THERMOREGULATORY THRESHOLDS DURING EPIDURAL AND SPINAL-ANESTHESIA

Background: There are significant physiologic differences between spin al and epidural anesthesia. Consequently, these two types of regional anesthesia may influence thermoregulatory processing differently. Acco rdingly, in volunteers and in patients, we tested the null hypothesis that the core-temperature thresholds triggering thermoregulatory sweat ing, vasoconstriction, and shivering are similar during epidural and s pinal anesthesia. Methods: Six male volunteers participated on three c onsecutive study days: epidural or spinal anesthesia were randomly ass igned on the 1st and 3rd days (approximate to T10 level); no anesthesi a was given on the 2nd day. On each day, the volunteers were initially warmed until they started to sweat, and subsequently cooled by centra l venous infusion of cold fluid until they shivered. Mean skin tempera ture was kept constant near 36 degrees C throughout each study. The ty mpanic membrane temperatures triggering a sweating rate of 40 g.m(-2). h(-1), a finger flow less than 0.1 ml/min, and a marked and sustained increase in oxygen consumption (approximate to 30%) were considered th e thermoregulatory thresholds for sweating, vasoconstriction, and shiv ering, respectively. Twenty-one patients were randomly assigned to rec eive epidural (n = 10) or spinal (n = 11) anesthesia for knee and calf surgery (approximate to T10 level). As in the volunteers, the shiveri ng threshold was defined as the tympanic membrane temperature triggeri ng a sustained increase in oxygen consumption. Results: The thresholds and ranges were similar during epidural and spinal anesthesia in the volunteers. However, the sweating-to-vasoconstriction (interthreshold) range, the vasoconstriction-to-shivering range, and the sweating-to-s hivering range all were significantly increased by regional anesthesia . The shivering thresholds in patients assigned to epidural and spinal anesthesia were virtually identical. Conclusions: Comparable sweating , vasoconstriction, and shivering thresholds during epidural and spina l anesthesia suggest that thermoregulatory processing is similar durin g each type of regional anesthesia. However, thermoregulatory control was impaired during regional anesthesia, as indicated by the significa ntly enlarged interthreshold and sweating-to-shivering ranges.