Patients with single ventricle anatomy undergo staged surgical palliation. The result is an "in series" circulation with pulmonary blood flow and cardiac output directly related to pulmonary vascular resistance. While surgical outcomes have improved, the physiology of the single ventricle palliation results in continued long term attrition. Elevated pulmonary vascular resistance and impaired systemic ventricular function are important risk factors for failure of single ventricle palliation.

Sildenafil is a pulmonary vasodilator and has been shown to improve cardiac contractility in the pressure overloaded right ventricle.

The investigators will assess the safety, pharmacokinetics and hemodynamic efficacy of sildenafil in single ventricle patients following stage II and III surgical palliation.

Inclusion Criteria:

1. Age ≥ 3 months; ≤120 months.
2. History of congenital heart disease with severe hypoplasia of a right or left ventricle.
3. Undergoing cardiac catheterization as part of standard clinical care.
4. Availability and willingness of the parent/legally authorized representative to provide written informed consent.

Exclusion Criteria:

1. History of serious adverse event related to sildenafil administration.

2. History of sildenafil exposure within 48 hours of the study.

3. Presence of pulmonary venous obstruction.

4. Treatment with organic nitrates or alpha blockade therapy.

5. Contraindication to cardiac catheterization as determined by the attending cardiologist and including:

   1. Significant hemodynamic instability.
   2. Sepsis.
   3. Need for Extra-Corporeal Membrane Oxygenation (ECMO) support.
   4. Venous occlusion precluding adequate access.
   5. Recent systemic illness.

6. Renal failure defined as serum creatinine > 2 times higher than the upper limit of normal.

7. Liver dysfunction defined as alanine aminotransferase or aspartate aminotransferase > 3 times higher than the upper limit of normal.

8. Thrombocytopenia defined as a platelet count < 50 000 cells/µL.

9. Leukopenia defined as white blood cells < 2500 cells/µL.

Overall 21 subjects were enrolled and all subjects received a baseline assessment followed by sildenafil administration.