Microdialysis

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Microdialysis is a technique used to determine the chemical components of the fluid in the extracellular space of tissues. A microdialysis probe, which is inserted into the tissue, is a tiny tube made of a semi-permeable membrane. A semi-permeable membrane has tiny "pores" in it through which molecules can pass, if they are small enough. The technique has many uses; for example, researchers into traumatic brain injury use it to learn about how concentrations of ions change in the brain after injury (Richards et al, 2003). Other uses include primarily fat and muscle tissue.

Microdialysis works by slowly pumping a solution (the "dialysate") through the microdialysis probe. Molecules in the tissues diffuse into the dialysate as it is pumped through the probe; the dialysate is then collected and analysed to determine the identities and concentrations of molecules that were in the extracellular fluid (ECF). The concentration in the dialysate of any given substance will normally be much lower than the concentration present in the extracellular fluid, especially for substances of relatively high molecular weight. Typically the concentration of a peptide collected by microdialysis will be just 5-10% of the original concentration. This depends on the charge and size of the molecule in question as well as the dialysis speed. Different techniques can be used to assay the dialysis time needed to attain steady state including radioactive labelled probes. Analysis of the fluid can occur in a laboratory or at a patient's bedside, if microdialysis is being used in a clinical context.

[edit] Difficulties with the technique

While microdialysis is generally seen as a practical and useful analytic technique for elucidating the concentration of key low molecular weight components in the extracellular fluid, there are a number of practical considerations that need to be taken into account.

The absolute concentration of an analyte (the substance or substances being analysed) in the extracellular fluid is very difficult to determine using microdialysis. If the dialysate has a relatively high flow rate, then the analyte will most likely not fully equilibrate between perfusate (the exiting fluid from the probe) and ECF. Therefore high pressure liquid chromatography (HPLC), capillary electrophoresis or other analytic measurements of the dialysate will underestimate the concentration of the analyte in the ECF. There are a number of mathematical models available for quantifying the fraction of analyte in the dialysate, with Bungay et. al. (1990) providing a good basis of these techniques.

Due to this constraint, most efforts in this field have gone towards increasing the recovered analyte concentration in the dialysate. This can be done by either increasing the probe membrane surface area or reducing the flow rate (giving more time for molecules to diffuse into the dialysate). However, increasing the probe size means that it may cause more damage to the tissue and measurements will be less spatially precise. Reducing the flow rate reduces the temporal resolution of measurements

One way to overcome the difficulties in loss of temporal resolution is to develop better analytical techniques. Currently the most widely used analytical technique to determine analyte concentrations in the perfusate is high pressure liquid chromatography. This can be made very sensitive, in the low femtomolar range. Therefore an ultra-slow technique can be used to generate a high recovered fraction of analyte from the ECF while still pumping the dialysate to the HPLC. This generates very low volumes and concentrations of dialysate, but the relative amount of analyate more closely resembles the amount in the ECF. Another common technique is capillary electrophoresis. This technique has greater sensitivity (zeptomolar concentrations have been reported), leading to greater temporal resolution allowing a greater recovery of analyate. However, it is famously unreliable and prone to malfunction.