Temperature monitoring and management during neuraxial anesthesia.

F rank et al. (1) surveyed 102 members of the ASA to determine temperature-monitoring practices during neuraxial anesthesia. Their results indicate that only a third of the anesthesiologists in the United States routinely monitor temperature during neuraxial anesthesia. This conclusion is probably correct; nonetheless, it is worth considering some limitations of this study. For example, only 60 questionnaires were returned, which is a relatively low response rate. The authors did not attempt secondary questionnaires or telephone contact. Consequently, they were unable to use well established statistical methods to evaluate response bias. It is thus possible that a disproportionate fraction of the responses were from a self-selected group of anesthesiologists who were especially aware of and interested in the study topic. This possibility is supported by the fact that 60% of the returned questionnaires were from academic practitioners, which is hardly the typical ratio among ASA members. An additional concern is that respondents may have overestimated the proportion of neuraxial anesthetics during which they monitored temperature. Local anesthetics do not trigger malignant hyperthermia (2), nor do the sedatives typically used during neuraxial anesthesia. Unless fever is likely, detection of inadvertent hypothermia is the major reason for monitoring temperature during neuraxial anesthesia. The three major defenses against hypothermia in humans are vasoconstriction, shivering, and behavior (i.e., putting on a sweater, moving to a warmer environment). Neuraxial anesthesia impairs central autonomic thermoregulatory control (3), possibly by increasing apparent (as opposed to actual) leg skin temperature (4). This inhibition is proportional to block height (5) but is small compared with the central inhibition produced by general anesthetics (6). Neuraxial anesthesia also impairs behavioral thermoregulation with the result that patients often do not consciously perceive that they are hypothermic (7). Few surgical patients are in a position to much alter their environments, but lack of thermal complaints lulls anesthesiologists into believing that their patients are near-normothermic. Most importantly, however, major conduction anesthesia blocks autonomic control to the affected region, thus preventing vasoconstriction and shivering in the legs (8). Hypothermia during neuraxial anesthesia develops initially from a core-to-peripheral redistribution of body heat (9,10), with the amount depending on numerous factors, including the patient’s previous thermal environment (11) and medication use (12). Subsequent hypothermia, as during general anesthesia, results from heat loss exceeding heat production. The extent to which core temperature decreases during this phase depends largely on ambient temperature (13), the magnitude and duration of the surgical procedure (14), and the amount of unwarmed IV fluids that are given (15). At some point, reemergence of thermoregulatory defenses will moderate further cooling; however, defenses restricted to the upper body are often insufficient to prevent further hypothermia. Most studies evaluating the adverse consequences of mild hypothermia were performed in patients given general anesthesia; however, there is no reason to believe that neuraxial anesthesia in any way protects patients from hypothermia-induced complications. The major consequences of mild perioperative hypothermia (i.e., 1–2°C) include morbid myocardial outcomes (16), augmented blood loss and allogeneic transfusion requirement (17), reduced resistance to Major corporate funding for the Outcomes ResearchTM Laboratory is provided by Augustine Medical, Inc. The author does not consult for, accept honoraria from, or own stock or stock options in any company related to perioperative temperature monitoring or management. Accepted for publication November 10, 1998. Address correspondence and reprint requests to Dr. Sessler, Department of Anesthesia and Perioperative Care, University of California-San Francisco, 374 Parnassus Ave., 3rd Floor, San Francisco, CA 94143-0648. Address e-mail to [email protected].