Development of increased pulmonary vascular resistance and right-to-left shunting in patient with preexisting left-to-right shunt; most common causes are large ventricular septal defect and patent ductus arteriosus. Death results from endocarditis, thrombosis, or heart failure. Optimal treatment is closure of communication of connection at earliest possible date; once pulmonary hypertension develops, treatment is heart and/or lung transplantation.