Cerebral carbohydrate metabolism during severe ischemia in fetal sheep

The effect of cephalic hypotension on brain metabolism was studied in 10 unanesthetized, normoxic (P(a)O₂ > 17 mm Hg), late-gestation fetal lambs. Perfusion pressure (cephalic arterial minus sagittal venous pressure) was 40 ± 1 mm Hg (SEM) during control and was reduced to 10 ± 1 by occlusion of the Grachio-cephalic artery. Cerebral blood flow was measured with microspheres, and arterial and sagittal vein blood samples were analyzed for oxygen content, glucose, and lactate. During the occlusion, oxygen consumption decreased from 125 ± 8 to 95 ± 4 (p < 0.05) (all values μmol 100 g^-1 min^-1), and glucose uptake increased from 20 ± 3 to 25 ± 1 (p < 0.05). During the control period, there was no net lactate flux; during the occlusion, lactate excretion was 5.7 ± 1.4 (p < 0.005). The control glucose and oxygen uptakes demonstrated a normal 6:1 molar ratio; however, during the occlusion, 9.4 μmol 100 g^-1 glucose min^-1 were taken up in excess of expected aerobic glucose metabolism. If all of this glucose were anaerobically metabolized to lactate, three times the measured efflux would be produced. The transport properties of the fetal blood-brain barrier may be important factors in perinatal brain injury.