Study of Sildenafil to Treat Newborns With Persistent Pulmonary Hypertension

Inclusion Criteria: * Signed informed consent from legally acceptable guardian * PPHN or hypoxemic respiratory failure associated with: * Idiopathic PPHN * Meconium aspiration syndrome * Respiratory distress syndrome * Sepsis * Pneumonia * Greater than or equal to 35 weeks gestation * Age at enrollment less than 72 hours * Moderate hypoxemic respiratory failure, with 12\<OI\<35 (oxygenation index, calculated as FiO2 \* mean airway pressure \* 100 / postductal PaO2) * Absence of structural heart disease (except patent ductus arteriosus, atrial septal defect \<1cm, or muscular ventricular septal defect \< 2mm) * Absence of lethal congenital anomaly * Not participating in another concurrent experimental study Exclusion Criteria: * Prior or immediate need for iNO or ECMO * Profound hypoxemia: qualifying PaO2 \<30 mmHg, from a blood gas drawn within 30 minutes of starting study drug infusion. * Hypotension: Mean arterial pressure \<35 mmHg * Congenital heart disease, except patent ductus arteriosus, atrial septal defect \<1cm, or muscular ventricular septal defect \<2mm * Congenital diaphragmatic hernia or lung hypoplasia syndromes, diagnosed on the basis of prolonged oligohydramnios * Active seizures * Apgar score of \<3 at 5 minutes * Bleeding diathesis * Receipt of any other experimental drug or device * Receipt of any prohibited concurrent medication: * Potent cytochrome P450 3A4 inhibitors (e.g., erythromycin, ketoconazole, itraconazole and protease inhibitors) * Endothelin antagonists (e.g. Tracleer/bosentan) * Intravenous nitrates or nitric oxide donors * Known hereditary degenerative retinal disorders such as retinitis pigmentosa. * In the opinion of the investigator, a subject who is not likely to complete the study or would be considered inappropriate for the study, for any reason.