Monitoring metabolic parameters of the ischaemic brain

INTRODUCTION

The metabolic parameters of ischaemic brain are continuously monitored by ‘cerebral microdialysis’ (MD) which is a minimally invasive technique and one of the most recent additions to the neuromonitoring armamentarium. After Ungerstedt and Pycock improvised the design of MD probe and made it suitable for human use, MD was introduced to the clinical practice in the mid-1990s.[1] [2] It works on the ‘principle of dialysis’ where there is diffusion of water-soluble molecules from higher concentration to an area of lower concentration through a semipermeable membrane. The MD catheter consists of a fine double lumen probe, lined at its tip with the semi-permeable dialysis membrane. The catheter tip is placed into brain parenchyma and perfused via an inlet tube with fluid isotonic to the tissue interstitium known as ‘artificial CSF’ or perfusate. The perfusate passes along the membrane before exiting via outlet tubing into a special collecting chamber called ‘microvial’.[3] Diffusion drives the passage of molecules across the membrane along their concentration gradient. The MD catheter, therefore, acts an ‘artificial blood capillary’, and the concentration of substrate in the collected fluid (microdialysate) will depend in part on the balance between substrate delivery to and uptake from the extracellular fluid (ECF). Hence, any molecule small enough to pass through the membrane can be sampled in the microvial after a predefined period (usually 1 h). These microvials are then placed on the bedside MD monitor for brain tissue biochemical analysis.

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