The present invention relates to an implantable cardiac stimulus (ICS) system comprising a canister (42) housing operational circuitry for the ICS system. The canister (42) has a canister electrode (44) and a lead (46) having at least a first lead electrode (48, 50, 52) disposed thereon. The canister electrode (44) and the first lead electrode (48, 50, 52) are electrically coupled to the operational circuitry. The operational circuitry comprises detection means (10) for detecting electrical events (500) occurring within the patient using the electrodes (44, 48, 50, 52)； rate estimation means (408) for calculating a cardiac rate using a set of the detected electrical events (500)； overdetection analysis means (12) for identifying one or more detected events as likely occurring due to overdetection； and data correction means (20) for analyzing intervals around the events identified as likely occurring due to overdetection to determine whether the likely overdetection is likely a T-wave and a) if the likely overdetection is likely a T-wave, correcting data relating to the overdetection to reduce calculated cardiac rate； or b) if the likely overdetection is unlikely to be a T-wave, disabling data correction methods in the ICS system for at least the likely overdetection. The operational circuitry further comprises determining means for determining whether a likely cardiac arrhythmia is occurring and, if so, determining whether stimulus is needed； and therapy delivery means for delivering electrical stimulus to the patient from the ICS system if stimulus is needed.
