A method and apparatus for automatic rhythm analysis of digitized ECG signals. Candidate heartbeats are detected and the noise of the ECG signal portion containing the candidate is evaluated to determine whether the candidate is a valid nominal or noisy beat or an invalid noisy saturated signal. The timing of detected heartbeats, both noisy and nominal, are determined relative to prior validated heartbeats. Features of the detected heartbeats are automatically extracted including onset, offset and R wave locations; QRS complex width; R-R interval based on the onset points; instantaneous and averaged heartrates; and others. A 16 point template of the heartbeat starting at the onset is also determined. Beats are then classified in a two step process; first, by template comparison with a previously determined dominant template, and second, by a finite state machine process based on the results of the template comparison and relative timing of the beat. As a result of the two step classification beats are classified as belonging to a plurality of beat types including both single and sequence beat types. A class record for each beat type is created and stored, any portion of which is accessible for display upon demand.