opening in the partition between the two ventricles, or lower chambers, of the heart. Such defects are congenital and may be accompanied by other congenital defects of the heart, most commonly pulmonary stenosis.

The partition between the ventricles is thick and muscular except for a small fibrous section called the membranous septum. It is in this membranous portion that most septal defects are found. The condition is diagnosed by recognition of the characteristic heart sounds caused by the defect. If the opening is small, there may be no symptoms and no need for treatment. If it is large, with significant flow of blood from the left ventricle to the right, the treatment is surgical closure of the defect. If the blood flow is from the right ventricle to the left, as indicated by elevated pulmonary blood pressure, surgical repair is not indicated.