The location and course of the atrioventricular conduction system in common atrioventricular orifice and in its related anomalies with transposition of the great arteries--A histopathological study of six cases.

The location and course of the atrioventricular (AV) conduction system were studied histopathologically in 6 hearts by sectioning serially, 4 having common AV orifice (CAVO) with complete or partial transposition of the great arteries (TGA) and 2 having ventricular septal defect of the persistent common AV canal type (VSD-C) with complete TGA. Two cases of CAVO with TGA and asplenia syndrome (Cases 1 and 2) had 2 discrete AV conduction systems, being posterior and anterior to the site of the defect, respectively. In these 2 cases posterior AV conduction system well developed, whereas the anterior one was hypoplastic. In another case of CAVO with TGA and asplenia syndrome (Case 3), only the anterior AV conduction system existed near the base of the great arteries. In the other case of CAVO with TGA and polysplenia syndrome (Case 4), the posterior AV conduction system was found to have a congenital interruption of the AV bundle of His. Two cases of VSD-C with TGA but with no splenic anomaly (Cases 5 and 6) showed the posterior AV conduction system with communication-free accessory bundles. The posterior AV node, bundle of His and left bundle branch inevitably shifted postero-inferiorly, except in Case 3. The bundle branches were always distributed appropriately to their morphologically matched ventricles. The superiorly oriented vector in the mean frontal QRS axis in Cases 1, 5 and 6 seemed to be related to the postero-inferior displacement of the posterior AV conduction system, whereas those of the left-inferior oriented vector in Cases 2 and 3 were thought to be responsible for the excessively unbalanced size of ventricles. The complete AV block in Case 4 was correlated with the interruption of the bundle of His. The morphogenesis of the anterior AV conduction system was discussed in relation to the asplenia syndrome.