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.
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.