Microdialysis to Monitor Brain Metabolism

cMD was performed as part of the clinical routine at the Neurointensive Care Unit of the Department of Neurosurgery at the Karolinska Hospital to monitor brain metabolism.^{7–9 (link),26–28 (link)} A 0.6-mm-wide microdialysis catheter with a 10-mm dialysis membrane at its tip (100-kDa cutoff) was surgically introduced into the brain tissue of interest (in the border zone close to the injury). A pump perfused the interior of the catheter with a perfusion fluid, which equilibrated with the interstitial tissue surrounding the catheter. Equilibration occurred by diffusion of chemicals over the dialysis membrane. Using a perfusion flow of 0.3 μL/min, the recovery of glucose, lactate, pyruvate, and glutamate in the dialysate was ∼70% of the concentration in the interstitial fluid.^{29 (link)} Samples were continuously collected into microvials analyzed at bedside by a CMA 600 microdialysis analyzer every hour for changes in glucose, pyruvate, lactate, glycerol, and glutamate. In the same area, a similar catheter with a 100-kDa cutoff was introduced to collect proteins. The perfusion fluid was the same as for the 20-kDa catheter, but samples were collected every sixth hour and frozen at −70°C. The final collection tubes contained a protease and phosphatase inhibitor cocktail.^{30 (link),31 (link)}Because of the low protein concentrations of bECF samples, we needed to combine four consecutive collections (e.g., 6, 12, 18, and 24 h) to be able to assay them using WES (see Table 2). To match the bECF samples, we also pooled equal volumes of CSF samples collected at time points matching the bECF collections. The final, combined bECF and CSF samples represent three post-injury time points: days 1, 3, and 5 (Table 2).