Merideth, John

A simple classification of atrioventricular block, criteria for diagnosis, and suggested therapy are discussed. The differences between Mobitz I and II second-degree A-V block are reviewed. Good management of patients who have bundle-branch or other forms of intraventricular block depends on informed judgment which includes consideration of (1) the effect of intraventricular conduction disturbances on the pumping action of the heart, and (2) the likelihood that a specific intraventricular conduction disturbance will be complicated by complete heart block. Evidence suggests that common forms of intraventricular disturbance, unlike ventricular fibrillation, do not alter significantly the pumping action of the heart. The role of intraventricular conduction disturbances in the genesis of ventricular fibrillation is assessed. We concluded that only when right bundle-branch block is combined with block of the anterior or posterior fascicle of the left bundle branch is complete heart block sufficiently imminent to warrant special concern. If bilateral block is associated with symptoms of episodic severe bradycardia, pacemaker therapy is indicated.

The morphology of the sinus and atrioventricular (A-V) nodes of human hearts was studied and connecting pathways between these nodes were sought. The cells of both nodes differed from ordinary atrial and ventricular myocardium in respect to size, shape, number, and arrangement of cytoplasmic organelles. Using the criteria of fiber-to-fiber connection, shown by means of serial histological sectioning, this study confirmed previous reports of three pathways between the sinus and A-V nodes in the human heart. It was not required that these pathways be composed of classic Purkinje-type fibers, though many of the fibers did possess some of the features of Purkinje fibers. While recent evidence reveals that specialized atrial pathways do demonstrate increased conduction velocity, their prime functional significance may be in the facilitation of orderly atrial depolarization, the maintenance of sinus node control of ventricular depolarization under various physiological conditions, and the provision of an orderly "input" into the A-V node, a tissue generally acknowledged to have a low margin of safety.