Heart block and atrioventricular septal defect.

The cardiac conduction system was examined in 2 hearts which had repair of atrioventricular septal defect. The first was from a child known to have had a prolonged P-R interval and the second was from a girl who developed complete heart block before submission to surgery. The atrioventricular conduction axis was histologically discontinuous in the first case, but preserved and surrounded by hemorrhage in the second case. This study draws attention to the inherent vulnerability of the cardiac conduction axis which must also be recognized before surgical intervention.