ECG Challenge: Feeling Tired and Lightheaded

Philip J. Podrid, MD


March 16, 2020

The correct diagnosis is sinus rhythm, second-degree AV block with 2:1 conduction, Mobitz type II (Figure 2).

Figure 2. Courtesy of Dr Podrid.


There is a regular rhythm at a rate of 42 beats/min, although the last QRS complex is early (↓). The QRS complex has a normal duration (0.08 sec) and morphology. The axis is normal between 0° and +90° (positive QRS complex in leads I and aVF). The QT/QTc intervals are normal (450/375 msec). A P wave occurs before each QRS complex (+) with a stable PR interval (0.20 sec). The P wave is positive in leads I, II, aVF, and V4-V6. Hence, this is a sinus P wave.

A second P wave is seen after each QRS complex (v) but is not associated with a QRS complex; therefore, it is nonconducted. The PP interval is constant (└┘) and the atrial rate is 75 beats/min. This is a second-degree AV block with a pattern of 2:1 conduction.

The pattern of 2:1 AV block may be due to either Mobitz type I or Mobitz type II. The etiology can be established only if there is a change in the pattern of AV conduction—for example, two sequentially conducted P waves. As seen, the last two P waves (*) are conducted, accounting for the last QRS complex (↓) being early. The PR interval associated with these two QRS complexes is the same (^), and it is the same PR interval as all of the other conducted P waves (ie, 0.20 sec). Therefore, the 2:1 AV block is due to Mobitz type II.

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.

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