Hypoxia, the subsequent systemic metabolic acidosis, and their relationship with cerebral metabolite concentrations: An in vivo study in fetal lambs with proton magnetic resonance spectroscopy

Objective: The relationship among decreased fetal arterial oxygen saturation, the subsequent systemic metabolic acidosis, and changes in cerebral metabolite concentrations in the fetal lamb brain was investigated by means of quantitative proton magnetic resonance spectroscopy. Study Design: Fetal hypoxia was induced in 6 fetal lambs by gradual reduction of the oxygen supply to the anesthetized pregnant ewe. In vivo proton magnetic resonance spectroscopy was performed on the fetal lamb brain simultaneously with repeated measurements of fetal arterial oxygen saturation and acid-base balance. Results: Proton magnetic resonance spectra showed metabolites such as inositol, choline compounds, creatine, and N -acetylaspartate. A signal for cerebral lactate was below the detection level under normoxic conditions and increased during hypoxia to indicate concentrations varying from 2.8 to 11.1 mmol/kg wet weight brain tissue. N -Acetylaspartate signals decreased during hypoxia, whereas signals of inositol, choline compounds, and creatine remained constant. Conclusion: These results support the view that fetal cerebral anaerobic metabolism in fetal lambs does not start under hypoxic conditions if the arterial blood pH is >7.28 or the base excess is >–8 mmol/L. (Am J Obstet Gynecol 1999;181:1537-45.)