Hypoxemia Decreases the Shivering Threshold in Rabbits Anesthetized with 0.2 Minimum Alveolar Anesthetic Concentration Isoflurane

Abstract

Shivering has been proposed as an etiology of postoperative hypoxemia.The difficulty with this theory is that hypoxemia inhibits shivering in unanesthetized cats, rats, and humans. However, anesthesia inhibits many protective reflexes, including the ventilatory response to hypoxemia. We therefore tested the hypothesis that arterial hypoxemia fails to inhibit shivering in lightly anesthetized rabbits. Rabbits were intubated and instrumented during exposure to surgical concentrations of anesthesia, and anesthesia was then maintained with 0.2 minimum alveolar anesthetic concentration isoflurane. The core was cooled at a rate of 2-3[degree sign]C/h by perfusing water at 10[degree sign]C through a colonic thermode. Core temperatures were recorded from the distal esophagus. Sustained, vigorous shivering was considered physiologically significant. The core temperature that triggering significant shivering identified the thermoregulatory threshold for this response. Arterial blood was sampled for gas analysis at the shivering threshold in each rabbit. Hypoxemia linearly reduced the shivering threshold from 36.7[degree sign]C at 130 mm Hg to 35.4[degree sign]C at 50 mm Hg (threshold = PaO2 [center dot] 0.019 + 34.3; r2 = 0.49). We failed to confirm our hypothesis: instead, even mild hypoxemia reduced the shivering threshold >1[degree sign]C. A 1[degree sign]C decrease in the shivering threshold is likely to prevent or stop most postoperative shivering because it exceeds the reduction produced by many effective anti-shivering drugs. These data do not support the theory that shivering causes postoperative hypoxemia. Implications: Shivering has been proposed as an etiology of postoperative hypoxemia. Our data, in contrast, show that mild hypoxemia inhibits shivering. Shivering is thus unlikely to be a cause of postoperative hypoxemia. (Anesth Analg 1998;87:1408-11)