Effects of halothane and enflurane on the peripheral vasoconstriction and shivering induced by internal body cooling in rabbits.

General anesthetic agents have central and peripheral effects on body temperature regulation, and its alterations are related to the depth of anesthesia. To evaluate the effect of halothane and enflurane on thermoregulation, we investigated the threshold of body core temperatures to induce peripheral vasoconstriction and shivering in spontaneously breathing rabbits. Rabbits were anesthetized with halothane or enflurane at 0.0 (control), 0.2, and 0.4 MAC (minimum alveolar concentration). Internal whole body cooling was performed by perfusion with cold water (10°C) through a U-shaped intestinal thermode placed in the colon. Core (esophageal) and peripheral (ear skin)_temperatures were measured with thermistor probes. The esophageal temperatures at the beginning of peripheral vasoconstriction and shivering induced by internal whole body cooling were determined. Core temperature threshold values for peripheral vasoconstriction were significantly higher than those for shivering in both groups. The incidence of peripheral vasoconstriction was not significantly affected by halothane or enflurane. However, the incidence of shivering decreased in a dose-dependent fashion with both anesthetic agents. At 0.2 MAC, the incidence of shivering in the enflurane group was significantly higher than that in the halothane group, suggesting that suppression of shivering by halothane is stronger than enflurane.