ECG Challenge: Runner With an Irregular Beat

Philip J. Podrid, MD

Disclosures

July 01, 2019

The correct diagnosis is sinus bradycardia, interpolated junctional premature complexes in a trigeminal pattern, and retrograde concealed conduction.

Figure 2. Courtesy of Dr Podrid

Discussion

The rhythm is irregular, but there is a pattern. All of the long RR intervals are the same (1.16 sec), the intermediate RR intervals are the same (0.64 sec), and the short RR intervals are the same (0.58 sec). Thus, the rhythm is regularly irregular.

There is also a pattern of group beating: three QRS complexes followed by a pause. The average rate is 72 beats/min. The first and third QRS complexes have the same width (0.08 sec) and morphology, with a physiologic left axis of about -30° (positive QRS complex in lead I, negative complex in lead aVF, and biphasic in lead II). The QT/QTc intervals are normal (360/390 msec). The first and third QRS complexes have a P before them (+,*), and the PP intervals (П) are constant, at a rate of 50 beats/min. The P waves are positive in leads I, II, aVF, and V4-V6. Hence, there is an underlying sinus bradycardia.

The PR interval of each of the first QRS complexes is stable (0.26 sec), consistent with a first-degree atrioventricular (AV) block or first-degree AV conduction delay. The PR interval of the third QRS complexes is also constant but is longer (^) (0.32 sec). The second of the QRS complexes has a normal width (0.08 sec) and a morphology similar to the first and third QRS complexes. However, there is no P wave before this QRS complex. This is indicative of a premature junctional complex.

The junctional complex has a normal axis between 0° and +90° (positive QRS complex in leads I and aVF), in contrast to the leftward axis of the sinus complex. In addition, these complexes have amplitudes that vary slightly among several leads.

It is not uncommon for junctional complexes to show subtle differences in their axes and/or amplitudes compared with the sinus complexes. That is because the junctional complex enters the bundle of His—which is a series of tracts—at a different location compared with an impulse originating in the atrial myocardium and conducting via the AV node into the bundle of His. The premature junctional complex is considered "interpolated" because it is not associated with a pause and does not alter the PP interval. Because every third QRS complex is a premature junctional complex, this is called "junctional trigeminy."

The PR interval that follows the premature junctional complex is longer than the baseline PR interval because of retrograde concealed conduction. The junctional premature complex results in ventriculoatrial conduction, which only partially conducts through the AV node—ie, it is concealed within the AV node, causing it to be partially refractory. The next sinus P wave conducts through the AV node, but because the node is partially refractory this conduction slows, resulting in a slightly longer PR interval.

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