Clinical cerebral microdialysis--determining the true extracellular concentration.

Current monitoring of the cerebral extracellular chemistry of neurosurgical patients using microdialysis does not provide the true extracellular concentration because full equilibration across the membrane is not achieved. By varying the flow rate and extrapolating to zero flow, the relative recovery i.e. the concentration of the substance in the microdialysate as a proportion of the true concentration in the extracellular space may be calculated. The disadvantage of this method is that it depends on the underlying baseline chemistry being constant during measurements for the calculations, which is not the case in the changing environment of a neuro-intensive unit. We have therefore designed a modification of the extrapolation to zero flow method using an adjacent constant flow rate catheter to monitor the baseline. The results demonstrate that the relative recovery varies considerably with flow rate, and for the CMA70 10 mm membrane catheter, is approximately 70% at a rate of 0.3 microliter/min and 30% at a rate of 1.0 microliter/min for glucose, lactate, pyruvate and glutamate.