ECG Challenge: What Triggered This AF Patient's Shortness of Breath?

Philip J. Podrid, MD


April 22, 2021

The correct diagnosis is atrial fibrillation with complete heart block and an escape ventricular rhythm (Figure 2).

Figure 2. Courtesy of Dr Podrid.


The underlying rhythm is atrial fibrillation. The first three and the last two QRS complexes (+) are regular (↔) at a rate of 38 beats/min. The fourth and fifth QRS complexes (^) have different RR intervals (⊔) and are therefore irregularly irregular. The intermittent regularization of atrial fibrillation is caused by complete heart block. The irregular fourth and fifth QRS complexes (^) result from intact atrioventricular (AV) conduction. Therefore, the complete heart block is intermittent.

The two conducted complexes have a widened QRS duration (0.12 sec) and resemble a left bundle branch block (LBBB). However, there is a small septal R wave in lead V1 (↓) and a prominent S wave in V5 (←) which are not characteristic of LBBB because septal forces — resulting from initial septal activation via a septal or median branch from the left bundle — and terminal impulse moving from left to right (ie, terminal S wave in V5) are not seen with a left bundle. The terminal S wave in lead V5 is more consistent with a right bundle branch block. Hence, this is an intraventricular conduction delay to the left ventricle.

The QRS complexes resulting from complete heart block (+) are wider (0.16 sec) and have a different morphology, as seen in the lead V1 and V5 rhythm strips. Therefore, the escape rhythm is ventricular. This means that the complete heart block is not caused by AV nodal blockade or AV nodal-blocking agents. Rather, it is the result of conduction block within the His-Purkinje system, a result of His-Purkinje disease.

Philip Podrid, MD, is an electrophysiologist, a professor of medicine and pharmacology at Boston University School of Medicine, and a lecturer in medicine at Harvard Medical School. Although retired from clinical practice, he continues to teach clinical cardiology and especially ECGs to medical students, house staff, and cardiology fellows at many major teaching hospitals in Massachusetts. In his limited free time he enjoys photography, music, and reading.

You can follow Dr Podrid on Twitter @PPodrid

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