Which of the following statements about VSDs is/are correct?

A. Perimembranous lesions are located in the region of the membranous portion of the interventricular septum near the anteroseptal commissure of the tricuspid valve.

C. VSD, in its isolated form, is the most commonly recognized congenital heart defect.

D. The conduction bundle runs along the posteroinferior rim of a perimembranous VSD.

DISCUSSION: Perimembranous VSDs occupy the area of the membranous portion of the interventricular septum adjacent to the anteroseptal commissure of the tricuspid valve. Often a remnant of the membranous portion of the interventricular septum (the membranous flap) is left hanging on the posteroinferior rim of the defect. The annulus of the tricuspid and aortic valves often form a part of the rim of the defect, but in some patients they are separated from the VSD by a thin rim of muscle tissue that protects the conduction bundle. Muscular VSDs have exclusively muscular rims on all four sides. VSDs in the outlet septum that extend to the annuluses of the aortic and pulmonary valves are called doubly committed or juxta-arterial defects. Isolated VSDs occur at an approximate rate of 2 per 1000 live births and represent 30% to 40% of all congenital heart malformations at birth. The conduction bundle in patients with perimembranous VSDs does run along the posteroinferior rim of the defect on the left ventricular side. Sutures used for repair of a perimembranous VSD should be placed well away from this area to avoid the creation of surgically induced complete heart block.