Histopathological correlation between the QRS axis and disposition of the atrioventricular conduction system in common atrioventricular orifice and in its related anomalies.

Serial sections of the atrioventricular (AV) conduction system were microscopically studied in 12 autopsied hearts: four with complete type common AV orifice (CAVO), two with atrial septal defect of the ostium primum type (ASD-I), two with ventricular septal defect of the persistent common AV canal type (VSD-C) and four without heart disease. The anatomic findings were semiquantitatively compared with the normal control using Feldt's method and correlated to the mean frontal QRS axis on ECG. The cases of CAVO or VSD-C with left axis devation invariably showed similar patterns regarding the location and course of the AV conduction system: (1) posterior displacement of the AV node, (2) relatively short distance between the AV node and the beginning of the left bundle branch (LBB), and (3) postero-inferior displacement of the bundle of His and the LBB. The postero-inferior displacement of the LBB seemed to be responsible for left axis deviation. The cases of ASD-I showed some additional findings: (1) impaired contiguity between the AV node and the bundle of His might have caused the occasional advanced AV block in one case, and (2) the posterior radiation of the LBB traversed down through the branch-free muscle bundle, which was assumed to be related to the right axis deviation in the other case. These findings suggest that the disposition of the AV conduction system in CAVO and in its related anomalies were basically the same whatever the type of defect, and this was considered to be correlated to the established ECG pattern.