Multi-threshold sensing of cardiac electrical signals in an implantable medical device

1. A medical device, comprising:

a control circuit configured to:

set a drop time interval; and

detect an expiration of the drop time interval; and

a sensing circuit coupled to the control circuit and configured to:

receive a cardiac electrical signal;

hold a cardiac event sensing threshold at a first threshold value until the expiration of the drop time interval;

adjust the cardiac event sensing threshold from the first threshold value to a minimum threshold value upon the expiration of the drop time interval;

sense a cardiac event attendant to a myocardial depolarization in response to the cardiac electrical signal crossing the cardiac event sensing threshold; and

generate a sensed event signal in response to sensing the cardiac event.

2. The medical device of claim 1 , wherein the control circuit is further configured to set the drop time interval based on an R-P time interval extending from an R-wave to a P-wave.

3. The medical device of claim 2 , wherein the control circuit is further configured to:

determine the R-P time interval from the cardiac electrical signal; and

set the drop time interval to be greater than the determined R-P time interval.

4. The medical device of claim 1 , wherein the control circuit is further configured to:

determine a heart rate; and

adjust the drop time interval based on the heart rate.

5. The medical device of claim 4 , further comprising a therapy delivery circuit configured to generate cardiac pacing pulses;

wherein the control circuit is configured to:

schedule a pacing pulse to be generated by the therapy delivery circuit at a pacing interval;

determine the heart rate by determining the pacing interval; and

adjust the drop time interval based on the pacing interval.

6. The medical device of claim 1 , wherein:

the sensing circuit is configured to:

set a blanking interval in response to the cardiac electrical signal crossing the cardiac event sensing threshold; and

determine a peak amplitude of the cardiac electrical signal during the blanking interval; and

the control circuit is further configured to determine at least one of the first threshold value and the minimum threshold value based on the peak amplitude.

7. The medical device of claim 1 , wherein the control circuit is further configured to:

determine a baseline noise amplitude of the cardiac electrical signal; and

set the minimum threshold value to be greater than the baseline noise amplitude.

8. The medical device of claim 1 , further comprising a therapy delivery circuit configured to generate cardiac pacing pulses,

wherein the control circuit is coupled to the therapy delivery circuit and configured to:

start a pacing interval in response to the generated sensed event signal;

detect an expiration of the pacing interval; and

the therapy delivery circuit is configured to generate a cardiac pacing pulse in response to the expiration of the pacing interval.

9. The medical device of claim 1 , further comprising a therapy delivery circuit configured to deliver an electrical stimulation therapy,

wherein the control circuit is coupled to the therapy delivery circuit and configured to:

determine a cardiac event interval in response to the sensed event signal generated by the sensing circuit;

determine that the cardiac event interval is less than a tachyarrhythmia detection interval;

increase a tachyarrhythmia interval count in response to determining that the cardiac event interval is less than the tachyarrhythmia interval;

determine that the tachyarrhythmia interval count reaches a threshold number of intervals to detect tachyarrhythmia; and

detect a tachyarrhythmia in response to the tachyarrhythmia interval count reaching the threshold number of intervals to detect tachyarrhythmia;

wherein the therapy delivery circuit is configured to deliver the electrical stimulation therapy in response to the control circuit detecting the tachyarrhythmia.

10. The medical device of claim 1 further comprising a connector block configured to receive an extra-cardiovascular implantable lead.

11. A method, comprising:

receiving a cardiac electrical signal;

setting a drop time interval;

detecting an expiration of the drop time interval;

holding a cardiac event sensing threshold at a first threshold value until the expiration of the drop time interval;

adjusting the cardiac event sensing threshold from the first threshold value to a minimum threshold value upon the expiration of the drop time interval;

sensing a cardiac event attendant to a myocardial depolarization in response to the cardiac electrical signal crossing the cardiac event sensing threshold; and

generating a sensed event signal in response to sensing the cardiac event.

12. The method of claim 11 , further comprising setting the drop time interval based on an R-P time interval extending from an R-wave to a P-wave.

13. The method of claim 12 , further comprising:

determining the R-P time interval from the cardiac electrical signal; and

setting the drop time interval to be greater than the determined R-P time interval.

14. The method of claim 11 , further comprising:

determining a heart rate; and

adjusting the drop time interval based on the heart rate.

15. The method of claim 14 , further comprising:

scheduling a cardiac pacing pulse at a pacing interval;

determining the heart rate by determining the pacing interval; and

adjusting the drop time interval based on the pacing interval.

16. The method of claim 11 , further comprising:

setting a blanking interval in response to the cardiac electrical signal crossing the cardiac event sensing threshold; and

determining a peak amplitude of the cardiac electrical signal during the blanking interval; and

determining at least one of the first threshold value and the minimum threshold value based on the peak amplitude.

17. The method of claim 11 , further comprising:

determining a baseline noise amplitude of the cardiac electrical signal; and

setting the minimum threshold value to be greater than the baseline noise amplitude.

18. The method of claim 11 , further comprising:

starting a pacing interval in response to the generated sensed event signal;

detecting an expiration of the pacing interval; and

generating a cardiac pacing pulse in response to the expiration of the pacing interval.

19. The method of claim 11 , further comprising

determining a cardiac event interval in response to the sensed event signal generated by the sensing circuit;

determining that the cardiac event interval is less than a tachyarrhythmia detection interval;

increasing a tachyarrhythmia interval count in response to determining that the cardiac event interval is less than the tachyarrhythmia interval;

determining that the tachyarrhythmia interval count reaches a threshold number of intervals to detect tachyarrhythmia;

detecting a tachyarrhythmia in response to the tachyarrhythmia interval count reaching the threshold number of intervals to detect tachyarrhythmia;

delivering an electrical stimulation therapy in response to detecting the tachyarrhythmia.

20. A non-transitory computer-readable medium storing instructions, which when executed by a control circuit of a medical device cause the device to:

receive a cardiac electrical signal;

set a drop time interval;

detect an expiration of the drop time interval;

hold a cardiac event sensing threshold at a first threshold value until the expiration of the drop time interval;

adjust the cardiac event sensing threshold from the first threshold value to a minimum threshold value upon the expiration of the drop time interval;

sense a cardiac event attendant to a myocardial depolarization in response to the cardiac electrical signal crossing the cardiac event sensing threshold; and

generate a sensed event signal in response to sensing the cardiac event.