ECG Challenge: Lightheadedness in the ED

The correct diagnosis is normal sinus rhythm, complete heart block (third-degree heart block), escape ventricular rhythm (Figure 2).

Figure 2. Courtesy of Dr Podrid.

Discussion

There is a regular rhythm with a rate of 36 beats/min. The fifth QRS complex (*) is early, has a different QRS morphology from the first four, and has a shorter RR interval. The initial four QRS complexes are wide (0.14 sec) and have a left bundle branch block morphology, with a broad R wave in lead I and V6 and a QS complex in lead V1.

P waves are present (+); occasionally they are on top of the T wave (v) and therefore not obvious. The P waves are unassociated with the QRS complexes with variable PR intervals; this is AV dissociation. The PP intervals are regular at a rate of 95 beats/min. The P waves are positive in leads I, II, aVF, and V4-V6. Therefore, this is a normal sinus rhythm with AV dissociation.

There are two causes for AV dissociation. One is complete heart block in which the atrial rate is faster than the rate of the QRS complexes. The other cause is an accelerated lower pacemaker in which the atrial rate is slower than the rate of the QRS complexes. In this case, the atrial rate is faster than the ventricular rate; this is complete heart block or third-degree AV block.

The etiology of the escape rhythm is based on the morphology of the QRS complex and not the rate of the escape rhythm. It is uncertain whether this is an escape junctional rhythm with a left bundle branch block or a ventricular rhythm. However, the fifth QRS complex (*) is early and there is a P wave before it (↑). Because the QRS complex is early, it is responding to the P wave that precedes it.

The PR interval is prolonged (0.36 sec). Therefore, this is a conducted complex (with a first-degree AV block) and the QRS complex has a different morphology, suggestive of a right bundle branch block with a terminal S wave in leads V5-V6 (^). The conducted complex has a different morphology from the dissociated or escape QRS complexes, so the escape rhythm is ventricular and not junctional. The QT/QTc intervals are normal (520/400 msec).

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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Any views expressed above are the author's own and do not necessarily reflect the views of WebMD or Medscape.

Cite this: ECG Challenge: Lightheadedness in the ED - Medscape - Sep 22, 2020.

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