Microdialysis--a new technique for monitoring local tissue events in the clinic.

Microdialysis is a technique for sampling the chemistry of essentially all organs and tissues of the body. It was conceived of in the early 70th by Delgado's group in the USA and our group in Sweden. The idea was to implant an "artificial blood capillary" in the tissue, perfuse it with a physiological solution, recover molecules that diffuse over the membrane, and analyse the dialysate. The technique is now universally accepted for investigations of the brain and peripheral organs in animals and more than 4000 papers are published using the technique in essentially every organ of the body. The first implantation of microdialysis catheters in human subcutaneous and brain tissue were made ten years ago at the Karolinska institute in order to follow tissue metabolism. The lack of available instruments slowed the development of the technique. However, two years ago suitable catheters, a portable microdialysis pump and a bedside analyser became available. Today more than 200 papers are published on human investigations mainly in the field of diabetes, adipous tissue metabolism, neuro intensive and neonatal care. The microdialysis catheter is a flexible concentric catheter covered with a 10-30 mm dialysis membrane distally. An inlet tube connects to a small portable pump and the outlet tube to a microvial holder. There is one catheter intended for implantation into subcutaneous tissue or resting muscle and another for implantation into the brain. In the clinical situation the microvial is usually changed every 1W min and the sample analysed in the bedside analyser and then often brought to the laboratory for a more extensive analysis of various analytes. The usefulness of microdialysis rests on two features of the technique: 1) It samples the chemistry in individual tissues and organs as opposed from blood samples and 2) It allows very frequent sampling without any removal of blood or tissue from the body. In the first case microdialysis represent a new and unique possibility to follow tissue metabolism, for example during hypoxia or ischemia as well as the local concentration of drugs and other exogenuous substances. In the second case it is a well tolerated technique for monitoring, for example, a diabetes patient in order to adjust insulin treatment or a neonate where frequent blood sampling is a severe problem. There is a strong interest for applying microdialysis in various clinical situations: In vascular and plastic surgery for monitoring preand postsurgical tissue metabolism. In intensive care for monitoring metabolism after multi trauma and sepsis. In neurointensive care for monitoring brain trauma and hemorrhage. In neonates for minimizing blood samp ling and during and between hemodialysis treatments for monitoring tissue urea. The analytes of immediate interest for bedside monitoring are glucose, lactate, pyruvate, glycerol, glutamate and urea. The lactate/pyruvate ratio is of particular interest for distinguishing hypermetabolism from hypoxia and ischemia. Glycerol, being the end point of lipolysis, is an indirect measure of sympathetic activity but, especially in the brain, an interesting marker for cell membrane disintegation. Glycerol is an integral component of the double layer of most membranes in the body and cell damage and calcium influx activates phospholipases which start the membrane break down and the liberation of glycerol into the extracellular fluid. We hope that microdialysis will become a valuable tool for the anaesthesiologist in an number of clinical situations, in the operating room and in intensive care and in countless research applications on tissue and drug metabolism.