Guller, Barbara

The morphology of the atrioventricular (A-V) conduction system was studied by serial histologic section in two cases of tricuspid atresia type IB (without transposition of the great arteries, with subpulmonary stenosis). The A-V node was adjacent to the central fibrous body, and its location was indicated by a "dimple" in the floor of the right atrium. The left bundle branches originated unusually close to the nodal-bundle junction. The right bundle branch was markedly elongated in its course to the right septal endocardium. It was situated along the inferior aspect of the channel-like ventricular septal defect as it traversed the septum. The early origin of the left bundle branches and the markedly elongated course of the right bundle branch were such that, theoretically, abnormal spread of the activation wave might result.

Fourteen hearts with tricuspid atresia were reviewed at autopsy. The study included examples of normally related great arteries and of transposition of the great arteries; instances of pulmonary stenosis were present in both groups. In these examples of tricuspid atresia the size of the right ventricle depended on the size of the ventricular septal defect. The ventricular septal defect and the right ventricle usually were smaller in proportion to the size of the heart in specimens with normally related great arteries. In the absence of transposition of the great arteries, the ventricular septal defect was found to be the site of obstruction to pulmonary flow, and its size in relation to the size of the aorta determined the size of the pulmonary artery. Morphologic abnormalities exist in the region in which the proximal portions of the major atrioventricular conduction system (A-V node, bundle of His, origin of bundle branches) normally are located; thus, anatomic abnormalities in the course of the atrioventricular conduction tissue could be expected in tricuspid atresia. Based on histologic evidence, severe elevation of pulmonary vascular resistance, in general, does not occur in infancy in tricuspid atresia.