Volume overload due to left-to-right shunting in patients with atrial septal defect type secundum causes pulmonary vascular disease over a long period of time. Pulmonary vascular resistance can be assessed non-invasively using bicycle stress echocardiography. By measuring cardiac output and pulmonary artery pressures at different stages of exercise, a pressure-output plot can be obtained. The slope of the pressure-output plot reflects pulmonary vascular resistance. In patients undergoing ASD repair after the age of 40 years, pulmonary vascular resistance was higher when compared to age-matched controls, indicating the presence of mild pulmonary vascular disease. Bosentan has been shown to decrease pulmonary vascular resistance.