NEUROMUSCULAR BLOCKING-AGENTS IN BURNED P ATIENTS

Bums of more than 15 % of the body surface area result in major physio logical changes with an alteration of cardiovascular, pulmonary, hepat ic and renal functions, as well as modifications in the pharmacokineti cs and the pharmacodynamics of many drugs. Among these, a major change in the activity of muscle relaxants occurs which can be specific to t his pathology. Succinylcholine is contraindicated during recovery from a burn trauma because of a possible hyperkaliemic response, directly related to the dose, the post-burn delay and the area of burned body s urface. The kaliemic response and the related cardiac complications re main unpredictable. The height of twich depression with small doses of succinylcholine such as 0,1 to 0,2 mg.kg(-1), demonstrates the hypers ensitivity to this agent and does cause neither metabolic disturbances nor cardiac arrest. Nevertheless, the administration of succinylcholi ne is contra-indicated for from the 5(th) day on at least two years af ter the burn injury. Conversely, the action of non-depolarizing muscle relaxants is characterized by a resistance, which is correlated to bo th the posttraumatic delay and the extent of the burned area. It start s an about the seventh day, reaches peak intensity between day 15 and day 40 and can persist up two years after the thermal injury. In the c ourse of a burn, the so-called ''immature'' acetylcholine receptors, c haracterized by the substitution of the sub-unit epsilon by a protein gamma, increase at the level of the end plate areas and the extrasynap tic muscle membrane. These receptors explain both the hyperkaliemic re sponse and the hypersensitivity to succinlycholine. The reduction in t he activity of non-depolarizing muscle relaxants can be explained, in part, by the increase of nicotinic receptors. However some other facto rs have a part in this resistance. Elements such as increased protein binding, alteration of renal excretion and an ''anti-curare'', plasmat ic substance all participate in this phenomenon.