Cerebral blood flow and oxygenation in ovine fetus: Responses to superimposed hypoxia at both low and high altitude

For the fetus, although the roles of arterial blood gases are recognized to be critical in the regulation of cerebral blood flow (CBF) and cerebral oxygenation, the relation of CBF, cortical tissue P(t P_O2), sagittal sinus P_O2, and related indices of cerebral oxygenation to arterial blood gases are not well defined. This is particularly true for that fetus subjected to long-term hypoxia (LTH). In an effort to elucidate these interrelations, we tested the hypothesis that in the fetus acclimatized to high altitude, cerebral oxygenation is not compromised relative to that at low altitude. By use of a laser Doppler flowmeter with a fluorescent O₂ probe, in near-term fetal sheep at low altitude (n=8) and those acclimatized to high altitude hypoxia (3801 m for 90 ± 5 days; n=6), we measured laser Doppler CBF (LD-CBF), t P_O2, and related variables in response to 40 min superimposed hypoxia. At both altitudes, fetal LD-CBF, cerebral O₂ delivery, t P_O2, and several other variables including sagittal sinus P_O2, correlated highly with arterial P_O2 (P_a,O2). In response to superimposed hypoxia (P_a,O2=11 ± 1 Torr), LD-CBF was significantly blunted at high altitude, as compared with that at low altitude. In the two altitude groups fetal cerebral oxygenation was similar under both control conditions and with superimposed hypoxia, cortical t P_O2 decreasing from 8 ± 1 and 6 ± 1 Torr, respectively, to 2 ± 1 Torr. Also, for these conditions sagittal sinus P_O2 and [HbO₂ ]values were similar. In response to superimposed hypoxia, cerebral metabolic rate for O₂ decreased ∼50% in each group (P < 0.05). For both the fetus at low altitude and that acclimatized to high altitude LTH, we present the first dose-response data on the relation of LD-CBF, cortical t P_O2, and sagittal sinus blood gas values to P_a,O2. In addition, despite differences in several variables, the fetus at high altitude showed evidence of successful acclimatization, supporting the hypothesis that such fetuses demonstrate no compromise in cerebral oxygenation.