Fig. 7.1. Schematic representation of a microdialysis catheter in brain tissue. Fluid isotonic to the brain extracellular fluid (ECF) is pumped through the microdialysis catheter at a rate of 0.3 |jl min-1. Molecules at high concentration in the brain ECF equilibrate across the semi-permeable microdialysis membrane and can be analysed in the collected perfusate (the microdiasylate).
Fig. 7.2. Components of a clinical microdialysis system: (a) microdialysis pump; (b) microdialysis catheter; (c) microdialysis catheter tip showing exchange of molecules across the dialysis membrane; (d) microvial for collection of the microdialysate; (e) bedside analyser.
Fig. 7.3. Cranial CT scan showing the locations of an intracranial pressure monitor (ICP) and microdialysis catheter (MD). Microdialysis catheters have a small incorporated gold tip allowing their position to be clearly identified on a CT scan.
Fig. 7.4. Schematic representation of a blood capillary and microdialysis catheter in brain tissue. The concentration of substrate in the collected fluid (microdiasylate) is related to the balance between substrate delivery to, and uptake/excretion from, the brain extracellular fluid. L/P ratio, lactate to pyruvate ratio.
Fig. 7.5. Glucose metabolic pathways.
Fig. 7.6. Changes in lactate/pyruvate (L/P) ratio in ‘at-risk’ (a) and normal (b) brain tissue during a period of low and normal cerebral perfusion pressure (CPP). The normal range for the L/P ratio is shown by the shaded area. Note the rise in L/P ratio in the ‘at-risk’ tissue during a period of cerebral hypoperfusion but normal values measured by the catheter in normal brain.
Fig. 8.4. Typical EEG traces and descriptors.
