Small VSD (Figs. 13.14, 13.6, and 13.11) is a small defect bordered by white fibrous tissue with a diameter less than 5 mm. The predilection site in the embryonic period is the commissural line of the ventricular septum, especially the inferior edge of the supraventricular crest (equivalent to the commissural line of the anterior and posterior ventricular septum in the embryonic period) and the bulbar ridge (equivalent to the commissural line of the conus artery ridge in the embryonic period). These small VSDs can be sutured directly without patch due to their strong edge. In many cases of small single membranous VSDs, the defects are bordered by the TV and fibrous tissue. The external aperture of the defect seems to be surrounded by a fibrous canal when it is viewed from the RV. The suture line should be placed on the base of the VSD, rather than on the aperture of the canal, to avoid having a loose stitch. Sometimes a fibrous stripe is found across the defect. If it is mistaken for the edge of the defect, a residual defect may be left.

Large subarterial VSD (Figs. 13.12, 13.15, and 13.16) is tear shaped rather than circular and has no muscular upper margin. Parts of the aortic and pulmonary annuli form the upper margin. The front of the right coronary cusp can be seen through the defect. The AV is easily injured, and the conduction system remains at a distance from the defect. The AV should be protected during the operation.

Above the large infracristal VSD (Figs. 13.8, 13.9, and 13.2) is the right coronary cusp of the AV, and the posterior edge of the defect, which is either fibrous or muscular, extends to the tricuspid annulus. A conduction bundle runs along the posterior-inferior edge of the defect. The papillary muscle of the conus artery begins posterior to the defect, and the supraventricular crest lies anterior to the defect. The defect usually is large and should be repaired with a patch rather than direct suture. The conduction system should be protected when repairing the posterior inferior edge of defect.

The defect beneath the septal leaflet of the TV and the AV canal type of VSD (Figs. 13.13, 13.19, 13.20, 13.7, and 13.24) are usually long and slender, located below the septal leaflet of the TV. The tricuspid annulus forms the upper margin of the defect and is located posterior to the papillary muscle of the conus artery. The anterior margin remains at a distance from the AV. Most of the defect can be obscured by the septal leaflet of the TV.

A residual, fistula, small membranous defect may be divided into two holes by a fibrous stripe or be surrounded by the fibrous canal below the TV. The hole may be mistaken for the entire defect during operation, resulting in residual shunting and murmur.

AV injury leads to aortic insufficiency. In the case of a large membranous VSD, especially a subarterial VSD, the AV may be injured during surgery, resulting in avulsion and perforation of the cusp. Traction from the suture line may cause deformation of the valve. When these situations arise, reoperation should be performed decisively to repair the injured AV (Fig. 13.10).

Complete atrioventricular block may occur. An atrioventricular bundle usually runs along the posteroinferior edge of a membranous VSD (including an infracristal VSD ), close to the right ventricular side. Sutures that are too deep may injure the perforating branch or the left and right bundle branch of the bundle of His, resulting in heart block. Violent traction of local tissue may cause temporary or permanent atrioventricular block (Fig. 13.25).

Traction of the TV leads to tricuspid insufficiency. A large membranous and perimembranous VSD may be obscured by the chordae of the TV and the papillary muscle of the conus artery. Part of the leaflet or the chordae may be sutured onto the patch so that the TV closure is disturbed, leading to tricuspid insufficiency.