A method and medical device for discriminating an arrhythmia that includes an input circuit sensing cardiac signals and a microprocessor identifying an event associated with the sensed cardiac signals as a first arrhythmia event. A first circuit delivers a first therapy, substantially simultaneous with coupling of a charging circuit and an energy storage device to generate stored energy on the energy storage device, in response to the first arrhythmia event, and a control circuit controls delivery of the first therapy by the first circuit. The microprocessor evaluates a physiologic response to the delivered first therapy and determines the event is other than the first arrhythmia event in response to the evaluated physiologic response.
