An ambulatory medical device includes a cardiac activity sensing circuit and a processing circuit. The processor circuit includes a correlation circuit and a rhythm discrimination circuit. The correlation circuit generates an indication of correlation between each of at least a portion of the cardiac depolarizations and a stored template representative of a normal sinus rhythm. The rhythm discrimination circuit is configured to compare the indications of correlation to a specified correlation threshold value, classify the information representative of cardiac activity as a specific cardiac rhythm using the comparison, and identify at least one indication of correlation that determines the classification. The processing circuit provides the identified indication of correlation to a user or process.
