Electrocardiographic criteria for the diagnosis of left anterior fascicular block: left axis deviation and delayed intraventricular conduction

The two current criteria for diagnosis of left anterior fascicular block (LAFB) were evaluated; they are marked left axis deviation (LAD) and a delay in the time of inscription of the intrinsicoid deflection (ID) in lead aVL asynchronous to V₆. From 400 electrocardiograms with a LAD of -30° or greater, 62% showed asynchronous activation of the left ventricle. There was only a general relationship between the degree of LAD and delayed ID in aVL. The incidence of delayed ID in aVL was as follows: 2% with mean frontal QRS axis at 0°; 9% at -15°; 41% at -30°; 69% at -45°; 82% at -60°; and 100% at -75° or greater. The lack of correlation between both criteria in many instances questions their validity. The LAD alone should not be considered synonymous with LAFB. Recognition of delayed inscription of the ID in aVL is a useful supplemental criterion for diagnosis.