On mathematical models of microdialysis: geometry, steady-state models, recovery and probe radius.

Commonly used methods for microdialysis recovery measurement are reviewed and the zero flow and no net flux methods are suggested as the most robust in practice. Six different mathematical models of microdialysis assumptions are investigated and compared for varying dialysis probe radius. One transmitter (dopamine), three metabolites (DOPAC, HVA and 5HIAA) and two drugs (caffeine and theophylline) were studied. Histology and functional response to a drug were measured. Deficiencies were demonstrated for several of the models, the one best explaining experimental data includes both passive diffusion and active tissue regulation in a cylindrical symmetric geometry. The recovery decreased with decreasing probe radius but smaller probes caused less tissue injury. It is concluded that a mathematical model of microdialysis must include diffusional and physiological processes in order to accurately account for experimentally observed phenomena. The experiments also demonstrated that, for small brain nuclei, the size of the nucleus may influence the recovery.