Antenatal betamethasone augments lung perfusion but lowers upper body blood flow and O2 delivery with delayed cord clamping at birth in preterm lambs.

Although the corticosteroid betamethasone is routinely administered to accelerate lung and cardiovascular maturation in the preterm fetus prior to birth, and use of delayed cord clamping (DCC) is recommended at birth by professional bodies, it is unknown whether antenatal betamethasone alters perinatal pulmonary or systemic arterial blood flow accompaniments of DCC. To address this issue, preterm fetal lambs [gestation 127 (1) days, term = 147 days] with (n = 10) or without (n = 10) antenatal betamethasone treatment were acutely instrumented under general anaesthesia with flow probes to obtain left (LV) and right ventricular (RV) outputs, major central arterial blood flows and shunt flow across both the ductus arteriosus and foramen ovale (FO). After delivery, lambs underwent initial ventilation for 2 min prior to DCC. During initial ventilation and after DCC, betamethasone (1) augmented rises in pulmonary arterial blood flow, with this greater increase supported during initial ventilation by enhanced pulmonary distribution of a higher RV output that was largely underpinned by newly emergent and substantial left-to-right (L → R) shunting across the FO, and after DCC, by an added contribution from more pronounced L → R ductal shunting; (2) increased a redistribution of LV output away from the upper body region, accompanied by lowering of upper body blood flow and O₂ delivery; and (3) accentuated a progressive systemic-to-pulmonary arterial shift in the distribution of the combined LV and RV output that occurred in conjunction with more pronounced perinatal L → R shunting. These findings suggest that antenatal betamethasone substantially alters arterial blood flow effects of initial ventilation and DCC in the preterm birth transition. KEY POINTS: Betamethasone is given to increase fetal lung and cardiovascular maturation prior to preterm birth, while delayed cord clamping (DCC) is recommended at birth. Whether antenatal betamethasone alters perinatal arterial blood flow responses to DCC is unknown. Anaesthetized preterm fetal lambs with or without betamethasone pretreatment were instrumented with central arterial flow probes and, at birth, underwent ∼2 min of ventilation before DCC. Betamethasone augmented perinatal rises in pulmonary arterial blood flow, related to enhanced pulmonary distribution during initial ventilation of a higher right ventricular output largely underpinned by left-to-right (L → R) shunting across the foramen ovale, with an added contribution from more pronounced L → R ductal shunting after DCC. Betamethasone increased a redistribution of left ventricular output away from the upper body region, with lowering of upper body blood flow and O₂ delivery. Betamethasone accentuated a systemic-to-pulmonary arterial shift in the distribution of combined ventricular output occurring with greater perinatal L → R shunting.