Man’s Heart Rhythm Has Been “Strange”

ANSWER
The ECG reveals sinus bradycardia with second-degree atrioventricular (AV) block (Mobitz I), also known as Wenckebach block.

Mobitz I heart block often occurs with reversible reasons of conduction block at the level of the AV node. While the P-P intervals remain constant, conduction fatigue within the AV node results in the P-R interval becoming progressively longer, until the AV node completely blocks conduction from the atria to the ventricles. The process then repeats itself in a pattern of P to QRS groups.

In this case, there are three P waves for every two QRS complexes, resulting in a 3:2 pattern. The PR interval is longest prior to the blocked QRS and shortest immediately after it. The diagnosis of sinus bradycardia results from a constant P-P interval of 58 beats/min.

Further questioning of the patient revealed that he had inadvertently doubled his dose of metoprolol. Correcting this resulted in the return of normal sinus rhythm.