Method and apparatus for detecting and discriminating tachycardia

1. A method, comprising:

obtaining a plurality of cardiac event intervals, at least a portion of the plurality of intervals corresponding to a tachycardia interval;

sensing a first cardiac event having a morphology corresponding to a supraventricular beat;

obtaining evidence of a rhythm breaking point preceding the first cardiac event; and

detecting a non-treatable rhythm in response to the plurality of cardiac event intervals and the evidence of the rhythm breaking point, wherein obtaining evidence of a rhythm breaking point comprises:

sensing a preceding cardiac event, the preceding cardiac event and the first cardiac event being consecutive cardiac events;

measuring an interval between the first cardiac event and the preceding cardiac event;

generating an event interval threshold;

comparing the measured interval to the event interval threshold; and

detecting a long pause in response to the interval being longer than the event interval threshold, the method further comprising:

obtaining evidence of an undersensed cardiac event in response to detecting the long pause; and

cancelling the long pause detection in response to obtaining the evidence of an undersensed cardiac event.

2. The method of claim 1 , wherein the first cardiac event and the preceding cardiac event are sensed using a first electrode pair and the evidence of an undersensed cardiac event is obtained using a second electrode pair different than the first electrode pair.

3. The method of claim 2 , wherein obtaining evidence of an undersensed cardiac event comprises sensing an intervening cardiac event on the second electrode pair, the intervening cardiac event sensed during the long pause.

4. The method of claim 3 , wherein sensing the intervening cardiac event comprises:

setting an interval during the long pause that is truncated from the beginning of the long pause and truncated from the end of the long pause; and

sensing the intervening cardiac event using the second electrode pair during the truncated interval.

5. The method of claim 1 , wherein obtaining evidence of an undersensed cardiac event comprises:

measuring a feature of a cardiac signal during the long pause;

comparing the feature to a feature of a sensed cardiac event; and

detecting an undersensed event in response to the comparison.

6. The method of claim 5 , wherein the feature comprises one of an amplitude and a slew rate.

7. A method, comprising:

obtaining a plurality of cardiac event intervals, at least a portion of the plurality of intervals corresponding to a tachycardia interval;

sensing a first cardiac event having a morphology corresponding to a supraventricular beat;

obtaining evidence of a rhythm breaking point preceding the first cardiac event; and

detecting a non-treatable rhythm in response to the plurality of cardiac event intervals and the evidence of the rhythm breaking point, wherein obtaining evidence of a rhythm breaking point comprises:

sensing a preceding cardiac event, the preceding cardiac event and the first cardiac event being consecutive cardiac events;

measuring an interval between the first cardiac event and the preceding cardiac event;

generating an event interval threshold;

comparing the measured interval to the event interval threshold; and

detecting a long pause in response to the interval being longer than the event interval threshold, the method further comprising storing a normal morphology template, wherein determining whether the morphology of the preceding cardiac event corresponds to a supraventricular heartbeat comprises computing a morphology matching score by comparing the morphology of the preceding cardiac event starting the long pause to the stored template.

8. The method of claim 7 , further comprising:

generating a morphology threshold based on a duration of the long pause; and

comparing the morphology matching score to the morphology threshold, and wherein the evidence of the rhythm breaking point is obtained in response to the morphology matching score exceeding the morphology threshold.

9. The method of claim 8 , wherein generating the morphology threshold comprises:

setting the morphology threshold to a first threshold value in response to the long pause being a first length; and

setting the morphology threshold to a second threshold value less than the first threshold value in response to the long pause being a second length longer than the first length.

10. A medical device for monitoring a patient's heart rhythm, comprising:

a first detector to sense a plurality of cardiac events intervals, at least a portion of the plurality of intervals corresponding to a tachycardia interval;

a second detector to sense a first cardiac event having a morphology corresponding to a supraventricular beat and to obtaining evidence of a rhythm breaking point preceding the first cardiac event; and

a processor configured to detect a non-treatable rhythm in response to the plurality of cardiac event intervals and the evidence of a rhythm breaking point, wherein obtaining the evidence of a rhythm breaking point comprises:

sensing a preceding cardiac event, the preceding cardiac event and the first cardiac event being consecutive cardiac events;

measuring an interval between the first cardiac event and the preceding cardiac event;

generating an event interval threshold;

comparing the measured interval to the event interval threshold; and

detecting a long pause in response to the interval being longer than the event interval threshold, and wherein the second detector obtains evidence of an undersensed cardiac event in response to detecting the long pause, and

cancels the long pause detection in response to obtaining the evidence of an undersensed cardiac event.

11. The device of claim 10 , further comprising a first electrode pair and a second electrode pair different than the first electrode pair, wherein the first cardiac event and the preceding cardiac event are sensed using the first electrode pair, and wherein the evidence of an undersensed cardiac event is obtained using the second electrode pair.

12. The device of claim 11 , wherein obtaining the evidence of an undersensed cardiac event by the second detector comprises sensing an intervening cardiac event on the second electrode pair, the intervening cardiac event sensed during the long pause.

13. The device of claim 12 , wherein the second detector sets an interval during the long pause that is truncated from the beginning of the long pause and truncated from the end of the long pause, and senses the intervening cardiac event using the second electrode pair during the truncated interval.

14. The device of claim 10 , wherein obtaining the evidence of an undersensed cardiac event by the second detector comprises:

measuring a feature of a cardiac signal during the long pause;

comparing the feature to a feature of a sensed cardiac event; and

detecting an undersensed event in response to the comparison.

15. The device of claim 14 , wherein the feature comprises one of an amplitude and a slew rate.

16. The device of claim 10 , wherein obtaining the evidence of a rhythm breaking point by the second detector comprises determining whether the preceding cardiac event ending the long pause has a morphology corresponding to a supraventricular heartbeat, and further comprising a storage device for storing a normal morphology template, wherein determining whether the morphology of the preceding cardiac event corresponds to a supraventricular heartbeat comprises computing a morphology matching score by comparing the morphology of the preceding cardiac event starting the long pause to the stored template.

17. The device of claim 16 , wherein the second detector generates a morphology threshold based on a duration of the long pause, and compares the morphology matching score to the morphology threshold, and wherein the evidence of the rhythm breaking point is obtained in response to the morphology matching score exceeding the morphology threshold.

18. The device of claim 17 , wherein generating the morphology threshold comprises:

setting the morphology threshold to a first threshold value in response to the long pause being a first length; and

setting the morphology threshold to a second threshold value less than the first threshold value in response to the long pause being a second length longer than the first length.