The morphology of ventricular septal defects

Ventricular septal defect (VSD) is the most common congenital cardiac lesion, occurring either in isolation or in hearts containing more complex lesions. Usually, the defect is between two ventricles, each of which is connected to a separate atrium and to a separate great artery, and surgical or spontaneous closure of the VSD basically corrects the circulation if the effects of any associated lesion are ignored. In other situations, the VSD is an integral part of the circulation, either because the atria connect to only one ventricle or because the ventricles give rise to only one patent great artery or else both great arteries arise from the same ventricle. When in such circumstances the circulation is VSD-dependent and the defect cannot be surgically closed without bypassing it with a conduit or similar device. In all of these situations, the VSD takes one of three basic forms. Usually, the VSD abuts directly upon the fibrous skeleton of the heart formed by the conjoined rings of the cardiac valves. The membranous part of the ventricular septum is an integral part of this skeleton, and these defects are termed perimembranous. These defects do not always occupy the same part of the septum. They may extend mostly into either the inlet, trabecular, or outlet parts of the muscular septum, or else be confluent extending into two or all these parts. Less commonly, VSD may be exclusively contained within the muscular septum. Such muscular defects can also be confined to either the inlet, trabecular, or outlet parts of the septum. They may be multiple or coexist with one of the other types. The third and least common type is a VSD which is roofed by the conjoined rings of the aortic and pulmonary valves because of absence of the outer septum. Such a defect may have a muscular posteroinferior rim or may extend to become perimembranous. Identifying a defect as perimembranous or muscular, together with its location relative to the different parts of the muscular septum, gives at the same time information concerning the site of the conduction axis in relation to the defect and its chances of spontaneous closure.