Non-ventilatory events substantially contribute to rise in pulmonary arterial blood flow with early or delayed cord clamping at birth in preterm lambs.

A widely held view is that an increased pulmonary arterial (PA) blood flow at birth is not triggered until the onset of lung aeration, but experimental data indicate that the non-ventilatory events of a reduction in lung liquid volume, complete fetal delivery, and umbilical cord clamping can also increase fetal PA flow. However, the effect of cord clamping strategy on the contribution of these non-ventilatory events to birth-related rises in PA flow is unknown. Accordingly, PA blood flow was measured via transit-time flow probe in anaesthetized, acutely instrumented preterm fetal lambs at baseline, after a ∼35% reduction in lung liquid volume, following complete fetal delivery, and then after (1) delayed cord clamping (DCC) preceded by ventilation lasting ∼100 s (n = 11), or (2) early cord clamping (ECC) followed by either a non-asphyxial (∼35 s, n = 10) or an asphyxial interval (∼100 s,  < 10 mmHg, n = 10) before ventilation. PA flow rose stepwise after reduction of lung liquid volume (P < 0.001) and fetal delivery (P < 0.001), as well as initial ventilation (P < 0.001) and subsequent DCC (P = 0.002). PA flow also rose after ECC (P < 0.001), with flow maintained in the non-asphyxial group, but markedly reduced to near-baseline fetal levels by pulmonary vasoconstriction in the asphyxial group (P = 0.009), before rising with ventilation (P < 0.001). Overall, non-ventilatory events cumulatively accounted for ∼30% of the fetal baseline-to-peak newborn increment in PA flow. These findings suggest that (1) non-ventilatory events substantially contribute to a perinatal rise in PA blood flow with ECC or DCC, and (2) this contribution is negated if an asphyxial level of arterial oxygenation develops after ECC. KEY POINTS: Although a widely held view is that an increased pulmonary blood flow (PBF) at birth is not triggered until onset of lung aeration, reduction of lung liquid volume, complete fetal delivery and umbilical cord clamping also increase fetal PBF. The contribution of these non-ventilatory events to birth-related rises in PBF is unknown, particularly with different cord clamping strategies. Anaesthetized preterm fetal lambs instrumented with central arterial flow probes underwent birth via delayed cord clamping (DCC) preceded by ventilation, or early cord clamping (ECC) followed by either a non-asphyxial or asphyxial interval before ventilation. PBF rose with reduction of lung liquid volume, fetal delivery, ECC and DCC. However, while an increased PBF after ECC was maintained with a non-asphyxial interval, it fell markedly after ECC with an asphyxial interval, before rebounding with ventilation. Cumulatively, non-ventilatory events accounted for ∼ 30% of the perinatal increase in PBF occurring with DCC or ECC birth strategies.