Abstract: A method and apparatus for detecting cardiac arrhythmias in a patient's heart is disclosed. The monitoring method and apparatus sense cardiac electrical signals when the heart is functioning in a known cardiac state, then characterize this known cardiac state by storing a temporally compressed template of time sequence samples. The method and apparatus allow testing during multiple different cardiac states and provide for storage of templates associated with each state. Subsequently, when the heart is functioning in an unknown cardiac state, the method and apparatus monitor cardiac electrical signals by temporally compressing samples and scan correlating these samples with the previously stored template sequences to derive correlation coefficients. The method and apparatus then use these correlation coefficients to characterize cardiac function.