Unfortunately, no history is provided with the rhythm in the figure below. How should one proceed in analyzing this challenging rhythm strip?

Unless there are calipers readily available, it is unlikely one could be certain of the rhythm diagnosis. That said, what observations can one make that should greatly limit diagnostic possibilities? The rhythm in the figure is supraventricular because all QRS complexes are narrow.

First, it is important to recognize the group beating in the figure because it immediately suggests that some form of Wenckebach conduction may be present. Specifically, there are three groups of two beats each (beats 1-2, 6-7, and 8-9) and two groups of three beats each (beats 3, 4, and beats 5 and 10, 11, and 12). Further, there are five short pauses (the R-R intervals between beats 2-3, 5-6, 7-8, 9-10, and 12-13). Each pause is terminated by a sinus-conducted P wave with an identical (and slightly prolonged) PR interval. Thus, each group in this tracing begins with a sinus-conducted P wave that manifests the same PR interval

In addition to the sinus P waves that precede beats 1, 3, 6, 8, 10, and 13, there are other signs of regularly occurring atrial activity. For example, there is notching at approximately the same point in the upslope of the ST segment of beats 2, 4, 7, 9, and 11. This is not due to chance. Also, there appears to be extra peaking of the T waves of beats 1, 3, 6, 8, and 10. Even more subtly, there appears to be angulation in the upslope of the remaining two T waves (the T waves of beats 5 and 12). While impossible to rule out frequent premature atrial contractions, the regularity of the above-described deflections should suggest the possibility of an underlying regular atrial rhythm. (This could be confirmed easily if calipers were available for precise measurement.)

Still, even without calipers, a final observation should suggest second-degree AV block, Mobitz Type I (AV Wenckebach) as the most likely diagnosis. This observation is the presence of Wenckebach “periodicity.” In addition to group beating and an underlying regular (or at least almost regular) atrial rhythm, progressively decreasing R-R intervals within the two three-beat groups and the finding that duration of each of the short pauses is less than twice the shortest R-R interval are highly characteristic features of AV Wenckebach.

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ECG Review