Abstract:
A method and device for detecting cardiac signals that includes a first plurality of electrodes that senses cardiac signals and delivers therapy, and a second plurality of electrodes that senses the cardiac signals. A microprocessor detects a cardiac event in response to the sensing by the first plurality of electrodes, and verifies the cardiac event in response to the sensing by the second plurality of electrodes.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to medical devices, and, more particularly, to a method and apparatus for sensing and detecting cardiac signals in a medical device.  
       BACKGROUND OF THE INVENTION  
       [0002]     Implantable medical devices (IMDs) have many functions including the delivery of therapies to cardiac patients, neuro-stimulators, muscular stimulators, and so forth. For purposes of this application reference will be made only to implantable cardiac devices, it being understood that the principles herein may have applicability to other implantable medical devices as well.  
         [0003]     An implantable cardiac device (ICD) may be a device commonly referred to as a pacemaker, which is used to stimulate the heart into a contraction if the sinus node of the heart is not properly timing, or pacing, the contractions of the heart. Modern cardiac devices also perform many other functions beyond that of pacing. For example, some cardiac devices may also perform therapies such as defibrillation and cardioversion as well as providing several different pacing therapies, depending upon the needs of the user and the physiologic condition of the user&#39;s heart. For convenience, all types of implantable cardiac devices will be referred to herein as ICDs, it being understood that the term, unless otherwise indicated, is inclusive of an implantable device capable of administering any of a number of therapies to the heart of the user.  
         [0004]     In typical use, an ICD is implanted in a convenient location usually under the skin of the user and in the vicinity of the one or more major arteries or veins. One or more electrical leads connected to the pacemaker are inserted into or on the heart of the user, usually through a convenient vein or artery. The ends of the leads are placed in contact with the walls or surface of one or more chambers of the heart, depending upon the particular therapies deemed appropriate for the user.  
         [0005]     One or more of the leads is adapted to carry a current from the pacemaker to the heart tissue to stimulate the heart in one of several ways, again depending upon the particular therapy being delivered. The leads are simultaneously used for sensing the physiologic signals provided by the heart to determine when to deliver a therapeutic pulse to the heart, and the nature of the pulse, e.g., a pacing pulse or a defibrillation shock.  
         [0006]     There has been recent interest in development of implantable defibrillators that may be inserted entirely subcutaneously or sub-muscularly, having no leads or electrodes positioned within the thoracic cavity. The elimination of transvenous or epicardial leads is believed likely to allow for implant of the devices by a wider range of physicians, in some cases at a lower cost than traditional ICDs. Absence of transvene or epicardial leads may reduce acute and long term complications. Such devices, are therefore believed to offer the opportunity for increased levels of use, particularly for prophylactic implant. US Application Publication Nos. 2002/0042634, 200200068958 and 2002/0035377 to Bardy et al., are exemplary of current thinking with regard to such subcutaneous ICDs. Additional subcutaneous ICDs are disclosed in US Application Publication No. 20020082658 by Heinrich et al. and PCT publication WO/04043919A2 by Olson. All of the above cited applications and publications are incorporated herein by reference in their entireties.  
         [0007]     One potential problem associated with the sensing of the physiologic signal from the heart in both the transvenous systems and the subcutaneous systems relates to what is often referred to as “false positive” and “false negative” detections. The most widely accepted detection algorithm is based on the rate of depolarizations of the ventricles, or simply on “heart rate”. Such algorithms rely on detecting events based upon signals obtained between two electrodes positioned within or on the heart. If the number of detected events per a given time is greater than a preset value, then the device charges an energy storage capacitor and then shocks the heart; otherwise no shock is delivered. However, cutaneous and subcutaneous ECG signals can sometimes be corrupted by muscle noise and/or other artifacts, such as baseline wander, for example, making reliable R-wave sensing problematic. Accordingly, what is needed is a method and apparatus for improving detection of arrhythmias in a medical device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     Aspects and features of the present invention will be readily appreciated as they become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a schematic diagram of a an exemplary medical device of a type in which the present invention may usefully be practiced;  
         [0010]      FIG. 2  is a schematic diagram of an exemplary medical device of a type in which the present invention may usefully be practiced;  
         [0011]      FIG. 3  is a schematic diagram of an exemplary medical device of a type in which the present invention may usefully be practiced;  
         [0012]      FIG. 4  is a functional schematic diagram of the medical device of  FIG. 1 , in which the present invention may usefully be practiced;  
         [0013]      FIG. 5  is a functional schematic diagram of the medical device of  FIG. 2  according to an embodiment of the present invention;  
         [0014]      FIG. 6  is a functional schematic diagram of the medical device of  FIG. 3  according to an embodiment of the present invention;  
         [0015]      FIG. 7  is a schematic diagram of an exemplary medical device according to an embodiment of the present invention; and  
         [0016]      FIG. 8  is a flowchart of a method for detecting cardiac signals in a medical device according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  is a schematic diagram of an exemplary medical device of a type in which the present invention may usefully be practiced. As illustrated in  FIG. 1 , an exemplary medical device  10  according to an embodiment of the present invention includes a ventricular lead  3 , an atrial/SVC lead  5  and a coronary sinus lead  7  coupled to a device housing  11  via a connector block  12  positioned along housing  11 . Ventricular lead  3  includes an elongated insulative lead body  16 , carrying three mutually insulated conductors. Located adjacent the distal end of lead  3  are a ring electrode  24 , an extendable helix electrode  26 , mounted retractably within an insulative electrode head  28 , and an elongated coil electrode  20 . Each of electrodes  24 - 28  is coupled to one of the conductors (not shown) within the lead body  16 . Electrodes  24  and  26  are employed for cardiac pacing and for sensing ventricular depolarizations. A bifurcated connector  14  at the proximal end of lead  3  carries electrical connectors coupled to the coiled conductors.  
         [0018]     Atrial/SVC lead  5  includes an elongated insulative lead body  15 , also carrying three mutually insulated conductors (not shown). Located adjacent the J-shaped distal end of the lead are a ring electrode  21  and an extendible helix electrode  17 , mounted retractably within an insulative electrode head  19 . Each of electrodes  17  and  21  is coupled to one of the conductors within the lead body  15 . Electrodes  17  and  21  are employed for atrial pacing and for sensing atrial depolarizations. An elongated coil electrode  23  is provided, proximal to electrode  21  and coupled to the third conductor within the lead body  15 . A bifurcated connector  13  is positioned at the proximal end of lead  5  and includes three electrical connectors, each coupled to one of the coiled conductors, to connect the conductors to circuitry within housing  11  of device  10 .  
         [0019]     Coronary sinus lead  7  includes an elongated insulative lead body  6 , carrying one conductor, coupled to an elongated coiled defibrillation electrode  8 . Electrode  8 , illustrated in broken outline, is located within the coronary sinus and great vein of the heart. At the proximal end of the lead is a connector plug  4 , which carries an electrical connector, coupled to the coiled conductor.  
         [0020]     According to an embodiment of the present invention, medical device  10  of  FIG. 1  is a pacemaker/cardioverter/defibrillator having electronic circuitry used for generating cardiac pacing pulses for delivering cardioversion and defibrillation shocks and for monitoring the patient&#39;s heart rhythm located within housing  11 . Medical device  10  is shown with the lead connector assemblies  4 ,  13  and  14  inserted within connector block  12 , which serves as a receptacle and electrical connector for receiving connectors  4 ,  13  and  14  and interconnecting the leads to the circuitry within housing  11 . As will be described in more detail, medical device  10  includes one or more secondary sensors that are utilized during detection of cardiac rhythms. The sensor or sensors may either be positioned along a lead or a device housing, such as along a header block of the medical device  10 , or, as illustrated schematically in  FIG. 1  by broken outline, a secondary sensor  30  is positioned along housing  11  of medical device  10 , for example. According to the present invention, the secondary sensor(s) are used for verifying detection of cardiac rhythms, as will be described in detail below.  
         [0021]     Optionally, device may include an uninsulated portion of housing  11  that serves as a subcutaneous defibrillation electrode, used to defibrillate either the atria or ventricles. Other lead configurations and electrode locations may of course be substituted for the lead set illustrated. For example, atrial defibrillation and sensing electrodes might be added to either the coronary sinus lead or the right ventricular lead instead of being located on a separate atrial lead, allowing for a two-lead system.  
         [0022]      FIG. 2  is a schematic diagram of an exemplary medical device of a type in which the present invention may usefully be practiced. As illustrated in  FIG. 2 , an exemplary medical device  120  according to an embodiment of the present invention may be a cardiac pacemaker that includes a hermetically sealed housing  124  containing the electronic circuitry used for generating cardiac pacing pulses and for monitoring the patient&#39;s heart rhythm. Mounted to housing  124  is a header  122  which serves as a receptacle and electrical connector for receiving the connectors  132  and  134  of pacing leads  128  and  130  and interconnecting the leads to the circuitry within housing  124 . Lead  128  is a ventricular lead provided with electrodes  140  and  142  for monitoring right ventricular heart signals. Atrial lead  130  carries electrodes  136  and  138  and is employed for sensing and pacing the patient&#39;s atrium.  
         [0023]     Medical device  120  includes one or more secondary sensors positioned along either one of leads  128 ,  130  or along housing  124  or header  122 , or along any combination of the lead, housing  124 , or header  122 , as described above. The device  120  of  FIG. 2  is shown having a secondary sensor  126  illustrated schematically by broken outline along housing  124  and a secondary sensor  144  positioned along lead  128 . Sensor  144  may be used in conjunction with or as an alternative to sensor  126  for verifying the detection of cardiac rhythms, as described below.  
         [0024]      FIG. 3  is a schematic diagram of an exemplary medical device of a type in which the present invention may usefully be practiced. As illustrated in  FIG. 3  an exemplary medical device  100  according to an embodiment of the present invention may be a subcutaneously implantable monitor that includes a hermetically sealed housing  104  containing the electronic circuitry used for generating cardiac pacing pulses and for monitoring the patient&#39;s heart rhythm and which carries a molded plastic header  108 . Housing  104  and header  108  each carry an electrode  102  and  106 , respectively for monitoring heart rhythm. Also mounted in the header  108  is an antenna  110  for use in communicating between the device and an external programmer. Medical device  100  includes one or more secondary sensors positioned along housing  104  that may be used to verify detection of cardiac rhythms, as described below. Medical device  100  is shown in  FIG. 3 , for example, as having a sensor  112 , illustrated in broken outline, positioned along housing  104 . Signals are detected between the electrodes  102  and  106  and normal detection algorithms are utilized, with signals detected by secondary sensor  112  being utilized to verify detected cardiac rhythms, as will be described below.  
         [0025]      FIG. 4  is a functional schematic diagram of the medical device of  FIG. 1 , in which the present invention may usefully be practiced. This diagram should be taken as exemplary of one type of anti-tachyarrhythmia device in which the invention may be embodied, and not as limiting, as it is believed that the invention may usefully be practiced in a wide variety of device implementations, including devices providing therapies for treating atrial arrhythmias instead of or in addition to ventricular arrhythmias, cardioverters and defibrillators which do not provide anti-tachycardia pacing therapies, anti-tachycardia pacers which do not provide cardioversion or defibrillation, and devices which deliver different forms of anti-arrhythmia therapies such nerve stimulation or drug administration.  
         [0026]     The device is provided with a lead system including electrodes, which may be as illustrated in  FIG. 1 . Alternate lead systems may of course be substituted. If the electrode configuration of  FIG. 1  is employed, the correspondence to the illustrated electrodes is as follows. Electrode  311  corresponds to the electrode associated with the uninsulated portion of the housing  11  of the medical device that serves as a defibrillation electrode. Electrode  320  corresponds to electrode  20  and is a defibrillation electrode located in the right ventricle. Electrode  310  corresponds to electrode  8  and is a defibrillation electrode located in the coronary sinus. Electrode  318  corresponds to electrode  23  and is a defibrillation electrode located in the superior vena cava. Electrodes  324  and  326  correspond to electrodes  24  and  26 , and are used for sensing and pacing in the ventricle. Electrodes  317  and  321  correspond to electrodes  19  and  21  and are used for pacing and sensing in the atrium. Finally, sensor  344  corresponds to the secondary sensor or sensors  30  positioned along housing  11  and/or a lead included with the device.  
         [0027]     Electrodes  310 ,  311 ,  318  and  320  are coupled to high voltage output circuit  234 . Electrodes  324  and  326  are coupled to the R-wave amplifier  200 , which preferably takes the form of an automatic gain controlled amplifier providing an adjustable sensing threshold as a function of the measured R-wave amplitude. A signal is generated on R-out line  202  whenever the signal sensed between electrodes  324  and  326  exceeds the present sensing threshold.  
         [0028]     Electrodes  317  and  321  are coupled to the P-wave amplifier  204 , which preferably also takes the form of an automatic gain controlled amplifier providing an adjustable sensing threshold as a function of the measured R-wave amplitude. A signal is generated on P-out line  206  whenever the signal sensed between electrodes  317  and  321  exceeds the present sensing threshold. The general operation of the R-wave and P-wave amplifiers  200  and  204  may correspond to that disclosed in U.S. Pat. No. 5,117,824 to Keimel, et al., incorporated herein by reference in its entirety. However, any of the numerous prior art sense amplifiers employed in implantable cardiac pacemakers, defibrillators and monitors may also usefully be employed in conjunction with the present invention. Switch matrix  208  is used to select which of the available electrodes are coupled to wide band amplifier  210  for use in digital signal analysis. Selection of electrodes is controlled by the microprocessor  224  via data/address bus  218 , which selections may be varied as desired. Signals from the electrodes selected for coupling to bandpass amplifier  210  are provided to multiplexer  220 , and thereafter converted to multi-bit digital signals by A/D converter  222 , for storage in random access memory  226  under control of direct memory access circuit  228 . Microprocessor  224  may employ digital signal analysis techniques to characterize the digitized signals stored in random access memory  226  to recognize and classify the patient&#39;s heart rhythm employing any of the numerous signal-processing methodologies known to the art.  
         [0029]     Telemetry circuit  330  receives downlink telemetry from and sends uplink telemetry to the patient activator by means of antenna  332 . Data to be uplinked to the activator and control signals for the telemetry circuit are provided by microprocessor  224  via address/data bus  218 . Received telemetry is provided to microprocessor  224  via multiplexer  220 . The atrial and ventricular sense amp circuits  200 ,  204  produce atrial and ventricular EGM signals, which also may be digitized, and uplink telemetered to an associated programmer on receipt of a suitable interrogation command. The device may also be capable of generating so-called marker codes indicative of different cardiac events that it detects. A pacemaker with marker-channel capability is described, for example, in U.S. Pat. No. 4,374,382 to Markowitz, which patent is hereby incorporated by reference herein in its entirety. The particular telemetry system employed is not critical to practicing the invention, and any of the numerous types of telemetry systems known for use in implantable devices may be used. In particular, the telemetry systems as disclosed in U.S. Pat. No. 5,292,343 issued to Blanchette et al., U.S. Pat. No. 5,314,450, issued to Thompson, U.S. Pat. No. 5,354,319, issued to Wybomy et al. U.S. Pat. No. 5,383,909, issued to Keimel, U.S. Pat. No. 5,168,871, issued to Grevious, U.S. Pat. No. 5,107,833 issued to Barsness or U.S. Pat. No. 5,324,315, issued to Grevious, all incorporated herein by reference in their entireties, are suitable for use in conjunction with the present invention. However, the telemetry systems disclosed in the various other patents cited herein which are directed to programmable implanted devices, or similar systems may also be substituted. The telemetry circuit  330  is of course also employed for communication to and from an external programmer, as is conventional in implantable anti-arrhythmia devices.  
         [0030]     Signals detected by the one or more secondary sensors  344  are processed by sensor processing circuitry  342  and utilized by microprocessor  224  to verify detection of cardiac rhythms, as will be described below. In addition, while the present invention may be utilized in a device that does not including pacing therapy, in those devices in which pacing is included, sensors  344  and circuitry  342  may be employed in the conventional fashion described in U.S. Pat. No. 4,428,378 issued to Anderson et al, incorporated herein by reference in its entirety, to regulate the underlying pacing rate of the device in rate responsive pacing modes in addition to functioning as a secondary sensor for verifying the detection of cardiac rhythms.  
         [0031]     The remainder of the circuitry is dedicated to the provision of cardiac pacing, cardioversion and defibrillation therapies, and, for purposes of the present invention may correspond to circuitry known in the prior art. An exemplary apparatus is disclosed for accomplishing pacing, cardioversion and defibrillation functions as follows, although other embodiments may not include pacing. The pacer timing/control circuitry  212  includes programmable digital counters which control the basic time intervals associated with DDD, WI, DVI, VDD, AAI, DDI, DDDR, WIR, DVIR, VDDR, AAIR, DDIR and other modes of single and dual chamber pacing well known to the art. Circuitry  212  also controls escape intervals associated with anti-tachyarrhythmia pacing in both the atrium and the ventricle, employing, any anti-tachyarrhythmia pacing therapies known to the art.  
         [0032]     Intervals defined by pacing circuitry  212  may include atrial and ventricular pacing escape intervals, the refractory periods during which sensed P-waves and R-waves are ineffective to restart timing of the escape intervals and the pulse widths of the pacing pulses. The durations of these intervals are determined by microprocessor  224 , in response to stored data in memory  226  and are communicated to the pacing circuitry  212  via address/data bus  218 . Pacer circuitry  212  also determines the amplitude of the cardiac pacing pulses under control of microprocessor  224 .  
         [0033]     During pacing, the escape interval counters within pacer timing/control circuitry  212  are reset upon sensing of R-waves and P-waves as indicated by signals on lines  202  and  206 , and in accordance with the selected mode of pacing on time-out trigger generation of pacing pulses by pacer output circuits  214  and  216 , which are coupled to electrodes  317 ,  321 ,  324  and  326 . The escape interval counters are also reset on generation of pacing pulses, and thereby control the basic timing of cardiac pacing functions, including anti-tachyarrhythmia pacing.  
         [0034]     The durations of the intervals defined by the escape interval timers are determined by microprocessor  224 , via data/address bus  218 . The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used to measure the durations of R-R intervals, P-P intervals, PR intervals and R-P intervals, which measurements are stored in memory  226 .  
         [0035]     Microprocessor  224  operates as an interrupt driven device, and is responsive to interrupts from pacer timing/control circuitry  212  corresponding to the occurrences of sensed P-waves and R-waves and corresponding to the generation of cardiac pacing pulses. These interrupts are provided via data/address bus  218 . Any necessary mathematical calculations to be performed by microprocessor  224  and any updating of the values or intervals controlled by pacer timing/control circuitry  212  take place following such interrupts. Microprocessor  224  includes associated ROM in which the stored program controlling its operation as described below resides. A portion of the memory  226  may be configured as a plurality of re-circulating buffers, capable of holding series of measured intervals, which may be analyzed in response to the occurrence of a pace or sense interrupt to determine whether the patient&#39;s heart is presently exhibiting atrial or ventricular tachyarrhythmia.  
         [0036]     The cardiac rhythm detection method of the present invention may include any of the numerous available prior art tachyarrhythmia detection algorithms. One preferred embodiment may employ all or a subset of the rule-based detection methods described in U.S. Pat. No. 5,545,186 issued to Olson et al. or in U.S. Pat. No. 5,755,736 issued to Gillberg et al., both incorporated herein by reference in their entireties. However, any of the various other arrhythmia detection methodologies known to the art might also be utilized.  
         [0037]     In the event that an atrial or ventricular tachyarrhythmia is detected, and an anti-tachyarrhythmia pacing regimen is desired, timing intervals for controlling generation of anti-tachyarrhythmia pacing therapies are loaded from microprocessor  224  into the pacer timing and control circuitry  212 , to control the operation of the escape interval counters therein and to define refractory periods during which detection of R-waves and P-waves is ineffective to restart the escape interval counters.  
         [0038]     In the event that generation of a cardioversion or defibrillation pulse is required, microprocessor  224  employs the escape interval counter to control timing of such cardioversion and defibrillation pulses, as well as associated refractory periods. In response to the detection of atrial or ventricular fibrillation or tachyarrhythmia requiring a cardioversion pulse, microprocessor  224  activates cardioversion/defibrillation control circuitry  230 , which initiates charging of the high voltage capacitors  246 ,  248  via charging circuit  236 , under control of high voltage charging control line  240 . The voltage on the high voltage capacitors is monitored via VCAP line  244 , which is passed through multiplexer  220  and in response to reaching a predetermined value set by microprocessor  224 , results in generation of a logic signal on Cap Full (CF) line  254 , terminating charging. Thereafter, timing of the delivery of the defibrillation or cardioversion pulse is controlled by pacer timing/control circuitry  212 . Following delivery of the fibrillation or tachycardia therapy the microprocessor then returns the device to cardiac pacing and awaits the next successive interrupt due to pacing or the occurrence of a sensed atrial or ventricular depolarization. In the illustrated device, delivery of the cardioversion or defibrillation pulses is accomplished by output circuit  234 , under control of control circuitry  230  via control bus  238 . Output circuit  234  determines whether a monophasic or biphasic pulse is delivered, whether the housing  311  serves as cathode or anode and which electrodes are involved in delivery of the pulse.  
         [0039]      FIG. 5  is a functional schematic diagram of the medical device of  FIG. 2 . The medical device of  FIGS. 2 and 5  is essentially a set of subcomponents of the medical device illustrated in  FIGS. 1 and 4 . Like the medical device of  FIG. 4 , the medical device of  FIG. 5  is a microprocessor-controlled device with microprocessor  189  operating under control of programming stored in Read Only Memory (ROM)  191 . In the device as illustrated, electrodes  136  and  138 , intended for location in the atrium of the patient&#39;s heart are coupled to an atrial amplifier  181  which may correspond to atrial amplifier  204  in  FIG. 4 . Similarly, ventricular electrodes  140  and  142  are coupled to ventricular amplifier  182 , which may correspond to ventricular amplifier  200  in  FIG. 4 . The outputs of atrial and ventricular amplifiers  181  and  182  are input into timing and control circuitry  183  which conforms generally to the pacer timing and control circuitry  212  of  FIG. 4 , and which measures intervals between detected depolarizations and controls intervals between delivered pacing pulses as well as generating interrupts via data/address  192  to awake microprocessor  189  in response to delivery of a pacing pulse or sensing of a cardiac depolarization. Intervals between depolarizations measured by timing/control circuitry  183  are stored in Random Access Memory (RAM)  190  until processed by microprocessor  189  to derive average heart rate values. Atrial and ventricular pacing pulses delivered according to one or more of the standard pacing modes described in conjunction with  FIG. 4  are produced by atrial and ventricular pulse generator circuits  184  and  185  which may correspond to pulse generator circuits  215  and  216  in  FIG. 4 .  
         [0040]     As described above, signals detected by the one or more secondary sensors  144 ,  126  are processed by sensor processing circuitry  186  and utilized by microprocessor  189  to verify detection of cardiac rhythms, as will be described below. In addition, sensors  144 ,  126  and circuitry  186  may be employed in the conventional fashion described in U.S. Pat. No. 4,428,378 issued to Anderson et al, incorporated herein by reference in its entirety, to regulate the underlying pacing rate of the device in rate responsive pacing modes and also serves as a secondary sensor for verifying the detection of an arrhythmia.  
         [0041]      FIG. 6  is a functional schematic diagram of the medical device of  FIG. 3 . As illustrated in  FIG. 6 , device  100  consists essentially of a set of subcomponents of the more complex embodiment of the invention disclosed in  FIGS. 4 and 5 , and includes a sense amplifier  152  coupled to electrodes  102  and  106 , illustrated in  FIG. 3 . Sense amplifier  152  may correspond to sense amplifier  204  or  200  in  FIG. 5 . Like the device of  FIGS. 4 and 5 , medical device  100  may be a microprocessor control device operating under control microprocessor  156  with its functionality controlled primarily by software stored in the read only memory associated therein. In this context, amplifier  152  detects the occurrence of heart depolarizations, with timing/control circuitry  154  serving to measure the durations between the detected heart depolarizations and to generate interrupts awakening microprocessor  156  so that it may store, analyze and process the detected intervals. Random Access Memory (RAM)  158  serves to store measured and calculated parameters including the calculated average heart rate values for later telemetry to an external device. Like the device in  FIGS. 4 and 5 , timing and control circuitry communicates with the microprocessor and the remaining circuitry by means of the address/data bus  168 . Telemetry system  162  may correspond to telemetry system  330  in  FIG. 4 , with antenna  110  transmitting and receiving information from the external programmer, including information stored in RAM  158 .  
         [0042]     As described above, signals detected by the one or more secondary sensors  112  are processed by sensor processing circuitry  166 , which may correspond to sensor processing circuitry  342  in  FIG. 4 , and utilized by microprocessor  224  to verify detection of cardiac rhythms, as will be described below.  
         [0043]      FIG. 7  is a schematic diagram of an exemplary medical device according to an embodiment of the present invention. As illustrated in  FIG. 7 , a medical device  400  in which the present invention may be usefully practiced may include a subcutaneous cardioverter/defibrillator, for example. Such a medical device  400  includes a housing  402  having an electrode  404  positioned along a side wall  406  of housing  402  that is to be positioned subcutaneously within the patient. A header  405  of housing  402  is coupled to a subcutaneous lead  408  carrying conventional conductors (not shown) extending therethrough to electrically couple circuitry located within housing  402  to an electrode  410  positioned distally on the lead  408 . The housing  402  and the lead  408  are positioned subcutaneously within the patient so that electrodes  404  and  410  are directed towards the patient&#39;s heart. In particular, according to an embodiment of the present invention, the lead  408  extends posteriorly around the patient&#39;s back and the housing  402  is subcutaneously implanted outside the patient&#39;s ribcage anterior to the cardiac notch, for example, to provide a desired defibrillation vector. Sensing may be performed between electrodes  404  and  410 , or between electrode  404  on the housing  402  and a ring electrode positioned along the lead  408  distally from electrode  410 . Sensing may also be accomplished using two electrodes positioned along the housing  402 .  
         [0044]     As illustrated in  FIG. 7 , medical device  400  includes one or more secondary sensors  407  in combination with any desired number of sensing and/or therapy delivering electrodes, positioned either along lead  408 , housing  402 , or header  405 , or along any combination of lead  408 , housing  402  and header  405 . As illustrated in  FIG. 7 , secondary sensor  407  is positioned along header  407  of housing  402 , although, although other locations may also be utilized for the one or more secondary sensors, such as being positioned along lead  408  or along a separate patch coupled to housing  402  via a separate subcutaneous lead. Secondary sensor(s) may be used in conjunction with or as an alternative to sensor  404  and  410  for verifying the detection of cardiac rhythms, as described below.  
         [0045]     Other subcutaneous cardioverter/defibrillator medical devices in which the present invention may be usefully practiced may include, for example, the devices described in commonly assigned U.S. patent application Ser. No. 11/004,498, entitled “Subcutaneous Implantable Cardioverter/Defibrillator, by Ghanem et al., Dec. 3, 2004, or commonly assigned U.S. patent application Ser. No. ______, entitled “Method and Apparatus for Detection in a Medical Device” (Attorney Docket No. P-10987), by Mitrani et al. and filed Jan. 18, 2005, both incorporated herein by reference in their entireties.  
         [0046]     According to an embodiment of the present invention, the secondary sensor or sensors may correspond to an acoustic sensor, formed, for example, from a piezoelectric material, which may be a piezoelectric ceramic, film, or polymer. In an alternative embodiment, heart sound sensor may be provided as a miniaturized microphone. However, a piezoelectric material does not require an energizing power supply, allowing the battery size required by the medical device to be minimized, reducing the overall size of the device. As described above, the sensor or sensors may be positioned on or within the housing of the medical device, and electrically coupled to a circuit board within device housing, or positioned along one or more leads, or positioned within a header block on the device housing. The sensors are preferably hermetically sealed against body fluids, and mounted on a diaphragm or other component that stabilizes the position of sensor  22  while providing good acoustical coupling. A monitor housing including a microphone diaphragm, for example, which may be adapted for use with the present invention, is generally disclosed in U.S. Pat. No. 6,409,675, issued to Turcott, incorporated herein by reference in its entirety.  
         [0047]     The sensor or sensors may be provided as a hard piezoelectric ceramic, a relatively soft piezoelectric ceramic, or a flexible piezoelectric film formed from a piezoelectric polymer such as polyvinylidene fluoride. For example, a soft piezoelectric ceramic such as Model PZT-5A available from Morgan Electro Ceramics, may utilized.  
         [0048]     In addition, the sensor or sensors may correspond to an activity sensor, a respiration sensor, for example as disclosed in U.S. Pat. No. 5,562,711 issued to Yerich et al or a hemodynamic sensor  140 , a hemodynamic sensor, such as an oxygen saturation sensor in conjunction with associated processing circuitry as described in U.S. Pat. No. 5,903,701 issued to Moore or U.S. Pat. No. 6,198,952 issued to Miesel, a pressure or temperature sensor and associated sensor processing circuitry as described in U.S. Pat. No. 5,564,434 issued to Halperin et al. or an impedance sensor and associated sensor processing circuitry as described in U.S. Pat. No. 5,824,029 issued to Weijand et al., all incorporated herein by reference in their entireties, or may correspond to other types of physiologic sensors, as may be appropriate.  
         [0049]      FIG. 8  is a flowchart of a method for detecting cardiac signals in a medical device according to an embodiment of the present invention. As illustrated in  FIG. 8 , utilizing a medical device according to the present invention, such as those described above, for example, cardiac signals are sensed, Block  400 , using appropriate electrodes associated with sensing. The sensed signals are used to generate an ECG signal and processed using known detection algorithms to determine whether an abnormality, such as a tachyarrhythmias is detected. Once an arrhythmia is detected using known arrhythmia detection algorithms, YES in Block  402 , secondary sensing via the secondary sensor(s) is initiated, Block  404 . Microprocessor  224  receives the signals detected by the one or more secondary sensors and determines whether the secondary signal is consistent with the detected abnormality, i.e., whether the secondary signal is indicative of an arrhythmia, Block  406 . If the secondary signal is consistent with the detected abnormality, detection is confirmed, Block  408 . If the secondary signal is not consistent with the detected abnormality, the likelihood that the detected arrhythmia was noise related or that oversensing has occurred increases, and therefore appropriate adjustments are made, Block  410 .  
         [0050]     In particular, according to an embodiment of the present invention, if the abnormality detected corresponds to a tachycardia event and the signals generated as a result of sensing by the secondary sensor or sensors during the same time period as the signals sensed by the ECG sensing electrodes are slow and/or regular compared with the ECG signal, the likelihood that the event is associated with oversensing on the ECG signal, rather than a tachycardia event, increases. Therefore additional analysis is needed and appropriate adjustments should be made. On the other hand, if the concomitant signals generated as a result of sensing by the secondary sensor or sensors are consistent with the signals sensed from the ECG sensing electrodes (e.g., have approximately the same timing as signals sensed by the ECG sensing electrodes), or if the secondary sensor indicates directly or indirectly a change in patient status, such as decreased patient activity, decreased hemodynamic output, or decreased local tissue perfusion, the identification of the tachycardia event is confirmed.  
         [0051]     According to another embodiment of the present invention, if the abnormality detected by the ECG signal corresponds to a slow, asystolic event and the signals generated as a result of sensing by the secondary sensor or sensors during the same time period as the signals sensed by the ECG sensing electrodes are fast and indicative of a fast heart rate (e.g., for an acoustic sensor, an indication of fast heart rate with S1 heart sounds located approximately coincident with R-waves identified on the ECG signal), the likelihood that the detected event is associated with undersensing on the ECG signal, rather than a slow, asystolic event, increases. Therefore additional analysis is needed and appropriate adjustments should be made. Similarly, if the signals generated as a result of sensing by the secondary sensor or sensors during the same time period indicates directly or indirectly a change in patient status (e.g., decreased patient activity, decreased hemodynamic output, decreased local tissue perfusion), the likelihood of a tachycardia event needing intervention is increased, and additional analysis of the ECG and/or sensor signals is warranted to determine if and/or when therapy should be delivered. On the other hand, if the concomitant signals generated as a result of sensing by the secondary sensor or sensors are determined to corroborate the slow, asystolic rate measured by the ECG signal (e.g., an acoustic signal will have substantial heart sounds occurring with approximately the same timing as signals sensed by the ECG sensing electrodes), then the diagnosis of slow heart rate is confirmed.  
         [0052]     Some of the techniques described above may be embodied as a computer-readable medium comprising instructions for a programmable processor such as a microprocessor. The programmable processor may include one or more individual processors, which may act independently or in concert. A “computer-readable medium” includes but is not limited to any type of computer memory such as floppy disks, conventional hard disks, CR-ROMS, Flash ROMS, nonvolatile ROMS, RAM and a magnetic or optical storage medium. The medium may include instructions for causing a processor to perform any of the features described above for initiating a session of the escape rate variation according to the present invention.  
         [0053]     The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, therefore, that other expedients known to those of skill in the art or disclosed herein may be employed without departing from the invention or the scope of the appended claim. It is therefore to be understood that the invention may be practiced otherwise than as specifically described, without departing from the scope of the present invention. As to every element, it may be replaced by any one of infinite equivalent alternatives, only some of which are disclosed in the specification.