THERMOREGULATORY VASOCONSTRICTION DOES NOT IMPEDE CORE WARMING DURINGCUTANEOUS HEATING

Background: Although forced-air warming rapidly increases intraoperati ve core temperatures, it is reportedly ineffective postoperatively. A major difference between these two periods is that arteriovenous shunt s are usually dilated during surgery, whereas vasoconstriction is unif orm in hypothermic postoperative patients. Vasoconstriction may decrea se efficacy of warming because its major physiologic purposes are to r educe cutaneous heat transfer and restrict heat transfer between the t wo thermal compartments. Accordingly, we tested the hypothesis that th ermoregulatory vasoconstriction decreases cutaneous transfer of applie d heat and restricts peripheral-to-core now of heat, thereby delaying and reducing the increase in core temperature. Methods: Eight healthy male volunteers anesthetized with propofol and isoflurane were studied . Volunteers were allowed to cool passively until core temperature rea ched 33 degrees C. On one randomly assigned day, the isoflurane concen tration was reduced, to provoke thermoregulatory arteriovenous shunt v asoconstriction; on the other study day, a sufficient amount of isoflu rane was administered to prevent vasoconstriction. On each day, forced -ah warming was then applied for 2 h. Peripheral (arm and leg) tissue heat contents were determined from 19 intramuscular needle thermocoupl es, 10 skin temperatures, and ''deep'' foot temperature. Core (trunk a nd head) heat content was determined from core temperature, assuming a uniform compartmental distribution. Time-dependent changes in periphe ral and core tissue heat contents were evaluated using linear regressi on. Differences between the vasoconstriction and vasodilation study da ys, and between the peripheral and core compartments, were evaluated u sing two-tailed, paired t tests. Data are presented as means +/- SD; P < 0.01 was considered statistically significant. Results: Cutaneous h eat transfer was similar during vasoconstriction and vasodilation. For ced-air warming increased peripheral tissue heat content comparably wh en the volunteers were vasodilated and vasoconstricted: 48 +/- 7 versu s 53 +/- 10 kcal/h. Core compartment tissue heat content increased sim ilarly when the volunteers were vasodilated and vasoconstricted: 51 +/ - 8 versus 44 +/- 11 kcal/h. Combining the two study days, the increas e in peripheral and core heat contents did not differ significantly: 5 1 +/- 8 versus 48 +/- 10 kcal/h, respectively. Core temperature increa sed at essentially the same rate when the volunteers remained vasodila ted (1.3 degrees C/h) as when they were vasoconstricted (1.2 degrees C /h). Conclusions: The authors failed to confirm their hypothesis that thermoregulatory vasoconstriction decreases cutaneous transfer of appl ied heat and restricts peripheral-to-core how of heat in anesthetized subjects. The reported difference between intraoperative and postopera tive rewarming efficacy may result from nonthermoregulatory anesthetic -induced vasodilation.