Lower-Body Warming Mimics the Normal Epidural-Induced Reduction in the Shivering Threshold

Abstract

Neuraxial anesthesia reduces the shivering threshold approximately 0.6 degrees C. This effect might be mediated by an apparent (as opposed to actual) increase in lower body temperature. Accordingly, sufficient lower body warming should result in thermoregulatory inhibition comparable to that exerted by epidural anesthesia. We tested the hypothesis that increasing leg skin temperature to 38 degrees C mimics the normal approximately 0.6 degrees C reduction in the shivering threshold during epidural anesthesia. Shivering threshold during internal body cooling was determined in nine female volunteers on two separate days: one unanesthetized control day, and one day with a T10-11 epidural block. On each study day, lower body skin temperature was maintained near 38 degrees C and upper body skin temperature near 33 degrees C. We assessed equivalency of the shivering thresholds on the control and epidural days using the two one-sided tests method. The thresholds on the control (35.8 degrees C +/- 0.5 degrees C; mean +/- sd) and epidural (35.8 degrees C +/- 0.5 degrees C) days were shown to be equivalent because the 95% CI for the difference in means, 0.0 (-0.4, 0.4), was within our prespecified limits of -0.6 degrees C to +0.6 degrees C (P < 0.025 for both one-sided equivalency tests). Lower body warming mimics the normal epidural-induced reduction in the shivering threshold. Our results support a mechanism based on increased apparent lower body skin temperature during neuraxial anesthesia.