Mobitz I following Acute Inferior MI

Mobitz I is a common rhythm which is seen in approximately 10% of myocardial infarctions, but it is usually transient, resolving within 72 hours post infarction (1,2). The block is located within the AV node and is most often caused by an increase in vagal tone or, less commonly, ischemia of the AV junction. It is more commonly seen with inferior wall myocardial infarction than with anterior wall involvement.

Mobitz IIoccurs in less than 1% of patients and usually indicates damage to the AV junction or bundle of His. It is therefore more common with anterior wall infarction. The QRS complexes are usually narrow, but when widened, they reflect concomitant damage to the bundle branches or fascicles and hence more extensive myocardial involvement. Risk of progression to complete heart block is high.

Subendocardial ischemic events do not usually cause these arrhythmias mainly because they do not by definition penetrate deep enough into the myocardium to cause significant ischemia to the conducting system fibers. Thus rhythm disturbances are much more of a problem with transmural ischemic events.

There was one study that tried to assess the risk for progression to complete heart block with an acute myocardial infarction (3). The Multicenter Investigation of the Limitation of Infarct Size developed a scoring system to predict the occurrence of complete heart block in patients with acute myocardial infarction. The incidence of new complete heart block was 5.4%, and it occurred a mean of 2.6 days after myocardial infarction. Risk of complete heart block was not associated with the location of the infarction or the left ventricular ejection fraction.

The following factors were considered to be predictive of or associated with increased risk of complete heart block and were each assigned one point: development of PR prolongation, occurrence of second-degree AV block, left anterior or posterior fascicular block, left bundle branch block, and right bundle branch block. The risk of progression to complete heart block was 1% to 7% with a point score of 0, 8% to 10% with a score of 1, 25% to 30% with a score of 2, and 36% with a score of 3 or more.

REFERENCES:

  1. Tuns AL, Lie KI, Durrer D. Clinical setting and prognostic significance of high grade atrioventricular block in acute inferior myocardial infarction: a study of 144 patients. Am Heart J 1980;99:4
  2. Nicod P, Gilpin E, Dittrich H, et al. Factors associated with acute onset of atrioventricular block in acute Q-wave inferior infarction. J Am Coll Cardiol 1988;12:589
  3. Lamas G. A simplified approach to predicting the occurrence of complete heart block during acute myocardial infarction. Am J Cardiol 1986;57:1213

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