Ventricular septal defect: Ventricular septal defect is a defect in the wall between the two lower chambers of the heart – left and right ventricles. As a result, oxygenated blood from the left ventricle which has a higher pressure enters the right ventricle. This in turn gets pumped to the right ventricle, increasing the pulmonary blood flow. The extra blood then returns from the left ventricle to the left atrium and back to the left ventricle. This vicious cycle increases the work of the left ventricle, which can fail in early infancy.