Sildenafil to Repair Brain Injury Secondary to Birth Asphyxia

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