A medical device and associated method for discriminating cardiac events includes determining whether a cardiac evidence counter is greater than a predetermined detection threshold, advancing from a concerned state to a convinced state in response to the evidence counter being greater than the predetermined detection threshold, determining whether a reduction in the cardiac evidence counter occurs while in the convinced state, determining whether one of a first rate corresponding to the first sensing vector and a second rate corresponding to the second sensing vector is less than a predetermined rate limit, and determining whether to advance from the convinced state to one of a therapy delivery state, the concerned state and the unconcerned state in response to determining whether one of the first rate and the second rate is less than a predetermined rate limit.