The significance and prognosis of chronic bifascicular block.

The clinical course of 303 patients with bifascicular block was reviewed. Initially, 62% of the total group had evidence of heart disease. The highest frequency of heart disease was in patients with left bundle-branch block and first-degree atrioventricular block (78% or 22/28), while the lowest was in those with right bundle-branch block and left axis deviation (56% or 55/98). During the subsequent clinical course the occurrence of cardiovascular morbidity was greatest in patients who had left bundle-branch block and heart disease (55% or 49/89). Complete heart block occurred in only 11 patients. The highest incidence of complete heart block occurred in right bundle-branch block and left axis deviation when associated with heart disease, but the annual rate was only 4%/yr. Mortality was highest in those with left bundle-branch block, first-degree atrioventricular block, and left axis deviation (43%/yr). A one-year mortality of 65% (11/17) was noted for patients who had bifascicular block prior to the time of acute myocardial infarction.