Fetal cerebral oxygen consumption at different levels of oxygenation

Cerebral oxygen consumption (VCO2) was measured in 10 unanesthetized, chronically catheterized fetal sheep at 130-140 days of gestation. The VCO2 was calculated using cerebral blood flow (QC) measured with radioactive microspheres and the cerebral arteriovenous difference of O2 content (C(a-V)O2) calculated from preductal arterial and sagittal sinus venous blood. The ewe was exposed to varying concentrations of oxygen, resulting in fetal arterial oxygen contents (CaO2) of 0.89-5.58 mM, arterial oxygen tension (PaO2) values of 14-36 Torr, and cerebral venous oxygen tension (PVO2) values of 9-25 Torr. Although there was a clear relationship between the fetal CaO2 and C(a-V)O2, this was shown to be the result of changes in Qc rather than changes in VCO2. There was not a statistically significant correlation between either CaO2 or PVO2 and VCO2 over this range of oxygenation. On the other hand, C(a-V)O2 was highly correlated with Qc. There is no evidence that VCO2 is a function of oxygen tension (PO2) in the unanesthetized fetal sheep as long as the sagittal sinus PO2 is greater than or equal to 9 Torr.