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Devices and methods for analyzing cardiac signal data. An illustrative method includes identifying a plurality of detected events and measuring intervals between the detected events for use in rate estimation. In the illustrative embodiment, a set of intervals is used to make the rate estimation by

Devices and methods for analyzing cardiac signal data. An illustrative method includes identifying a plurality of detected events and measuring intervals between the detected events for use in rate estimation. In the illustrative embodiment, a set of intervals is used to make the rate estimation by first discarding selected intervals from the set. The remaining intervals are then used to calculate an estimated interval, for example by averaging the remaining intervals.

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1. A method of cardiac signal analysis in an implantable cardiac system for monitoring or treating a patient's heart, the implantable cardiac system including at least a plurality of implantable electrodes, the method comprising: sensing a signal using the implantable electrodes;detecting a series o

1. A method of cardiac signal analysis in an implantable cardiac system for monitoring or treating a patient's heart, the implantable cardiac system including at least a plurality of implantable electrodes, the method comprising: sensing a signal using the implantable electrodes;detecting a series of events in the sensed signal, the series of events separated by intervals, to obtain a set of intervals;within the set of intervals, selecting a longest interval and generating a limited set of intervals that excludes the longest interval; andestimating a cardiac rate of the patient using the intervals in the limited set of intervals;wherein generating the limited set of intervals further comprises selecting the two shortest intervals within the set of intervals and excluding the two shortest intervals from the limited set of intervals. 2. The method of claim 1 wherein the limited set of intervals excludes the longest interval based only upon its status as longest among the set of intervals. 3. The method of claim 1 wherein the set of intervals includes 8 intervals, and the limited set of intervals includes 5 intervals, excluding the two shortest intervals from the 8 intervals and the longest interval from the set of 8 intervals. 4. The method of claim 1 wherein the set of intervals includes 7 intervals and the limited set of intervals includes 4 intervals and excludes the shortest two of the 7 intervals and the longest of the 7 intervals. 5. A method of cardiac signal analysis in an implantable cardiac system for monitoring or treating a patient's heart, the implantable cardiac system including at least a plurality of implantable electrodes, the method comprising: sensing a signal using the implantable electrodes;detecting events in the sensed signal, the detected events separated by intervals;at a first time, using a first method to estimate a heart rate for the patient using information related to the detected events;comparing the heart rate for the patient estimated using the first method to a rate threshold and determining that the threshold has been crossed;at a second time after determining that the rate threshold has been crossed, using a second, different method to estimate the heart rate for the patient. 6. The method of claim 5 wherein: the first method to estimate heart rate comprises averaging a set of intervals between detected events; andthe second method to estimate heart rate comprises choosing intervals according to a set of interval selection rules and calculating a rate based upon the chosen intervals. 7. The method of claim 6 further comprising: performing noise detection on the detected events and marking an interval before and an interval after a detected event that fails noise detection as suspect;performing overdetection identification on any detected events that pass noise detection and combining an interval before and an interval after any detected events found to be overdetected into combined intervals and treating as discarded the two intervals used to form the combined interval;wherein the interval selection rules comprise the following: include any combined interval and exclude all suspect or discarded intervals;exclude the shortest two intervals that are not combined intervals; andexclude the longest interval that is not a combined interval. 8. A method of cardiac signal analysis in an implantable cardiac system for monitoring or treating a patient's heart, the implantable cardiac system including at least a plurality of implantable electrodes, the method comprising: sensing a signal using the implantable electrodes;detecting a series of events in the sensed signal, the series of events separated by intervals, to obtain a set of intervals;within the set of intervals, selecting a shortest interval and generating a limited set of intervals that excludes the shortest interval; andestimating a cardiac rate of the patient using the intervals in the limited set of intervals;wherein the step of estimating a cardiac rate includes calculating a weighted average of the intervals in the limited set of intervals. 9. The method of claim 8 wherein the limited set of intervals excludes the shortest interval based only upon its status as shortest among the set of intervals. 10. The method of claim 8 wherein generating the limited set of intervals further comprises selecting a longest interval within the set of intervals and excluding the longest interval from the limited set of intervals. 11. The method of claim 8 wherein the limited set of intervals excludes the longest interval based only upon its status as longest among the set of intervals. 12. A method of cardiac signal analysis in an implantable cardiac system for monitoring or treating a patient's heart, the implantable cardiac system including at least a plurality of implantable electrodes, the method comprising: sensing a signal using the implantable electrodes;detecting a series of events in the sensed signals, the series of events separated by intervals, to obtain a set of intervals;performing a noise detection analysis on the series of events to identify any suspect events and labeling an interval before and an interval after any suspect event as a suspect interval;performing an overdetection analysis on the series of events to identify any overdetected events and combining an interval before and an interval after each overdetected event into a single combined interval;performing a first rate calculation by taking a first subset of the set of intervals and reducing the first subset by excluding at least one shortest interval and at least one longest interval from the first subset to generate a reduced first subset and then calculating a first rate based on the reduced first subset;performing a second rate calculation by taking a second subset consisting of a selected N intervals from the set of intervals where the N intervals are selected by excluding any suspect intervals and then taking N combined intervals and intervals that have not been combined from those that remain after excluding the suspect intervals, and calculating a second rate based on the second subset; andperforming rhythm analysis using either the first rate or the second rate. 13. A method of cardiac signal analysis in an implantable cardiac system for monitoring or treating a patient's heart, the implantable cardiac system including at least a plurality of implantable electrodes, the method comprising: sensing a signal using the implantable electrodes;detecting a series of events in the sensed signals, the series of events separated by intervals, to obtain a set of intervals;selecting a first set of intervals for a first rate calculation;performing a first rate calculation by obtaining a mean and error measure for the first set of intervals and using the mean less the error measure to estimate cardiac rate for the patient. 14. The method of claim 13 wherein the error measure is selected from one of standard deviation and variance. 15. The method of claim 14 further comprising performing rhythm analysis by determining whether the error measure is less than a predetermined level and, if so, determining that the patient is not experiencing a malignant arrhythmia. 16. The method of claim 15 further comprising, if the error measure is not less than the predetermined level, analyzing one or more of rate and morphology of the cardiac signal to determine whether the patient is experiencing a malignant arrhythmia. 17. An implantable cardiac system for monitoring or treating a patient's heart, the implantable cardiac system including at least a plurality of implantable electrodes and a canister housing operational circuitry, the operational circuitry configured to perform a method of cardiac signal analysis: sensing a signal using the implantable electrodes;detecting a series of events in the sensed signal, the series of events separated by intervals, to obtain a set of intervals;within the set of intervals, selecting a longest interval and generating a limited set of intervals that excludes the longest interval; andestimating a cardiac rate of the patient using the intervals in the limited set of intervals;wherein the operational circuitry is further configured such that, within the method of cardiac signal analysis, generating the limited set of intervals further comprises selecting the two shortest intervals within the set of intervals and excluding the two shortest intervals from the limited set of intervals. 18. The implantable cardiac stimulus system of claim 17 wherein the operational circuitry is further configured such that, within the method of cardiac signal analysis, the limited set of intervals excludes the longest interval based only upon its status as longest among the set of intervals. 19. The implantable cardiac stimulus system of claim 17 wherein the operational circuitry is further configured such that, within the method of cardiac signal analysis, the set of intervals includes 8 intervals, and the limited set of intervals includes 5 intervals, excluding the two shortest intervals from the 8 intervals and the longest interval from the set of 8 intervals. 20. The implantable cardiac stimulus system of claim 17 wherein the operational circuitry is further configured such that, within the method of cardiac signal analysis, the set of intervals includes 7 intervals and the limited set of intervals includes 4 intervals and excludes the shortest two of the 7 intervals and the longest of the 7 intervals.