PROPOFOL LINEARLY REDUCES THE VASOCONSTRICTION AND SHIVERING THRESHOLDS

Background: Skin temperature is best kept constant when determining re sponse thresholds because both skin and core temperatures contribute t o thermoregulatory control. In practice, however, it is difficult to e valuate both warm and cold thresholds while maintaining constant cutan eous temperature. A recent study shows that vasoconstriction and shive ring thresholds are a linear function of skin and core temperatures, w ith skin contributing 20 +/- 6% and 19 +/- 8%, respectively. (Skin tem perature has long been known to contribute approximate to 10% to the c ontrol of sweating.) Using these relations, we were able to experiment ally manipulate both skin and tore temperatures, subsequently compensa te for the changes in skin temperature, and finally report the results in terms of calculated core-temperature thresholds at a single design ated skin temperature. Methods: Five volunteers were each studied on 4 days: (1) control; (2) a target blood propofol concentration of 2 mu g/ml; (3) a target concentration of 4 mu g/ml; and (4) a target concen tration of 8 mu g/ml. On each day, we increased skin and core temperat ures sufficiently to provoke sweating. Skin and core temperatures were subsequently reduced to elicit peripheral vasoconstriction and shiver ing. We mathematically compensated for changes in skin temperature by using the established linear cutaneous contributions to the control of sweating (10%) and to vasoconstriction and shivering (20%). From thes e calculated core-temperature thresholds (at a designated skin tempera ture of 35.7 degrees C), the propofol concentration-response curves fo r the sweating, vasoconstriction, and shivering thresholds were analyz ed using linear regression. We validated this new method by comparing the concentration-dependent effects of propofol with those obtained pr eviously with an established model. Results: The concentration-respons e slopes for sweating and vasoconstriction were virtually identical to those reported previously. Propofol significantly decreased the core temperature triggering vasoconstriction (slope = -0.6 +/- 0.1 degrees C.mu g(-1).ml(-1); r(2) = 0.98 +/- 0.02) and shivering (slope = -0.7 /- 0.1 degrees C.mu g(-1).ml(-1); r(2) = 0.95 +/- 0.05). In contrast, increasing the blood propofol concentration increased the sweating thr eshold only slightly (slope = 0.1 +/- 0.1 degrees C.mu g(-1).ml(-1); r (2) = 0.46 +/- 0.39). Conclusions: Advantages of this new model includ e its being nearly noninvasive and requiring relatively Little core-te mperature manipulation. Propofol only slightly alters the sweating thr eshold, but markedly reduces the vasoconstriction and shivering thresh olds. Reductions in the shivering and vasoconstriction thresholds are similar; that is, the vasoconstriction-to-shivering range increases on ly slightly during anesthesia.