PROPOFOL CAUSES A DOSE-DEPENDENT DECREASE IN THE THERMOREGULATORY THRESHOLD FOR VASOCONSTRICTION BUT HAS LITTLE EFFECT ON SWEATING

Background: Volatile anesthetics increase the core temperature require d to trigger sweating and decrease the core temperature required to tr igger vasoconstriction. However, little is known about the effects of intravenous anesthetics on thermoregulation. We therefore tested the h ypothesis that propofol increases the sweating threshold and decreases the vasoconstriction threshold, thereby increasing the interthreshold range (core temperatures not triggering autonomic thermoregulatory re sponses). The study was conducted using a new model in which thermal m anipulations were restricted to insensate skin, and sensate skin tempe rature was controlled. Methods: Six healthy, male volunteers were stud ied on 3 randomly ordered days: no propofol, target propofol blood con centration 2 mu g/ml, and target blood propofol concentration 4 mu g/m l. Each day, epidural anesthesia (approximate to T11 level) was induce d, using 2% 2-chloroprocaine (one volunteer received bupivacaine). The rmal manipulations were confined to the legs, and we attempted to main tain upper-body (sensate) skin temperature constant. Propofol was infu sed by a computer-controlled infusion pump. Volunteers were heated unt il sweating was observed, then cooled until fingertip vasoconstriction was observed. The sweating threshold was defined as the tympanic memb rane temperature triggering sustained evaporative heat loss >40 g.m(-2 ).h(-1). Similarly, the vasoconstriction threshold was defined as the tympanic membrane temperature triggering a sustained reduction in fing ertip blood now to <0.25 ml/min. Central venous blood was assayed for propofol blood concentration. Results:Increasing propofol concentratio n produced a Linear decrease the vasoconstriction threshold (slope = - 0.53 +/- 0.34 degrees C.mu g(-1).ml(-1); R(2) = 0.98 +/- 0.04 [mean +/ - SD]), but had little effect on the sweating threshold. The interthre shold range was 0.51 +/- 0.46 degrees C during epidural anesthesia alo ne, and increased significantly, by 0.49 +/- 0.31 degrees C.mu g(-1).m l(-1) during propofol administration. Conclusions: Like volatile anest hetics, propofol reduces the vasoconstriction threshold and increases the interthreshold range. However, propofol differs in leaving the swe ating threshold unchanged.