Anatomic substrate for congenital atrioventricular block in middle-aged adults

Congenital atrioventricular block is usually a benign disorder not necessitating pacing. In some patients slowing of rate and/or mortality have been noted with aging. However an anatomic substrate has not been established for the progressive slowing of the escape rate. In this study we report an anatomic substrate in two such patients who were dying in congestive heart failure, ages 49 and 42, respectively. Multiple pre-mortem ECG's in both cases revealed wide QRS escape rhythms, and escape rates of approximately 35 and 28 beats/minute, respectively. Conduction system examination by serial section in both cases revealed lack of connection between the atrial septum with the peripheral conduction system with total replacement by fat of the AV nodal approaches and AV node, and advanced sclerosis of the summit of the ventricular septum which was more marked on the right side. In addition, the His bundle showed marked septation in case one and fragmentation in case two. Sclerosis of the summit of the ventricular septum involved the branching bundle and the bundle branches in both cases. In conclusion, both patients had the characteristic lesions of congenital atrioventricular block, namely replacement of the AV node and AV nodal approaches by fat, with lack of connection to the peripheral conduction system, and one also had a fragmented His bundle. In addition premature aging of the summit of the ventricular septum may have reflected the long-standing hemodynamic stresses of chronic bradycardia. This in turn resulted in trifascicular involvement of the conduction system leading to a shifting of the escape rhythm distally eventuating in a slower idioventricular escape rhythm.