Inclusion Criteria:  Gestational age ≥ 36 weeks Birth weight between 2.5 and 4 kg. Post-natal age between 6 and 72 hours. PPHN confirmed by echocardiography Oxygenation index (OI) > 30 on two occasions at least 15 minutes apart Connected to Mechanical Ventilation  Exclusion Criteria:  Failure to obtain informed consent Newborns to mothers who received magnesium sulfate within 48 hours before labor. Congenital heart diseases, other than patent ductus arteriosus and foramen ovale. Major congenital anomalies (including congenital diaphragmatic hernia and lung hypoplasia). Prior need for cardiopulmonary resuscitation. Mean arterial blood pressure (MABP) < 35 mmHg despite therapy with volume infusions and vasoactive inotropes. Impaired kidney function. Prior administration of pulmonary vasodilators. Gastrointestinal intolerance or bleeding