Cerebrovascular autoregulation during fetal development in sheep

There are scant data regarding the development of cerebrovascular autoregulation in fetuses. We tested the hypothesis that a decrease in cerebrovascular resistance (CVR) at reduced cerebral perfusion pressure (CPP) is absent in midgestation and near-term fetal sheep. Catheters were chronically implanted for microsphere determination of cerebral blood flow (CBF) in 9 fetuses at 92 days and in 10 fetuses at 132 days gestation (full term = 145 days). CPP was reduced by ventricular infusion of artificial cerebrospinal fluid. In 92-day fetuses, CPP was reduced stepwise from 35 to 25 and 18 mmHg and CBF decreased from 52 ± 5 to 43 ± 4 and 27 ± 5 (SE) ml · min^-1 · 100 g^-1, respectively. Half of the immature fetuses showed some reduction in CVR at moderate reduction in CPP; however, there was no significant change in CVR in the group as a whole (from 0.72 ± 0.06 to 0.61 ± 0.04 and 0.89 ± 0.20 mmHg · ml^-1 · 100 g). In 132-day fetuses, CPP was reduced from 45 to 33 and 28 mmHg and CBF was unchanged from 105 ± 7 to 97 ± 11 and 89 ± 8 ml · min^-1 · 100 g^-1). CVR decreased from 0.45 ± 0.05 to 0.41 ± 0.08 and 0.33 ± 0.03 mmHg · ml^-1 · min · 100 g. There were no significant changes in arterial blood gases at reduced CPP in either age group. We conclude that cerebrovascular autoregulation at reduced CPP is not well developed at 92 days (0.63 gestation) in fetal sheep but that autoregulatory capacity is evident near term. We speculate that poor autoregulation may place the premature fetal brain at risk for injury.