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GENERAL ARTICLES

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

Anthony G. Doufas, MD, PhD^*, Nobutada Morioka, MD, Adel N. Maghoub, MD, Edward Mascha, PhD, and Daniel I. Sessler, MD

From the *Department of Anesthesia, Stanford University School of Medicine; Outcomes Research Group, Department of Anesthesia and Perioperative Care, University of California, San Francisco; Department of Quantitative Health Sciences, and Department of Outcomes Research, Cleveland Clinic, Cleveland, Ohio.

Address correspondence and reprint requests to Anthony G. Doufas, MD, PhD, Department of Anesthesia, Stanford University School of Medicine, 300 Pasteur Dr., H3590 – Stanford, CA 94305-5640. Address e-mail to agdoufas{at}stanford.edu.

BACKGROUND: Neuraxial anesthesia reduces the shivering threshold 0.6°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°C mimics the normal 0.6°C reduction in the shivering threshold during epidural anesthesia.

METHODS: 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°C and upper body skin temperature near 33°C. We assessed equivalency of the shivering thresholds on the control and epidural days using the two one-sided tests method.

RESULTS: The thresholds on the control (35.8°C ± 0.5°C; mean ± sd) and epidural (35.8°C ± 0.5°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°C to +0.6°C (P < 0.025 for both one-sided equivalency tests).

CONCLUSIONS: 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.