ATRIAL BRADYCARDIA:

Rhythm originating in the atrium at a rate of < 55 beats per minute.

(These rates reflect the rate of atrial beats and not necessarily the rate of the ventricular beats (e.g., There may be atrial fibrillation with a rapid ventricular response indicating the atrial rate is > 350 bpm, and the ventricular rate is > 100 bpm)).