Oxygen concentration and pulmonary hemodynamics in newborn lambs with pulmonary hypertension

The effect of oxygen concentration on lowering pulmonary vascular resistance (PVR) during resuscitation in a model of persistent pulmonary hypertension of the newborn (PPHN) is not known. PPHN was induced in fetal lambs by ductal ligation 9 d before delivery. After delivery by cesarean section, resuscitation of PPHN lambs with 21%, 50%, or 100% O₂ (n = 6 each) for 30 min produced similar decreases in PVR. Lambs were then ventilated with 50% O₂ for 60 min and exposed to inhaled nitric oxide (iNO, 20 ppm). Initial resuscitation with 100% O₂ significantly impaired the subsequent response to iNO compared with 21% O₂ (42 ± 9% vs 22 ± 4% decrease from baseline PVR). Finally, each lamb was randomly and sequentially ventilated with 10%, 21%, 50%, or 100% O₂. PVR decreased with increased concentrations of inhaled O₂ up to 50%, there being no additional decrease in PVR with 100% O₂. When PVR was correlated with Pao₂, the maximal change in PVR was achieved at Pao₂ values <60 mm Hg. We conclude that resuscitation with 100% O₂ does not enhance pulmonary vasodilation compared with 21% and 50% O₂, but impairs the subsequent response to iNO in PPHN lambs. Hypoxia increases PVR but hyperoxia does not confer significant additional pulmonary vasodilation in lambs with PPHN.