Lack of improvement in cerebral metabolism after hyperoxia in severe head injury : a microdialysis study

Object. The authors investigated the effects of hyperoxia on brain tissue PO2 and on glucose metabolism in cerebral and adipose tissue after traumatic brain injury (TBI). Methods. After 3 hours of ventilation with pure O-2, 18 tests were performed on different days in eight comatose patients with TBI. Lactate, pyruvate, glucose, glutamate, and brain tissue PO2 were measured in the cerebral extracellular fluid (ECF) by using microdialysis. Analytes were also measured in the ECF of abdominal adipose tissue. After 3 hours of increase in the fraction of inspired O-2, brain tissue PO2 rose from the baseline value of 32.7 +/- 18 to 122.6 +/- 45.2 mm Hg (p < 0.0001), whereas brain lactate dropped from its baseline (3.21 +/- 2.77 mmol/L), reaching its lowest value (2.90 +/- 2.58 mmol/L) after 3 hours of hyperoxia (p < 0.01). Pyruvate dropped as well, from 153 +/- 56 to 141 +/- 56 mumol/L (p < 0.05), so the lactate/pyruvate ratio did not change. No significant changes were observed in glucose and glutamate. The arteriovenous difference in O-2 content dropped, although not significantly, from a baseline of 4.52 +/- 1.22 to 4.15 +/- 0.76 ml/100 ml. The mean concentration of lactate in adipose tissue fell significantly as well (p < 0.01), but the lactate/pyruvate ratio did not change. Conclusions. Hyperoxia slightly reduced lactate levels in brain tissue after TBI. The estimated redox status of the cells, however, did not change and cerebral O-2 extraction seemed to be reduced. These data indicate that oxidation of glucose was not improved by hyperoxia in cerebral and adipose tissue, and might even be impaired.