Arrhythmias and Conduction Defects

The most frequent effect of MI is the development of cardiac arrhythmias. PVCs, monomorphic ventricular tachycardia, ventricular fibrillation, all degrees of AV blocks, and polymorphic ventricular tachycardia are commonly caused by an MI.

These arrhythmias may evolve over time. For example, a patient with both an inferior wall MI and a right ventricular infarction tends to develop a complete supra-His AV block that regresses into first-degree or second-degree AV block before returning to normal AV conduction within a few days. Pacing is rarely required in these patients. 

Complete infra-Hisian AV block is rare but is seen in patients with an extensive anterior wall MI. The presence of complete infra-Hisian AV block indicates severe damage to the left ventricle.

Fascicular blocks may develop in the presence of an MI. RBBBs occur more frequently than LBBBs.

Other Complications

Ventricular aneurysm, ventricular rupture, and septal perforation are other complications of an MI.

Ventricular aneurysm caused by an anterior wall MI may present on ECG as a persistent ST elevation > 1 mm in more than three anterolateral leads (I, aVL, V₃ through V₆). However, the specificity of the 12-lead ECG in detecting aneurysms from anterior wall MI is low. This may be caused by the proximity effect in the limb leads. An inferior aneurysm, on the other hand, is more likely to be detectable by persistent ST elevations in leads II, III, and aVF.

Ventricular rupture or septal perforation is not detected by ECG. A second ST elevation may warn of imminent rupture.