By Ken Grauer, MD

Professor Emeritus in Family Medicine, College of Medicine, University of Florida

How would one interpret the rhythm disturbance shown in the figure below? Unfortunately, no history was available. Does the rhythm in the figure represent Mobitz II second-degree AV block, complete AV block, or Wenckebach? Does the patient need a pacemaker?

This is a challenging tracing. That said, there are several observations that can be made that significantly narrow the diagnostic possibilities. Group beating is present in the form of alternating short-long intervals. Although several arrhythmias (e.g., atrial bigeminy or trigeminy) also may produce group beating, recognition of this phenomenon should prompt one to consider some form of Wenckebach conduction. Several beats are non-conducted. For example, two P waves in a row (without any intervening QRS complex) are seen within the R-R intervals of beats 2-3, 4-5, 6-7, and 8-9.

At least some beats are conducted because one can see an identical PR interval preceding the first QRS complex at the end of each brief pause (i.e., the PR interval preceding beats 1, 3, 5, 7, and 9 is the same). The atrial rhythm appears to be regular. This is not easy to appreciate without using calipers. This is because on-time P waves are hiding within the ST segments of beats 1, 3, 5, and 7. But three P waves in a row in each group are not visible. For example, a P wave with a short PR interval immediately precedes beat 2, another P wave immediately follows the ST-T wave of beat 2, and a third consecutive P wave appears before beat 3. This appearance of consecutive P waves reveals what the P-P interval is and allows one to verify that subtle notching within the ST segments of beats 1, 3, 5, and 7 represents on-time, regular atrial activity.

These observations strongly suggest some form of Wenckebach conduction is present. The rhythm in the figure is not complete AV block because the constant PR intervals preceding the first QRS complex at the end of each pause proves there is at least some conduction. The rhythm also is not Mobitz II second-degree AV block because the PR interval never remains constant for two conducted beats in a row. Finally, a pacemaker will not necessarily be needed because the rhythm appears to be AV Wenckebach, in which the overall ventricular response is not overly slow. Instead, clinical correlation will be needed to determine the best course of management.

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