By Ken Grauer, MD
Professor Emeritus in Family Medicine, College of Medicine, University of Florida
The ECG in the figure below was obtained from a 40-year-old man with an irregular heartbeat. How would one interpret this rhythm?
This is a difficult tracing. The QRS complex is narrow in all 12 leads. This tells us that the rhythm is supraventricular. Although the rhythm is irregular, there is group beating — that is, a definite pattern of three-beat groups can be seen. The sequence and relative duration of the R-R intervals between beats is similar in each of the four groups. A short pause of similar duration separates each of the three-beat groups. Some P waves also are visible. The fact that the PR interval preceding beats 4, 7, and 10 is the same suggests an underlying sinus rhythm is present.
The complex part of this tracing is the realization that a number of additional P waves are present. The difficulty is trying to figure out how many additional P waves are present and what this means. Definite P waves precede beats 3, 6, 9, and 12, each with a similar PR interval that is longer than the PR interval of sinus beats 4, 7, and 10. In addition, it appears P waves also are present, notching the terminal part of the T waves of beats 1, 4, 7, and 10. The PR interval of these four P waves is similar and prolonged. Note the T waves of beats 2-3, 5-6, 8-9, and 11 all appear to be taller and more peaked than the T wave of beats 1, 4, 7, and 10. The reason for this could be the P waves are hidden within each peaked T wave. Because these P waves occur so early in the refractory period, they are not conducted to the ventricles. Thus, the reason for the short pause that follows each of the three-beat groups is there is a non-conducted PAC that peaks the T waves of beats 3, 6, and 9.
Sometimes, it is simply not possible to be certain of the etiology of a rhythm from a single tracing. This ECG illustrates one of those times. What can be said is the rhythm is not AV Wenckebach, because the atrial rhythm is clearly not regular (as it should be for second- or third-degree AV block). Therefore, no pacemaker is needed. Instead, my hunch is repetitive PACs are visible, or perhaps short runs of atrial tachycardia with intermittent PR interval prolongation or non-conduction of atrial activity arising when P waves occur early in the cardiac cycle.
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