Left anterior hemiblock in acute myocardial infarction. Incidence and clinical significance in relation to the presence of bundle branch block and to the absence of intraventricular conduction defects.

The incidence of intraventricular conduction defects was examined retrospectively in 449 consecutive patients with acute myocardial infarction (AMI). The incidence of left anterior hemiblock (LAH), right bundle branch block (RBBB), left bundle branch block (LBBB) and RBBB+LAH was 12.2, 4.2, 3.8 and 2.5%, respectively. At least 24 patients (5.8%) developed LAH as a result of the AMI. LAH was present in 20% (33/164) of patients with anterior infarction, in 14% (18/131) of those with infarction of undetermined localization, and in 3% (4/143) of patients with diaphragm infarction. The incidence of complete atrioventricular (AV) block in patients with LAH was 6% and in patients with no intraventicular conduction defects 7%. In patients with RBBB, RBBB+LAH and LBBB, the incidence of complete AV block was 37, 45 and 18%, respectively. Severe pump failure occurred with the same low incidence in patients with LAH as in patients without intraventricular conduction defects, but was much more common in patients with complete bundle branch block (BBB). The mortality rate for patients with LAH was 22% and for patients with no intraventricular conduction defects 21%. In patients with RBBB, RBBB+LAH and LBBB, the mortality rates were 53, 55 and 53%, respectively. Patients with complete BBB had a higher age and a higher incidence of previous AMI than the others. Compared to patients with no intraventricular conduction defects, the presence of LAH did not increase the mortality rate, or the risk of developing severe heart failure or complete AV block, in contrast to the serious prognosis in patients with complete BBB.