Observations on the control of cerebral blood flow in the sheep fetus and newborn lamb

Cerebral blood flow was measured with xenon 133 in 21 fetal sheep and 14 newborn lambs under 48 hours old. When the ewe breathed air, the average gray matter blood flow in the fetuses was 70.6 ml/100 g/min, the average white matter blood flow 15.4 ml/100 g/min and average gray matter oxygen consumption 2.06 ml/100 g/min. The corresponding values in the newborn lambs were 87.2, 17.8, and 2.98 ml/100 g/min. A reduction of fetal Pao 2 by 7 to 11 mm Hg at a constant Paco 2 or an increase of Paco 2 at constant Pao 2 caused a rise in blood flow and arterial pressure and a fall in vascular resistance. Intravenous catecholamines caused variable changes in blood flow but no significant change in gray matter oxygen consumption. Occlusion of the umbilical cord caused an average increase in gray matter blood flow of 50% of control, in white matter blood flow of 30%, and in gray matter oxygen consumption of 90%. By the end of the first hour, these values had fallen and were then similar to those seen in the newborn lamb. These changes were not seen in fetuses in which both vagi had been cut nor in two fetuses with a failing circulation.