The most common of all congenital heart defects are those that occur in the ventricular septum—ventriculoseptal defect (VSD). These lesions are most frequent in the membranous portion of the septum and they allow blood to flow from the left ventricle (higher pressure) to the right ventricle (lower pressure), leading to an abnormal communication between the systemic and pulmonary circulation. This leads to right ventricular hypertrophy, increased pulmonary blood flow, elevated arterial pulmonary pressure, and increased blood volume returning to the left ventricle, causing its dilation. Increased pulmonary pressure in most severe cases may cause pulmonary edema. If large enough and left untreated, VSDs can produce marked clinical problems that might require surgery. VSD may be an isolated abnormality or part of a syndromic constellation, such as the tetralogy of Fallot.