Comparison of the effects between mild hypothermia and normothermia on propofol or isoflurane anesthesia

Dae Woo Kim, MD, Jin Deok Joo, MD

Department of Anesthesiology, College of Medicine, The Catholic University of Korea, Seoul, Korea

Background and Goal of Study: Although mild intraoperative hypothermia is common and prolonged duration of postanesthetic recovery, the effects of mild hypothermia on anesthetic drug pharmacology are not well known. The purpose of the present study was to evaluated the effects of core normothermia and mild hypothermia on recovery profile whether mild intraoperative hypothermia prolongs the duration of postanesthetic recovery. In addition, we compared the BIS₅₀ (the value of 50 on Bispectral index monitor) of effect site concentrations(C_E) or expired tidal concentrations(C_ET) on normothermia and hypothermia during propofol or isoflurane anesthesia, respectively.

Materials and Methods: Sixty unpremedicated patients were scheduled to undergo endoscopic sinus surgery. Patients were randomized to four groups as follows: Group 1 (n=15) received propofol anesthesia with normothermia, Group 2 (n=15) received propofol anesthesia with mild hypothermia, Group 3 (n=15) received isoflurane anesthesia with normothermia, Group 4 (n=15) received isoflurane anesthesia with mild hypothermia. In cases of propofol anesthesia, first of all, fentanyl target controlled infusion(TCI) started with a target concentration(C_T) 1 ng/ml using a Stelpump (J. F. Coetzee, University of Stellenbosch, South Africa) and propofol infusion was started at a C_T 6 mg/ml using a Diprifusorä (Master TCI, Fresenius Vial S.A., Brezins, France) with flash mode. Anesthesia was maintained with propofol C_T 4 mg/ml supplemented with fentanyl C_T 1 ng/ml and 50% nitrous oxide in oxygen, however, the target concentration of propofol could be adjustable in order to set BIS 40-60. Meanwhile, in cases of isoflurane anesthesia, which were consisting of thiopental 3-5 mg/kg intravenously for induction and isoflurane 0.6-1 vol% and nitrous oxide 50% in oxygen for maintenance of anesthesia. The time of eye opening and orientation (giving name and date of birth) of each patient was assessed and recorded the BIS₅₀ of C_E or C_ET during propofol or isoflurane anesthesia, respectively. In the mild hypothermia group, body temperature was allowed to decrease during surgery and recovery, and no measures were taken to maintain normothermia. In contrast, patients in the normothermia group had their core and skin temperatures maintained at preoperative levels throughout the perioperative period. This was achieved by active warming of the intravenous fluids at 37℃ with a blood warmer, application of a Bair Hugger forced-air warmer (Model 505, Augustine Medical, Inc., Eden Prairie, MN), and airway humidification circuit.

Results: See Fig.1 and 2

Conclusions: In contrast to isoflurane anesthesia, mild hypothermia was associated with increased effect site concentrations for the BIS₅₀ and more drug requirements in propofol anesthesia, even though recovery time were similar in all cases of both anesthesia.

Figure 1. Changes of core (distal esophageal) and skin temperatures on normothermia and mild hypothermia during anesthesia. * : P<0.05 compared to intubation.

Figure 2. Mild hypothermia was associated with increased effect site concentrations (C_E) for the BIS₅₀ in propofol anesthesia, while, it was associated with decreased expired tidal concentrations (C_ET) for the BIS₅₀ in isoflurane anesthesia. * : P<0.05 vs normothermia.