Cerebral blood flow, oxygenation, and carbohydrate metabolism in immature fetal sheep in utero.

Studies of cerebral blood flow (CBF) and metabolism in fetal sheep have been largely confined to late gestation, a time when brain development in this species is largely complete. Few studies have been done at a time when the fetal sheep brain is in the midst of rapid differentiation and development. We studied seven fetal sheep in utero at 91 days of gestation (term = 145-150 days) 24 h after catheters were placed into the sagittal sinus, axillary artery, and inferior vena cava. We measured CBF by the microsphere method and used arteriovenous differences of O2, lactate, and glucose to calculate cerebral O2 consumption (CMRo2), fractional O2 extraction, glucose consumption, O2-glucose index (OGI), and cerebral lactate production. Compared with near-term fetal sheep, we found lower CBF (33.9 +/- 5.3 ml.100 g-1.min-1), lower glucose consumption (8.5 +/- 1.25 mumol.100 g-1.min-1), and lower CMRo2 (41.8 +/- 8.8 mumol.100 g-1.min-1). Fractional O2 extraction was 0.29 +/- 0.04, which is similar to near-term fetal sheep. There was consistent cerebral lactate production (2.45 +/- 1.58 mumol.100 g-1.min-1). The OGI was 81 +/- 16%, i.e., oxidative metabolism could account for 81% of glucose uptake. Lactate production accounts for virtually all glucose uptake exceeding that required for oxidation.