Preferential blood flow increase to the brain stem in moderate neonatal hypoxia: reversal by naloxone.

Previous studies have shown that severe neonatal asphyxia and hypoxia lead to a redistribution of cerebral blood flow (CBF) with a preferential perfusion of the brain stem. The present study shows that this mechanism is operative also in moderately hypoxic newborn lambs (oxygen saturation 32.7-65.2) with a threshold of about 25% reduction in oxygen saturation. In hypoxia, the mean increase in total CBF, brain stem and telencephalic blood flow was 44%, 68% and 43%, respectively (five lambs). We found that naloxone reverses this redistribution, and that the effects of naloxone on telencephalic perfusion and cerebral metabolic rate of oxygen (CMRO2) were proportional. In hypoxia + naloxone (1 mg/kg), a further increase in total CBF, brain stem, and telencephalic blood flow of 30%, 7% and 31% was noted. We therefore suggest that the redistribution of CBF is an important opioid-mediated homeostatic mechanism, which diminishes the metabolic requirements of the newer part of the brain in hypoxia and allows a preferential perfusion of the vital structures of the brain stem.