threshold at block  314  and to VT and VF detection thresholds at block  332 . If a VT or VF detection interval counter has reached an R-sense confirmation threshold, “yes” branch of block  314 , the second cardiac electrical signal from sensing channel  85  is analyzed to detect P-wave oversensing that may be causing false R-wave sensed event signals to be produced by the first sensing channel  83 , resulting in VT and/or VF counters being increased at block  312 . The R-sense confirmation threshold may be a VT or VF interval count value that is greater than one or another higher threshold count value. Different R-sense confirmation thresholds may be applied to the VT interval counter and the VF interval counter. For example, the R-sense confirmation threshold may be a count of two on the VT interval counter and a count of three on the VF interval counter. In other examples, the R-sense confirmation threshold is a higher number, for example five or higher, but may be less than the number of VT or VF intervals required to detect VT or VF. In addition or alternatively to applying an R-sense confirmation threshold to the individual VT and VF counters, an R-sense confirmation threshold may be applied to a combined VT/VF interval counter. It is recognized that in some examples, VT detection may not be enabled and VF detection may be enabled. In this case, only a VF interval counter is updated at block  312  in response to RRI determinations and the R-sense confirmation threshold may be applied to the VF interval counter at block  314 . 
     If the R-sense confirmation threshold is not reached by any of the tachyarrhythmia interval counters at block  314 , the control circuit  80  waits for the next R-wave sensed event signal at block  308  to buffer the next segment of the second cardiac electrical signal. If the R-sense confirmation threshold is reached at block  314 , e.g., when the VF interval counter is greater than 2, the control circuit  80  begins analysis of the second cardiac electrical signal segments for detecting P-wave oversensing. 
     At block  316 , control circuit  80  may retrieve one or more notch filtered signal segments stored in memory  82 . In some examples, the stored second cardiac electrical signal segments are notch filtered by control circuit  80  at block  316 , e.g., by a firmware implemented notch filter, after the R-sense confirmation threshold is reached. In other examples, the notch-filtered signal is received from the second sensing channel  85  as shown in  FIG. 4  and buffered in memory  82  for retrieval by control circuit  80 . From each consecutive non-rectified, notch-filtered, second cardiac electrical signal segment buffered in response to an R-wave sensed event signal after the R-sense confirmation threshold is reached, control circuit  80  determines the maximum peak-to-peak amplitude. The maximum peak-to-peak amplitude is determined for at least three consecutively buffered cardiac signal segments to enable determination of two amplitude differences between two consecutive pairs of maximum peak-to-peak amplitudes as described above in conjunction with  FIG. 6A . At block  318 , control circuit  80  determines the differences between consecutively determined peak-to-peak amplitudes. 
     Once three consecutively determined peak-to-peak amplitudes are available, so that two consecutive amplitude differences can be determined at block  318 , control circuit  80  may apply criteria for detecting an alternating pattern of the peak amplitudes at block  320 . The criteria applied at block  320  may correspond to the examples described above in conjunction with  FIG. 6A . It is to be understood that, in some cases, when the R-sense confirmation threshold is first reached, the peak-to-peak amplitudes of three consecutively buffered cardiac electrical signal segments may not be available since the determination of the peak-to-peak amplitudes from a buffered segment may not begin until the R-sense confirmation threshold is reached. Two more R-wave sensed event signals and corresponding buffered second cardiac electrical signal segments may be required before the first determination of an alternating pattern can be made at block  320 . 
     As described above, control circuit  80  may detect an alternating pattern of maximum peak-to-peak amplitude by determining the maximum peak-to-peak amplitudes of three consecutive segments, identifying the highest maximum amplitude of two consecutively determined peak-to-peak amplitudes, set a difference threshold as a percentage of the highest maximum peak-to-peak amplitude, then compare the difference in the two consecutively determined maximum peak-to-peak amplitudes to the difference threshold. Two consecutive peak-to-peak amplitude differences are determined from three consecutively determined peak-to-peak amplitudes. The two consecutive differences may be compared to a first set of criteria for detecting a high-low-high pattern as described in conjunction with example  151   a  of  FIG. 6A . If the high-low-high pattern is not detected, the two consecutive differences may be compared to a second set of criteria for detecting a low-high-low pattern as described in conjunction with example  151   b  of  FIG. 6A . In other examples, control circuit  80  may apply the low-high-low amplitude difference criteria first and, if unmet, apply the high-low-high amplitude difference criteria second or apply both low-high-low and high-low-high criteria in parallel for detecting an alternating pattern of the maximum peak-to-peak amplitudes of three consecutive second cardiac electrical signal segments. 
     When an alternating pattern is not detected at block  320 , control circuit  80  may determine if the number of VT and/or VF intervals required to detect VT or VF has been reached at block  332 . When a threshold number of intervals to detect (NID) is not reached by the VT interval counter, VF interval counter, or combined VT/VF interval counter, control circuit  80  returns to block  310  to continue determining RRIs and analyzing second cardiac electrical signal segments as long as the R-sense confirmation threshold is satisfied (block  314 ). 
     When an alternating pattern is detected at block  320 , control circuit  80  identifies which of the three consecutive segments correspond to suspected true sensed R-waves at block  322 , based on the alternating pattern identified. For example, when a high-low-high amplitude pattern is identified, the first and third cardiac electrical signal segments are identified as corresponding to suspected true sensed R-waves and the second, middle cardiac electrical signal segment is identified as a suspected oversensed P-wave. When a low-high-low amplitude pattern is detected at block  320 , the middle cardiac electrical signal segment is identified as corresponding to a suspected true sensed R-wave, and the first and third cardiac electrical signal segments are identified as suspected P-wave oversensing segments, corresponding to R-wave sensed event signals that are suspected to be false. 
     At block  326 , gross morphology metrics are determined for the cardiac electrical signal segment(s) identified as corresponding to a suspected true sensed R-wave. The gross morphology metrics may be determined by processing and analyzing one or both the rectified, notch-filtered segment of the second cardiac electrical signal and/or the non-rectified, notch-filtered segment as described above in conjunction with  FIGS. 7 and 8 . Accordingly, the notch-filtered signal segment may be rectified at block  324  (e.g., by the second sensing channel  85  of  FIG. 4 ), and control circuit  80  may use the rectified, notch-filtered signal segment at block  326  for determining the gross morphology metrics. The gross morphology metrics may include an amplitude metric and a signal width metric as described above in conjunction with  FIGS. 7 and 8 , respectively. When the suspected true R-wave cardiac electrical signal segment(s) of the alternating signal feature pattern are determined to include a tachyarrhythmia morphology at block  328 , based on tachyarrhythmia morphology criteria being met, the suspected true R-wave(s) is(are) not verified as true sensed R-waves. In response to detecting at least one tachyarrhythmia morphology at block  328  in the alternating pattern of signal segments, withholding of a VT or VF detection due to suspected PWOS is inhibited by withholding detection of PWOS evidence at block  330  even though an alternating pattern of signal segments may be detected. For example, as described in conjunction with  FIGS. 7 and 8 , a suspected true sensed R-wave may not be verified when one of the gross morphology amplitude metric or the gross morphology signal width metric is greater than a respective true sensed event threshold value. In this case, a tachyarrhythmia morphology is detected at block  328  and PWOS evidence detection is not made at block  330 , even when the alternating pattern of peak-to-peak amplitude is detected at block  320 . An oversensing evidence flag may be set to 0 at block  330 , or a PWOS counter may be adjusted according to the number of times PWOS evidence has been detected over the last Y R-wave sensed event signals. 
     When tachyarrhythmia morphology criteria are not met, e.g., when both of the gross morphology amplitude feature and the gross morphology signal width feature are less than the true sensed event threshold, a tachyarrhythmia morphology is not detected at block  328  for the given signal segment. The suspected true R-wave sensed event is verified. In response to the suspected true R-wave sensed event(s) being verified in the alternating pattern of signal features, PWOS evidence is detected at block  334 . A PWOS evidence flag may be set to 1 at block  334 , or a PWOS counter may be adjusted to increase the number of times PWOS evidence has been detected over the last Y R-wave sensed event signals. 
     The suspected true R-wave sensed event signal segments are not necessarily determined to be false R-waves when the gross morphology parameters meet tachyarrhythmia morphology criteria at block  328 . However, when the tachyarrhythmia morphology criteria are met, e.g., when one or both gross morphology metrics exceed the true sensed event signal threshold, the suspected true sensed R-wave signal segments cannot be verified and may represent true ventricular tachyarrhythmia signal segments. Withholding of a VT or VF detection due to possible PWOS is inhibited in the presence of a relatively high gross morphology amplitude metric and/or relatively high gross morphology signal width metric, both correlated to a tachyarrhythmia morphology. When the gross morphology metrics are relatively low, the suspected R-wave sensed event signals may be true R-waves but relatively low in amplitude, which may lead to frequent PWOS when the R-wave sensing threshold is set based on the maximum peak amplitude of a sensed signal. In this case, a PWOS evidence detection is made at block  334 . 
     Control circuit  80  continues to analyze segments of the second cardiac electrical signal for detecting PWOS evidence (blocks  316 - 328 ) as long as the R-sense confirmation threshold is met at block  314 . If the VT and VF interval counters no longer meet the R-sense confirmation threshold at block  314 , the PWOS evidence counter or buffer may be cleared or reset. The PWOS evidence counter or buffer may begin counting PWOS detections, or setting PWOS evidence buffer flags in a first-in-first-out basis, the next time the R-sense confirmation threshold is reached. When the NID is reached at block  332 , based on the values of the VT and/or VF interval counters, control circuit  80  determines whether PWOS is detected at block  336 . In order to detect PWOS and cause a VT or VF detection to be withheld, the number of PWOS evidence flags, e.g., stored in a first-in-first out buffer, or value of a PWOS evidence counter, is required to reach a threshold number. For example, if a PWOS event flag is set to “1” for at least eight out of the most recent 32 buffered second cardiac electrical signal segments, PWOS is detected at block  336 . The oversensing threshold applied to the PWOS evidence counter or flags may be a fixed threshold or adjustable based on the NID. For example, control circuit  80  may set the oversensing threshold to a relatively lower value when the NID is relatively low and increase the oversensing threshold when the NID is relatively high. 
     The VT or VF detection is withheld at block  342  and no ventricular tachyarrhythmia therapy is delivered in response to the PWOS detection at block  336 . As long as the NID continues to be met, control circuit  80  may continue to update the PWOS evidence counter as new R-waves are sensed to determine if the oversensing threshold is still being met at block  336 . In some examples, control circuit  80  may determine if termination criteria are met at block  344  when PWOS detection does not occur. Termination of the fast rhythm may be detected based on a predetermined number of RRIs that are greater than a tachyarrhythmia detection interval or when a mean, median or other metric of RRIs determined over predetermined time interval is greater than a tachyarrhythmia detection interval. For example, when a threshold number of RRIs longer than the VT detection interval (e.g., when VT detection is enabled) or longer than the VF detection interval (e.g., when VT detection is not enabled) are detected subsequent to the NID being met, tachyarrhythmia termination may be detected at block  344 . In one example, termination is detected at block  344  when at least eight consecutive long RRIs, e.g., greater than the VT detection interval, are detected. In another example, control circuit  80  may detect termination at block  344  when a predetermined time interval elapses and a median RRI is greater than the VT detection interval. For instance, when the median RRI of the most recent 12 RRIs is always greater than the VT detection interval for at least 20 seconds, or other predetermined time period, control circuit  80  may detect termination at block  344 . Control circuit  80  may reset the VT and VF interval counters and return to block  310  in response to detecting termination. 
     When the NID is met at block  332  and PWOS is not detected at block  336 , the VT or VF episode is detected at block  338 . Therapy delivery circuit  84  may deliver a VT or VF therapy at block  340  in response to the VT/VF detection. It is to be understood that other criteria may be applied before detecting the VT or VF at block  338 . For example, various noise rejection criteria, T-wave oversensing rejection criteria, supraventricular tachycardia (SVT) rejection criteria, etc. may be required to be unmet before detecting VT/VF at block  338 . 
     It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the method). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. In addition, while certain aspects of this disclosure are described as being performed by a single circuit or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or circuits associated with, for example, a medical device. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer). 
     Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPLAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     Thus, a medical device has been presented in the foregoing description with reference to specific examples. It is to be understood that various aspects disclosed herein may be combined in different combinations than the specific combinations presented in the accompanying drawings. It is appreciated that various modifications to the referenced examples may be made without departing from the scope of the disclosure and the following claims.