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 includes detection means (10) for detecting electrical events (500) occurring within a patient using the electrodes (44, 48, 50, 52), event sampling means for constructing event representations (504) for the detected electrical events (500), the event representations (504) each comprising a series of samples； and comparing means (506) for performing a plurality of comparisons between a selected event representation (202) and a template (200) by comparing the selected event representation (202) to the template (200) having a template fiducial point by identifying an event fiducial point in the selected event representation (202) and aligning the template fiducial point and the selected event fiducial point to one another and calculating a first correlation between the selected event representation (202) and the template (200) when so aligned, and again comparing the selected event representation (202) to the template (200), this time by selecting a different alignment of the fiducial points of the template (200) and the selected event representation (202) and calculating a second correlation between the selected event representation (202) and the template (200). The operational circuitry further includes correlation selection means for selecting from at least the first correlation or the second correlation whichever indicates greater correlation as the correlation result for the first event representation (202), correction means (20) for using a series of correlation results from the correlation selection means to determine whether acc
