Effect of Tadalafil on Exercise Capacity in Pediatric Fontan Patients

Inclusion Criteria: * Patients who have undergone the Fontan procedure (regardless of type of Fontan connection or presence or absence of a fenestration) * Age 8.0 to 35.0 years * Stable clinical condition over the last 3 months (i.e. No change in medication, treatments, or development of new symptoms) * Ability to perform exercise testing * Consent and assent (as appropriate to participate in the study after receiving information concerning procedures, risks, and possible clinical benefits) o Patients will be enrolled without regard to gender, race, and ethnicity. All patients meeting study eligibility will be approached for consent. Exclusion Criteria: * Severe heart failure (New York Heart Association functional class III or IV) * Presence of liver or renal dysfunction based on the latest lab test results * Presence of hearing or visual deficit * Transcutaneous arterial blood oxygen saturation (SaO2) \<80% at rest * History of echocardiographic, MRI, or angiographic evidence of Fontan pathway obstruction * History of exercise-induced life-threatening arrhythmias or unstable rhythm(i.e. atrial flutter) * Known or suspected pregnancy. Females in the reproductive age group will be screened for pregnancy using serum beta hCG prior to administering study drug. * Patients on open label sildenafil or tadalafil