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In a subcutaneous implantable cardioverter/defibrillator, cardiac arrhythmias are detected to determine necessary therapeutic action. Cardiac signal information is sensed from far field electrodes implanted in a patient. The sensed cardiac signal information is then amplified and filtered. Parameter

In a subcutaneous implantable cardioverter/defibrillator, cardiac arrhythmias are detected to determine necessary therapeutic action. Cardiac signal information is sensed from far field electrodes implanted in a patient. The sensed cardiac signal information is then amplified and filtered. Parameters such as rate, QRS pulse width, cardiac QRS slew rate, amplitude and stability measures of these parameters from the filtered cardiac signal information are measured, processed and integrated to determine if the cardioverter/defibrillator needs to initiate therapeutic action.

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1. A method of discriminating cardiac rhythms in an implantable defibrillator, the implantable defibrillator including a plurality of electrodes for use in sensing and/or therapy delivery, as well as operational circuitry coupled to the plurality of electrodes, the method comprising: the operational

1. A method of discriminating cardiac rhythms in an implantable defibrillator, the implantable defibrillator including a plurality of electrodes for use in sensing and/or therapy delivery, as well as operational circuitry coupled to the plurality of electrodes, the method comprising: the operational circuitry detecting a number of cardiac events in an electrical signal using at least two of the plurality of electrodes;the operational circuitry calculating a rate using the number of detected cardiac events;the operational circuitry determining a width for a plurality of the detected cardiac events and further determining a width stability therefrom;the operational circuitry applying a decision boundary to a combination of the rate and width stability observed from the detected events; andthe operational circuitry determining, from the application of the decision boundary, whether a supraventricular condition which should not receive therapy is occurring or a ventricular arrhythmia which should receive therapy is occurring. 2. The method of claim 1 wherein the implantable defibrillator is a subcutaneous-only device in which all of the electrodes are implanted subcutaneously in the patient, such that the electrical signal is captured from electrodes implanted subcutaneously. 3. The method of claim 1 wherein the decision boundary is set such that, at a given rate, a width stability that leads to a determination that a supraventricular condition is occurring is a higher width stability than a width stability that leads to a determination that a ventricular arrhythmia is occurring. 4. The method of claim 1 wherein the decision boundary is set such that, at a given width stability, a rate that leads to a determination that a ventricular arrhythmia is occurring is a higher rate than a rate that leads to a determination that a supraventricular condition is occurring. 5. The method of claim 1 wherein the operational circuitry includes both a wide-band filter and a narrow band filter, and method is performed such that: the operational circuitry uses the electrical signal as filtered by the narrow band filter to detect cardiac events; andthe operational circuitry uses the electrical signal as filtered by the wide band filter to determine width. 6. An implantable defibrillator comprising: a plurality of electrodes for use in sensing and/or therapy delivery;a canister having at least one of the electrodes thereupon; andoperational circuitry housed by the canister and coupled to the plurality of electrodes, wherein the operational circuitry includes one or more batteries for power, one or more high power capacitors for use in therapy delivery, and a processor and associated memory to enable the operational circuitry to perform the following:the operational circuitry detecting a number of cardiac events in an electrical signal using at least two of the plurality of electrodes;the operational circuitry calculating a rate using the number of detected cardiac events;the operational circuitry determining a width for a plurality of the detected cardiac events and further determining a width stability therefrom;the operational circuitry applying a decision boundary to a combination of the rate and width stability observed from the detected events; andthe operational circuitry determining, from the application of the decision boundary, whether a supraventricular condition which should not receive therapy is occurring or a ventricular arrhythmia which should receive therapy is occurring. 7. The implantable defibrillator of claim 6 wherein the operational circuitry is configured to apply the decision boundary such that, at a given rate, a width stability that leads to a determination that a supraventricular condition is occurring is a higher width stability than a width stability that leads to a determination that a ventricular arrhythmia is occurring. 8. The implantable defibrillator of claim 6 wherein the operational circuitry is configured to apply the decision boundary such that, at a given width stability, a rate that leads to a determination that a ventricular arrhythmia is occurring is a higher rate than a rate that leads to a determination that a supraventricular condition is occurring. 9. The implantable defibrillator of claim 6 wherein the operational circuitry further comprises both a wide-band filter and a narrow band filter, and: the operational circuitry is configured to use the electrical signal as filtered by the narrow band filter to detect cardiac events; andthe operational circuitry is configured to use the electrical signal as filtered by the wide band filter to determine width. 10. An implantable defibrillator comprising: a plurality of electrodes for use in sensing and/or therapy delivery;a canister having at least one of the electrodes thereupon; andoperational circuitry housed by the canister and coupled to the plurality of electrodes and including one or more batteries for power, one or more high power capacitors for use in therapy delivery, a wide band filter, a narrow band filter, a QRS detector, an analog-to-digital (A/D) converter, and a processor and associated memory to enable the operational circuitry to perform the following:in hardware and without activating the processor, applying the narrow band filter to generate a signal to feed to a QRS detector and detect a given event for use in determining R-R intervals and rate;in hardware and without activating the processor, applying the wideband filter to generate a signal to feed to the A/D convertor for use in calculating slew rate for the given detected event; andactivating the processor to perform post-processing of one or more of the slew rate, R-R interval of the given detected event.