The purpose of this study is to determine if the medication, sildenafil (also known as Revatio), can slow or stop the progression of liver disease in patients who previously had a Fontan operation.

1. All participants will undergo a baseline liver magnetic resonance elastography. The patients with liver stiffness score >2.5 kiloPascal (KPa) [Normal: ≤2.5 KPa] will be enrolled in the study.
2. In addition to a baseline liver MRE, all participants will undergo cardiac MRI, transthoracic echocardiogram (TTE), FibroSure® (alpha-2 macroglobulin, haptoglobulin, gamma-glutamyltransferase, bilirubin, apolipoprotein A1, and alanine transaminase), and chemistry panel.
3. This will be a double blinded placebo control study design. All participants will be randomized 1:1 to sildenafil or placebo for a total of 12 months therapy.
4. Sildenafil will be initiated at 5 mg 3 times per day for the first week, and titrated to 10 mg 3 times per day for the second week and 20 mg 3 times per day from the 3rd week to the end of the study period. The patients will be required to check their pulse rate and blood pressure daily during the first month of drug therapy. Patient who experience hypotension (blood pressure < 90/50 plus symptoms such as dizziness) during dose titration will be asked to remain on the previous tolerated dose. Patients who cannot tolerate 10 mg 3 times daily will be asked to withdraw from the study and will be asked to continue checking their blood pressure for three days after stopping the medication.
5. After 12 months (+/- 2 weeks) of therapy, all imaging studies (liver MRE, cardiac MRI, TTE) and blood tests (FibroSure® and chemistry panel) will be repeated. A final liver MRE and FibroSure will be performed at 18 months (+/- 2 weeks) for the participants whose 18 months follow-up still falls within the study period.
6. Adverse event and compliance monitoring: During the first month of enrollment (initiation and dose titration), adverse events will be collected by subject report and by weekly telephone interview with dedicated research personnel. For the rest of the study period, adverse events will be collected by subject report and by monthly telephone interview with dedicated research personnel. The research personnel will be responsible for sending out the monthly supply of medications and obtaining a count of the remaining number pills as a measure of compliance. All participants will be provided with a pamphlet containing all the side effects of sildenafil, and contact information of research team for reporting any adverse event of concerns about the study.

Inclusion criteria:

• All adult Fontan patients who have no contraindications for magnetic resonance imaging (MRI) will be eligible for the study.

Exclusion criteria:

• Subjects with implantable pacemakers
• Residual cardiac lesions (severe ventricular dysfunction, severe atrioventricular valve regurgitation, Fontan baffle or conduit obstruction)
• Viral hepatitis
• Severe renal dysfunction
• History of sildenafil use in the six months prior to study enrollment
• Ongoing sildenafil therapy
• Patients currently taking nitrates
• Hypotension at baseline (BP <90/50 mmHg)
• Pulmonary veno-occlusive disease
• Hearing/vision impairment
• Pulmonary hypertension due to sickle cell disease
• Women of child-bearing potential with a positive pregnancy test will additionally be excluded

The safety of the subject will be monitored throughout the duration of the study. During the first month of enrollment (initiation and dose titration), adverse events will be collected by subject report and by weekly telephone interview with dedicated research personnel. For the rest of the study period, adverse events will be collected by subject report and by monthly telephone interview with dedicated research personnel. All participants will be provided with a pamphlet containing all the side effects of sildenafil, and contact information of research team for reporting any adverse event of concerns about the study.

The patients will be required to check their pulse rate and blood pressure daily during the first month of drug therapy. Patient who experience hypotension (blood pressure < 90/60 plus symptoms such as dizziness) during dose titration will be asked to remain on the previous tolerated dose

Adverse events were collected from baseline to end of study for a total of approximately 24 months on all participants.

Both the treatment arm (sildenafil) and the placebo arm will receive monthly package of similar pills. Equal number of patients will be randomly assigned to each arm.