Comparison of brain tissue oxygen tension to mfcrooialysisbased measures of cerebral ischemia in head-injured humans

Introduction: Although experimental models of traumatic brain injury may be used to examine the brain's responses to extreme physiologic conditions, such manipulations are obviously not possible in the clinical setting. One way to study such events in humans is to focus on patients with uncontrollable elevations in intracranial pressure. In this report, we compare decreases in brain tissue oxygen tension (PbtO2) during impending brain death to changes in cerebral extracellular lactate and glucose levels as measured by mtcrodialysis. Methods: Of the 90 patients in whom cerebral microdialysis has been performed, five patients eventually died from severe head injury white PbtO2 was being monitored. The technique used for cerebral microdialysis has been described previously. LJcox pO2 probes (GMS, KJet-Mielkendorf, Germany) were used to measure PbtO2. Glucose and lactate were analyzed on a YSI 2700 Select Biochemistry Analyzer (Yellow Springs Instruments, Yellow Springs, Ohio). Results: When PbtO2 decreased to approximately 5 mm Hg, (tie lactate/glucose rate, which is a gauge of anaerobic metabolism, began to exhibit minor increases. A dramatic increase in the lactate/glucose ratio was not seen until PbtO₂ reached zero. When either lactate or glucose was plotted against PbtO₂, such a distinct relationship was not seen. Conclusions: These results demonstrate that the lactate/glucose ratio does not increase significantly unti PbtO₂ drops to zero. They also emphasize the importance of changes in tissue tortuosity, a term which refers to the potential for variations in tissue geometry or other characteristics to affect the diffusion of analyte to the microdialysis probe. By using ratios of similarly sized compounds to analyze mJcrodialysis data, one can minimize erroneous interpretation of changes in the retrieved concentrations of these compounds.