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A male newborn is delivered at 35 -6/7 weeks’ gestational age with a birthweight of 2,700 g (50th percentile) to a gravida 3, para 2, 33-year old mother by elective lower segment cesarean section for prolonged rupture of membranes. Fetal ultrasound is unremarkable; a complete course of antenatal steroids and 2 doses of benzylpenicillin are administered. Apgar scores are 7 and 9 at 1 and 5 minutes, respectively. Maternal history is unremarkable except for a treated urinary tract infection at 33 weeks. Group B streptococcal screening is negative. Umbilical arterial blood gas shows a pH of 7.30; base deficit of 1.5 mmol/L; and lactate of 3.6 mmol/L. At 1 hour of age, the infant is noted to have cyanosis and tachypnea, with respiratory rates of 80 to 90 breaths per minute, requiring 50% of incubator oxygen and subsequently nasal continuous positive airway pressure to maintain oxygen saturation of 91% to 95%. There is no pre- and postductal saturation gradient, and thoracic transillumation excludes a significant pneumothorax. On examination, the precordial impulse and femoral pulses are normal, heart sounds are dual, and a 1/6 systolic murmur at the lower left sternal edge is heard. There is no hepatosplenomegaly. Capillary blood gas shows the following: pH, 7.26; partial pressure of carbon dioxide, 62 mm Hg (8.2 kPa); partial pressure of oxygen (Pao2), 34 mm Hg (4.5 kPa); base excess, –1.8; plasma lactate, 30.6 mg/dL (3.4 mmol/L); and serum glucose, 45 mg/dL (2.5 mmol/L). Intravenous access is obtained with a full blood count and blood culture, and benzylpenicillin and gentamicin are administered. Arterial blood sampling from the right radial artery demonstrates Pao2 of 49 mm Hg (6.5 kPa) in 50% inspired oxygen (Fio2), which does not increase in 90% Fio2. An echocardiogram at 5 …