The trifascicular scheme of intraventricular conduction has gained wide clinical acceptance during the past decade.1,2 Anatomical distribution of specialized conducting fibers in the left ventricle does not correspond well with the discrete hypothetical fascicles that are postulated in normal activation,3 but the concept of three fascicles serves as a useful functional classification of conduction disturbances. When strict diagnostic criteria are applied, electrocardiographic findings of left anterior or posterior fascicular block are reasonably specific signs of true conduction disturbances. When criteria are applied loosely or signs of confounding conditions such as inferior myocardial infarction or right ventricular hypertrophy are not taken into account, the diagnosis is suspect.
Clinical implications of fascicular block, particularly bifascicular block, have generated much controversy. In 1968, Lasser and associates4 reported that a high proportion of patients with complete atrioventricular heart block had right bundle-branch block (RBBB) and left anterior fascicular block on earlier