A method and medical 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, during first processing of a first interval sensed along the first sensing vector during a predetermined sensing window and a second interval sensed along the second sensing vector during the predetermined sensing window, whether one or both of the first interval and the second interval is within one of a ventricular tachycardia shock zone and a ventricular fibrillation shock zone, identifying the cardiac event as a shockable event in response to one or both of the first interval and the second interval determined as being within the ventricular tachycardia shock zone, identifying the cardiac event as a shockable event in response to both of the first interval and the second interval determined as being within the ventricular fibrillation shock zone, and determining whether to confirm the cardiac event being identified as a shockable event in response to the identifying.
