The invention relates to an implantable cardiac stimulus device (ICSD) comprising a canister housing operational circuitry and a lead electrode assembly including a plurality of electrodes disposed thereon, with the lead electrode assembly configured to couple to the canister to electrically couple the operational circuitry to the electrodes on the lead electrode assembly, the operational circuitry comprising detection means for detecting a series of events using electrical signals captured with the electrodes interval means for identifying intervals between consecutive detected events characterized by the operational circuitry further comprising overdetection analysis means using morphology analysis to identify overdetections from the series of events, interval correction means for, whenever an overdetection is identified, combining an interval preceding the overdetection and an interval following the overdetection to form a combined interval, certifying means for, following the morphology analysis to identify overdetections, certifying combined intervals and intervals that are not adjacent to overdetections, rate estimation means for estimating a cardiac rate using only certified intervals, and rhythm classification means for using the estimated cardiac rate to make therapy decisions.
