We and others have previously shown that asymptomatic patients with the Fontan operation have a mean flow-mediated dilation of approximately 4% compared to 8-9% in controls. In order to detect a 25% change in FMD, with a power of 0.80, the current study would require a patient population of 13 cases. There are currently over 40 patients with single ventricle who are followed in the adult congenital clinic at Emory University.