Transplacental oxygen transfer during physiological-based cord clamping in preterm lambs

Physiological-based cord clamping (PBCC) involves aeration of the newborn lung before umbilical cord clamping. As the infant can continue to receive oxygen from the mother during PBCC, the dynamics of transplacental oxygen transfer are unknown, particularly following the onset of pulmonary gas exchange. We have investigated the effects of pulmonary ventilation with supplemental oxygen during PBCC on transplacental oxygen transfer in preterm lambs. Pregnant ewes and their fetuses (<i>n</i> = 8; 127 days of gestation; term 147 days) were instrumented with catheters and flow probes under general anesthesia to measure oxygen transfer across the placenta. Before umbilical cord clamping, lambs were intubated and mechanically ventilated with a fraction of inspired oxygen ([Formula: see text]) that increased every 10 min, from 0.21 to 0.5 and then 1.0. Data were analyzed using a mixed-effects analysis with a Holm-Šídák post hoc test. During PBCC, ventilation of lambs significantly decreased oxygen uptake across the placenta from 317.7 ± 15.0 to 166.5 ± 26.9 (<i>P</i> = 0.0028) and to 73.6 ± 34.3 mL/min/kg (<i>P</i> = 0.0009) at [Formula: see text] levels of 0.21 and 0.5, respectively. Ventilation with a [Formula: see text] of 1.0 reversed oxygen uptake across the placenta (-37.2 ± 14.0 mL/min/kg; <i>P</i> < 0.0001), resulting in oxygen transfer from lamb to ewe. In contrast, oxygen delivery to the lamb, via the umbilical vein, remained unchanged with increasing [Formula: see text] (<i>P</i> = 0.4485). During PBCC, pulmonary oxygen uptake by the newborn reduces oxygen uptake across the placenta, and when oxygen levels are in excess, the mother acts as an "oxygen sink," reducing the risk of hyperoxemia of the newborn.<b>NEW & NOTEWORTHY</b> This study investigated transplacental oxygen transfer during physiological-based cord clamping in preterm lambs. During physiological-based cord clamping, ventilating newborn lambs reduced oxygen uptake across the placenta and that ventilation with high oxygen (1.0 [Formula: see text]) reversed the oxygen gradient, resulting in newborn to mother oxygen transfer. Our findings indicate that the mother can act as an "oxygen sink" that protects the newborn from excess oxygen during physiological-based cord clamping.