The effect of oxygen content during an initial sustained inflation on heart rate in asphyxiated near-term lambs

Objective At birth, an initial sustained inflation (SI) uniformly aerates the lungs, increases arterial oxygenation and rapidly improves circulatory recovery in asphyxiated newborns. We hypothesised that lung aeration, in the absence of an increase in arterial oxygenation, can increase heart rate (HR) in asphyxiated near-term lambs. Interventions Lambs were delivered and instrumented at 139±2 days of gestation. Asphyxia was induced by umbilical cord clamping and then delaying the onset of ventilation until mean carotid arterial pressures (CAPs) had decreased 2 ; n=6), 5% oxygen (O 2 ; n=6), 21% O 2 (n=6) or 100% O 2 (n=6) followed by ventilation in air for 30 min. Main outcome measures HR, CAP and pulmonary blood flow (PBF) were continuously recorded. Results HR and PBF increased more quickly in lambs resuscitated with 100% and 21% O 2 than with 5% O 2 or N 2 . HR and PBF recovery in the 5% O 2 group was delayed relative to all other oxygen SI groups. HR in 5%, 21% and 100% O 2 groups reached 100 bpm before the SI was complete. HR and PBF in the N 2 group did not increase until 10 s after the SI was completed and ventilation was initiated with air. CAP tended to increase quicker in all O 2 groups than in N 2 group. Conclusions Oxygen content during an SI is important for circulatory recovery in asphyxiated lambs. This increase in HR is likely driven by the increase in PBF and venous return to the heart.