ECG Challenge: A Swimmer With an Irregular Pulse

Philip J. Podrid, MD


October 13, 2017


The diagnosis is normal sinus rhythm, second-degree atrioventricular (AV) block-Mobitz type I or Wenckebach, and sinus node exit block (Figure 2).

Figure 2. Courtesy of Dr Podrid.

The rhythm is regular except for two long RR intervals or pauses. The rate of the regular intervals is 90 beats/min. The QRS complex duration is normal (0.08 sec) and there is a normal morphology. The axis is extremely leftward between -30° and -90° (positive QRS complex in leads I and negative QRS complex in leads II and aVF). As the QRS complexes have an rS morphology in leads II and aVF, the left axis is due to a left anterior fascicular block. There is poor R-wave progression from leads V1 to V5, which is often seen with a left anterior fascicular block and represents clockwise rotation of the electrical axis in the horizontal plane. This is established by imagining the heart as if viewed from under the diaphragm. With clockwise rotation, left ventricular forces develop later and are more posteriorly directed. Although P waves are not apparent before most of the QRS complexes, P waves are seen after each pause (+). A P wave can be seen before the next three QRS complexes (+) and then the P wave is less apparent as it is superimposed on the T wave (*). The PP intervals are regular at a rate of 90 beats/min, and there is progressive lengthening of the PR intervals (┌┐). Therefore, this is a Mobitz type I or Wenckebach. The pause (↔) is not the result of an on-time, nonconducted P wave, however. The P wave is not seen (↓). Therefore, the pause is not the end of the Wenckebach cycle. As the P wave is not present, this is a sinus pause. Because the PP interval around the pause is twice the underlying sinus interval (↔), this is a sinus node exit block. In this situation, there has been an on-time sinus node discharge, but the impulse does not get out of the sinus node area to stimulate the atria—hence, no P wave.


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