ECG Review

Not Just 2:1 AV Block — Mobitz II?

By Ken Grauer, MD, Professor Emeritus in Family Medicine, College of Medicine, University of Florida. Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.

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Scenario: Interpret the rhythm strip shown above. Does it represent 2nd degree AV block, Mobitz Type II? Can you be sure?

Interpretation: The ventricular rhythm in the Figure is slow and irregular. Nevertheless — the QRS complex is narrow, and the atrial rate is regular at ~115/minute. The P waves immediately preceding each QRS complex manifest a fixed (and normal) PR interval. Thus, these P waves are conducting. This means that the rhythm is not complete AV block. Since there are many non-conducted P waves on the tracing — the rhythm must represent some form of high-grade 2nd degree AV block.

As opposed to last month's ECG Review (which showed the regular 2-to-1 AV conduction form of 2nd degree AV block) — the tracing here manifests an irregular and slower ventricular rate. There are features of both Mobitz I (AV Wenckebach) and Mobitz II on this tracing. In favor of Mobitz II is the low conduction ratio and high grade of AV block. No less than three P waves in a row are non-conducted in the middle of the tracing. However, the QRS complex is narrow — which is highly unusual for Mobitz II.

Clinically — the importance of distinguishing AV Wenckebach (Mobitz I) from Mobitz II relates to the much better prognosis of Mobitz I, a generally better response to treatment with atropine, and a much lower likelihood of needing a pacemaker. In this particular case it is impossible to be certain which form of 2nd degree AV block is present from this tracing alone, since one never sees two consecutively conducted P waves. Thus, one cannot tell if the PR interval is progressively increasing until the point of non-conduction. Although unusual for Mobitz I — more than one P wave in a row may be blocked on occasion with this conduction disturbance. That said — from a practical treatment perspective — distinguishing between Mobitz I and Mobitz II appears to be less important since a pacemaker may be needed in either case if the high-grade degree of AV block does not improve.