A method and medical device for determining a cardiac episode that includes sensing a cardiac signal, identifying the signal sensed during a predetermined time interval as one of a cardiac event, a non-cardiac event, and an unclassified event, determining a number of identified cardiac events, determining a number of identified unclassified events, and determining whether the cardiac episode is occurring in response to the number of identified cardiac events being greater than a cardiac event count threshold and the number of identified unclassified events being less than an unclassified event count threshold.