256 HYPOXIC HYBOXIA (HH) AND CEREBRAL O2 DELIVERY IN THE FETAL AND NEWBORN SHEEP

Abstract

The effects of HH on cerebral O2 delivery and on the ratio between O2 delivery and cerebral O2 consumption, i.e. the cerebral fractional O2 extraction, has been studied in 20 unanesthetized fetal sheep in utero and 7 newborn lambs. Cerebral O2 delivery was calculated as the product of cerebral blood flow (CBF), measured with the radioactive microsphere technique, and arterial O2 content (CaO2). Fetuses were studied as PO2 varied from 12 to 35mmHg; lambs, as PaO2 varied from 30 to 150mmHg. All data were corrected to a constant arterial PCO2. Neither cerebral O2 delivery nor fractional O2 extraction varied with PaO2 (unless hypoxia was extreme) in either fetuses or lambs, despite wide differences in their physiologic circumstances. This suggests that the regulated variable during HH is (CBFX CaO2), not CBF. Since PaO2 and CaO2 are not linearly related, CBF must respond to changes in CaO2 rather than PaO2. This indicates regulation of cerebral blood flow at the tissue level. Cerebral fractional O2 extraction is relatively constant between fetuses and lambs despite considerable differences in cerebral O2 consumption and delivery. Thus, the most important determinant of cerebral O2 delivery is not developmental stage, but O2 need.