Acute myocardial infarction and left bundle branch block.

Complete left bundle branch block often masks old as well as acute myocardial infarctions. However, a diagnosis of acute myocardial infarction in the presence of complete left bundle branch block can be made when the acute injury current is large enough to modify the secondary repolarization abnormalities of left bundle branch block. Under these circumstances the classical ST-T changes of an acute infarction may evolve in serial electrocardiograms.