Cardiac activity is sensed over a plurality of heart beats defining a beat set. For each beat in the set, it is determined whether the beat is a non-classified beat (e.g., paced beat, a beat outside of a specified heart rate range or a PVC), or a classified beat. For each classified beat, it is determined whether the beat is a non-detect beat, a minor beat or a major beat. Counts of classified beats, non-classified beats, major beats, minor beats, and non-detect beats are maintained. The beat set is declared to be one of a non-classified set, a major set, a minor set or a non-detect set based on the relative counts of classified beats, non-classified beats, major beats, minor beats, and non-detect beats. Over a period of time, counts of beat-set types are maintained and entry into and exit from ST episodes are determined based on these beat-set counts.