An apparatus comprises an implantable cardiac signal sensing circuit and a controller circuit. The implantable cardiac signal sensing circuit provides a sensed depolarization signal from a ventricle and a sensed depolarization signal from an atrium. The controller circuit includes an onset detection circuit and a classification circuit. The onset detection circuit detects an onset episode that includes fast cardiac depolarizations and identifies a depolarization that initiates the onset episode. The classification circuit classifies the onset episode as supra-ventricular tachycardia (SVT) when the initiating onset episode is identified in the atrium and the number of atrial depolarizations is greater than or equal to the number of ventricular depolarizations during the onset episode, and as ventricular tachycardia (VT) when the initiating onset depolarization is identified in the ventricle and the number of ventricular depolarizations is greater than the number of atrial depolarizations during the onset episode.