Methods and devices for tachyarrhythmia sensing and high-pass filter bypass

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; and

selecting 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, and

during 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; and

if 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; and

in 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, and

during 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; and

if 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; and

if so, delivering cardioversion/defibrillation stimulus.

16. The method of claim 9 , further comprising filtering DC components of the captured signal.