ECG Review

2nd Degree AV Block, Mobitz Type 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? Clinically — why is it important to distinguish between Mobitz I and Mobitz II 2nd degree AV block?

Interpretation: The rhythm in the Figure is slow but regular. The QRS complex is narrow, and a regular atrial rate is seen at 100/minute (arrows). Every other P wave conducts (as evidenced by the fact that a P wave does precede each QRS complex with a fixed PR interval!).

Traditionally — the AV blocks are divided into three degrees based on severity of the conduction disturbance:

• 1st degree AV block — in which all atrial impulses are conducted to the ventricles, albeit with delay (so that the PR interval exceeds 0.20 second).

• 2nd degree AV block — in which some (but not all) atrial impulses are conducted to the ventricles.

• 3rd degree (or "complete") AV block — in which none of the atrial impulses are conducted to the ventricles, despite having more than adequate opportunity for conduction to occur.

Second degree AV blocks are further classified into three types:

• Mobitz I (AV Wenckebach) — in which the PR interval progressively lengthens until a beat is dropped. This is by far the most common form of 2nd degree AV block. Mobitz I usually occurs at the level of the AV node. As a result, the QRS complex is typically narrow. Mobitz I is generally associated with inferior infarction; it often spontaneously resolves, and typically responds to atropine (which works on the AV node).

• Mobitz II — in which there is a constant PR interval for consecutively conducted beats until one or more beats are dropped. Because Mobitz II typically occurs low down in the conduction system — the QRS complex is generally wide. This less common form of 2nd degree AV block is generally associated with anterior infarction; it usually does not respond to atropine — and is important to recognize because pacing will probably be needed.

• 2-to-1 AV Block — in which one never sees two consecutively conducted beats, so that you cannot tell if the PR interval is lengthening or not. As a result, it is impossible to know for sure whether this form of 2nd degree AV block represents Mobitz I or Mobitz II. This is precisely the situation seen in the Figure. We suspect this rhythm represents 2nd degree AV block, Mobitz Type I (Wenckebach) because: 1) Mobitz I is so much more common than Mobitz II; and 2) the QRS complex is narrow, as it almost always is with Mobitz I. Finding additional rhythm strips on this patient that clearly showed progressive lengthening of consecutively conducted QRS complexes before dropping a beat would strongly support our suspicion. Clinically the distinction is important because no treatment (other than perhaps atropine) is likely to be needed for Mobitz I (especially given that the ventricular rate in the above example is not overly slow at 50/minute). In contrast, pacing would probably be needed if the rhythm was Mobitz II.