Sildenafil After the Fontan Operation

Inclusion Criteria: * 8 years of age or older * All participants must have had Fontan completion Exclusion Criteria: * Height less than 132 cm * Unable to participate in exercise testing due to medical restrictions or physical limitations * Fontan baffle obstruction or single lung physiology * Coarctation of the aorta or neo-aorta (BP gradient \> 20 mmHg) * Severe ventricular dysfunction assessed qualitatively by echocardiography * Severe atrioventricular valvar regurgitation assessed qualitatively by echocardiography * Presence of electronic pacemaker * History of treatment with sildenafil in the six weeks prior to enrollment in study * Patients with severe renal impairment * Patients with severe hepatic impairment * Patients taking medications that inhibit or induce Cytochrome P450 3A4 (CYP3A4) (including grapefruit juice and St. John's Wort) * Patients taking alpha-blockers and nitrates * Parents/guardians or subjects who, in the opinion of the investigator, may be non-compliant with study schedules or procedures