Glucose metabolism in traumatic brain injury: a combined microdialysis and [18F]-2-fluoro-2-deoxy-D-glucose-positron...

Following traumatic brain injury, as a consequence of ionic disturbances and neurochemical cascades, glucose metabolism is affected. [18F]-2-Fluoro-2-deoxy-D-glucose (FDG) Positron Emission Tomography (FDG-PET) provides a measure of global and regional cerebral metabolic rate of glucose (rCMRglc), but only during the time of the scan. Microdialysis monitors energy metabolites over extended time periods, but only in a small focal volume of the brain. Our objective in this study is to assess the association of parameters derived from these techniques when applied to patients with traumatic brain injury. Eleven sedated, ventilated patients receiving intracranial pressure monitoring and managed using Addenbrooke's Neurosciences Critical Care Unit protocols were monitored. Dialysate values for glucose, lactate, pyruvate, and glutamate, and the lactate to glucose (L/G), lactate to pyruvate (L/P) and pyruvate to glucose (P/G) ratios were determined and correlated with rCMRglc. FDG-PET scans were performed within 24 hours (five patients), or between 1 and 4 days (two patients) or after 4 days (six patients). Two patients were rescanned 4 and 7 days after their initial scan. A 20 mm region of interest (ROI) was defined on co-registered CT scan on two contiguous slices around the microdialysis catheter. Mean (+/-sd) for rCMRglc was 19.1 +/- 5.5 micromol/100 g/min, and the corresponding microdialysis values were: glucose 1.4 +/- 1.4 mmol/ L; lactate 5.3 +/- 3.6 mmol/L; pyruvate 164.1 +/- 142.3 micromol/L; glutamate 15.0 +/- 14.7 micromol/L; L/G 11.0 +/- 16.0; L/P 27.3 +/- 7.9 and P/G 381 +/- 660. There were significant relations between rCMRglc and dialysate lactate (r = 0.58, P = 0.04); pyruvate (r = 0.57, P = 0.04), L/G (r = 0.55, P = 0.05), and the P/G (r = 0.56, P = 0.05) but not between rCMRglc and dialysate glucose, L/P or glutamate in this data set. The results suggest that increases in glucose utilization as assessed by FDG-PET in these patients albeit in mainly healthy tissue are associated with increases in dialysate lactate, pyruvate, L/G and the P/G ratio perhaps indicating a general rise in metabolism rather than a shift towards non-oxidative metabolism. Further observations are required with regions of interest (microdialysis catheters positioned) adjacent to mass lesions notably contusions.