Implantable medical device with real time T-wave oversensing detection

1. A method of controlling ventricular tachycardia (VT) and ventricular fibrillation (VF) detection in an implantable medical device, the method comprising:

sensing R-waves in a ventricular electrogram (EGM) signal;

obtaining time intervals between successive sensed R-waves;

monitoring a plurality of characteristics indicative of T-wave oversensing;

wherein monitoring the plurality of characteristics comprises maintaining a first count to track a number of occurrences of large successive interval differences and maintaining a second count to track a number of occurrences of a current time interval being approximately equal to a sum of two preceding time intervals, and

withholding detection of a VT or VF in response to the first count and second count indicating presence of oversensed T-waves among the sensed R-waves.

2. The method of claim 1 wherein the large differences between successive time intervals are derived from a current time interval and at least one preceding time interval.

3. The method of claim 1 , further comprising maintaining a third count to track a number of occurrences of alternating waveform morphologies, wherein withholding detection of a VT or VF in response to the characteristics comprises withholding detection of a VT or VF in response to the third count and one of the first count and second count indicating presence of oversensed T-waves among the sensed R-waves.

4. The method of claim 1 wherein if at least one of the first count and second count does not indicate presence of oversensed T-waves among the sensed R-waves, the method further comprising withholding detection in response to the other one of the first count and the second count reaching a value strongly indicating T-wave oversensing.

5. The method of claim 1 , further comprising:

analyzing a waveform morphology of each sensed R-wave;

maintaining a third count to track a number of instances of alternating waveform morphologies; and

withholding detection of the VT or VF if the third count and one of the first count and second count indicate a likelihood of T-wave oversensing.

6. The method of claim 1 wherein maintaining the first count comprises incrementing the first count in response to a large successive difference between the current interval and an immediately preceding interval being present and decrementing the first count in response to a large successive difference not being present between the current interval and an immediately preceding interval.

7. The method of claim 6 wherein maintaining the second count comprises incrementing the second count in response to a current interval being approximately equal to a sum of two preceding intervals, the second count maintained over a predetermined number of intervals.

8. The method of claim 7 wherein monitoring a plurality of characteristics of T-wave oversensing further comprises maintaining a third count to track a number of an alternating waveform morphology, the third count maintained over a second predetermined number of sensed events by incrementing the third count in response to a current sensed R-wave and two immediately preceding R-waves having alternating morphologies.

9. The method of claim 8 further comprising:

comparing the first count to a first count threshold, comparing the second count to a second count threshold, comparing the third count to a third count threshold, and

withholding detection of the VT or VF in response to two of the first count, the second count, and the third count exceeding the respective first count threshold, the second count threshold and the third count threshold.

10. The method of claim 9 further comprising:

comparing one of the first count, the second count and the third count to a next threshold in response to the other two of the first count, the second count, and the third count not exceeding the respective first count threshold, the second count threshold and the third count threshold; and

withholding detection of the VT or VF in response to the one of the first count, the second count and the third count exceeding the next threshold.

11. The method of claim 1 wherein maintaining the first count comprises:

determining a first difference between the current interval and the immediately preceding interval;

determining a second difference between the current interval and a next immediately preceding interval;

determining a third difference between the current interval and a sum of the immediately preceding interval and the next immediately preceding interval;

determining a fourth difference between the current interval and a previous successive difference;

determining a minimum of one of the first, second, third and fourth differences; and

adjusting the first count in response to a comparison of the first difference to the minimum.

12. The method of claim 1 wherein the first count and second count are updated with each sensed R-wave.

13. The method of claim 3 , further comprising:

comparing at least three successive sensed events to a template R-wave;

classifying each of the three successive sensed events as either a high percentage match or low percentage match based on the comparison;

detecting an occurrence of alternating waveform morphologies when the classification of the three successive sensed events are high-low-high or low-high-low; and

incrementing the third count upon detecting the occurrence of alternating waveform morphologies.

14. An implantable medical device comprising:

at least one sensing electrode to sense an EGM signal from a heart of a patient;

a sense circuit for sensing events representing R-waves in the EGM signal; and

a processor to identify T-wave oversensing by the implantable medical device based on counts of T-wave oversensing characteristics, the processor obtaining time intervals between successive sensed events, maintaining a first count of T-wave oversensing characteristics to track a number of occurrences of large successive interval differences, maintaining a second count of T-wave oversensing characteristics to track a number of occurrences of a current time interval being approximately equal to a sum of two preceding time intervals, and withholding detection of a ventricular tachycardia (VT) or ventricular fibrillation (VF) in response to the first count and the second count indicating presence of oversensed T-waves among the sensed R-waves.

15. The implantable medical device of claim 14 wherein at least one of the first count and second count does not indicate presence of oversensed T-waves among the sensed R-waves and the processor withholds detection of a ventricular tachycardia or ventricular fibrillation if one characteristic count has reached a value strongly indicating T-wave oversensing.

16. The device of claim 14 wherein maintaining the first count comprises incrementing the first count in response to a large successive difference between the current interval and an immediately preceding interval being present and decrementing the first count in response to a large successive difference not being present between the current interval and an immediately preceding interval.

17. The device of claim 16 wherein maintaining the second count comprises incrementing the second count in response to a current interval being approximately equal to a sum of two preceding intervals, the second count maintained over a predetermined number of intervals.

18. The device of claim 17 wherein monitoring a plurality of characteristics of T-wave oversensing further comprises maintaining a third count to track a number of an alternating waveform morphology, the third count maintained over a predetermined number of sensed events by incrementing the third count in response to a current sensed R-wave and two immediately preceding R-waves having alternating morphologies.

19. The device of claim 18 further comprising:

comparing the first count to a first count threshold, comparing the second count to a second count threshold, comparing the third count to a third count threshold, and

withholding detection of the VT or VF in response to two of the first count, the second count, and the third count exceeding the respective first count threshold, the second count threshold and the third count threshold.

20. The device of claim 19 further comprising:

comparing one of the first count, the second count and the third count to a next threshold in response to the other two of the first count, the second count, and the third count not exceeding the respective first count threshold, the second count threshold and the third count threshold; and

withholding detection of the VT or VF in response to the one of the first count, the second count and the third count exceeding the next threshold.

21. The device of claim 14 wherein the first count and second count are updated with each sensed R-wave.