Look for the Pattern

By Ken Grauer, MD

How would you interpret the two-lead rhythm strip shown in the figure? Hint: Is there a pattern?

Interpretation: For readers who have not seen this arrhythmia before, today’s tracing presents a challenge. The “good news” is that, once you become familiar with this entity, it becomes much easier to rapidly recognize other examples of this same phenomenon.

• The rhythm is fairly fast (about 100 beats/minute) and slightly irregular.
• The QRS complex looks to be wider than usual in lead I, but it does not appear to be more than half a large box wide. Therefore, it can be suspected that this rhythm is supraventricular (especially because the irregularity seen in the figure is more than typically is observed with ventricular tachycardia). Clearly, we would need to see a complete 12-lead electrocardiogram before being able to conclude that the QRS is not wide (and, thus, that the rhythm is supraventricular). However, assuming the patient is hemodynamically stable, a supraventricular etiology can be presumed.
• P waves are present. Without calipers, it is difficult to determine if the underlying atrial rhythm is regular. That said, multiple P waves are seen at different points within the R-R interval throughout the entire rhythm strip, with many of these P waves producing a notch within the ST segment or an extra peaking to some of the T waves (i.e., P waves are almost certain to be present in lead I within the peaked T waves of beats 1, 5, 9, and 13).

The key to interpreting today’s rhythm is that there is a pattern that continually repeats! That is, we see four groups of four beats each — with each of these groups separated by a short pause of equal duration (i.e., the R-R intervals between beats 4 and 5, 8 and 9, and 12 and 13 are all equal).

• Note that the QRS complex at the end of each of these three short pauses is preceded by a P wave with the same normal PR interval (i.e., the PR intervals preceding beats 5, 9, and 13 are normal and equal to each other). This is not by chance — and serves as proof that there is a regular pattern of “group beating” with definite conduction of the P wave that appears at the end of each pause.
• Start from this first conducted P wave at the beginning of each group. Even without calipers, doesn’t it seem like the underlying atrial rhythm appears to be regular? In other words, don’t the distances look to be about equal from the P wave before beat 5 until the peaked T wave in lead I that appears after beat 5, until the P wave notching the ST segment of beat 6, and until the P wave occurring immediately after the QRS of beat 7?
• Doesn’t there also appear to be another P wave of equal P-P distance producing a slightly rounded deflection at the end of the QRS of beat 8?

Impression: Today’s rhythm is atrial tachycardia with Wenckebach conduction. The repetitive pattern of group beating in which there are pauses that are less than twice the shortest R-R interval — and in which there is definite conduction with a relatively shorter PR interval at the end of each pause — is characteristic of Wenckebach conduction.

• Awareness that atrial tachycardia very commonly manifests “group beating” as a result of Wenckebach conduction facilitates rapid recognition of this arrhythmia.
• The use of calipers would enable confirmation of the underlying rapid and regular atrial rhythm in a matter of seconds.

Note: For more information about this case, visit https://tinyurl.com/KG-Blog-420.