| Summary: | <b>Background:</b> Currently, 21–30% supplemental oxygen is recommended during resuscitation of preterm neonates. Recent studies have shown that 58% of infants < 32 week gestation age are born with a heart rate (HR) < 100 bpm. Prolonged bradycardia with the inability to achieve a preductal saturation (SpO<sub>2</sub>) of 80% by 5 min is associated with mortality and morbidity in preterm infants. The optimal oxygen concentration that enables the achievement of a HR ≥ 100 bpm and SpO<sub>2</sub> of ≥80% by 5 min in preterm lambs is not known. <b>Methods:</b> Preterm ovine model (125–127 d, gestation equivalent to human neonates < 28 weeks) was instrumented, and asphyxia was induced by umbilical cord occlusion until bradycardia. Ventilation was initiated with 30% (OX30), 60% (OX60), and 100% (OX100) for the first 2 min and titrated proportionately to the difference from the recommended preductal SpO<sub>2</sub>. Our primary outcome was the incidence of the composite of HR ≥ 100 bpm and SpO<sub>2</sub> ≥ 80%, by 5 min. Secondary outcomes were to evaluate the time taken to achieve the primary outcome, gas exchange, pulmonary/systemic hemodynamics, and the oxidative injury. <b>Results:</b> Eighteen lambs (OX30-6, OX60-5. OX100-7) had an average HR < 91 bpm with a pH of <6.92 before resuscitation. Sixty seven percent achieved the primary outcome in OX100, 40% in OX60, and none in OX30. The time taken to achieve the primary outcome was significantly shorter with OX100 (6 ± 2 min) than with OX30 (10 ± 3 min) (* <i>p</i> = 0.04). The preductal SpO<sub>2</sub> was highest with OX100, while the peak pulmonary blood flow was lowest with OX30, with no difference in O<sub>2</sub> delivery to the brain or oxidative injury by 10 min. <b>Conclusions:</b> The use of 30%, 60%, and 100% supplemental O<sub>2</sub> in a bradycardic preterm ovine model did not demonstrate a significant difference in the composite primary outcome. The current recommendation to use 30% oxygen did not achieve a preductal SpO<sub>2</sub> of 80% by 5 min in any preterm lambs. Clinical studies to optimize supplemental O<sub>2</sub> in depressed preterm neonates not requiring chest compressions are warranted.
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