Effect of General Anesthesia on Heat Loss Responses during Hyperthermic Treatment

Abstract

The effects of general anesthesia on changes in temperature of the tissue heated in hyperthermic treatment were investigated in 9 rabbits. The thermosensitive tissue of the preoptic area and anterior hypothalamus (PO/AH) was locally heated by Argon Laser irradiation guided with glass fiber (0.4 mm in diameter). After local heating of PO/AH at 45 mW for 10 min under unanesthesia, PO/AH was locally heated at 45, 63, 90, 125 mW for 10 min under anesthesia with sodium pentobarbital (2.5 mg/kg/hr i.v.) in each rabbit. The heat loss responses of increase in ear skin temperature (Tea') by vasodilatation and increase in respiratory rate (RR) for evaporative heat loss were induced by local heating of PO/AH, and temperatures of the rectum (Tre) and brain (Tbr) at 10 mm apart from the irradiated center decreased following these heat loss responses in unanesthetic condition. But under anesthesia, unchanging Tre and increase in Tbr were observed because of no heat loss responses induced by local heating of PO/AH. Under unanesthesia, changes in Tea', RR, Tre and Tbr at 45 mW were +5.48 ± 0.57 °C, +56 ± 9min^-1, -0.33 ± 0.04 °C and -0.47 ± 0.08 °C, respectively. Under anesthesia, the changes in Tea', RR, Tre and Tbr at 45 mW were +0.22 ± 0.19 °C, +8 ± 2 min^-1, +0.01 ± 0.03 °C and +0.17 ± 0.06 °C, respectively. Furthermore, changes in Tea', RR, Tre and Tbr at 63, 90 and 125 mW were -0.45±0.25 °C, +12±2 min^-1, -0.02±0.03 °C and +0.19±0.04 °C, respectively. In cases of all values obtained under anesthesia, there were significant differences to compare with each value under unanesthesia.
Heat loss responses are likely to be induced in hyperthermic treatment, and these responses act on decreasing of core temperature. This phenomenon is obvious disadvantage to hyperthermic treatment under unanesthesia. One disadvantage is that the excess heat energy should be supplied to keep the temperature of the target tissue at the effective level. And the other is that the tissue temperature is variable depending on change in temperature of circulating blood as well as change in blood flow of the tissue. Inhibiting heat loss responses of thermoregulation with anesthesia is significantly effective in hyperthermic treatment. But the greatest attention should be paied to the temperature of heated tissue and heat load in the time course of hyperthermic treatment, since the unpredictable tissue damage might be caused by excess heating in consequence of inhibited physiological functions under anesthesia.