The investigators will determine the maximum tolerable dose of sildenafil and establish the pharmacokinetic and pharmacodynamic profile of sildenafil in human asphyxiated neonates treated with hypothermia. They will use a 3+3 design to escalate the sildenafil dose up to 6 mg/kg/day (3mg/kg/dose q12h) in asphyxiated neonates demonstrating brain injury despite hypothermia treatment and assess whether we observe any beneficial effects of sildenafil on their brain and cardiopulmonary hemodynamics, without causing serious adverse events

Inclusion Criteria:

• Male and female neonates meeting the criteria for induced hypothermia:

  • Gestational age ≥ 36 weeks and birth weight ≥ 1800 g;
  • Evidence of fetal distress, i.e., history of an acute perinatal event, cord pH ≤ 7.0 or base deficit ≤ - 16 mEq/L;
  • Evidence of neonatal distress, such as an Apgar score ≤ 5 at 10 minutes, postnatal blood gas pH obtained within the first hour of life ≤ 7.0 or base deficit ≤ - 16 mEq/L, or a continued need for ventilation initiated at birth and continued for at least 10 minutes;
  • Evidence of moderate to severe neonatal encephalopathy by an abnormal neurological exam and/or an amplitude-integrated electroencephalogram (aEEG).

• Evidence of brain injury on a brain magnetic resonance imaging (MRI) performed on day 2 of life.

Exclusion Criteria:

• Neonates with complex congenital heart disease
• Neonates with cerebral malformations
• Neonates with genetic syndrome
• Neonates with intraventricular and/or intraparenchymal hemorrhage on MRI performed on day 2 of life

No plan to make individual participant data (IPD) available to other researchers