Left anterior fascicular block: Electrocardiographic criteria for its recognition in the presence of inferior myocardial infarction

Although the vectorcardiographic criteria for recognizing left anterior fascicular block in the presence of inferior myocardial infarction are well established, comparable electrocardiographic criteria have not been studied. From vectorcardiographic criteria, it was hypothesized that in patients with left axis deviation but without bundle branch block the presence of a deep negative terminal deflection (S wave) in lead II accompanied by a positive terminal deflection (r wave) in lead aVR should indicate left anterior fascicular block whether or not inferior infarction is present. The electrocardiograms of 75 patients with unequivocal vectorcardiographic evidence of either left anterior fascicular block or inferior infarction, or both, were reviewed. Of the 47 patients who met strict vectorcardiographic criteria for left anterior fascicular block, 44 (94 percent) showed the predicted electrocardiographic pattern, including 24 of 26 (92 percent) who had both this conduction defect and inferior myocardial infarction. There was only one patient with vectorcardiographic evidence of inferior myocardial infarction alone with the findings of left axis deviation and the electrocardiographic pattern of combined infarction and fascicular block (that is, only one false positive). Thus, if bundle branch block is excluded, the proposed electrocardiographic pattern permits recognition of left anterior fascicular block whether or not there is coexistent inferior myocardial infarction.