Conoventricular defects are the most common isolated VSDs, comprising about 70% to 80% in most series. They are situated in the junctional area between the conal or outlet and inlet portions of the septum and may extend into the inlet, outlet, or both parts of the septum. They extend up to the tricuspid valve annulus in the region of the anteroseptal commissure of the tricuspid valve. The aortic valve is easily visible through the defect when viewed through the tricuspid valve at operation. Alternatively, these defects may be partially occluded or rendered restrictive by overlapping tricuspid valve tissue. Occasionally, the noncoronary aortic leaflet may prolapse through the defect resulting in progressive aortic incompetence.

The conduction system is intimately associated with this VSD. The bundle of His penetrates the right trigone of the central fibrous body at the anteroseptal commissure of the tricuspid valve. From there, it is closely related to the lower half (posteroinferior edge) of the defect, giving off the left fascicles along its course to the medial papillary muscle (also called the muscle of Lancisi or the papillary muscle of the conus). As it passes just inferior to the latter, only the right bundle remains, and this continues into the trabecular septomarginalis away from the edge of the defect. Conoventricular septal defects are usually repaired through the right atrium.

Conotruncal defects are also called supracristal, subarterial, subpulmonary, juxtaarterial, or infundibular. They account for 5% to 10% of isolated defects, except in the Asian population in whom they are more common accounting for up to 30% of VSDs. Typically, the defects are oval in shape and extend up to the pulmonary and aortic annuli. Because the normal subpulmonary conus is deficient, the pulmonary and aortic valves are separated only by a very thin rim of fibrous tissue that maintains fibrous continuity between the aortic and pulmonary valves. This results in aortic leaflet prolapse, which occurs in 40% to 50% of conal defects. The right aortic leaflet most commonly is sucked into the defect, resulting in aortic insufficiency. The conduction tissue is remote from the borders of this type of VSD, which is most easily repaired through a short transverse incision in the right ventricular outflow tract, or through the main pulmonary artery (Fig. 80.1). These defects will not close spontaneously
but can be significantly restricted by a prolapsed aortic valve leaflet.

Inlet septal defects account for about 5% of isolated defects. They are situated beneath the septal leaflet of the tricuspid valve, with the tricuspid valve annulus forming their posterior border. The tricuspid valve may straddle across the defect with attachments into the left ventricle. The conduction tissue is closely related to the posteroinferior border of the defect, up to the muscle of Lancisi, at which point the right bundle branch diverges down the trabecular septomarginalis to form the moderator band. The AV septum is intact, although occasionally the anterior leaflet of the mitral valve may be partially cleft. Inlet septal defects will not close spontaneously and are best repaired through the tricuspid valve.

Muscular VSDs account for 10% to 20% of isolated VSDs, may be single or multiple, and may occur in any part of the muscular septum. They may be associated with other types of VSDs. They are generally classified depending upon their location in the septum and may be divided into midmuscular defects (most common), apical or posterior muscular, and anterior muscular defects (Fig. 80.1). Muscular defects may have numerous openings of variable size on the right ventricular side but only a single opening on the left ventricular side of the septum. This is referred to as a “Swiss cheese” defect. The conduction tissue is generally remote from the edges of a muscular defect, with two notable exceptions: (1) when associated with a conoventricular defect, the penetrating bundle usually runs in the muscle bridge separating the two defects and may easily be injured if each are closed individually and (2) if a muscular defect is present in the inlet septum (i.e., the defect is separated from the tricuspid valve by a thin rim of muscle), the conduction tissue runs along the defect superior and anterior (leftward) margin— the AV node penetrates the interventricular septum at the anteroseptal commissure of the tricuspid valve and takes the most direct route to the medial papillary muscle along the superior border of the defect. Muscular defects often close spontaneously. Surgical approach is variable based upon the location of the defect.