Patent Publication Number: US-7715916-B2

Title: Multi-parameter arrhythmia discrimination

Description:
RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/804,471 filed on Mar. 19, 2004, to issue as U.S. Pat. No. 7,218,966 on Mar. 15, 2007, which claims the benefit of Provisional Patent Application Ser. No. 60/462,272, filed on Apr. 11, 2003, to which Applicant claims priority under 35 U.S.C. §120 and 35 U.S.C. §119(e), respectively, and which are incorporated herein by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to implantable cardiac monitoring and stimulation devices and, more particularly, to multi-parameter arrhythmia discrimination using electrocardiogram information and non-electrophysiological cardiac activity information. 
   BACKGROUND OF THE INVENTION 
   The healthy heart produces regular, synchronized contractions. Rhythmic contractions of the heart are normally controlled by the sinoatrial (SA) node, which is a group of specialized cells located in the upper right atrium. The SA node is the normal pacemaker of the heart, typically initiating 60-100 heartbeats per minute. When the SA node is pacing the heart normally, the heart is said to be in normal sinus rhythm. 
   If the heart&#39;s electrical activity becomes uncoordinated or irregular, the heart is denoted to be arrhythmic. Cardiac arrhythmia impairs cardiac efficiency and can be a potential life-threatening event. Cardiac arrhythmias have a number of etiological sources, including tissue damage due to myocardial infarction, infection, or degradation of the heart&#39;s ability to generate or synchronize the electrical impulses that coordinate contractions. 
   Bradycardia occurs when the heart rhythm is too slow. This condition may be caused, for example, by impaired function of the SA node, denoted sick sinus syndrome, or by delayed propagation or blockage of the electrical impulse between the atria and ventricles. Bradycardia produces a heart rate that is too slow to maintain adequate circulation. 
   When the heart rate is too rapid, the condition is denoted tachycardia. Tachycardia may have its origin in either the atria or the ventricles. Tachycardias occurring in the atria of the heart, for example, include atrial fibrillation and atrial flutter. Both conditions are characterized by rapid contractions of the atria. Besides being hemodynamically inefficient, the rapid contractions of the atria may also adversely affect the ventricular rate. 
   Ventricular tachycardia occurs, for example, when electrical activity arises in the ventricular myocardium at a rate more rapid than the normal sinus rhythm. Ventricular tachycardia can quickly degenerate into ventricular fibrillation. Ventricular fibrillation is a condition denoted by extremely rapid, uncoordinated electrical activity within the ventricular tissue. The rapid and erratic excitation of the ventricular tissue prevents synchronized contractions and impairs the heart&#39;s ability to effectively pump blood to the body, which is a fatal condition unless the heart is returned to sinus rhythm within a few minutes. 
   Implantable cardiac rhythm management systems have been used as an effective treatment for patients with serious arrhythmias. These systems typically include one or more leads and circuitry to sense signals from one or more interior and/or exterior surfaces of the heart. Such systems also include circuitry for generating electrical pulses that are applied to cardiac tissue at one or more interior and/or exterior surfaces of the heart. For example, leads extending into the patient&#39;s heart are connected to electrodes that contact the myocardium for sensing the heart&#39;s electrical signals and for delivering pulses to the heart in accordance with various therapies for treating arrhythmias. 
   Typical Implantable cardioverter/defibrillators (ICDs) include one or more endocardial leads to which at least one defibrillation electrode is connected. Such ICDs are capable of delivering high-energy shocks to the heart, interrupting the ventricular tachyarrhythmia or ventricular fibrillation, and allowing the heart to resume normal sinus rhythm. ICDs may also include pacing functionality. 
   Although ICDs are very effective at preventing Sudden Cardiac Death (SCD), most people at risk of SCD are not provided with implantable defibrillators. Primary reasons for this unfortunate reality include the limited number of physicians qualified to perform transvenous lead/electrode implantation, a limited number of surgical facilities adequately equipped to accommodate such cardiac procedures, and a limited number of the at-risk patient population that may safely undergo the required endocardial or epicardial lead/electrode implant procedure. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to cardiac monitoring and/or stimulation methods and systems that, in general, provide transthoracic monitoring, defibrillation therapies, pacing therapies, or a combination of these capabilities. Embodiments of the present invention are directed to subcutaneous cardiac monitoring and/or stimulation methods and systems that detect and/or treat cardiac activity or arrhythmias. 
   An embodiment of the invention is directed to an arrhythmia discrimination method that involves sensing electrocardiogram signals at a subcutaneous non-intrathoracic location. The electrocardiogram signals may include a cardiac signal and one or both of noise and electrocardiographic artifacts. Signals associated with a non-electrophysiological cardiac source are also received. A sensed electrocardiogram signal is verified to be a cardiac signal using a non-electrophysiological signal. A cardiac arrhythmia is detected using one or both of the sensed electrocardiogram signal and the verified cardiac signal. Treatment of the cardiac arrhythmia is withheld if the sensed signal is not verified to be the cardiac signal. 
   An arrhythmia may be detected using the electrocardiogram signals, and the presence of the arrhythmia may be verified or refuted using the non-electrophysiologic signals. Temporal relationships between the electrocardiogram signals and non-electrophysiologic signals may be determined. A detection window may be initiated in response to receiving the electrocardiogram signal, and used to determine whether the non-electrophysiologic signal is received at a time falling within the detection window. 
   Heart rates may be computed based on both a succession of electrocardiogram signals and a succession of non-electrophysiologic signals. The rates may be used to discriminate between normal sinus rhythm and the arrhythmia. The rates may be compared with arrhythmia thresholds, and used to determine absence of an arrhythmia, such as in response to a first rate exceeding a first arrhythmia threshold and a second rate failing to exceed a second arrhythmia threshold. The presence of an arrhythmia may be determined using a morphology of the electrocardiogram signals, and then verified using the non-electrophysiologic signals. Examples of non-electrophysiologic signals include heart sound signals, subsonic acoustic signals indicative of cardiac activity, pulse pressure signals, impedance signals indicative of cardiac activity, and pulse oximetry signals. 
   In another embodiment of the present invention, defibrillation therapy delivery may be inhibited in response to detecting an arrhythmia using the electrocardiogram signals but not detecting the arrhythmia using the non-electrophysiologic signal. A method of sensing an arrhythmia and inhibiting therapy may involve sensing an electrocardiogram signal at a subcutaneous non-intrathoracic location. A detection window may be defined with a start time determined from the electrocardiogram signal. A signal associated with a non-electrophysiological cardiac source may be received and evaluated within the detection window. The presence or non-presence of a cardiac arrhythmia may be determined using the electrocardiogram signal, and confirmed by the presence of the cardiac arrhythmia as detected by the non-electrophysiological cardiac signal. The start time of a detection window used for confirmation may be associated with an inflection point of the electrocardiogram signal, such as a maxima or a minima. A correlation may be performed between the electrocardiogram signal and the non-electrophysiological cardiac signal. 
   An embodiment of the present invention is directed to an implantable cardiac device including a housing and an electrode arrangement configured for subcutaneous non-intrathoracic placement. Detection circuitry is provided in the housing and coupled to the electrode arrangement. The detection circuitry is configured to detect electrocardiogram signals comprising a cardiac signal and one or both of noise and electrocardiographic artifacts. A sensor configured to sense non-electrophysiologic signals associated with a non-electrophysiological cardiac source is coupled to the detection circuitry. A processor is provided in the housing and coupled to the detection circuitry, sensor, and energy delivery circuitry, and discriminates between normal sinus rhythm and arrhythmia using the electrocardiogram and non-electrophysiologic signals. The processor verifies that the sensed electrocardiogram signal is a cardiac signal using the non-electrophysiological signal. The processor withholds treatment of the cardiac arrhythmia if the sensed signal is not verified to include the cardiac signal. 
   The energy delivery circuitry may include one or both of defibrillation therapy circuitry and pacing therapy circuitry. The sensor may be provided in or on the housing, and/or in or on a lead coupled to the housing. Appropriate sensors include an accelerometer, a microphone, an acoustic transducer, a blood-flow transducer, photoplethysmography circuitry, and a pulse oximeter. 
   The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are views of a transthoracic cardiac sensing and/or stimulation device as implanted in a patient in accordance with an embodiment of the present invention; 
       FIG. 1C  is a block diagram illustrating various components of a transthoracic cardiac sensing and/or stimulation device in accordance with an embodiment of the present invention; 
       FIG. 1D  is a block diagram illustrating various processing and detection components of a transthoracic cardiac sensing and/or stimulation device in accordance with an embodiment of the present invention; 
       FIG. 2  is a diagram illustrating components of a transthoracic cardiac sensing and/or stimulation device including an electrode array in accordance with an embodiment of the present invention; 
       FIG. 3  is a pictorial diagram of a carotid pulse waveform, a phonocardiogram (PCG) waveform, an electrocardiogram (ECG) waveform, and a filtered transthoracic impedance signal for two consecutive heartbeats; 
       FIG. 4  is a graph illustrating two consecutive PQRS complexes and their associated pseudo accelerometer signals, and a detection window for correlation of the signals in accordance with an embodiment of the present invention; 
       FIG. 5  is a flow chart illustrating methods of multi-parameter arrhythmia discrimination in accordance with the present invention; and 
       FIG. 6  is a flow chart illustrating a method of multi-parameter arrhythmia discrimination in accordance with the present invention. 
   

   While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail below. It is to be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS 
   In the following description of the illustrated embodiments, references are made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration, various embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized, and structural and functional changes may be made without departing from the scope of the present invention. 
   An implanted device according to the present invention may include one or more of the features, structures, methods, or combinations thereof described hereinbelow. For example, a cardiac monitor or a cardiac stimulator may be implemented to include one or more of the advantageous features and/or processes described below. It is intended that such a monitor, stimulator, or other implanted or partially implanted device need not include all of the features described herein, but may be implemented to include selected features that provide for unique structures and/or functionality. Such a device may be implemented to provide a variety of therapeutic or diagnostic functions. 
   In general terms, a cardiac signal discrimination arrangement and method may be used with a subcutaneous cardiac monitoring and/or stimulation device. One such device is an implantable transthoracic cardiac sensing and/or stimulation (ITCS) device that may be implanted under the skin in the chest region of a patient. The ITCS device may, for example, be implanted subcutaneously such that all or selected elements of the device are positioned on the patient&#39;s front, back, side, or other body locations suitable for sensing cardiac activity and delivering cardiac stimulation therapy. It is understood that elements of the ITCS device may be located at several different body locations, such as in the chest, abdominal, or subclavian region with electrode elements respectively positioned at different regions near, around, in, or on the heart. 
   The primary housing (e.g., the active or non-active can) of the ITCS device, for example, may be configured for positioning outside of the rib cage at an intercostal or subcostal location, within the abdomen, or in the upper chest region (e.g., subclavian location, such as above the third rib). In one implementation, one or more electrodes may be located on the primary housing and/or at other locations about, but not in direct contact with the heart, great vessel or coronary vasculature. 
   In another implementation, one or more leads incorporating electrodes may be located in direct contact with the heart, great vessel or coronary vasculature, such as via one or more leads implanted by use of conventional transvenous delivery approaches. In a further implementation, for example, one or more subcutaneous electrode subsystems or electrode arrays may be used to sense cardiac activity and deliver cardiac stimulation energy in an ITCS device configuration employing an active can or a configuration employing a non-active can. Electrodes may be situated at anterior and/or posterior locations relative to the heart. Examples of useful subcutaneous electrodes, electrode arrays, and orientations of same are described in commonly owned U.S. Pat. No. 7,499,750, and U.S. Publication No. 2004/0230230, which are hereby incorporated herein by reference. 
   Certain configurations illustrated herein are generally described as capable of implementing various functions traditionally performed by an implantable cardioverter/defibrillator (ICD), and may operate in numerous cardioversion/defibrillation modes as are known in the art. Examples of ICD circuitry, structures, and functionality, aspects of which may be incorporated in an ITCS device in accordance with the present invention, are disclosed in commonly owned U.S. Pat. Nos. 5,133,353; 5,179,945; 5,314,459; 5,318,597; 5,620,466; and 5,662,688, which are hereby incorporated herein by reference in their respective entireties. 
   In particular configurations, systems and methods may perform functions traditionally performed by pacemakers, such as providing various pacing therapies as are known in the art, in addition to cardioversion/defibrillation therapies. Examples of pacemaker circuitry, structures, and functionality, aspects of which may be incorporated in an ITCS device in accordance with the present invention, are disclosed in commonly owned U.S. Pat. Nos. 4,562,841; 5,284,136; 5,376,106; 5,036,849; 5,540,727; 5,836,987; 6,044,298; and 6,055,454, which are hereby incorporated herein by reference in their respective entireties. It is understood that ITCS device configurations may provide for non-physiologic pacing support in addition to, or to the exclusion of, bradycardia and/or anti-tachycardia pacing therapies. 
   An ITCS device in accordance with the present invention may implement diagnostic and/or monitoring functions as well as provide cardiac stimulation therapy. Examples of cardiac monitoring circuitry, structures, and functionality, aspects of which may be incorporated in an ITCS device in accordance with the present invention, are disclosed in commonly owned U.S. Pat. Nos. 5,313,953; 5,388,578; and 5,411,031, which are hereby incorporated herein by reference in their respective entireties. 
   An ITCS device may be used to implement various diagnostic functions, which may involve performing rate-based, pattern and rate-based, and/or morphological tachyarrhythmia discrimination analyses. Subcutaneous, cutaneous, and/or external sensors may be employed to acquire physiologic and non-physiologic information for purposes of enhancing tachyarrhythmia detection and termination. It is understood that configurations, features, and combination of features described in the present disclosure may be implemented in a wide range of implantable medical devices, and that such embodiments and features are not limited to the particular devices described herein. 
   Referring now to  FIGS. 1A and 1B  of the drawings, there is shown a configuration of a transthoracic cardiac sensing and/or stimulation (ITCS) device having components implanted in the chest region of a patient at different locations. In the particular configuration shown in  FIGS. 1A and 1B , the ITCS device includes a housing  102  within which various cardiac sensing, detection, processing, and energy delivery circuitry may be housed. It is understood that the components and functionality depicted in the figures and described herein may be implemented in hardware, software, or a combination of hardware and software. It is further understood that the components and functionality depicted as separate or discrete blocks/elements in the figures may be implemented in combination with other components and functionality, and that the depiction of such components and functionality in individual or integral form is for purposes of clarity of explanation, and not of limitation. 
   Communications circuitry is disposed within the housing  102  for facilitating communication between the ITCS device and an external communication device, such as a portable or bed-side communication station, patient-carried/worn communication station, or external programmer, for example. The communications circuitry may also facilitate unidirectional or bidirectional communication with one or more external, cutaneous, or subcutaneous physiologic or non-physiologic sensors. The housing  102  is typically configured to include one or more electrodes (e.g., can electrode and/or indifferent electrode). Although the housing  102  is typically configured as an active can, it is appreciated that a non-active can configuration may be implemented, in which case at least two electrodes spaced apart from the housing  102  are employed. 
   In the configuration shown in  FIGS. 1A and 1B , a subcutaneous electrode  104  may be positioned under the skin in the chest region and situated distal from the housing  102 . The subcutaneous and, if applicable, housing electrode(s) may be positioned about the heart at various locations and orientations, such as at various anterior and/or posterior locations relative to the heart. The subcutaneous electrode  104  is coupled to circuitry within the housing  102  via a lead assembly  106 . One or more conductors (e.g., coils or cables) are provided within the lead assembly  106  and electrically couple the subcutaneous electrode  104  with circuitry in the housing  102 . One or more sense, sense/pace, or defibrillation electrodes may be situated on the elongated structure of the electrode support, the housing  102 , and/or the distal electrode assembly (shown as subcutaneous electrode  104  in the configuration shown in  FIGS. 1A and 1B ). 
   In one configuration, the lead assembly  106  is generally flexible and has a construction similar to conventional implantable, medical electrical leads (e.g., defibrillation leads or combined defibrillation/pacing leads). In another configuration, the lead assembly  106  is constructed to be somewhat flexible, yet has an elastic, spring, or mechanical memory that retains a desired configuration after being shaped or manipulated by a clinician. For example, the lead assembly  106  may incorporate a gooseneck or braid system that may be distorted under manual force to take on a desired shape. In this manner, the lead assembly  106  may be shape-fit to accommodate the unique anatomical configuration of a given patient, and generally retains a customized shape after implantation. Shaping of the lead assembly  106  according to this configuration may occur prior to, and during, ITCS device implantation. 
   In accordance with a further configuration, the lead assembly  106  includes a rigid electrode support assembly, such as a rigid elongated structure that positionally stabilizes the subcutaneous electrode  104  with respect to the housing  102 . In this configuration, the rigidity of the elongated structure maintains a desired spacing between the subcutaneous electrode  104  and the housing  102 , and a desired orientation of the subcutaneous electrode  104 /housing  102  relative to the patient&#39;s heart. The elongated structure may be formed from a structural plastic, composite or metallic material, and includes, or is covered by, a biocompatible material. Appropriate electrical isolation between the housing  102  and subcutaneous electrode  104  is provided in cases where the elongated structure is formed from an electrically conductive material, such as metal. 
   In one configuration, the rigid electrode support assembly and the housing  102  define a unitary structure (e.g., a single housing/unit). The electronic components and electrode conductors/connectors are disposed within or on the unitary ITCS device housing/electrode support assembly. At least two electrodes are supported on the unitary structure near opposing ends of the housing/electrode support assembly. The unitary structure may have an arcuate or angled shape, for example. 
   According to another configuration, the rigid electrode support assembly defines a physically separable unit relative to the housing  102 . The rigid electrode support assembly includes mechanical and electrical couplings that facilitate mating engagement with corresponding mechanical and electrical couplings of the housing  102 . For example, a header block arrangement may be configured to include both electrical and mechanical couplings that provide for mechanical and electrical connections between the rigid electrode support assembly and housing  102 . The header block arrangement may be provided on the housing  102  or the rigid electrode support assembly. Alternatively, a mechanical/electrical coupler may be used to establish mechanical and electrical connections between the rigid electrode support assembly and housing  102 . In such a configuration, a variety of different electrode support assemblies of varying shapes, sizes, and electrode configurations may be made available for physically and electrically connecting to a standard ITCS device housing  102 . 
   It is noted that the electrodes and the lead assembly  106  may be configured to assume a variety of shapes. For example, the lead assembly  106  may have a wedge, chevron, flattened oval, or a ribbon shape, and the subcutaneous electrode  104  may include a number of spaced electrodes, such as an array or band of electrodes. Moreover, two or more subcutaneous electrodes  104  may be mounted to multiple electrode support assemblies  106  to achieve a desired spaced relationship amongst subcutaneous electrodes  104 . 
   An ITCS device may incorporate circuitry, structures, and functionality of the subcutaneous implantable medical devices disclosed in commonly owned U.S. Pat. Nos. 5,203,348; 5,230,337; 5,360,442; 5,366,496; 5,397,342; 5,391,200; 5,545,202; 5,603,732; and 5,916,243, which are hereby incorporated herein by reference in their respective entireties. 
     FIG. 1C  is a block diagram depicting various components of an ITCS device in accordance with one configuration. According to this configuration, the ITCS device incorporates a processor-based control system  205 , which includes a micro-processor  206  coupled to appropriate memory (volatile and non-volatile)  209 , it being understood that any logic-based control architecture may be used. The control system  205  is coupled to circuitry and components to sense, detect, and analyze electrical signals produced by the heart and deliver electrical stimulation energy to the heart under predetermined conditions to treat cardiac arrhythmias. In certain configurations, the control system  205  and associated components also provide pacing therapy to the heart. The electrical energy delivered by the ITCS device may be in the form of low energy pacing pulses or high-energy pulses for cardioversion or defibrillation. 
   Cardiac signals are sensed using the subcutaneous electrode(s)  214  and the can or indifferent electrode  207  provided on the ITCS device housing. Cardiac signals may also be sensed using only the subcutaneous electrodes  214 , such as in a non-active can configuration. As such, unipolar, bipolar, or combined unipolar/bipolar electrode configurations as well as multi-element electrodes and combinations of noise canceling and standard electrodes may be employed. The sensed cardiac signals are received by sensing circuitry  204 , which includes sense amplification circuitry and may also include filtering circuitry and an analog-to-digital (A/D) converter. The sensed cardiac signals processed by the sensing circuitry  204  may be received by noise reduction circuitry  203 , which may further reduce noise before signals are sent to the detection circuitry  202 . 
   Noise reduction circuitry  203  may also be incorporated after sensing circuitry  204  in cases where high power or computationally intensive noise reduction algorithms are required. The noise reduction circuitry  203 , by way of amplifiers used to perform operations with the electrode signals, may also perform the function of the sensing circuitry  204 . Combining the functions of sensing circuitry  204  and noise reduction circuitry  203  may be useful to minimize the necessary componentry and lower the power requirements of the system. 
   In the illustrative configuration shown in  FIG. 1C , the detection circuitry  202  is coupled to, or otherwise incorporates, noise reduction circuitry  203 . The noise reduction circuitry  203  operates to improve the signal-to-noise ratio (SNR) of sensed cardiac signals by removing noise content of the sensed cardiac signals introduced from various sources. Typical types of transthoracic cardiac signal noise includes electrical noise and noise produced from skeletal muscles, for example. 
   Detection circuitry  202  typically includes a signal processor that coordinates analysis of the sensed cardiac signals and/or other sensor inputs to detect cardiac arrhythmias, such as, in particular, tachyarrhythmia. Rate based and/or morphological discrimination algorithms may be implemented by the signal processor of the detection circuitry  202  to detect and verify the presence and severity of an arrhythmic episode. Examples of arrhythmia detection and discrimination circuitry, structures, and techniques, aspects of which may be implemented by an ITCS device in accordance with the present invention, are disclosed in commonly owned U.S. Pat. Nos. 5,301,677 and 6,438,410, which are hereby incorporated herein by reference in their respective entireties. 
   The detection circuitry  202  communicates cardiac signal information to the control system  205 . Memory circuitry  209  of the control system  205  contains parameters for operating in various sensing, defibrillation, and, if applicable, pacing modes, and stores data indicative of cardiac signals received by the detection circuitry  202 . The memory circuitry  209  may also be configured to store historical ECG and therapy data, which may be used for various purposes and transmitted to an external receiving device as needed or desired. 
   In certain configurations, the ITCS device may include diagnostics circuitry  210 . The diagnostics circuitry  210  typically receives input signals from the detection circuitry  202  and the sensing circuitry  204 . The diagnostics circuitry  210  provides diagnostics data to the control system  205 , it being understood that the control system  205  may incorporate all or part of the diagnostics circuitry  210  or its functionality. The control system  205  may store and use information provided by the diagnostics circuitry  210  for a variety of diagnostics purposes. This diagnostic information may be stored, for example, subsequent to a triggering event or at predetermined intervals, and may include system diagnostics, such as power source status, therapy delivery history, and/or patient diagnostics. The diagnostic information may take the form of electrical signals or other sensor data acquired immediately prior to therapy delivery. 
   According to a configuration that provides cardioversion and defibrillation therapies, the control system  205  processes cardiac signal data received from the detection circuitry  202  and initiates appropriate tachyarrhythmia therapies to terminate cardiac arrhythmic episodes and return the heart to normal sinus rhythm. The control system  205  is coupled to shock therapy circuitry  216 . The shock therapy circuitry  216  is coupled to the subcutaneous electrode(s)  214  and the can or indifferent electrode  207  of the ITCS device housing. Upon command, the shock therapy circuitry  216  delivers cardioversion and defibrillation stimulation energy to the heart in accordance with a selected cardioversion or defibrillation therapy. In a less sophisticated configuration, the shock therapy circuitry  216  is controlled to deliver defibrillation therapies, in contrast to a configuration that provides for delivery of both cardioversion and defibrillation therapies. Examples of ICD high energy delivery circuitry, structures, and functionality, aspects of which may be incorporated in an ITCS device of a type that may benefit from aspects of the present invention are disclosed in commonly owned U.S. Pat. Nos. 5,372,606; 5,411,525; 5,468,254; and 5,634,938, which are hereby incorporated herein by reference in their respective entireties. 
   In accordance with another configuration, an ITCS device may incorporate a cardiac pacing capability in addition to cardioversion and/or defibrillation capabilities. As is shown in dotted lines in  FIG. 1C , the ITCS device may include pacing therapy circuitry  230 , which is coupled to the control system  205  and the subcutaneous and can/indifferent electrodes  214 ,  207 . Upon command, the pacing therapy circuitry delivers pacing pulses to the heart in accordance with a selected pacing therapy. Control signals, developed in accordance with a pacing regimen by pacemaker circuitry within the control system  205 , are initiated and transmitted to the pacing therapy circuitry  230  where pacing pulses are generated. A pacing regimen may be modified by the control system  205 . 
   A number of cardiac pacing therapies may be useful in a transthoracic cardiac monitoring and/or stimulation device. Such cardiac pacing therapies may be delivered via the pacing therapy circuitry  230  as shown in  FIG. 1C . Alternatively, cardiac pacing therapies may be delivered via the shock therapy circuitry  216 , which effectively obviates the need for separate pacemaker circuitry. 
   The ITCS device shown in  FIG. 1C  is configured to receive signals from one or more physiologic and/or non-physiologic sensors in accordance with embodiments of the present invention. Depending on the type of sensor employed, signals generated by the sensors may be communicated to transducer circuitry coupled directly to the detection circuitry  202  or indirectly via the sensing circuitry  204 . It is noted that certain sensors may transmit sense data to the control system  205  without processing by the detection circuitry  202 . 
   Non-electrophysiological cardiac sensors may be coupled directly to the detection circuitry  202  or indirectly via the sensing circuitry  204 . Non-electrophysiological cardiac sensors sense cardiac activity that is non-electrophysiological in nature. Examples of non-electrophysiological cardiac sensors include blood oxygen sensors, transthoracic impedance sensors, blood volume sensors, acoustic sensors and/or pressure transducers, and accelerometers. Signals from these sensors are developed based on cardiac activity, but are not derived directly from electrophysiological sources (e.g., R-waves or P-waves). A non-electrophysiological cardiac sensor  261 , as is illustrated in  FIG. 1C , may be connected to one or more of the sensing circuitry  204 , detection circuitry  202  (connection not shown for clarity), and the control system  205 . 
   Communications circuitry  218  is coupled to the microprocessor  206  of the control system  205 . The communications circuitry  218  allows the ITCS device to communicate with one or more receiving devices or systems situated external to the ITCS device. By way of example, the ITCS device may communicate with a patient-worn, portable or bedside communication system via the communications circuitry  218 . In one configuration, one or more physiologic or non-physiologic sensors (subcutaneous, cutaneous, or external of patient) may be equipped with a short-range wireless communication interface, such as an interface conforming to a known communications standard, such as Bluetooth or IEEE 802 standards. Data acquired by such sensors may be communicated to the ITCS device via the communications circuitry  218 . It is noted that physiologic or non-physiologic sensors equipped with wireless transmitters or transceivers may communicate with a receiving system external of the patient. 
   The communications circuitry  218  may allow the ITCS device to communicate with an external programmer. In one configuration, the communications circuitry  218  and the programmer unit (not shown) use a wire loop antenna and a radio frequency telemetric link, as is known in the art, to receive and transmit signals and data between the programmer unit and communications circuitry  218 . In this manner, programming commands and data are transferred between the ITCS device and the programmer unit during and after implant. Using a programmer, a physician is able to set or modify various parameters used by the ITCS device. For example, a physician may set or modify parameters affecting sensing, detection, pacing, and defibrillation functions of the ITCS device, including pacing and cardioversion/defibrillation therapy modes. 
   Typically, the ITCS device is encased and hermetically sealed in a housing suitable for implanting in a human body as is known in the art. Power to the ITCS device is supplied by an electrochemical power source  220  housed within the ITCS device. In one configuration, the power source  220  includes a rechargeable battery. According to this configuration, charging circuitry is coupled to the power source  220  to facilitate repeated non-invasive charging of the power source  220 . The communications circuitry  218 , or separate receiver circuitry, is configured to receive RF energy transmitted by an external RF energy transmitter. The ITCS device may, in addition to a rechargeable power source, include a non-rechargeable battery. It is understood that a rechargeable power source need not be used, in which case a long-life non-rechargeable battery is employed. 
     FIG. 1D  illustrates a configuration of detection circuitry  302  of an ITCS device, which includes one or both of rate detection circuitry  310  and morphological analysis circuitry  312 . Detection and verification of arrhythmias may be accomplished using rate-based discrimination algorithms as known in the art implemented by the rate detection circuitry  310 . Arrhythmic episodes may also be detected and verified by morphology-based analysis of sensed cardiac signals as is known in the art. Tiered or parallel arrhythmia discrimination algorithms may also be implemented using both rate-based and morphologic-based approaches. Further, a rate and pattern-based arrhythmia detection and discrimination approach may be employed to detect and/or verify arrhythmic episodes, such as by use of the approaches disclosed in U.S. Pat. Nos. 6,487,443; 6,259,947; 6,141,581; 5,855,593; and 5,545,186, which are hereby incorporated herein by reference. 
   The detection circuitry  302 , which is coupled to a microprocessor  306 , may be configured to incorporate, or communicate with, specialized circuitry for processing sensed cardiac signals in manners particularly useful in a transthoracic cardiac sensing and/or stimulation device. As is shown by way of example in  FIG. 1D , the detection circuitry  302  may receive information from multiple physiologic and non-physiologic sensors. Transthoracic acoustics, for example, may be monitored using an appropriate acoustic sensor. Heart sounds, for example, may be detected and processed by non-electrophysiologic cardiac sensor processing circuitry  318  for a variety of purposes. The acoustics data is transmitted to the detection circuitry  302 , via a hardwire or wireless link, and used to enhance cardiac signal detection and/or arrhythmia detection. For example, acoustic information may be used in accordance with the present invention to corroborate ECG rate-based discrimination of arrhythmias. 
   The detection circuitry  302  may also receive information from one or more sensors that monitor skeletal muscle activity. In addition to cardiac activity signals, transthoracic electrodes readily detect skeletal muscle signals. Such skeletal muscle signals may be used to determine the activity level of the patient. In the context of cardiac signal detection, such skeletal muscle signals are considered artifacts of the cardiac activity signal, which may be viewed as noise. Processing circuitry  316  receives signals from one or more skeletal muscle sensors, and transmits processed skeletal muscle signal data to the detection circuitry  302 . This data may be used to discriminate normal cardiac sinus rhythm with skeletal muscle noise from cardiac arrhythmias. 
   As was previously discussed, the detection circuitry  302  is coupled to, or otherwise incorporates, noise-processing circuitry  314 . The noise processing circuitry  314  processes sensed cardiac signals to improve the SNR of sensed cardiac signals by reducing noise content of the sensed cardiac signals. 
   The components, functionality, and structural configurations depicted in  FIGS. 1A-1D  are intended to provide an understanding of various features and combination of features that may be incorporated in an ITCS device. It is understood that a wide variety of ITCS and other implantable cardiac monitoring and/or stimulation device configurations are contemplated, ranging from relatively sophisticated to relatively simple designs. As such, particular ITCS or cardiac monitoring and/or stimulation device configurations may include particular features as described herein, while other such device configurations may exclude particular features described herein. 
   In accordance with embodiments of the invention, an ITCS device may be implemented to include a subcutaneous electrode system that provides for one or both of cardiac sensing and arrhythmia therapy delivery. According to one approach, an ITCS device may be implemented as a chronically implantable system that performs monitoring, diagnostic and/or therapeutic functions. The ITCS device may automatically detect and treat cardiac arrhythmias. 
   In one configuration, an ITCS device includes a pulse generator and one or more electrodes that are implanted subcutaneously in the chest region of the body, such as in the anterior thoracic region of the body. The ITCS device may be used to provide atrial and/or ventricular therapy for bradycardia and tachycardia arrhythmias. Tachyarrhythmia therapy may include cardioversion, defibrillation and anti-tachycardia pacing (ATP), for example, to treat atrial or ventricular tachycardia or fibrillation. Bradycardia therapy may include temporary post-shock pacing for bradycardia or asystole. Methods and systems for implementing post-shock pacing for bradycardia or asystole are described in commonly owned U.S. Patent Publication No. 2004/0172066, which is incorporated herein by reference in its entirety. 
   In one configuration, an ITCS device according to one approach may utilize conventional pulse generator and subcutaneous electrode implant techniques. The pulse generator device and electrodes may be chronically implanted subcutaneously. Such an ITCS may be used to automatically detect and treat arrhythmias similarly to conventional implantable systems. In another configuration, the ITCS device may include a unitary structure (e.g., a single housing/unit). The electronic components and electrode conductors/connectors are disposed within or on the unitary ITCS device housing/electrode support assembly. 
   The ITCS device contains the electronics and may be similar to a conventional implantable defibrillator. High voltage shock therapy may be delivered between two or more electrodes, one of which may be the pulse generator housing (e.g., can), placed subcutaneously in the thoracic region of the body. 
   Additionally or alternatively, the ITCS device may also provide lower energy electrical stimulation for bradycardia therapy. The ITCS device may provide brady pacing similarly to a conventional pacemaker. The ITCS device may provide temporary post-shock pacing for bradycardia or asystole. Sensing and/or pacing may be accomplished using sense/pace electrodes positioned on an electrode subsystem also incorporating shock electrodes, or by separate electrodes implanted subcutaneously. 
   The ITCS device may detect a variety of physiological signals that may be used in connection with various diagnostic, therapeutic, or monitoring implementations in accordance with the present invention. For example, the ITCS device may include sensors or circuitry for detecting pulse pressure signals, blood oxygen level, heart sounds, cardiac acceleration, and other non-electrophysiological signals related to cardiac activity. In one embodiment, the ITCS device senses intrathoracic impedance, from which various respiratory parameters may be derived, including, for example, respiratory tidal volume and minute ventilation. Sensors and associated circuitry may be incorporated in connection with an ITCS device for detecting one or more body movement or body position related signals. For example, accelerometers and GPS devices may be employed to detect patient activity, patient location, body orientation, or torso position. 
   The ITCS device may be used within the structure of an advanced patient management (APM) system. Advanced patient management systems may allow physicians to remotely and automatically monitor cardiac and respiratory functions, as well as other patient conditions. In one example, implantable cardiac rhythm management systems, such as cardiac pacemakers, defibrillators, and resynchronization devices, may be equipped with various telecommunications and information technologies that enable real-time data collection, diagnosis, and treatment of the patient. Various embodiments described herein may be used in connection with advanced patient management. Methods, structures, and/or techniques described herein, which may be adapted to provide for remote patient/device monitoring, diagnosis, therapy, or other APM related methodologies, may incorporate features of one or more of the following references: U.S. Pat. Nos. 6,221,011; 6,270,457; 6,277,072; 6,280,380; 6,312,378; 6,336,903; 6,358,203; 6,368,284; 6,398,728; and 6,440,066, which are hereby incorporated herein by reference. 
   An ITCS device according to one approach provides an easy to implant therapeutic, diagnostic, or monitoring system. The ITCS system may be implanted without the need for intravenous or intrathoracic access, providing a simpler, less invasive implant procedure and minimizing lead and surgical complications. In addition, this system would have advantages for use in patients for whom transvenous lead systems cause complications. Such complications include, but are not limited to, surgical complications, infection, insufficient vessel patency, complications associated with the presence of artificial valves, and limitations in pediatric patients due to patient growth, among others. An ITCS system according to this approach is distinct from conventional approaches in that it may be configured to include a combination of two or more electrode subsystems that are implanted subcutaneously in the anterior thorax. 
   In one configuration, as is illustrated in  FIG. 2 , electrode subsystems of an ITCS system are arranged about a patient&#39;s heart  510 . The ITCS system includes a first electrode subsystem, comprising a can electrode  502 , and a second electrode subsystem  504  that includes at least two electrodes or at least one multi-element electrode. The second electrode subsystem  504  may include a number of electrodes used for sensing and/or electrical stimulation, and may also include non-electrophysiologic sensors. 
   In various configurations, the second electrode subsystem  504  may include a combination of electrodes. The combination of electrodes of the second electrode subsystem  504  may include coil electrodes, tip electrodes, ring electrodes, multi-element coils, spiral coils, spiral coils mounted on non-conductive backing, screen patch electrodes, and other electrode configurations. A suitable non-conductive backing material is silicone rubber, for example. 
   The can electrode  502  is positioned on the housing  501  that encloses the ITCS device electronics. In one embodiment, the can electrode  502  includes the entirety of the external surface of housing  501 . In other embodiments, various portions of the housing  501  may be electrically isolated from the can electrode  502  or from tissue. For example, the active area of the can electrode  502  may include all or a portion of either the anterior or posterior surface of the housing  501  to direct current flow in a manner advantageous for cardiac sensing and/or stimulation. 
   In accordance with one embodiment, the housing  501  may resemble that of a conventional implantable ICD, is approximately 20-100 cc in volume, with a thickness of 0.4 to 2 cm and with a surface area on each face of approximately 30 to 100 cm 2 . As previously discussed, portions of the housing may be electrically isolated from tissue to optimally direct current flow. For example, portions of the housing  501  may be covered with a non-conductive, or otherwise electrically resistive, material to direct current flow. Suitable non-conductive material coatings include those formed from silicone rubber, polyurethane, or parylene, for example. 
   In addition, or alternatively, all or portions of the housing  501  may be treated to change the electrical conductivity characteristics thereof for purposes of optimally directing current flow. Various known techniques may be employed to modify the surface conductivity characteristics of the housing  501 , such as by increasing or decreasing surface conductivity, to optimize current flow. Such techniques may include those that mechanically or chemically alter the surface of the housing  501  to achieve desired electrical conductivity characteristics. 
   As was discussed above, cardiac signals collected from subcutaneously implanted electrodes may be corrupted by noise. In addition, certain noise sources have frequency characteristics similar to those of the cardiac signal. Such noise may lead to over sensing and spurious shocks. Due to the possibility of relatively high amplitude of the noise signal and overlapping frequency content, filtering alone does not lead to complete suppression of the noise. In addition, filter performance is not generally sufficiently robust against the entire class of noises encountered. Further, known adaptive filtering approaches require a reference signal that is often unknown for situations when a patient experiences VF or high amplitude noise. 
   In accordance with one approach of the present invention, an ITCS device may be implemented to discriminate cardiac signals within a group of separated signals, such as those obtained from a blind source separation (BSS) technique. Devices and methods of blind source separation are further described in commonly owned U.S. Pat. No. 7,236,819, hereby incorporated herein by reference. Devices and methods associated with another useful signal separation approach that uses noise canceling electrodes are further described in commonly owned U.S. Pat. No. 7,499,750, hereby incorporated herein by reference. 
   Information from a non-electrophysiologic sensor  503 , such as those described previously, may be used to improve the accuracy of arrhythmia discrimination, such as ECG or other rate-based arrhythmia discrimination. A signal independent of cardiac electrical activity, such as an acoustic signal of cardiac heart-sounds, an accelerometer, a blood sensor, or other non-electrophysiologic source sensor, may be used to improve the detection and discrimination of arrhythmia from normal sinus rhythm (NSR) in the presence of noise. The non-electrophysiologic sensor  503  may be provided in or on the housing  501 , as illustrated in  FIG. 2 , or may be provided as part of the second electrode subsystem  504 , as described earlier. It is also contemplated that the non-electrophysiologic sensor  503  may be directly coupled to the housing  501  using an additional lead, or be wirelessly coupled as described with reference to  FIGS. 1C and 1D . 
   In an embodiment of the present invention, heart sounds are used to aid in signal discrimination when detecting various heart rhythms in the presence of electrical noise and/or electrocardiographic artifacts. Because the additional discriminating non-electrophysiologic signal is time correlated with respect to the cardiac electrophysiological signals, the non-electrophysiologic signal may provide information about a patient&#39;s rhythm state even in the presence of electrical noise and/or electrocardiographic artifacts. For example, the non-electrophysiologic signal may be used to verify that the ECG signal contains a cardiac signal having a QRS complex, and only ECG signals with QRS complexes are verified ECG signals. Subsequent analysis may require that only verified ECG signals are used for calculations of, for example, heart rate. This provides for more robust algorithms that are less susceptible to contamination from electrical interference and noise. 
   In one embodiment, a subcutaneous sensor, such as an accelerometer or acoustic transducer, may be used to detect heart sounds. The heart sounds may be used together with rate, curvature, and other ECG information to discriminate normal sinus with electrical noise from potentially lethal arrhythmias such as ventricular tachycardia and ventricular fibrillation. An ITCS device may utilize one or more of the presence, characteristics, and frequency of occurrence of the heart sound combined with ECG information when performing signal or rhythm discrimination. A heart rate determined from the ECG signal may, for example, be analyzed along with heart sound information for diagnostic purposes. High ECG heart rate detection along with normal rate heart sounds would indicate the presence of noise in the ECG signal. High ECG heart rate detection along with modified heart sounds would indicate a potentially lethal arrhythmia. It is noted that ECG morphology or other techniques could replace rate in the example above. It should also be noted that other sensor derived signals could replace heart sounds. For example, impedance, pulse pressure, blood volume/flow, or cardiac accelerations could be used. 
   Various types of acoustic sensors may be used to detect heart sounds. Examples of such acoustic sensors include diaphragm based acoustic sensors, MEMS-based acoustic sensors such as a MEMS-based acoustic transducer, fiber optic acoustic sensors, piezoelectric sensors, and accelerometer based acoustic sensors and arrays. These sensors may be used to detect the audio frequency pressure waves associated with the heart sounds, and may also be used to detect other non-electrophysiologic cardiac related signals. 
   The presence of cardiac pulse, or heartbeat, in a patient is generally detected by palpating the patient&#39;s neck and sensing changes in the volume of the patient&#39;s carotid artery due to blood pumped from the patient&#39;s heart. A graph of a carotid pulse signal  810 , representative of the physical expansion and contraction of a patient&#39;s carotid artery during two consecutive pulses, or heartbeats, is shown at the top of  FIG. 3 . When the heart&#39;s ventricles contract during a heartbeat, a pressure wave is sent throughout the patient&#39;s peripheral circulation system. The carotid pulse signal  810  shown in  FIG. 3  rises with the ventricular ejection of blood at systole and peaks when the pressure wave from the heart reaches a maximum. The carotid pulse signal  810  falls off again as the pressure subsides toward the end of each pulse. 
   The opening and closing of the patient&#39;s heart valves during a heartbeat causes high-frequency vibrations in the adjacent heart wall and blood vessels. These vibrations can be heard in the patient&#39;s body as heart sounds, and may be detected by sensors, as described earlier. A conventional phonocardiogram (PCG) transducer placed on a patient converts the acoustical energy of the heart sounds to electrical energy, resulting in a PCG waveform  820  that may be recorded and displayed, as shown by the graph in the upper middle portion of  FIG. 3 . 
   As indicated by the PCG waveform  820  shown in  FIG. 3 , a typical heartbeat produces two main heart sounds. A first heart sound  830 , denoted S 1 , is generated by vibration generally associated with the closure of the tricuspid and mitral valves at the beginning of systole. Typically, the heart sound  830  is about 14 milliseconds long and contains frequencies up to approximately 500 Hz. A second heart sound  840 , denoted S 2 , is generally associated with vibrations resulting from the closure of the aortic and pulmonary valves at the end of systole. While the duration of the second heart sound  840  is typically shorter than the first heart sound  830 , the spectral bandwidth of the second heart sound  840  is typically larger than that of the first heart sound  830 . 
   An electrocardiogram (ECG) waveform  850  describes the electrical activity of a patient&#39;s heart. The graph in the lower middle portion of  FIG. 3  illustrates an example of the ECG waveform  850  for two heartbeats and corresponds in time with the carotid pulse signal  810  and PCG waveform  820  also shown in  FIG. 3 . Referring to the first shown heartbeat, the portion of the ECG waveform  850  representing depolarization of the atrial muscle fibers is referred to as the “P” wave. Depolarization of the ventricular muscle fibers is collectively represented by the “Q.” “R,” and “S” waves of the ECG waveform, referred to as the QRS complex. Finally, the portion of the waveform representing repolarization of the ventricular muscle fibers is known as the “T” wave. Between heartbeats, the ECG waveform  850  returns to an isopotential level. 
   Fluctuations in a patient&#39;s transthoracic impedance signal  860  also correlate with blood flow that occurs with each cardiac pulse wave. The bottom graph of  FIG. 3  illustrates an example of a filtered transthoracic impedance signal  860  for a patient in which fluctuations in impedance correspond in time with the carotid pulse signal  810 , the PCG waveform  820 , and ECG waveform  850 , also shown in  FIG. 3 . 
   Referring now to  FIG. 4 , in another embodiment of the present invention involving heart sounds, such sounds may be used for discrimination of arrhythmia from normal sinus rhythm.  FIG. 4  is a graph depicting two consecutive PQRS complexes in the ECG signal  850  and their associated non-electrophysiological components developed from an accelerometer signal  835 . Also illustrated is a detection window  870  that is used to evaluate correlation of the signals in accordance with an embodiment of the present invention. As is illustrated in  FIG. 4 , an S 1  heart sound  832 , and an S 1  heart sound  834  are, in general, closely time correlated with a QRS complex  852  and a QRS complex  854 , respectively. The S 1  heart sound  832 , an S 2  heart sound  833 , and the S 1  heart sound  834  are illustrated as detected from an internally implanted accelerometer. The S 1  heart sound may provide a close time correlation with cardiac signals but not with noise and artifact signals. As such, heart sounds may be used to discriminate an arrhythmia from NSR. 
   In an embodiment of a method in accordance with the present invention, a method of arrhythmia detection uses the ECG signal to define a detection window. A non-electrophysiological source signal is then evaluated within the detection window for cardiac information. If the non-electrophysiological source signal includes a cardiac event within the window, then the ECG signal is corroborated as corresponding to a cardiac event. This may be used, for example, in a rate-based arrhythmia detection algorithm to provide a more robust rate than the rate calculated if only ECG information is used. The algorithm may, for example, only count ECG identified heart beats if the heart beats are corroborated by an associated non-electrophysiologically sensed heart beat. 
   An ITCS device may be implemented to include signal processing circuitry and/or signal processing software as illustrated in  FIGS. 1C and 1D . With continued reference to  FIG. 4 , signal processing may be used to correlate heart sounds, such as the S 1  heart sound, with R-wave peaks or other QRS complex features to provide discrimination of arrhythmias from NSR in the presence of noise. 
   In the approach illustrated in  FIG. 4 , an examination or detection window  870  is defined to start at a start time  875 , based on the Q point of the QRS complex  852 . The ITCS algorithm then searches the accelerometer signal  835  within the detection window  870  for the S 1  heart sound  832 . The algorithm may also look for time correlation between peak amplitudes of the S 1  heart sound  832  and the peak R of the QRS complex  852 . For example, the ECG signal  850  has an R-wave peak  856  falling within the examination window  870 , and an R-wave peak  858  falling within an examination window  872 . The R-wave peak  856  falling within the examination window  870  produces a large correlation value, indicating that the ECG signal  850  is time correlated to the S 1  heart sound signal  832  within the examination window  870 . Similarly, the R-wave peak  858  falling within the examination window  872  produces a large correlation value, indicating that the ECG signal  850  is time correlated to the S 1  heart sound signal  834  within the examination window  872 . Heart rate, for example, may be determined between successive heart beats with large correlation values of the QRS complexes  852 ,  854  and their associated S 1  heart sounds  832 ,  834 . 
   Referring now to  FIG. 5 , methods of signal discrimination in accordance with the present invention are illustrated in a flowchart  900 . Electrocardiogram signals  902  are received at a subcutaneous non-intrathoracic location. The electrocardiogram signals  902  may include a cardiac signal and one or both of noise and electrocardiographic artifacts. Non-electrophysiologic signals  904  associated with a non-electrophysiological cardiac source are also received to provide cardiac function information that is non-electrophysiologic in nature, such as heart sound information, blood flow information, blood oxygen information, and information from other non-electrophysiologic sensors described earlier. Both the electrocardiogram signals  902  and non-electrophysiologic signals  904  are used to discriminate between normal sinus rhythm and an arrhythmia via several optional paths, as is illustrated in the flowchart  900  of  FIG. 5 . 
   An arrhythmia may be detected using the electrocardiogram signals  902 , and the presence of the arrhythmia may be verified using a comparison  903  of the electrocardiogram signals  902  to the non-electrophysiologic signals  904 . Temporal relationships between the electrocardiogram signals  902  and non-electrophysiologic signals  904  may be determined such as by using a comparison  905  of morphologies  907 , 909  of the electrocardiogram signals  902  and the non-electrophysiologic signals  904 , respectively. 
   A detection window  906  may be initiated in response to receiving the electrocardiogram signal  902 , and used to determine whether the non-electrophysiologic signal  904  is received at a time falling within the detection window  906 , such as by using a correlation  911 . The start time of the detection window  906  used for confirmation may be associated with an inflection point of the electrocardiogram signal, such as a maxima or a minima or any other suitable morphological attribute. 
   Heart rates may be computed based on both a succession of electrocardiogram signals  902  and a succession of non-electrophysiologic signals  904 . An ECG rate  908  and a signal rate  916  may be used to discriminate between normal sinus rhythm and the arrhythmia. The ECG rate  908  may be compared with an arrhythmia threshold  910 , and used to determine presence/absence of an arrhythmia, such as in response to a first rate exceeding a first arrhythmia threshold but having a signal rate  916 , at a second rate, failing to exceed a second arrhythmia threshold  918 . 
   Using any path in the flow-chart  900 , defibrillation therapy delivery may be inhibited  920  or treated  921  in response to detecting an arrhythmia using the electrocardiogram signals  902  and confirming or rejecting the arrhythmia using, for example, the comparison  903 , a comparison  941 , and/or the correlation  911 . 
     FIG. 6  is a flow chart illustrating a method of multi-parameter arrhythmia discrimination in accordance with another embodiment of the present invention. An arrhythmia discrimination method  950  is illustrated that involves to sensing  951  an electrocardiogram signal at a subcutaneous non-intrathoracic location. A signal associated with a non-electrophysiological cardiac source is received  952  and subject to verification  953  to determine whether or not the sensed electrocardiogram signal includes a cardiac signal. A cardiac arrhythmia is detected  954  using one of the sensed electrocardiogram signal and the verified cardiac signal. Treatment of the cardiac arrhythmia is withheld  955  if the sensed signal is not the cardiac signal. A verification methodology according to this and other embodiments advantageously reduces or eliminates delivery of unnecessary cardiac shocks, by ensuring that the sensed signal from which arrhythmia detection, confirmation, and therapy decisions are made is indeed a cardiac signal. 
   Approaches to cardiac signal discrimination described herein involve the use of a non-electrophysiologic signal to discriminate and/or verify an arrhythmia and its associated ECG signal. Because signals developed from non-electrophysiological cardiac sources, such as heart sounds, are not electrophysiological in nature, they are not susceptible to the same noise sources as electrocardiogram signals. 
   An ITCS device may operate in a batch mode or adaptively, allowing for on-line or off-line implementation. To save power, the system may include the option for a hierarchical decision-making routine that uses algorithms known in the art for identifying presence the of arrhythmias or noise in the collected signal and judiciously turning on and off the cardiac signal discrimination methods in accordance with the present invention. 
   Various modifications and additions can be made to the preferred embodiments discussed hereinabove without departing from the scope of the present invention. Accordingly, the scope of the present invention should not be limited by the particular embodiments described above, but should be defined only by the claims set forth below and equivalents thereof.