A method and medical device for detecting a cardiac event that includes sensing cardiac signals from a plurality of electrodes, the plurality of electrodes forming a first sensing vector and a second sensing vector, 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 the first sensing vector during the predetermined sensing window and a second R-wave morphology corresponding to R-waves of a next interval subsequently sensed along the 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.
