SYMPOSIUM Cardiac Arrhythmias (Part 1) Anatomy of the Conduction System

are explained on the basis of anatomic lesions in specific sites of the conduction system or its blood supply. Second-degree A-V block of Mobitz type II, which may be a precursor of complete (third-degree) heart block, commonly is associated with fibrotic lesions of uncertain etiology in the branching part of the bundle of His or the bundle branches. Ischemic lesions are found less often, and other pathologic processes rarely are present. Chronic complete heart block most often results from nonspecific, fibrotic interruption of the distal bundle of His, or of the first parts of the bundle branches after their origins. Ischemic lesions are uncommonly the cause of chronic block. High-grade A-V block complicating acute myocardial infarction may be associated with infarction of the A-V conduction system, but often morphologic evidence of ischemia cannot be identified. Congenital variants in anatomy of the conduction system are responsible for some relatively uncommon arrhythmias.