IT HAS been of concern that bundle-branch block is a precursor of complete heart block. The concept of the distal conduction system being composed of three discrete pathways is one reason for this concern. The bundle branches connect the atrioventricular node to the ventricle. Although this distal conduction system is a syncytium, it can be divided into three major fascicles: the right bundle and the two portions of the left bundle, the anterior fascicle, and the posterior fascicle.
Block of one of these fascicles, unifascicular block, can be recognized as either right bundle-branch block, left anterior fascicular block (left-axis deviation of at least —30 degrees with initial R waves in leads II, III, aVf, and Q wave in lead I, aVl), or left posterior fascicular block (right axis deviation of +105 degrees or greater, with Q waves in II, III, aVf, and initial R waves in I,
McAnulty JH, Rahimtoola SH. Chronic Bundle-Branch BlockClinical Significance and Management. JAMA. 1981;246(19):2202-2204. doi:10.1001/jama.1981.03320190060033