Effect of mild hypothermia and hypoxia on blood flow and oxygen consumption of the fetal sheep brain.

This study was undertaken to measure the effects of mild hypothermia on cerebral blood flow and metabolism and cardiovascular responses to hypoxia in the fetal sheep. Near-term fetal sheep were chronically instrumented with laser Doppler flowmetry in the parietal cortex for measurement of relative changes in cerebral blood flow, as well as with arterial and sagittal sinus catheters for measurement of oxygen extraction by the brain and a cooling coil around the fetal thorax. Fetuses were studied during cooling alone, cooling with superimposed maternal hypoxia to achieve a fetal arterial Po2 of 1.33 to 1.60 kPa, or hypoxia alone. In response to cooling alone [1.6 degrees +/- 0.1 degrees C (mean +/- SEM) decrease in brain temperature], fetal blood pressure and heart rate both increased significantly whereas cerebral blood flow decreased 14 +/- 4%, commensurate with a 24 +/- 8% decline in cerebral metabolic rate. Administration of moderate hypoxia during cooling resulted in a significant increase in cerebral blood flow, decreased heart rate, and no further increase in blood pressure. In response to hypoxia alone, fetal blood pressure was significantly increased, heart rate was decreased, and cerebral blood flow increased by 24 +/- 8%, whereas cerebral metabolic rate decreased by 38 +/- 13%. Arteriovenous oxygen extraction was unchanged by cooling alone but increased significantly in response to hypoxia administered during cooling. We therefore conclude that oxygen delivery to the fetal sheep brain remains coupled to metabolic rate during hypothermia and that hypothermia does not impair the compensatory cardiovascular responses of the fetus to acute moderate hypoxia.