Congenital absence of the normal valvular opening between the right atrium and the right ventricle. There is also a small right ventricle, a large left ventricle and decreased pulmonary circulation. Blood from the right atrium passes through an atrial septal defect into the left atrium, where it mixes with oxygenated blood returning from the lungs, then flows into the left ventricle and out into the systemic circulation. Blood reaches the lungs through an atrial septal defect and a ventricular septal defect.