Patterns of energy substrates during ischemia measured in the brain by microdialysis

The purpose of this study was to examine the patterns of change in microdialysate concentrations of glucose, lactate, pyruvate, and glutamate in the brain during periods of hypoxia/ischemia identified by monitoring brain tissue pO₂ (PbtO₂). Of particular interest was a better understanding of what additional information could be obtained by the microdialysis parameters that was not available from the PbtO₂. Fifty-seven patients admitted with severe traumatic brain injury who had placement of both a brain tissue pO₂ (PbtO₂) and microdialysis probe were studied. The microdialysis probe was perfused with Ringer's solution at 0.3 μL/min and dialysate was collected at 1-h intervals. The concentration of glucose, pyruvate, lactate, and glutamate were measured in each dialysate sample. Changes in the microdialysis parameters were examined during episodes where the PbtO₂ decreased to below 10 mm Hg. Ten episodes of tissue hypoxia/ischemia identified by a decrease in PbtO₂ below 10 mm Hg were observed during the period of monitoring. The concentration of the dialysate glucose closely followed the PbtO₂. The dialysate pyruvate concentration was more variable and in some patients transiently increased as the PbtO₂ dropped below 10 mm Hg. The dialysate concentration of lactate was significantly increased as the PbtO₂ decreased to less than 10 mm Hg. Dialysate glutamate was significantly elevated only when PbtO₂ decreased to very low levels. Although changes in the PbtO₂ provided the earliest sign of hypoxia/ischemia, the microdialysis assays provided additional information about the consequences that the reduced tissue pO₂ has on brain metabolism, which may be helpful in managing these critically ill patients.