WO2004091719A2

WO2004091719A2 - Multi-parameter arrhythmia discrimination

Info

Publication number: WO2004091719A2
Application number: PCT/US2004/010917
Authority: WO
Inventors: Paul Haefner; Darrell Orvin Wagner; Apurv Kamath; Marina Brockway
Original assignee: Cardiac Pacemakers, Inc.
Priority date: 2003-04-11
Filing date: 2004-04-09
Publication date: 2004-10-28
Also published as: JP2006524106A; JP4521396B2; EP1615692A2; WO2004091719A3

Abstract

An arrhythmia discrimination device and method involves receiving electrocardiogram signals and alternate signals at subcutaneous locations. Both the electrocardiogram signals and alternate signals are used to discriminate between normal sinus rhythm an arrhythmia. An arrhythmia may be detected using electrocardiogram signals, and verified using the alternate signals. A detection window may be initiated in response to receiving the electrocardiogram signal, and used to determine whether the alternate signal is received at a time falling with the detection window. Heart rates may be computed based on both the electrocardiogram signals and alternate signals. The rates may be used to discriminate between normal sinus rhythm and arrhythmia, and used to determining absence of an arrhythmia.

Description

MULTI-PARAMETER ARRHYTHMIA DISCRIMINATION

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 information from an alternate sensor.

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 S A 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'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'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'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's heart are connected to electrodes that contact the myocardium for sensing the heart'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. Embodiments of the invention are directed to arrhythmia discrimination methods that involve 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 an alternate sensor are also received. Alternate sensors include, but are not limited to, non-electrophysiological cardiac sensors, blood sensors, patient activity sensors, impedance sensors, plethysmographic sensors, blood oxygen sensors, transthoracic impedance sensors, blood volume sensors, acoustic sensors and/or pressure transducers, and accelerometers. A sensed electrocardiogram signal is verified to be a cardiac signal using an alternate 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 ar hytl mia may be verified or refuted using the alternate signals. Temporal relationships between the electrocardiogram signals and alternate signals may be determined. A detection window may be initiated in response to receiving the electrocardiogram signal, and used to determine whether the alternate signal is received at a time falling within the detection window.

Heart rate^'s may be computed based on both a succession of electrocardiogram signals and a succession of alternate 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 alternate signals. Examples of non-electrophysiologic alternate 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 alternate 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 arrhytl mia 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 alternate 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 alternate 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.

According to one embodiment of the invention, a medical system includes a housing with energy delivery circuitry and detection circuitry provided in the housing. One or more electrodes are coupled to the energy delivery and detection circuitry and used to sense cardiac and muscle activity. A processor is provided in the housing and coupled to the energy delivery and detection circuitry. The processor may detect a ventricular arrhythmia using a cardiac signal developed from the sensed cardiac activity and may also detect an activity state of the patient using an activity signal developed from the sensed muscle activity. The processor may modify delivery of a therapy to treat the arrhythmia in response to the muscle activity signal.

In another embodiment of the present invention, the processor inhibits delivery of the arrhythmia therapy in response to the activity signal exceeding an activity threshold, indicating patient consciousness or movement. The processor may inhibit delivery of the arrhythmia therapy for a predetermined time period in response to the activity signal exceeding an activity threshold, and withhold delivery of the arrhythmia therapy upon expiration of the predetermined time period and cessation of the arrhythmia. The processor may immediately deliver the arrhythmia therapy irrespective of the activity signal in response to detection of a life-threatening arrhythmia.

In another embodiment, the processor may receive an electrocardiogram from the detection circuitry and discriminate the cardiac signal and the activity signal from the electrocardiogram using an electrode arrangement configured for muscle signal detection. A method in accordance with the present invention involves detecting signals using one or more electrodes, and discerning a cardiac signal from the detected signals. An activity signal associated with patient activity is also discerned from the detected signals. An arrhythmia may be detected using the cardiac signal and patient activity level may be detected using the activity signal. Arrhythmia therapy may be modified to treat the arrhythmia in response to the activity signal. Delivery of the arrhythmia therapy may be inhibited in response to the activity signal exceeding an activity threshold, using a signal indicating patient consciousness or movement. Delivery of the arrhythmia therapy may be inhibited for a predetermined time period in response to the activity signal exceeding an activity threshold, and delivery of the arrhythmia therapy may be withheld upon expiration of the predetermined time period and cessation of the arrhythmia.

According to one embodiment of the invention, a medical device includes a housing configured for subcutaneous non-intrathoracic placement. Detection circuitry is provided in the housing and configured to produce a cardiac electrophysiologic signal. Energy delivery circuitry is also provided in the housing. At least one electrode configured for subcutaneous non-intrathoracic placement is coupled to the detection and energy delivery circuitry. An implantable blood sensor configured to produce a blood sensor signal is also provided with the device, and coupled to a processor provided in the housing. The processor is also coupled to the detection and energy delivery circuitry, and used to evaluate a cardiac rhythm using the cardiac electrophysiologic signal and the blood sensor signal. In one approach, the processor is configured to use a blood sensor signal to verify that the cardiac electrophysiologic signal comprises a cardiac signal, and configured to evaluate a cardiac rhythm using the blood sensor signal and the cardiac electrophysiologic signal comprising the cardiac signal.

The blood sensor may be configured for subcutaneous non-intrathoracic placement and provided in or on the housing, on a lead coupled to the housing, and/or separate from the housing and coupled to the processor via hardwire or wireless link. The blood sensor may include a sensor configured for optical signal sensing, such as a blood oxygen saturation sensor or a pulse oximeter. A suitable pulse oximeter may include two light- emitting diodes and one photodetector. The photodetector may include circuitry having a detection threshold that is periodically adjusted to account for signal variations.

In another configuration, a suitable pulse oximeter may include a first light-emitting diode having a peak light-emission wavelength witliin a range of about 550 nm and about 750 nm, and a second light-emitting diode having a peak light-emission wavelength within a range of about 750 nm and about 1050 nm. A photoplethysmography circuit may be included as a blood sensor and coupled to the processor. The processor may identify a cardiac rhythm as a tachyarrhythmia using the cardiac electrophysiologic signal and the blood sensor signal.

The processor may identify the cardiac rhythm as a tachyarrhythmia using the cardiac electrophysiologic signal and a relative change in the blood sensor signal, and may also selectively activate and deactivate the blood sensor in response to detecting a tachyarrhythmia. The processor may use the cardiac electrophysiologic signal to activate the blood sensor and evaluate the tachyarrhythmia using the cardiac electrophysiologic signal and the blood sensor signal. The processor may further confirm or refute the presence of the tachyarrhythmia using the cardiac electrophysiologic signal and the blood sensor signal.

The device may deliver a therapy to treat a tachyarrhythmia, and the processor may deactivate the blood sensor before or after delivery of the therapy. The processor may determine a heniodynamic state using the cardiac electrophysiologic signal and the blood sensor signal. In response to detecting an unidentifiable cardiac rhythm using the cardiac electrophysiologic signal, the processor may activate the blood sensor to facilitate identification of the unidentifiable cardiac rhythm using the blood sensor signal. The processor may use the blood sensor signal to assess cardiac function, assess oxygen saturation and changes in oxygen saturation, and/or assess afterload by, for example, analyzing the morphology of the blood sensor signal.

Embodiments of rhythm evaluation methods in accordance with the present invention may involve sensing an electrocardiogram signal at a subcutaneous non- intrathoracic location and acquiring a blood sense signal from a subcutaneous non- intrathoracic sensing location. A cardiac rhythm may be evaluated using the electrocardiogram signal and the blood sensor signal. One approach involves verifying that the electrocardiogram signal comprises a cardiac signal, and evaluating a cardiac rhythm using the blood sense signal and the electrocardiogram signal comprising the cardiac signal. A tachyarrhythmia may be detected using one or both of the electrocardiogram signal and the blood sense signal, such as by performing a rate based analysis or by performing a morphology based analysis.

An activation pattern of the electrocardiogram signal may be analyzed using a plurality of electrodes, and detected tachyarrhythmias may be treated after confirming presence of the tachyarrhythmia using the blood sense signal. The tachyarrhythmia may be discerned from noise using the blood sense signal. Evaluating the cardiac rhythm may also involve detecting a cardiac arrhythmia by performing a correlation (or computing a transfer function) between the electrocardiogram signal and the blood sense signal. Acquiring the blood sense signal may involve selectively powering-up and powering-down a blood sensor that produces the blood sense signal.

Evaluating a cardiac rhythm may involve detecting a tachyarrhythmia using the electrocardiogram signal, powering-up a blood sensor that produces the blood sense signal, confirming presence of the tachyarrhythmia using the blood sense signal, and then powering-down the blood sensor. The blood sense signal may include, for example, blood perfusion information, blood oxygen saturation information, photoplethysmographic information, pulse oximetry information, and/or other information from a blood sensor. 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 Figures 1 A and IB 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; Figure 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;

Figure ID 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;

Figure IE is a block diagram illustrating one configuration of various ITCS device components in accordance with an embodiment of the present invention;

Figure 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;

Figure 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; Figure 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;

Figure 5 is a flow chart illustrating methods of multi-parameter arrhythmia discrimination in accordance with the present invention; Figure 6 is a flow chart illustrating a method of multi-parameter arrhythmia discrimination in accordance with the present invention;

Figure 7 is a graph of an electrocardiogram signal and a skeletal muscle signal including a threshold in accordance with an embodiment of the present invention;

Figure 8 is a flow chart of a method of arrhythmia discrimination in accordance with an embodiment of the present invention; Figure 9 is a plan view of a subcutaneously implanted ICD with photoplethysmography capability in accordance with an embodiment of the present invention;

Figure 10 is a block diagram illustrating a two-color photoplethysmographic system in accordance with an embodiment of the present invention;

Figure 11 is a graph illustrating signals from normal sinus rhythni versus ventricular fibrillation;

Figure 12 is a graph illustrating RMS photoplethysmogram levels during normal sinus rhythm versus ventricular fibrillation; Figures 13 A and 13B are circuit diagrams of an LED transmission circuit and an LED detection circuit in accordance with an embodiment of the present invention; and Figure 14 is a flow chart of a method of arrhythmia discrimination in accordance with an embodiment of 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.

Embodiments of the invention are directed to arrhythmia discrimination methods that involve 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 an alternate sensor are also received. Alternate sensors include, but are not limited to, non-electrophysiological cardiac sensors, blood sensors, patient activity sensors, impedance sensors, plethysrnographic sensors, blood oxygen sensors, transthoracic impedance sensors, blood volume sensors, acoustic sensors and/or pressure transducers, and accelerometers. A sensed electrocardiogram signal may be verified to be a cardiac signal using an alternate signal. Additionally, a cardiac arrhythmia may be detected using one or both of the sensed electrocardiogram signal and the verified cardiac signal. Treatment of the cardiac arrhythmia may be withheld if the sensed signal is not verified to be the cardiac signal. 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'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 US Patent Application Serial No. 10/738,608 entitled "Noise Canceling Cardiac Electrodes," filed December 17, 2003, and US Patent Application Serial No. 10/465,520 filed June 19, 2003 entitled "Methods And Systems Involving Subcutaneous Electrode Positioning Relative To A Heart", 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. Patent 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. Patent 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. Patent 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 Figures 1 A and IB 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 Figures 1 A and IB, 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 Figures 1A and IB, 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 witliin 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 Figures 1A and IB).

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'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 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. Patent 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. Figure 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 202 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 Figure 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. Patent 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 circuitiy 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. Patent 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 Figure 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 Figure lC. 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 Figure 1C is configured to receive signals from one or more physiologic and/or non-physiologic alternate sensors in accordance with embodiments of the present invention. Depending on the type of sensor employed, signals generated by the alternate 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 alternate sensors may transmit sense data to the control system 205 without processing by the detection circuitry 202. Alternate 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 alternate sensors that are 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). An alternate sensor 261, as is illustrated in Figure 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 alternate 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 alternate sensors equippe 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.

Figure ID 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. Patent 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 Figure ID, the detection circuitry 302 may receive information from multiple physiologic and non-physiologic alternate sensors. Transthoracic acoustics, for example, may be monitored using an appropriate acoustic sensor. Heart sounds, for example, may be detected and processed by alternate 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 alternate sensors that monitor skeletal muscle activity, hi 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.

Turning now to Figure IE, there is illustrated a block diagram of various components of an ITCS device in accordance with one configuration. Figure IE illustrates a number of components that are associated with detection of various physiologic and non- physiologic parameters. As shown, the ITCS device includes a micro-processor 406, which is typically incorporated in a control system for the ITCS device, coupled to detection circuitry 402. Sensor signal processing circuitry 410 can receive sensor data from a number of different electrocardiogram and/or alternate sensors.

For example, an ITCS device may cooperate with, or otherwise incorporate, various types of non-physiologic sensors 421, external/cutaneous physiologic sensors 422, and/or internal physiologic sensors 424. Such sensors may include an acoustic sensor, an impedance sensor, an oxygen saturation sensor, a blood volume sensor, and a blood pressure sensor, for example. Each of these sensors 421, 422, 424 may be communicatively coupled to the sensor signal processing circuitry 410 via a short range wireless communication link 420. Certain sensors, such as an internal physiologic sensor 424, may alternatively be communicatively coupled to the sensor signal processing circuitry 410 via a wired connection (e.g., electrical or optical connection). A useful photoplethysmography sensor and techniques for using same that may be implemented in an ITCS device of the present invention are disclosed in US Patent No. 6,491,639, which is hereby incorporated herein by reference.

The components, functionality, and structural configurations depicted in Figures 1 A- 1E 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 Application entitled "Subcutaneous Cardiac Stimulator Employing Post- Shock Transthoracic Asystole Prevention Pacing, Serial Number 10/377,274, filed on February 28, 2003, 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 alternate 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

I 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: US Patent 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 Figure 2, electrode subsystems of an ITCS system are arranged about a patient's heart 510. The ITCS system includes a first electrode subsystem, comprising a can electrode 502, and a second electrode subsystem 504 that includes one or more electrodes and/or one or more multi-element electrodes. The second electrode subsystem 504 may include a number of electrodes used for sensing and/or electrical stimulation, and may also include alternate 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 a 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 cm². 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. 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 may not lead to complete suppression of the noise. In addition, filter performance may not be 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 use . an alternate signal to identify cardiac signals within a group of separated signals, such as those obtained. from a blind source separation (BSS) technique. It is understood that all or certain aspects of the signal identification technique described below may be implemented in a device or system (implantable or non-implantable) other than an ITCS device, and that the description of the BSS technique as the method of separation implemented in an ITCS device is provided for purposes of illustration, and not of limitation.

Signal separation techniques provide for separation of many individual signals from composite signals. For example, a composite signal detected on or within a patient may contain several signal components produced from a variety of signal sources, such signal components including cardiac signals, skeletal muscle movement related signals, electromagnetic interference signals, and signals of unknown origin. Signal separation techniques separate the composite signal into individual signals, but do not necessarily indicate the source of such signals.

The use of the largest eigenvalues produced by performing a principal component analysis on a composite signal matrix provides one method of identifying the separated signals most likely to be the cardiac signal of interest for ITCS devices. However, analyzing all the separated signals is a computationally intensive operation. By using an alternate signal to help differentiate cardiac signals for noise signals, the present invention provides an indication of the signals most likely to be the cardiac signal of interest in an efficient manner, thereby greatly decreasing the time necessary to identify the cardiac signal of interest from many possible separated signals.

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. Patent Application No. 10/741,814, filed December 19, 2003, 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. Patent Application No. 10/738,608, filed December 17, 2003, hereby incorporated herein by reference. Information from an alternate sensor 503, such as those described previously, may be used to improve the accuracy of arrhythmia discrimination, such as ECG or other rate-based aπ-hythmia 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 alternate sensor 503 may be provided in or on the housing 501, as illustrated in Figure 2, or may be provided as part of the second electrode subsystem 504, as described earlier. It is also contemplated that the alternate sensor 503 may be directly coupled to the housing 501 using an additional lead, or be wirelessly coupled as described with reference to Figures 1C and ID.

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 alternate signal is time correlated with respect to the cardiac electrophysiological signals, the alternate signal may provide information about a patient's rhythm state even in the presence of electrical noise and/or electrocardiographic artifacts. For example, the alternate 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's neck and sensing changes in the volume of the patient's carotid artery due to blood pumped from the patient's heart. A graph of a carotid pulse signal 810, representative of the physical expansion and contraction of a patient's carotid artery during two consecutive pulses, or heartbeats, is shown at the top of Figure 3. When the heart's ventricles contract during a heartbeat, a pressure wave is sent throughout the patient's peripheral circulation system. The carotid pulse signal 810 shown in Figure 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'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'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 Figure 3.

As indicated by the PCG waveform 820 shown in Figure 3, a typical heartbeat produces two main heart sounds. A first heart sound 830, denoted SI, 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 S2, 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's heart. The graph in the lower middle portion of Figure 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 Figure 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's transthoracic impedance signal 860 also correlate with blood flow that occurs with each cardiac pulse wave. The bottom graph of Figure 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 Figure 3.

Referring now to Figure 4, in another embodiment of the present invention involving heart sounds, such sounds may be used for discrimination of arrhythmia from normal sinus rhythm. Figure 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 Figure 4, an SI heart sound 832, and an SI heart sound 834 are, in general, closely time correlated with a QRS complex 852 and a QRS complex 854 respectively. The SI heart sound 832, an S2 heart sound 833, and the SI heart sound 834 are illustrated as detected from an internally implanted accelerometer. The SI 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. An alternate source signal is then evaluated within the detection window for cardiac information. If the alternate 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 alternately sensed heart beat.

An ITCS device may be implemented to include signal processing circuitry and/or signal processing software as illustrated in Figures 1C and ID. With continued reference to Figure 4, signal processing may be used to correlate heart sounds, such as the SI 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 Figure 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 SI heart sound 832. The algorithm may also look for time correlation between peak amplitudes of the SI 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 SI heart sound signal 832 within the examination window 870. Similarly, the R-wave peak 858 falling witliin the examination window 872 produces a large correlation value, indicating that the ECG signal 850 is time correlated to the SI 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 SI heart sounds 832, 834.

Referring now to Figure 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. Alternate signals, such as non-electrophysiologic signals 904 associated with a non- electrophysiological cardiac source are also received. The alternate signals 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 alternate 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 Figure 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 arrhytl mia 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.

Figure 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 sensing 951 an electrocardiogram signal at a subcutaneous non-intrathoracic location. An alternate 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. Figure 7 is a graph showing an electrocardiogram 1410 and an alternate signal, the alternate signal being a patient activity signal 1420. The graph illustrated in figure 7 includes a threshold 1450, in accordance with an embodiment of the present invention. The graph includes time as its abscissa and signal voltage level as its ordinate. The ECG signal 1410 and the patient activity signal 1420 shown in Figure 7 have been amplified and filtered, h this example, both the ECG signal 1410 and the patient activity signal 1420 are derived from cardiac electrodes. The patient activity signal 1420 is, in this case, derived from a cardiac electrode arrangement preferentially located to provide a signal indicative of skeletal muscle activity. Note that the patient activity signal 1420 includes a significant ECG component, but that muscle movement is clearly identifiable witliin at least a muscle noise detect window 1440.

Muscle movement, indicative of a conscious and active patient, may be defined, for example, when the patient activity signal 1420 exceeds the threshold 1450. The threshold 1450 may be adaptable, dynamic or fixed, and may be defined as an absolute value, as a percentage of a baseline, or using other known signal morphological or statistical methodologies. If, for example, the ECG signal 1410 indicates an arrhythmia is occurring necessitating a shock to the patient, but the patient activity signal 1420 indicates that the patient is mobile or active, then an algorithm in the ITCS device may delay shock to the patient's heart for a period of time, such as a delay 1460.

The delay 1460 provides a period of time during which the ITCS device evaluates whether there is a spurious signal in the ECG signal 1410, or whether there is actually a need to shock the patient. The duration of the delay 1460 is selected to provide the ITCS device additional time following initial detection of an arrhythmia to confirm the presence of the detected arrhythmia using one or more non-cardiac signals, such as skeletal muscle signals or patient movement signals. The delay period 1460 should be sufficient in duration to allow for re-evaluation of the detected arrhythmia, while not compromising patient wellbeing. The duration of the delay 1460 may range from 2 seconds to 60 seconds, for example. The device may also provide notification when the delay time is invoked. After the delay 1460, the ITCS device may begin charging the defibrillation capacitor(s) in preparation for delivering a shock to the patient, and may re-evaluate the patient activity signal 1420 prior to delivering the shock. The patient activity signal 1420 is re-evaluated to determine the activity status of the patient prior to delivering the shock at shock time 1470.

At the shock time 1470 shown in the graph in Figure 7, the patient activity signal 1420 has fallen below the threshold 1450, indicating that the patient is no longer active. This may be because the patient has succumbed to insufficient blood supply, and is possibly unconscious. Clearly, the shock 1470 is indicated, and is delivered to resuscitate the patient in this case. If, however, the ECG signal 1410 at the shock time 1470 indicates that the arrhythmia has terminated, a shock is not delivered to the patient, irrespective of the status of the patient activity signal 1420.

The delay 1460 may be used in a hierarchical manner, such that it is selectively used depending on the severity of the detected arrhythmia. For example, if the ECG signal 1410 clearly indicates presence of a dangerous or life-threatening arrhytlimia, then the delay 1460 may be bypassed and the patient may be shocked immediately. If, however, the ECG signal 1410 is inconclusive, but indicates a possibility of arrhythmia, then delivery of an arrhythmia therapy is delayed so that the patient activity signal 1420 may be evaluated.

Figure 8 illustrates various processes associated with one method of utilizing subcutaneous skeletal muscle signal detection in combination with ECG- or EGM-based rhythm detection. The skeletal muscle signal detection circuitrj'' may be enabled after other means of arrhythmia detection have been utilized, such as cardiac electrogram-based algorithms. To conserve energy, for example, skeletal muscle signal detection may be activated after detecting an arrhythmia using cardiac signal detection circuitry, and thereafter deactivated after delivering an arrhythmia therapy or cessation of the arrhythmia. Discriminating arrhythmia event from noise using skeletal muscle signal detection in this manner can reduce occurrences of inappropriate shock delivery and offers the potential to significantly improve patient comfort.

With continued reference to Figure 8, an ECG-based detection algorithm 600 is employed to detect cardiac arrhythmias in accordance with an embodiment of the invention. If a ventricular arrhythmia is detected 602 using ECG based detection 601 , a check 604 is made to determine the state of the skeletal muscle signal. If the current state of the skeletal muscle signal is not known or available, the skeletal muscle signal is acquired 606, which may involve activating (i.e., powering-up) a skeletal muscle sensor or detection circuitry. If a comparison 607 of the skeletal muscle signal to a threshold indicates patient inactivity, the defibrillation capacitor is charged 608 and a shock is delivered 610 to treat the arrhythmia. If, however, comparison 607 of the skeletal muscle signal to the threshold indicates patient activity or consciousness, a delay period is initiated and a recheck 614 of the electrocardiogram signal is made after expiration of the delay period. If the electrocardiogram signal indicates or confirms the continued presence of a ventricular arrhythmia after having previously checked the skeletal muscle signal at block 606, the defibrillation capacitor is charged 608 and a shock is delivered 610.

In this illustrative approach, re-evaluation of the detected ventricular arrhythmia using the skeletal muscle signal is performed once, so that treatment of a confirmed ventricular arrhythmia is not unduly delayed. It is noted that a ventricular arrhythmia re- verification routine may be performed during capacitor charging prior to shock delivery.

In another embodiment of the present invention, a blood sensor is used to provide an alternate signal for arrhythmia discrimination and verification. Electrocardiogram signals often contain noise signals and artifacts that mimic true cardiac signals and various aiThythmias. Using a blood sensor as an alternate sensor in accordance with the present invention provides the ability to discriminate true arrhythmia conditions from various noisy conditions. Moreover, using a blood sensor as an alternate sensor in accordance with the present invention provides the ability to confirm that the signals upon which arrhythmia detection and therapy delivery decisions are made contain a cardiac signal (e.g., a QRS complex), rather than a spurious signal that may have features similar to those of a true cardiac signal.

An ITCS device may be implemented to include multi-parameter cardiac signal verification and/or arrhythmia discrimination capabilities to improve noise rejection of cardiac ECG signals sensed by subcutaneous electrodes. This noise rejection/reduction approach advantageously reduces the risk of false positives for detection algorithms by providing multi-parameter arrhythmia discrimination.

For example, an alternate signal may be used to verify that the ECG signal contains a cardiac signal having a QRS complex, and that only ECG signals with QRS complexes are considered verified ECG signals. Subsequent cardiac rhythm analyses, including, in particular, arrhythmia analyses, may require that only verified ECG signals are used for computations of, for example, heart rate used for such analyses. This cardiac signal confirmation technique provides for more robust algorithms that are less susceptible to contamination from electrical interference and noise, thereby reducing incidences of inappropriate tachyarrhythmia therapy delivery.

One approach to cardiac signal confirmation involves determining temporal relationships between electrocardiogram signals and alternate signals. A detection window may, for example, be initiated in response to detecting an electrocardiogram signal, and used to determine whether a alternate signal is or is not received at a time falling within the detection window. For example, one arrhythmia detection approach uses the ECG signal to define a detection window. A non-electrophysiological source signal, such as a blood sensor 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.

Heart rates, for example, may be computed based on both a succession of electrocardiogram signals and a succession of alternate signals. These 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 alternate signals.

In another embodiment of the present invention, defibrillation therapy delivery may be inhibited or withheld in response to detecting an arrhythmia using the electrocardiogram signals but failing to detect the arrhythmia using the alternate signal, such as a blood sensor 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 deteπnined 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. According to one embodiment, photoplethysmography is used to provide an alternate signal that aids in noise discrimination when detecting various heart rhythms in the presence of electrical noise or artifacts. Because the additional discriminating signal is based on blood oxygen level or pulsatile blood volume level, and not based on electrical cardiac signals, this signal may provide information about a patient's rhythm state or hemodynamics even in the presence of electrical noise. A subcutaneous sensor may be used to detect blood oximetry. One such sensor is a pulse oximetry sensor, for example. The blood oxygen level information 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 the characteristics of blood oxygen information combined with typical ECG information for discrimination.

In accordance with an embodiment of the present invention, subcutaneous photoplethysmography may be employed to develop a non-electrophysiological cardiac signal as an alternate signal for detection and/or confirmation of cardiac rhythm. This

I feature employs a subcutaneous photoplethysmogram as part of a subcutaneous ICD system (e.g., ITCS device) as an alternative or additional signal to the electrocardiogram for detecting cardiac rhythm or hemodynamic state, particularly in the presence of electrical noise.

Photoplethysmography may be used subcutaneously for confirming patient cardiac arrhythmia detected by an implantable cardiovertor/defibrillator. Subcutaneous photoplethysmography may also be used to characterize patient hemodynamics for an implantable cardiovertor/defibrillator. For example, subcutaneous photoplethysmography may be used to evaluate afterload. Afterload is the systolic load on the left ventricle after it has started to contract. The resistance associated with afterload results from resistive forces of the vasculature that are overcome in order to push a bolus of blood into this vasculature during every heart beat. Hypertension or aortic stenosis could cause chronically increased afterload and lead to left ventricular hypertrophy and, subsequently, to heart failure.

Photoplethysmography may further be used subcutaneously for pulse oximetry to measure characteristics related to changes in patient oxygen saturation for an implantable cardiovertor/defibrillator. In general, it is desirable to reduce overall photoplethysmography energy, such as by utilizing it only for arrhythmia confirmation after other detection algorithms have been employed.

In one particular approach, a subcutaneous photoplethysmogram is used to confirm or verify that the cardiac signal used for cardiac rhythm analysis is indeed a cardiac signal, rather than a spurious signal, such as a skeletal noise signal. For example, the subcutaneous photoplethysmogram may be used to verify that the cardiac signal used to make tachyarrythmia therapy delivery decisions is an electrocardiogram indicative of the patient's actual heart rhythm. According to this approach, the subcutaneous photoplethysmogram is used primarily for verifying that the signal used for arrhythmia analysis and therapy delivery decisions is indeed the cardiac signal, which is distinct from using this signal to separately verify the presence or absence of an arrhythmia. It is understood, however, that the subcutaneous photoplethysmogram may be used as a signal to separately verify the presence or absence of an arrhythmia, exclusively or in addition to using tins signal for cardiac signal confirmation.

For example, the control system processor may inhibit delivery of a tachyarrhythmia therapy until the ECG signal used to detect presence of the arrhytlimia is confirmed to include a cardiac signal (e.g., QRS complex) using a photoplethysmic signal. The processor, for example, may inhibit delivery of the tachyarrhythmia therapy for a predetermined time period during which the verification processes is carried out, and withhold delivery of the tachyarrhythmia therapy upon expiration of the predetermined time period if such verification processes is unsuccessful or in response to cessation of the arrhythmia. The processor may deliver the tachyarrhythmia therapy in response to a successful outcome of the verification process. Also, the processor may immediately deliver the tachyarrhythmia therapy irrespective of the verification process in response to detection of a life-threatening arrhythmia.

Several benefits may be achieved through use of subcutaneous photoplethysmography. For example, subcutaneous photoplethysmography may be used to reduce the number of inappropriate shocks by improving shock specificity. It may also be used to provide for confirmation of ventricular arrhythmias based on the level of blood perfusion or relative change in blood perfusion. Further, subcutaneous photoplethysmography may be used to complement cardiac electrocardiogram-based algorithms by using a non-electric photo-based detection method. Subcutaneous photoplethysmography may also be used for redetection and reconfirmation of arrhythmias.

Figures 9 through 14 illustrate various embodiments and processes associated with the use of subcutaneous blood sensing that provides an alternate signal employed for detection and/or confirmation of a cardiac rhythm. Figure 9 illustrates one implementation of a photoplethysmographic sensing system 500 suitable for use in a subcutaneous cardiac stimulator 511 (e.g., an ITCS device). Figure 9 illustrates deployment of a subcutaneous photoplethysmographic sensor 520 in an orientation between a layer of skin 530 and a layer of muscle tissue 540. The illustrative example of Figure 9 depicts a light source 550 (i.e., LED) and a detector 560 facing towards the muscle tissue 540. This orientation advantageously reduces interference from ambient light sources, thus reducing noise artifacts on the plethysmogram, particularly if an opaque barrier 570 is used to direct light into the detector 560. Other configurations may have the light source 550 and the detector 560 on the side or facing towards the skin.

When the cardiac stimulator 511 encounters an electrocardiogram that it cannot interpret, or to confirm detection of a hemodynamically unstable arrhythmia, the light source 550 is activated and the output of the photodetector 560 is synchronously measured.

Algorithms in the cardiac stimulator 511 are then invoked to determine the pulse rate from the photoplethysmogram and inform therapy decisions. Measurements from this signal may also be used to inform or adapt electrocardiogram noise discrimination and/or arrhythmia detection algorithms. Use of subcutaneous photoplethysmography in accordance with this embodiment advantageously provides for detection of cardiac rhythm in the presence of electrical noise or artifacts. The algorithm is robust, in that the photoplethysmogram is an optical signal and therefore not susceptible to the same noise sources as the ECG.

An expanded view 580 illustrates a light path 570 from the light source 550 to the detector 560. The perfusion of blood in the muscle tissue 540 affects the character of the light as it is reflected from the tissue 540 to the detector 560 along the path 570, providing blood information such as blood oxygen saturation level, blood volume, pulse, and other blood characteristics.

The implementation shown in Figure 10 includes an optical source circuit 515 that includes an LED control 525 respectively coupled to a Red LED 535 and an Infrared (IR) LED 545. Changes in oxygen saturation levels in the tissue 542 may be measured using two light sources and one detector. Typically, one light source has absorption characteristics that are generally unaffected by blood color change (such as the IR LED 545, emitting at ~960 nm), while the other light source has absorption characteristics that are sensitive to color change in the blood (such as the Red LED 535, emitting at ~660 nm). Due to potential errors in calculating absolute oxygen saturation using reflectance in areas of low perfusion, the embodiments illustrated in Figures 10 through 13 only monitor changes in oxygen saturation, and not absolute levels. The information from changes in oxygen saturation of the blood is sufficient to discriminate between potentially lethal arrhythmias and noise artifacts that could otherwise lead to unnecessary shocking of the patient without the discrimination.

Still referring to Figure 10, an optical detection circuit 555 includes a detector 565 coupled to a photo diode 576. Processing ckcuitry 575 is coupled to the optical source circuit 515 and the optical detection circuit 555 in this configuration. The processing circuitry 575 includes a multiplexer 585 coupled to the LED control 525 and the detection circuit 555. A Red signal channel 5,86 and IR signal channel 587 are respectively coupled between the multiplexer 585 and the signal processing circuitry 575. The signal processing circuitry 575 operates on signals received from the Red signal channel 586 and the IR signal channel 587, and employs various algorithms to evaluate such signals for cardiac rhythm detection and/or confirmation, including arrhythmia detection and confirmation. A magnified view 582 illustrates a light path 572 from a first light source 552 and a light path 574 from a second light source 554 to a detector 562. The perfusion of blood in the muscle tissue 542 affects the character of the light as it is reflected from the tissue 542 to the detector 562 along paths 572 and 574, providing blood information such as blood oxygen saturation level, blood volume, pulse, or other blood characteristics. Figures 11 and 12 are graphs of data taken from a live porcine subject, and illustrate an example of combining electrocardiography and photoplethysmography to differentiate normal sinus rhythm from arrhythmia in accordance with an embodiment of the present invention. Figure 11 illustrates a cardiac electrocardiogram 700 and a photoplethysmogram 710 presented over a 2 second duration for a normal sinus rhythm condition 730 and a ventricular fibrillation condition 740. Figure 12 illustrates a cardiac electrocardiogram 760 and a time correlated photoplethysmogram 770 during a 38 second period 780 in which a normal sinus rhythm 762 is followed by a ventricular fibrillation event 764. Figures 11 and 12 demonstrate that both the cardiac electrocardiograms 700, 760 and photoplethysmograms 710, 770 change significantly in character when the normal sinus rhythm 730, 762 devolves into the ventricular fibrillation 740, 764 conditions.

Referring back to Figure 11, note that the scale of the normal sinus rhythm 730 graph and the ventricular fibrillation 740 graph are different. Although the photoplethysmogram 710 of the ventricular fibrillation 740 looks comparable to the photoplethysmogram 710 of the normal sinus rhythm 730, the peak-to-peak amplitude of the photoplethysmogram 710 in the ventricular fibrillation 740 graph is significantly smaller than the peak-to-peak amplitude of the photoplethysmogram 710 in the normal sinus rhythm 730 graph. The ordinate scale of the ventricular fibrillation 740 graph is equal to the ordinate scale of the normal sinus rhythm 730 graph.

Referring now to Figure 12, a RMS blood oxygen level 772 corresponds to the normal sinus rhythm 762, and a RMS blood oxygen level 774 corresponds to the ventricular fibrillation event 764. A threshold 776 may be predetermined or adaptively adjusted to help differentiate between the normal sinus rhythm 762 and the ventricular fibrillation event 764. The time period between the normal sinus rhythm 762 and the ventricular fibrillation event 764 indicates a loss of data in the electrocardiogram 760 during the intentional induction of the ventricular fibrillation 764.

Figure 13A is a schematic of an LED current source section 1810 of a photoplethysmography circuit in accordance with an embodiment of the present invention. As is illustrated in Figure 13 A, the' current source section 1810 is configured as a constant current source, using a source LED circuit 1811, and is driven by an oscillator 1812 that may produce drive pulses 1813 having a period of 1 ms and a pulse width of 0.1 ms, for example. Figure 13B is a schematic of a photo detector section 1820 of a photoplethysmography circuit in accordance with an embodiment of the present invention. The detector section shown in Figure 13B includes a photo diode 1821, a light current to voltage amplifier 1822, a high pass filter 1823, a voltage integrator 1824, and a low pass filter 1825. The circuits illustrated in Figures 13A and 13B are useful for providing a photoplethysmic signal, such as signal 770 shown in Figure 12.

Figure 14 illustrates various processes associated with one method of utilizing subcutaneous photoplethysmography in combination with electrocardiogram-based rhythm detection. The method illustrated in Figure 14 presents details concerning energy utilization. The photoplethysmography circuitry may be enabled only after other arrhythmia detection methods have been employed, such as cardiac electrocardiogram-based algorithms. Photoplethysmography may be used only before potentially delivering the shock, to conserve energy. The circuitry may be disabled when use of the photoplethysmogram is completed. According to one implementation, if photoplethysmography is used for 10 seconds, the additional energy required is about 0.5 joules. This energy is very low when compared with the energy used for defibrillation ( > 5 joules). Therefore, discriminating one electrocardiogram identified arrhythmia event as noise using photoplethysmography has the potential to save over 4.5 joules. It is understood that eliminating unnecessary shocks extends the useful life of the ITCS, while simultaneously improving patient comfort.

With reference to Figure 14, and with further reference to Figures 11 and 12, an ECG-based detection algorithm 1600 is employed to detect cardiac arrhythmias. If a ventricular arrhythmia is detected 1602 using ECG based detection 1601, a determination 1604 is performed to see if the photoplethysmogram has been checked. A check 1606 of an acquired photoplethysmogram is performed.

If the photoplethysmogram indicates or confirms the presence of a ventricular arrhythmia, such as by using a threshold 1607, the defibrillation capacitor is charged 1608 and a shock is delivered 1610. It is noted that a ventricular arrhythmia re- verification routine may be performed during capacitor charging prior to shock delivery. If the photoplethysmogram signal exceeds the predetermined threshold 1607, such as the threshold shown in Figure 12 (note that an RMS level of the photoplethysmogram may be used in this comparison), a recheck 1614 of the ECG signal is made after a predetermined time period. In the methodology depicted in Figure 14, the photoplethysmic sensor that produces the photoplethysmogram signal may be selectively powered-up and powered-down. For example, the photoplethysmic sensor may be in a powered-down state until a tachyarrhythmia is detected using the ECG signal, such as at blocks 1601 and 1602 in Figure 14. The photoplethysmic sensor may remain powered-on until completion of the cardiac signal and/or arrhythmia detection verification processes. For example, the photoplethysmic sensor may be powered-down after completing the processes associated with blocks 1606 and 1607, and prior to charging the defibrillation capacitor at block 1608, which may take as long as about 20 seconds to fully charge the capacitor(s).

Approaches to cardiac signal discrimination described herein involve the use of an alternate signal for a variety of purposes, including verifying the presence of a cardiac , signal, and discriminating and/or verifying an arrhythmia and its associated ECG signal. An ITCS device employing aspects of the present invention 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 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.

Claims

CLAIMSWhat is claimed is:

1. An implantable cardiac stimulation device, comprising: a housing configured for subcutaneous non-intrathoracic placement in a patient; energy delivery circuitry provided in the housing; detection circuitry provided in the housing; one or more electrodes coupled to the energy delivery and detection circuitry and configured for subcutaneous non-intrathoracic placement in a patient, the one or more electrodes sensitive to cardiac and muscle activity; and a processor provided in the housing and coupled to the energy delivery and detection circuitry, the processor detecting a arrhythmia using a cardiac signal developed from the sensed cardiac activity and detecting an activity state of the patient using an activity signal developed from the sensed muscle activity, the processor modifying delivery of a therapy that treats the arrhytlimia in response to the activity signal.

2. The device of claim 1, wherein the processor inhibits delivery of the arrhytlimia therapy in response to the activity signal exceeding an activity threshold.

3. The device of claim 1, wherein the processor inhibits delivery of the arrhythmia therapy in response to the activity signal indicating patient consciousness or movement.

4. The device of claim 1, wherein the processor inhibits delivery of the arrhythmia therapy for a predetermined time period in response to the activity signal exceeding an activity threshold, and withholds delivery of the arrhythmia therapy upon expiration of the predetermined time period and cessation of the arrhythmia.

5. The device of claim 1, wherein the processor inhibits delivery of the arrhytlimia therapy for a predetermined time period in response to the activity signal exceeding an activity tlireshold, and delivers the arrhythmia therapy upon expiration of the predetermined time period and non-cessation of the arrhythmia.

6. The device of claim 1 , wherein the processor inhibits delivery of the arrhythmia therapy in response to the activity signal exceeding an activity tlireshold, and wherein the processor provides a notification that the arrhythmia therapy is inhibited.

7. The device of claim 1, wherein: in response to the activity signal indicating patient consciousness or movement, the processor inhibits delivery of the arrhythmia therapy; and in response to detection of a life-threatening arrhythmia, the processor immediately delivers the arrhythmia therapy irrespective of the activity signal.

8. The device of claim 1 , wherein the processor receives an electrocardiogram using the detection circuitry and discriminates the cardiac signal and the activity signal from the electrocardiogram.

9. The device of claim 8, wherein the processor discriminates the cardiac and activity signals using a signal separation technique.

10. The device of claim 8, wherein the processor discriminates the cardiac and activity signals using blind source separation.

11. The device of claim 8, wherein the one or more electrodes comprises: a first combination of electrodes adapted to preferentially sense cardiac signals associated with the cardiac activity; and a second combination of electrodes adapted to preferentially sense noise signals associated with the muscle activity.

12. The device of claim 11, wherein: the one or more electrodes comprises a plurality of electrodes; the processor selects combinations of the plurality of electrodes; and the processor senses a cardiac signal component and a noise component of signals acquired by each of the controller selected electrode combinations.

13. The device of claim 1, wherein the one or more electrodes comprise a plurality of electrodes, and wherein the processor detects the aιτhythmia using an activation sequence over the plurality of electrodes.

14. The device of claim 1, wherein the processor detects the arrhythmia using a cardiac signal curvature.

15. A cardiac stimulation method, comprising: detecting signals at one or more subcutaneous non-intrathoracic locations; discerning a cardiac signal from the detected signals; discerning an activity signal associated with muscle activity from the detected signals; detecting a arrhythmia using the cardiac signal; detecting a patient activity state using the activity signal; and modifying delivery of a subcutaneous non-intrathoracic therapy that treats the arrhythmia in response to the activity signal.

16. The method of claim 15, wherein delivery of the arrhythmia therapy is inhibited in response to the activity signal exceeding an activity threshold.

17. The method of claim 15, wherein delivery of the arrhytlimia therapy is inhibited in response to the activity signal indicating patient consciousness or movement.

18. The method of claim 15 , wherein delivery of the arrhythmia therapy is inhibited for a predetermined time period in response to the activity signal exceeding an activity tlireshold, and delivery of the arrhythmia therapy is withlield upon expiration of the predetermined time period and cessation of the arrhythmia.

19. The method of claim 15, wherein delivery of the arrhythmia therapy is inhibited for a predetermined time period in response to the activity signal exceeding an activity threshold, and the arrhythmia therapy is delivered upon expiration of the predetermined time period and non-cessation of the arrhythmia.

20. The method of claim 15, wherein: in response to the activity signal indicating patient consciousness or movement, inhibiting delivery of the arrhythmia therapy; and in response to detection of a life-threatening arrhytlimia, immediately delivering the arrhythmia therapy irrespective of the activity signal.

21. The device of claim 15, wherein discerning the cardiac and activity signals comprises discriminating the cardiac and activity signals using a signal separation technique on the detected signals.

22. The method of claim 15, wherein discerning the cardiac and activity signals comprises performing a blind source separation on the detected signals.

23. The method of claim 15, wherein detecting the signals comprises detecting the signals using a plurality of electrodes provided at a plurality of non-intrathoracic locations, the electrodes selectively combinable to define a plurality of sensing vectors, and discerning the cardiac and activity signals comprises: selecting a first sensing vector of the plurality of sensing vectors which is preferentially sensitive to signals associated with cardiac activity; and selecting a second sensing vector of the plurality of sensing vectors which is preferentially sensitive to signals associated with the muscle activity.

24. The method of claim 15, wherein the muscle activity is indicative of patient movement, and discerning the activity signal comprises detecting an accelerometer signal indicative of the patient movement.

25. The method of claim 15, wherein the arrhythmia is detected using morphology based arrhythmia detection.

26. The method of claim 15, wherein the arrhythmia is detected using rate based arrhythmia detection.

27. The method of claim 15, wherein the arrhythmia is detected using an activation sequence detected by a plurality of electrodes.

28. The method of claim 15, wherein the arrhythmia is detected using a curvature of the cardiac signal.

29. An arrhythmia discrimination method, comprising: sensing electrocardiogram signals at a subcutaneous non-intrathoracic location; receiving signals associated with a non-electrophysiological cardiac source; verifying that the electrocardiogram signals comprise a cardiac signal using the non- electrophysiological signal; discriminating, using the electrocardiogram signals and non-electrophysiologic signals, between a normal sinus rhythm and a cardiac arrhythmia; and withholding delivery of subcutaneous non-intrathoracic cardiac stimulation therapy if the sensed signal is not the cardiac signal.

30. The method of claim 29, wherein discriminating between normal sinus rhythm and the arrhythmia comprises: detecting the arrhythmia using the electrocardiogram signals; and verifying presence of the arrhythmia using the non-electrophysiologic signals.

31. The method of claim 29, wherein discriminating between normal sinus rhythm and the arrhythmia comprises: detecting the arrhythmia using the electrocardiogram signals; determining temporal relationships between the electrocardiogram signals and non- electrophysiologic signals received while detecting the arrhythmia; and verifying presence of tlie arrhythmia based on the temporal relationships between the electrocardiogram signals and non-electrophysiologic signals.

32. The method of claim 29, wherein discriminating between normal sinus rhythm and the arrhytlimia comprises: initiating a detection window in response to receiving each electrocardiogram signal of a succession of the electrocardiogram signals; and determining whether each non-electrophysiologic signal of a succession of the non- electrophysiologic signals is received at a time falling within the detection window.

33. The method of claim 29, wherein discriminating between normal sinus rhythm and the arrhythmia comprises: computing a first rate based on successive electrocardiogram signals; computing a second rate based on successive non-electrophysiologic signals; and discriminating between normal sinus rhythm and the arrhythmia using tl e first and second rates.

34. The method of claim 29, wherein discriminating between normal sinus rhythm and the arrhythmia comprises: computing a first rate based on successive electrocardiogram signals; computing a second rate based on successive non-electrophysiologic signals; comparing the first rate with a first arrhythmia threshold; comparing the second rate with a second arrhythmia threshold; and determining presence of the arrhythmia in response to both the first and second rates exceeding the first and second arrhythmia thresholds, respectively.

35. The method of claim 29, wherein discriminating between normal sinus rhythm and the arrhytlimia comprises: computing a first rate based on successive electrocardiogram signals; computing a second rate based on successive non-electrophysiologic signals; comparing the first rate with a first arrhythmia threshold; comparing the second rate with a second arrhythmia tlireshold; and determining absence of the arrhythmia in response to the first rate exceeding the first arrhythmia threshold and the second rate failing to exceed the second arrhythmia threshold.

36. The method of claim 29, wherein discriminating between normal sinus rhythm and the arrhythmia comprises: determining presence of the arrhytlimia using a morphology of the electrocardiogram signals; and verifying presence of the arrhythmia using the non-electrophysiologic signals.

37. The method of claim 29, wherein the non-electrophysiologic signals comprise heart sound signals.

38. The method of claim 29, wherein the non-electrophysiologic signals comprise subsonic acoustic signals indicative of cardiac activity.

39. The method of claim 29, wherein the non-electrophysiologic signals comprise pulse pressure signals.

40. The method of claim 29, wherein the non-electrophysiologic signals comprise impedance signals indicative of cardiac activity.

41. The method of claim 29, wherein the non-electrophysiologic signals comprise pulse oximetry signals.

42. The method of claim 29, further comprising declaring an arrhythmic episode in response to detecting the arrhythmia using the electrocardiogram signals and detecting the arrhythmia using the non-electrophysiologic signals.

43. The method of claim 29, further comprising enabling defibrillation therapy delivery in response to detecting the arrhythmia using the electrocardiogram signals and detecting the arrhythmia using the non-electrophysiologic signals.

44. The method of claim 29, further comprising inhibiting defibrillation therapy delivery in response to detecting the arrhythmia using the electrocardiogram signals but not detecting the arrhythmia using the non-electrophysiologic signals.

45. An arrhythmia discrimination method, comprising: sensing an electrocardiogram signal at a subcutaneous non-intrathoracic location; receiving a signal associated with a non-electrophysiological cardiac source; verifying that the sensed electrocardiogram signal comprises a cardiac signal using the non-electrophysiological signal; detecting a cardiac arrhythmia using one of the sensed electrocardiogram signal and the verified cardiac signal; and withholding treatment of the cardiac arrhythmia if the sensed signal is not the cardiac signal.

46. The method of claim 45, further comprising: defining a detection window with a start time associated with an inflection point of the electrocardiogram signal; and evaluating the received non-electrophysiological signal within the detection window.

47. The method of claim 46, wherein the start time of the detection window is associated with a maxima or a minima of the electrocardiogram signal.

48. The method of claim 45, further comprising: computing a first heart-rate based on intervals between successive electrocardiogram signals; and computing a second heart-rate based on intervals between successive non- electrophysiological cardiac signals; wherein confirming presence of the cardiac arrhythmia comprises comparing the first heart-rate to the second heart-rate.

49. The method of claim 45, wherein confirming presence of the cardiac arrhytlimia comprises performing a correlation between the electrocardiogram signal and the non-electrophysiological cardiac signal.

50. The method of claim 45, wherein the non-electrophysiological cardiac signal comprises acoustic emission information.

51. The method of claim 50, wherein the acoustic emission information comprises a temporal location of a peak heart-sound.

52. The method of claim 45, wherein the non-electrophysiological cardiac signal comprises cardiac acceleration information.

53. The method of claim 45, wherein the non-electrophysiological cardiac signal comprises one or more of pulse pressure information, blood-flow information, heart rate information, and pulse oximetry information.

54. The method of claim 45, wherein detecting presence or non-presence of the cardiac arrhythmia comprises performing a rate based analysis of the electrocardiogram signal.

55. The method of claim 45, wherein detecting presence or non-presence of the cardiac arrhythmia comprises performing a morphology based analysis of the electrocardiogram signal.

56. The method of claim 45, further comprising delivering a cardiac therapy to treat the cardiac arrhythmia.

57. An implantable cardiac device, comprising: an implantable housing; an electrode arrangement configured for subcutaneous non-intrathoracic placement; detection circuitry provided in the housing and coupled to the electrode arrangement, the detection circuitry configured to detect electrocardiogram signals; a sensor configured to sense signals associated with a non-electrophysiological cardiac source; energy delivery circuitry coupled to the electrode arrangement; and a processor provided in the housing and coupled to the detection circuitry, sensor, and energy delivery circuitry, the processor using the non-electrophysiological signals to verify that the detected electrocardiogram signals comprise a cardiac signal, the processor withholding treatment of the cardiac arrhythmia if the detected electrocardiogram signals do not comprise the cardiac signal.

58. The device of claim 57, wherein the energy delivery circuitry comprises defibrillation therapy circuitry.

59. The device of claim 57, wherein the energy delivery circuitry comprises pacing therapy circuitry.

60. The device of claim 57, wherein the sensor is provided in or on the housing.

61. The device of claim 57, wherein the sensor is provided in or on a lead coupled to the housing.

62. The device of claim 57, wherein the sensor comprises one or more of an accelerometer, a microphone, an acoustic transducer, a blood-flow transducer, a pulse oximeter, and photoplethysmography circuitry.

63. An implantable subcutaneous device, comprising: a housing configured for subcutaneous non-intrathoracic placement; detection circuitry provided in the housing and configured to produce a cardiac electrophysiologic signal; energy delivery circuitry provided in the housing; at least one electrode configured for subcutaneous non-intrathoracic placement and coupled to the detection and energy delivery circuitry; an implantable blood sensor configured to produce a blood sensor signal; and a processor provided in the housing and coupled to the blood sensor and the detection and energy delivery circuitry, the processor configured to use the blood sensor signal to verify that the cardiac electrophysiologic signal comprises a cardiac signal, and configured to evaluate a cardiac rhythm using the blood sensor signal and the cardiac electrophysiologic signal comprising the cardiac signal.

64. The device of claim 63, wherein the blood sensor is configured for subcutaneous non-intrathoracic placement.

65. The device of claim 63, wherein the blood sensor is provided in or on the housing.

66. The device of claim 63, wherein the blood sensor comprises a sensor configured for optical signal sensing.

67. The device of claim 63, wherein the blood sensor comprises a blood oxygen saturation sensor.

68. The device of claim 63, wherein the blood sensor comprises a pulse oximeter.

69. The device of claim 63, wherein the blood sensor comprises a pulse oximeter, the pulse oximeter comprising at least two light-emitting diodes and at least one photodetector.

70. The device of claim 69, wherein a photodetector detection threshold is periodically adjusted to account for signal variations.

71. The device of claim 63, wherein the blood sensor comprises a pulse oximeter, the pulse oximeter comprising a first light-emitting diode having a peak light- emission wavelength within a range of about 550 nm and about 750 nm, and a second light- emitting diode having a peak light-emission wavelength within a range of about 750 nm and about 1050 nm.

72. The device of claim 63, wherein the blood sensor includes photoplethysmography circuitry.

73. The device of claim 63, wherein the processor identifies the cardiac rhythm as a tachyarrhythmia using tlie cardiac electrophysiologic signal and the blood sensor signal.

74. The device of claim 63, wherein the processor identifies the cardiac rhythm as a tachyarrhythmia using the cardiac electrophysiologic signal and a relative change in the blood sensor signal.

75. The device of claim 63, wherein the processor selectively activates and deactivates the blood sensor.

76. The device of claim 63, wherein the processor, in response to detecting a tachyarrhythmia using the cardiac electrophysiologic signal, activates the blood sensor and evaluates the tachyarrhythmia using the cardiac electrophysiologic signal and the blood sensor signal.

77. The device of claim 76, wherein the processor confirms presence of the tachyarrhythmia using the cardiac electrophysiologic signal and the blood sensor signal.

78. The device of claim 77, wherein the processor, in response to confirming presence of the tachyarrhythmia, delivers a therapy that treats the tachyarrhythmia, the processor deactivating the blood sensor before or after delivery of the therapy.

79. The device of claim 63, wherein the processor inhibits delivery of a tachyarrhythmia therapy until the processor confirms presence of the tachyarrhythmia using the blood sensor signal.

80. The device of claim 63, wherein the processor determines a hemodynamic state using the cardiac electrophysiologic signal and the blood sensor signal.

81. The device of claim 63, wherein the processor, in response to detecting an unidentifiable cardiac rhythm using the cardiac electrophysiologic signal, activates the blood sensor facilitating identification of the unidentifiable cardiac rhythm using the blood sensor signal.

82. The device of claim 63, wherein the at least one electrode is coupled to the housing via a lead and wherein the housing further comprises at least one housing electrode.

83. The device of claim 63, wherein the processor uses the blood sensor signal for assessing cardiac function.

84. The device of claim 63, wherein the processor uses the blood sensor signal for assessing oxygen saturation.

85. The device of claim 63, wherein the processor uses the blood sensor signal for assessing afterload.

86. The device of claim 85, wherein the processor assesses afterload by analyzing the morphology of the blood sensor signal.

87. A rhythm evaluation method, comprising: sensing an electrocardiogram signal at a subcutaneous non-intrathoracic location; acquiring a blood sense signal from a subcutaneous non-intrathoracic sensing location; verifying that the electrocardiogram signal comprises a cardiac signal; and evaluating a cardiac rhythm using the blood sense signal and the electrocardiogram signal comprising the cardiac signal.

88. The method of claim 87, wherein evaluating the cardiac rhythm comprises detecting a tachyarrhythmia using one or both of the electrocardiogram signal and the blood sense signal.

89. The method of claim 88, wherein detecting the tachyarrhythmia comprises performing a rate based analysis of the electrocardiogram signal.

90. The method of claim 88, wherein detecting the tachyarrhythmia comprises performing a morphology based analysis of the electrocardiogram signal.

91. The method of claim 88, wherein detecting the tachyarrhythmia comprises analyzing an activation pattern of the electrocardiogram signal using a plurality of electrodes.

92. The method of claim 88, further comprising treating the tachyarrhythmia.

93. The method of claim 87, wherein evaluating the cardiac rhythm comprises confirming presence of a tachyarrhythmia using the electrocardiogram signal and the blood sense signal.

94. The method of claim 87, wherein evaluating the cardiac rhythm comprises detecting a tachyarrhythmia using the electrocardiogram signal, and confirming presence of the tachyarrhythmia using the blood sense signal.

95. The method of claim 87, wherein evaluating the cardiac rhythm comprises detecting a tachyarrhythmia using the electrocardiogram signal, and discerning the detected tachyarrhythmia from noise using the blood sense signal.

96. The method of claim 87, wherein evaluating the cardiac rhythm comprises detecting a cardiac arrhythmia by performing a correlation between the electrocardiogram signal and the blood sense signal.

97. The method of claim 87, wherein evaluating the cardiac rhythm comprises detecting a cardiac tachyarrhythmia, and the method further comprises inhibiting delivery of a tachyarrhythmia therapy until presence of the tachyarrhythmia is confirmed using the blood sense signal.

98. The method of claim 87, wherein acquiring the blood sense signal comprises selectively powering-up and powering-down a blood sensor that produces the blood sense signal.

99. The method of claim 87, wherein evaluating the cardiac rhythm comprises: detecting a tachyarrhythmia using the electrocardiogram signal; powering-up a blood sensor that produces the blood sense signal in response to detecting the tachyarrhythmia; confirming presence of the tachyarrhythmia using the blood sense signal; and powering-down the blood sensor after confirming presence of the tachyarrhythmia.

100. The method of claim 99, further comprising: treating the tachyarrhythmia; and powering-down the blood sensor before or after treating the tachyarrhythmia.

101. The method of claim 87, wherein the blood sense signal comprises one or more of blood perfusion information, blood oxygen saturation information, photoplethysmographic information, and pulse oximetry information.