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The present invention, in illustrative embodiments, includes methods and devices for analyzing cardiac signals in an implantable cardiac stimulus system. Within the analysis, a threshold may be defined related to a cardiac event rate. If the cardiac event rate does not exceed the threshold, filterin

The present invention, in illustrative embodiments, includes methods and devices for analyzing cardiac signals in an implantable cardiac stimulus system. Within the analysis, a threshold may be defined related to a cardiac event rate. If the cardiac event rate does not exceed the threshold, filtering of captured cardiac signals occurs, including attenuating T-waves. If the cardiac event rate does exceed the threshold, circuitry for analog filtering or programming for digital filtering is bypassed to avoid attenuating low frequency components of the captured cardiac signals.

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1. A method of operating an implantable cardiac stimulus device, the method comprising: capturing cardiac signals between implanted electrodes; observing whether a tachyarrhythmia is likely occurring; andselecting and implementing a mode of filtering based upon whether a tachyarrhythmia is likely oc

1. A method of operating an implantable cardiac stimulus device, the method comprising: capturing cardiac signals between implanted electrodes; observing whether a tachyarrhythmia is likely occurring; andselecting and implementing a mode of filtering based upon whether a tachyarrhythmia is likely occurring such that:a) if no tachyarrhythmia is likely occurring, using a first filtering, mode on the cardiac signal and using the cardiac signal, as filtered using the first filtering mode, to detect whether a cardiac event has occurred;b) if a tachyarrhythmia is likely occurring, using a second filtering mode on the cardiac signal and using the cardiac signal, as filtered using the second filtering mode, to detect whether a cardiac event has occurred;wherein the first filtering mode attenuates frequencies likely to contain T-waves of the cardiac signal more than the second filtering mode, and wherein the selecting and implementing a mode of filtering is performed based on whether a tachyarrythmia is likely occurring as follows:during a first time period, observing that a tachyarrhythmia is not likely occurring and the first filtering mode is used, andduring a second time period observing that a tachyarrhythmia is likely occurring and the second filtering mode is used instead of the first filtering mode. 2. The method of claim 1, wherein the cardiac signal is filtered to attenuate T-waves in the digital domain and, if a tachyarrhythmia is likely occurring, a portion of an instruction set for digital filtering is bypassed to avoid filtering to attenuate T-waves. 3. The method of claim 1, wherein the cardiac signal is filtered to attenuate T-waves in the analog domain and, if a tachyarrhythmia is likely occurring, a portion of an analog circuit is bypassed to avoid analog filtering to attenuate T-waves. 4. The method of claim 1, wherein: a first threshold event rate and a second threshold event rate are defined;the step of observing whether a tachyarrhythmia is likely occurring includes estimating a cardiac event rate;if the cardiac event rate exceeds the first threshold event rate but not the second threshold event rate, it is determined that a tachyarrhythmia is likely occurring; andif the cardiac event rate exceeds the second threshold event rate, it is determined that treatment stimulus is indicated. 5. The method of claim 1, wherein the step of observing whether a tachyarrhythmia is likely occurring includes observing a rate of cardiac events and, if the rate exceeds at least a first threshold, it is determined that a tachyarrhythmia is likely occurring. 6. The method of claim 1, wherein the step of observing whether a tachyarrhythmia is likely occurring includes analyzing the morphology of at least one cardiac signal. 7. The method of claim 1, further comprising: analyzing the cardiac signals to determine whether a malignant cardiac condition indicating treatment is likely occurring and, if so, delivering cardioversion/defibrillation stimulus. 8. The method of claim 1, further comprising filtering DC components of the captured signal. 9. A method of operating an implantable cardiac stimulus device comprising: capturing cardiac signals from implanted electrodes using a selectable filtering mode;analyzing the cardiac signals to determine whether a tachyarrhythmia is likely occurring; andin response to a determination of whether a tachyarrhythmia is likely occurring based on analysis of captured events, choosing the selectable filtering mode for use in analysis of future cardiac signals in the following manner:a) if no tachyarrhythmia is likely occurring, choosing a first filtering mode and filtering the cardiac signal using the first filtering mode and using the cardiac signal, as filtered by the first filtering mode, to detect whether a cardiac event has occurred;b) if a tachyarrhythmia is likely occurring, choosing a second filtering mode and filtering the cardiac signal using the second filtering mode and using the cardiac signal, as filtered by the second filtering mode, to detect whether a cardiac event has occurred; wherein:the first filtering mode, if applied to a T-wave of the cardiac signal, attenuates T-waves of the cardiac signal more than the second filtering mode would attenuate T-waves of the cardiac signal, and the method comprises:during a first time period, observing that a tachyarrhythmia is not likely occurring and applying the first filtering mode to the cardiac signal, andduring a second time period, observing that a tachyarrhythmia is likely occurring and applying the second filtering mode to the cardiac signal. 10. The method of claim 9, wherein the cardiac signal is filtered to attenuate T-waves in the digital domain and, if a tachyarrhythmia is likely occurring, a portion of an instruction set for digital filtering is bypassed to avoid filtering to attenuate T-waves. 11. The method of claim 9, wherein the cardiac signal is filtered to attenuate T-waves in the analog domain and, if a tachyarrhythmia is likely occurring, a portion of an analog circuit is bypassed to avoid analog filtering to attenuate T-waves. 12. The method of claim 9, wherein: a first threshold event rate and a second threshold event rate are defined;the step of observing whether a tachyarrhythmia is likely occurring includes estimating a cardiac event rate;if the cardiac event rate exceeds the first threshold event rate but not the second threshold event rate, it is determined that a tachyarrhythmia is likely occurring; andif the cardiac event rate exceeds the second threshold event rate, it is determined that treatment stimulus is indicated. 13. The method of claim 9, wherein the step of observing whether a tachyarrhythmia is likely occurring includes observing a rate of cardiac events and, if the rate exceeds at least a first threshold, it is determined that a tachyarrhythmia is likely occurring. 14. The method of claim 9, wherein the step of observing whether a tachyarrhythmia is likely occurring includes analyzing the morphology of at least one cardiac signal. 15. The method of claim 9, further comprising: analyzing the cardiac signals to determine whether a malignant cardiac condition indicating treatment is likely occurring; andif so, delivering cardioversion/defibrillation stimulus. 16. The method of claim 9, further comprising filtering DC components of the captured signal.