Inclusion Criteria:  Infants admitted to PCH NICU Diagnosis of congenital diaphragmatic hernia (CDH) Status post-surgical repair of diaphragmatic defect Has an echocardiogram 48-72 hours after repair with left ventricular eccentricity index (LVEI) ≥ 1.4 Parental consent obtained within 24 hours after the above echocardiogram  Exclusion Criteria:  Infants with CDH who do not undergo surgical repair Does not have an echocardiogram 48-72 hours post-repair Has LVEI < 1.4 on above echocardiogram Has concurrent severe congenital heart defect that requires neonatal cardiac repair Has a documented sildenafil allergy Concurrent therapy with fluconazole at time of study drug initiation Inability to obtain parental consent within 24 hours of the echocardiogram Receiving extracorporeal membrane oxygenation (ECMO) at the time of the study