ECG Challenge: Lightheadedness and Extremely Slow Pulse

Philip J. Podrid, MD


December 19, 2022

The correct diagnosis is sinus rhythm with high-degree atrioventricular (AV) block (ie, Mobitz II) (Figure 2).

Figure 2. Courtesy of Philip J. Podrid, MD.


The rhythm is regular, at a rate of 30 beats/min. The QRS complexes are wide (0.14 sec), and a left bundle branch block morphology occurs with a QS in lead V1 (←) and a broad R wave in leads I and V5-V6 (→). The axis is normal between 0° and +90° (positive QRS complex in leads I and aVF). The QT/QTc intervals are prolonged (680 msec/480 msec) but are normal when corrected for the prolonged QRS complex duration — ie, subtract 40 msec from the measured QT interval and correct for heart rate (640 msec/450 msec).

A P wave precedes each QRS complex (+), and the PR interval (^) is stable (0.28 sec). Between each QRS complex are two nonconducted P waves (*). The PP intervals are constant (⊔), with a rate of 90 beats/min. The P waves are positive in leads I, II, aVF, and V4-V6, indicating a normal sinus rhythm. Thus, this is a second-degree AV block (ie, an occasional nonconducted, on-time P wave).

With Mobitz type I, there is only one nonconducted P wave, whereas with Mobitz type II, there may be two or more nonconducted P waves. Hence, this is a Mobitz type II and is termed high-degree AV block because of the two nonconducted P waves. The PR interval of the conducted P waves is prolonged (0.28 sec), indicating a first-degree AV block or first-degree AV conduction delay.

Philip J. 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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