Effects of burns and vasoactive drugs on human skin Clinical and Experimental studies using microdialysis

Abstract: Patients who require critical care, including those with burns, are affected by a systemic inflammatory reaction, which at times has consequences such as multiple organ dysfunction and failure. It has become increasingly evident that other factors important in the development of organ dysfunction are disturbances at the tissue level, in the microcirculation. Such disturbances activate cascade systems including stress hormones, all of which have local effects on organ function.Despite this knowledge, monitoring and treatment in critical illness today relies mainly on central haemodynamics and blood sampling.Microdialysis is a minimally invasive technique that enables us to study the chemical composition and changes in biochemistry in the extracellular, extravascular space in living tissues. Most of our current experience is from animal models, but the technique has also been used in humans and has become routine in many neurosurgical intensive care units to monitor brain biochemistry after severe injury. In skin, this experience is limited. During the first half of this thesis we studied the injured and uninjured skin of severely burned patients. The results show that there are severe local metabolic disturbances in both injured and uninjured skin. Most interesting is a sustained tissue acidosis, which is not detectable in systemic (blood) sampling. We also recorded considerable alterations in the glucose homeostasis locally in the skin, suggesting a cellular or mitochondrial dysfunction. In parallel, we noted increased tissue glycerol concentrations, which indicated appreciable traumainduced lipolysis.We also examined serotonin kinetics in the same group of patients, as serotonin has been claimed to be a key mediator of the vasoplegia and permeability disturbances found in patients with burns. We have shown, for the first time in humans to our knowledge, that concentrations of serotonin in skin are increased tenfold, whereas blood and urine concentrations are just above normal. The findings support the need for local monitoring of substances with rapid local reabsorption, or degradation, or both. The results also indicate that serotonin may be important for the systemic response that characterises burn injuries.In the second half of the thesis we evaluated the effects of microdosing in skin on metabolism and blood flow of vasoactive, mainly stress-response-related, drugs by the microdialysis system. The objectives were to isolate the local effects of the drugs to enable a better understanding of the complex relation between metabolic effects and effects induced by changes in local blood flow. In the first of these two studies we showed that by giving noradrenaline and nitroglycerine into the skin of healthy subjects we induced anticipated changes in skin metabolism and blood flow. The results suggest that the model may be used to examine vascular and metabolic effects induced locally by vasoactive compounds. Data from the last study indicate that conventional pharmacodynamic models (Emax) for time and dose response modelling may be successfully used to measure the vascular and metabolic response in this microdosing model.We conclude that the microdialysis technique can be successfully used to monitor skin metabolism and iso late a mediator (serotonin) of the local skin response in burned patients. It was also feasible to develop a vascular model in skin based on microdialysis to deliver vasoactive substances locally to the skin of healthy volunteers. This model provided a framework in which the metabolic effects of hypoperfusion and reperfusion in skin tissues could be examined further.