Placental compliance during fetal extracorporeal circulation

The fetus requires large amounts of volume when weaning from cardiac bypass . This suggests that placental vasculature can act as a large capacitor in the fetal circulation. To assess placental compliance of fetal lambs, seven isolated in situ lamb placentas were placed on extracorporeal circulation. Umbilical artery blood flow was varied from 0 to 350 ml.min(-1).kg fetal w t(-1). Because the extracorporeal circuit is a closed system, volume change s in the placenta induced by umbilical artery pressure changes were measure d from reciprocal volume changes in the reservoir. There was a wide range o f change in absolute volume of blood within the fetal placental compartment (216.4 +/- 29.3 ml). Placental compliance was linear over the entire range of pressure changes exerted on the placental vasculature (r(2) = 0.83, P = 0.0001). This indicates that the placenta is a unique and sensitive capaci tor in the fetal circulation. This information is important clinically beca use it establishes that aggressive resuscitation of the fetus using volume may be necessary when weaning the fetus from cardiac bypass.