Cerebrovascular effects of rapid volume expansion in preterm fetal sheep

Preterm human infants are often treated with volume expansion during their initial stabilization. There are limited data regarding the cerebral vascular effects of this therapeutic approach. The effects of blood volume expansion on cerebral vascular reactivity and oxygen metabolism in very immature animals have not been determined. We examined the effects of volume expansion, with and without hypoxia, on cerebral blood flow and metabolism in unanesthetized, chronically catheterized, preterm fetal sheep. Rapid volume expansion with i.v. dextran increased circulating blood volume. Arterial blood pressure did not increase, nor did cerebral blood flow. However, volume expansion resulted in lower arterial Hb concentration and, consequently, oxygen content without a compensatory increase in cerebral blood flow. Cerebral oxygen delivery fell significantly. Induction of severe hypoxia after volume expansion resulted in an increase in cerebral blood flow, as expected, but the increase in flow was not enough to maintain cerebral oxygen delivery. Rapid volume expansion in normovolemic preterm fetal sheep is associated with decreased cerebral oxygen delivery, and this is further compromised when oxygen content is decreased.