A medical device and associated method for discriminating cardiac events includes determining whether a cardiac evidence counter is greater than a predetermined detection threshold, determining whether to advance from a current state to a next state in response to the evidence counter being greater than the predetermined detection threshold, determining whether advancing from a previous state to the current state occurred while in one of a low variability mode and a high variability mode during the previous state, and determining whether to advance from the current state to a previous state in response to determining whether advancing from a previous state to the current state occurred while operating in one of a low variability mode and a high variability mode during the previous state.
