A first analysis of data obtained from the registry of ASD showed that in the transcatheter closed ASD patients, mPAP was the only independent predictor of atrial arrhythmia after ASD repair. Moreover, our first prospective study showed that it is possible to identify patients with mild pulmonary vascular disease using stress echocardiography and that patients with an ASD closed at later age were unable to decrease pulmonary vascular resistance during exercise, resulting in a higher pulmonary vascular resistance at peak exercise when compared to a control group. This was reflected in a steeper pressure-flow plot when compared to healthy controls. Endothelin has shown to influence vasomotor tone, especially during exercise. Moreover, Faoro et al showed that bosentan decreased pulmonary vascular resistance as assessed with pressure-flow plots during hypoxia. Therefore, this study was designed to evaluate the effect of an dual endothelin receptor antagonist on total pulmonary vascular resistance during exercise in an older ASD patient population.