Effects of Hypoxic Hypoxia on Cerebral Phosphate Metabolites and pH in the Anesthetized Infant Rabbit

Abstract

The effects of hypoxic hypoxia on high-energy phosphate metabolites and intracellular pH (pH_i) in the brain of the anesthetized infant rabbit were studied in vivo using ³¹P nuclear magnetic resonance spectroscopy. Five 10- to 16-day-old rabbits were anesthetized with 1.5% halothane. Ventilation was controlled to maintain normocarbia. Inspired O₂ fraction was adjusted to produce three states of arterial oxygenation: hyperoxia (P_ao₂ > 250 mm Hg), normoxia (P_ao₂ ∼ 100 mm Hg), and hypoxia (P_ao₂ 25–30 mm Hg). During hypoxia, blood pressure was kept within 20% of control values with a venous infusion of epinephrine. During hyperoxia, the phosphocreatine-to-ATP ratio was 0.86, a value that is 2–2.5 times less than that reported for adults. During normoxia, ATP decreased by 20% and P_i increased by 90% from hyperoxia values. During 60 min of hypoxia, the concentrations of high-energy phosphate metabolites did not change, but intracellular and arterial blood pH (pH_a) decreased significantly. When hyperoxia was reestablished, pH_i returned to normal and pH_a remained low. These results suggest that during periods of hypoxemia, the normotensive infant rabbit maintains intracellular concentrations of cerebral high-energy phosphates better than has been reported for adult animals.