The present invention provides a system and a method for discriminating supraventricular tachyarrhythmias from ventricular arrhythmias during a tachycardia episode. First cardiac signals and second cardiac signals are sampled for cardiac complexes. A first feature on the first cardiac signal and a second feature on the second cardiac signal are utilized to determine an average time difference for a plurality of normal sinus rhythm complexes. A time difference between the first feature and the second feature is then determined for each cardiac complex of a tachycardiac rhythm. The cardiac complex is characterized as a ventricular tachycardia complex if the time difference exceeds the average time difference by a predetermined amount. Otherwise, it is classified as VT if its morphology after alignment is different from that during normal sinus rhythm.