Abstract: An implantable cardioverter including apparatus for measuring the width of a patient's R-wave in order to determine whether or not to deliver an antitachycardia therapy. The means for measuring the width of the R-wave comprises a digitizing means for converting electrical signals from the ventricle to digital signals and means for analyzing the digitized signals to determine the end points of sensed R-waves. End points are identified in response to the occurrence of a series of sequential digitized signals which differ from proceeding signals by more than a predetermined amount, and the width of the R-wave is defined as the interval between the first such identified end point associated with a detected R-wave and the last such detected end point associated with the same detected R-wave.