Modification of the precise relationship of the atrioventricular conduction bundle to the margins of the ventricular septal defects by the trabecula septomarginalis.

There is a widely recognized relationship between the atrioventricular conduction bundle and the margins of the ventricular septal defects according to whether the defect is perimembranous or muscular. We have shown that this relationship may in turn be markedly influenced by the precise topography of the trabecula septomarginalis. Fifteen hearts, each with a ventricular septal defect, were studied by serial sectioning. Four hearts had an isolated ventricular septal defect in the setting of normal chamber connections and relations. One heart had a truncus arteriosus, seven hearts had Fallot's tetralogy, and three hearts exhibited complete transposition. In those hearts with outlet defects in which a bar of muscle separated the rim of the defect from the position of the conduction bundle, deficiency of the posterior limb of the trabecula septomarginalis permitted the right-sided margin of the conduction axis to "surface" in directly subendocardial position. Moreover, in some of these outlet defects, the conduction axis in the region of the branching bundle was exposed directly upon the crest of the muscular septum. The position of the medial papillary muscle complex was the best landmark to this potential danger area. In some of the examples of Fallot's tetralogy, we observed a firm muscle bar overlying the posteroinferior margin of the defect, which nonetheless was perimembranous. This protected the nonbranching component of the conduction axis in the anticipated danger area. Paradoxically, a deficiency in the trabecula septomarginatis then permitted the branching bundle to sit directly astride the septum along the inferior rim of the defect. We noted particularly marked variability of the atrioventricular conduction axis among the hearts with tetralogy. Our results suggest that close inspection during the operation may reveal whether or not a well-developed trabecula septomarginalis is present and permit conclusions to be drawn concerning the precise position of the atrioventricular conduction axis.