ECG Challenge: A 45-year-old woman who runs on a regular basis presents to her physician for a routine physical examination. She has no known cardiac disease, but has mild hypertension for which she is taking hydrochlorothiazide. Her pulse is noted to be irregular and a 12-lead ECG is obtained.
The rhythm is irregular, but there is a repeating pattern as all of the long RR intervals are the same (1.16 s), the intermediate RR intervals are the same (0.64 s), and all the short RR intervals (0.58 s) are the same. Thus, the rhythm is regularly irregular. There is a pattern of group beating; there are 3 QRS complexes and then a pause. The average rate is 72 bpm. The first and third QRS complexes have the same width (0.08 s) and morphology with an axis of ≈–30° (positive QRS complex in lead I, negative complex in lead aVF, and biphasic in lead II). The QT/QTc intervals are normal (360 ms/390 ms). The first and third QRS complexes have a P before them (+,*) and the PP intervals are constant (┌┐) at a rate of 50 bpm. The P waves are positive in leads I, II, aVF, and V4 through V6. Hence, there is an underlying sinus bradycardia. The PR interval of each of the first QRS complexes is stable (0.26 s), 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 s). The second of the QRS complexes (↑) has a normal width (0.08 s) and a morphology that is the same as the first and third QRS complexes. However, there is no P wave before this QRS complex. Hence, it is a premature junctional complex. This 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 a slightly different amplitude in several leads. It is not uncommon for junctional complexes to have a slightly different axis and amplitude in comparison with the sinus complexes, because the junctional complex enters the bundle of His, which is a series of tracts, at a different location in comparison with an impulse that originates in the atrial myocardium and conducts through the AV node to conduct into the bundle of His. Because the premature junctional complex is not associated with a pause and does not alter the PP interval, the premature complex is said to be interpolated. Because every third QRS complex is a premature junctional complex, this is junctional trigeminy. The PR interval after the premature junctional complex is longer than the baseline PR interval as a result of retrograde concealed conduction. The junctional premature complex results in ventriculoatrial conduction that only partially conducts through the AV node (ie, it is concealed within the AV node and causes it to be partially refractory). The next sinus P wave conducts through the AV node, but, because the node is partially refractory, the conduction through the AV node is slower, resulting in a slightly longer PR interval.
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- © 2016 American Heart Association, Inc.