A case of sudden slow heart rate in a patient with atrial fibrillation - what could be the possible cause?

General practitioners regularly encounter atrial fibrillation in their daily practice. A crucial cornerstone of its management includes prescribing anticoagulants and controlling the heart rate. Tachycardia or bradycardia often reflects an ongoing pathological process that should be addressed. Clues are frequently revealed through comprehensive history-taking, complete physical assessment and 12-lead electrocardiogram (ECG) interpretation at the bedside. Thus, early recognition of potential life-threatening arrhythmia in the outpatient setting will lead to appropriate clinical management. In this paper, we illustrate a sudden decrease in the heart rate of a patient with atrial fibrillation. We discuss the ECG interpretation, potential differential diagnoses and approach to clinical management.