Safety of Sildenafil in Premature Infants

Inclusion Criteria: * Receiving positive airway pressure (nasal continuous airway pressure, nasal intermittent positive pressure ventilation, or nasal cannula flow \> 1LPM) or mechanical ventilation (high frequency or conventional) * \<29 weeks gestational age at birth * 7-28 (inclusive) days postnatal age at time of randomization Exclusion Criteria: * Currently receiving vasopressors * Currently receiving inhaled nitric oxide * Baseline mean arterial pressure \< gestational age (in weeks) plus postnatal age (in weeks) within 2 hours of sildenafil administration * Known allergy to sildenafil * Known sickle cell disease * Aspartate Aminotransferase (AST) \> 225 U/L \< 72 hours prior to randomization * Alanine Aminotransferase (ALT) \> 150 U/L \< 72 hours prior to randomization