EPIDURAL-ANESTHESIA IMPAIRS BOTH CENTRAL AND PERIPHERAL THERMOREGULATORY CONTROL DURING GENERAL-ANESTHESIA

Background: The authors tested the hypotheses that: (1) the vasoconstr iction threshold during combined epidural/general anesthesia is less t han that during general anesthesia alone; and (2) after vasoconstricti on, core cooling rates during combined epidural/general anesthesia are greater than those during general anesthesia alone. Vasoconstriction thresholds and heat balance were evaluated under controlled circumstan ces in volunteers, whereas the clinical importance of intraoperative t hermoregulatory vasoconstriction was evaluated in patients. Methods: f ive volunteers were each evaluated twice. On one of the randomly order ed days, epidural anesthesia (approximate to T9 dermatomal level) was induced and maintained with 2-chloroprocaine. On both study days, gene ral anesthesia was induced and maintained with isoflurane (0.7% end-ti dal concentration), and core hypothermia was induced by surface coolin g and continued for at least 1 h after fingertip vasoconstriction was observed. Patients undergoing colorectal surgery were randomly assigne d to combined epidural/enflurane anesthesia (n = 13) or enflurane alon e (n = 13). inappropriate patients, epidural anesthesia was maintained by an infusion of bupivacaine. The core temperature that triggered fi ngertip vasoconstriction identified the threshold. Results: In the vol unteers, the vasoconstriction threshold was 36.0 +/- 0.2 degrees C dur ing isoflurane anesthesia alone, but significantly less, 35.1 +/- 0.7 degrees C, during combined epidural/isoflurane anesthesia. Cutaneous h eat loss and the rates of core cooling were similar 30 min before vaso constriction with and without epidural anesthesia. in the 30 min after vasoconstriction, heat loss decreased 33 +/- 13 W when the volunteers were given isoflurane alone, but only 8 +/- 16 W during combined epid ural/isoflurane anesthesia. Similarly, the core cooling rates in the 3 0 min after vasoconstriction were significantly greater during combine d epidural/isoflurane anesthesia (0.8 +/- 0.2 degrees C/h) than during isoflurane alone (0.2 +/- 0.1 degrees C/h). In the patients, end-tida l enflurane concentrations were slightly, but significantly, less in t he patients given combined epidural/enflurane anesthesia (0.6 +/- 0.2% vs. 0.8 +/- 0.2%). Nonetheless, the vasoconstriction threshold was 34 .5 +/- 0.6 degrees C In the epidural/enflurane group, which was signif icantly less than that in the other patients, 35.6 +/- 0.8 degrees C. When the study ended after 3 h of anesthesia, patients given combined epidural/enflurane anesthesia were 1.2 degrees C more hypothermic than those given general anesthesia alone. The rate of core cooling during the last bout of the study was 0.4 +/- 0.2 degrees C/h during combine d epidural/enflurane anesthesia, but only 0.1 +/- 0.3 degrees C/h duri ng enflurane alone. Conclusions: These data Indicate that epidural ane sthesia reduces the vasoconstriction threshold during general anesthes ia. furthermore, the markedly reduced tate of core cooling during gene ral anesthesia alone illustrates the importance of leg vasoconstrictio n in maintaining core temperature.