CLONIDINE COMPARABLY DECREASES THE THERMOREGULATORY THRESHOLDS FOR VASOCONSTRICTION AND SHIVERING IN HUMANS

Background. Clonidine stops postoperative shivering, but its underlyin g mechanism of action is unknown. Clonidine may impair central control of thermoregulation or act on peripheral receptors. Accordingly, the authors tested the hypothesis that clonidine reduces both the vasocons triction and shivering thresholds, a pattern consistent with central t hermoregulatory impairment. Methods. Seven healthy volunteers particip ated in the study. Thermoregulatory vasoconstriction was evaluated usi ng forearm minus fingertip, skin-temperature gradients; values exceedi ng 4-degrees-C were considered to be significant vasoconstriction. Sys temic oxygen consumption (VO2) was measured with a canopy system. In a ddition, shivering was qualitatively evaluated using a simple scale, g raduated from 0 (no shivering) to 2 (intense shivering). The tympanic membrane temperatures triggering significant vasoconstriction and grad e 1 shivering were considered to be the thresholds for the two thermor egulatory responses. Measurements were performed after a 10-min steady state period and during cooling by central venous infusion of Ringer' s lactate solution at 4-degrees-C. Each subject was evaluated at two s essions, separated by at least 48 h. They were randomly and blindly as signed to received either an intravenous bolus of 75 mug clonidine or a placebo before cooling. When the shivering score equaled 2, 75 mug c lonidine was injected intravenously, and repeated if necessary, to com pletely stop shivering. Results: Clonidine significantly decreased the thermoregulatory threshold for shivering by 0.6 +/- 0.3-degrees-C (me an +/- SD). Similarly, the threshold for cutaneous vasoconstriction wa s significantly reduced by 0.5 +/- 0.2-degrees-C. Additional clonidine administration always stopped shivering, at whatever temperature it o ccurred. Conclusions: This study confirms that clonidine administratio n stops shivering, and suggests that it acts by impairing central ther moregulatory control. That an additional dose of clonidine stops shive ring in subjects already given one dose, indicates that the effect of clonidine is dose dependent.