Abstracts of Posters Presented at the International Anesthesia Research Society; 72nd Clinical and Scientific Congress; Orlando, FL; March 7-11, 1998: Anesthesia/OR Economics
Introduction: The incidence of postoperative shivering is a linear function of core temperature.  Furthermore, postanesthetic shivering occurs only in hypothermic volunteers.  Nonetheless, other studies and clinical observation suggest that shivering does occur in normothermic postoperative patients. Accordingly, we tested the hypothesis that some normothermic postoperative patients shiver.
Methods: Following IRB approval and consent, 61 patients undergoing major orthopedic surgery were anesthetized with 2 [micro sign]g/kg IV fentanyl, 0.2 mg/kg etomidate, and 0.5 mg/kg atracurium. Anesthesia was maintained by isoflurane (1.3 +/- 0.6%) in 70% N2 O, with an end-tidal PCO2 near 35 +/- 3 mmHg. Patients were randomly allocated to intraoperative passive insulation (n = 24) or active cutaneous heating (n = 37) (Warmtouch, Mallinckrodt, St. Louis, MO). Core temperature was recorded intraoperatively, and for one postoperative hour from the tympanic membrane, and arterio-venous shunt vasoconstriction was evaluated using forearm minus fingertip skin-temperature gradients; gradients < 0[degree sign]C identified vasodilation. Postanesthetic shivering was graded by a blinded investigator, using a four point scale (0 = no shivering, 1 = intermittent, low-intensity shivering, 2 = moderate shivering, 3 = continuous, intense shivering).
Results: Morphometric characteristics and hemodynamic responses were similar in the control and actively warmed patients. Core temperature decreased about 2.5[degree sign]C in the passively warmed group, but remained constant in normothermic patients. Population variability nonetheless produced initial postoperative core temperatures spanning the range from 34 to 37[degree sign]C. Shivering was observed in 18 of 24 unwarmed patients (75%), but only in 12 of 37 actively warmed patients (32%). Five of the actively warmed patients who shivered were vasodilated (14%) (Table 1).
Conclusion: Our results suggest that active intraoperative warming decreases the incidence of shivering after major orthopedic surgery, but fails to entirely obliterate this thermoregulatory defense. Shivering is likely to result because initial postoperative temperatures in some of the actively warmed patients were less than pre-operative values, thus triggering thermoregulatory shivering. However, shivering in some patients may have resulted from fever (due to release of pyrogenic mediators during surgery) or isoflurane-facilitated clonic muscular activity. 
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