Use of detection profiles in an implantable medical device

1. A method of operation in an implantable cardiac system comprising a plurality of electrodes coupled to operational circuitry configured to analyze cardiac signals, the method comprising:

the operational circuitry detecting a first cardiac event by comparing a cardiac signal received from the electrodes to a detection threshold, the detection threshold being defined by a detection profile chosen from at least first and second detection profiles, with the second detection profile having lesser sensitivity to cardiac signals than the first detection profile;

the operational circuitry detecting a close call after the first cardiac event, in which the close call is detected by applying a set of close call rules including a first rule in which:

the operational circuitry defines a detection threshold to apply after the first cardiac event is detected;

the operational circuitry defines a close call threshold below the detection threshold; and

the operational circuitry determines that the close call threshold has been crossed without the detection threshold being crossed; and

the operational circuitry determining that a close call has occurred in response to the set of close call rules being met, including finding that at least the first rule has been met, and, in response to determining that a close call has occurred, selects the second detection profile for use in detecting at least one cardiac event.

2. The method of claim 1 further comprising the operational circuitry analyzing a second rule in the close call rules including:

determining that the close call threshold is crossed after the first cardiac event;

analyzing a time at which the crossing of the close call threshold occurred to determine whether it corresponds to a likely T-wave using a formula for determining a QT Interval and assuming the first cardiac event represents a QRS complex;

wherein the step of the operational circuitry detecting the close call includes determining that the second rule has been met by finding that the crossing of the close call threshold corresponds to a likely T-wave.

3. The method of claim 2 wherein the operational circuitry is configured such that the formula is selected from the group consisting of Fridericia's formula, Bazett's formula, or the Sagie et al. regression formula.

4. The method of claim 1 further comprising the operational circuitry analyzing a second rule in the close call rules including:

determining a width of the first cardiac event; and

comparing the width of the first cardiac event to a width threshold; and

wherein the step of the operational circuitry detecting the close call includes determining that the second rule has been met by finding that the width of the first cardiac event is narrower than the width threshold.

5. The method of claim 1 further comprising the operational circuitry analyzing a second rule in the close call rules including comparing the first cardiac event to a template; and

wherein the step of the operational circuitry detecting the close call includes determining that the second rule has been met by finding that the first cardiac event matches the template.

6. The method of claim 1 further comprising:

the operational circuitry detecting a second cardiac event after the first cardiac event;

the operational circuitry determining that the close call occurred between the first and second cardiac events; and

in response to the close call, the operational circuitry applying the second detection profile following the second cardiac event in order to detect a third cardiac event after the second cardiac event;

wherein each cardiac event is intended to be a single cardiac cycle or beat.

7. The method of claim 1 wherein the operational circuitry is contained in a subcutaneous pulse generator, and the electrodes are on a lead for coupling to the subcutaneous pulse generator, in which the lead is configured for placement subcutaneously over the ribs of a patient approximately parallel to the sternum of the patient.

8. The method of claim 7 wherein the step of the operational circuitry detecting a first cardiac event comprises the operational circuitry receiving cardiac electrical signals from the electrodes while positioned subcutaneously along the sternum of the patient.

9. An implantable cardiac system comprising a plurality of electrodes coupled to operational circuitry configured to analyze cardiac signals, wherein:

the operational circuitry is configured to detect to cardiac events by the application of a detection threshold to a cardiac signal received from the electrodes, the detection threshold being defined by a detection profile selected from at least first and second detection profiles, with the second detection profile having lesser sensitivity to cardiac signals than the first detection profile;

the operational circuitry is configured to detect a first cardiac event and determine whether a close call has occurred by applying close call rules;

the operational circuitry is configured such that the close call rules comprise a first rule relying on a close call threshold defined relative to the detection threshold such that the first rule is met if the cardiac signal following the first cardiac event crosses the close call threshold without crossing the detection threshold; and

the operational circuitry is configured to select the second detection profile for use in detecting at least one cardiac event after the first cardiac event if it determines that a close call has occurred in which at least the first rule has been met.

10. The system of claim 9 wherein the operational circuitry is configured such that the close call rules comprise a second rule to determine whether, if the close call threshold is crossed after the first cardiac event, the time at which such crossing occurs corresponds to a likely T-wave using a formula for determining a QT Interval and assuming the first cardiac event represents a QRS complex, wherein if the close call threshold is crossed and the timing of the crossing corresponds to a likely T-wave, then it is found appropriate to declare a close call.

11. The system of claim 10 wherein the formula is selected from the group consisting of Fridericia's formula, Bazett's formula, or the Sagie et al. regression formula.

12. The system of claim 9 wherein the operational circuitry is configured such that the close call rules comprise a second rule to determine whether the first cardiac event has a width that is narrower than a width threshold such that, if the close call threshold is crossed, and the width of the first cardiac event is narrower than the width threshold, then it is found appropriate to declare a close call.

13. The system of claim 9 wherein the operational circuitry is configured such that the close call rules comprise a second rule to determine whether the first cardiac event matches a template, wherein if the close call threshold is crossed and the first cardiac event matches the template, then it is found appropriate to declare a close call.

14. The system of claim 9 wherein, the operational circuitry is configured to detect a second cardiac event after the first cardiac event and, if a close call has occurred between the first and second cardiac events, to apply the second detection profile following the second cardiac event in order to detect a third cardiac event after the second cardiac event, wherein each cardiac event is intended to be a single cardiac cycle or beat.

15. The system of claim 9 wherein the operational circuitry is contained in a subcutaneous pulse generator, and the electrodes are on a lead for coupling to the subcutaneous pulse generator, in which the lead is configured for placement subcutaneously over the ribs of a patient approximately parallel to the sternum of the patient.