Shivering After Outpatient Arthroscopy

Abstract

In Response: We appreciate Dr. Gupta's interest in our article on shivering after outpatient arthroscopy. We would agree that active warming during arthroscopic surgery does not appear to be the answer to preventing shivering after this ambulatory procedure. As stated in our discussion [1], heat is predominantly redistributed rather than lost to the environment during the early postoperative period. Therefore, forced-air warming has relatively little effect on postoperative shivering. Dr. Gupta is correct in pointing out that the incidence of shivering decreased from 35%-50% in the first phase of our study to 15%-20% during the second phase when active warming was continued into the postoperative period [1]. However, it should also be noted that the overall incidence of shivering was lower in both the warmed and the control patients during the second phase of the study compared with the earlier phase. Although there were fewer patients who shivered during the second phase of our study, the more prolonged warming period did not significantly reduce the proportion of patients who shivered after arthroscopic surgery. It is reassuring to note that the incidence of shivering in Dr. Gupta's unpublished study was similar to the incidence reported in our article. Although it is interesting to speculate on the possible contribution of the anesthetic technique (i.e., volatilc versus intravenous drugs) to the incidence of postoperative shivering, the results in Gupta's Table 1 suggest that 5 of 26 patients receiving isoflurane shivered compared with 7 of 24 patients receiving propofol. This very small difference was almost certainly due to chance (P = 0.51 using Fisher's exact test). Therefore, we do not feel that any meaningful conclusions can be drawn from these data relative to the contribution of the anesthetic technique. Finally, visual analog scale (VAS) scores were not a part of this study protocol. However, pain VAS scores (0 = no pain to 100 mm = worst pain imaginable) were available for approximately half of our study patients. In analyzing these data, we failed to find a difference in the pain VAS scores at 15 min and 30 min after the operation in those patients who shivered (mean, median, and range: 27, 19, 0-95 and 34, 24, 0-92 at 15 min and 30 min, respectively) compared with those who did not experience postoperative shivering (38, 43, 0-95 and 35, 38, 0-74). Similarly, the volume of room temperature intravenous fluid administered during the arthroscopy procedure did not differ between the patients who shivered and those who did not. In conclusion, we remain unconvinced that "active" intraoperative warming is beneficial in reducing shivering after short ambulatory operations. The primary benefit from forced-air warming in this patient population is that they feel more comfortable in the postanesthesia care unit (PACU). We would speculate that active warming may be just as effective if it is applied only in the PACU rather than both intraoperatively and postoperatively. However, our study did not address this particular issue. Until more convincing data are available, we feel that the use of warm cloth blankets remains the most cost-effective method for enhancing patient comfort in the early postoperative period. Ian Smith, MB, BCh, FRAC Paul F. White, PhD, MD Department of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75235-8894