A method and medical device for detecting a cardiac event that includes determining, in response to the cardiac event being identified as a shockable event, a first R-wave morphology corresponding to R-waves of a next interval subsequently sensed along a first sensing vector and a second R-wave morphology corresponding to R-waves of a next interval subsequently sensed along a second sensing vector, determining whether the next interval subsequently sensed along the first sensing vector and the next interval subsequently sensed along the second sensing vector satisfy a morphology metric, and identifying the cardiac event as one of a monomorphic ventricular tachycardia and a polymorphic ventricular tachycardia in response to the determined first R-wave morphology, second R-wave morphology, and the next interval subsequently sensed along the first sensing vector and the next interval subsequently sensed along the second sensing vector not satisfying the morphology metric.
