• Glenn R Pettifer and Giselle Hosgood.
• Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803, USA. gpettifer@vetmed.lsu.edu
• Vet Anaesth Analg. 2004 Apr 1;31(2):109-20.

ObjectivesTo determine whether moderate hypothermia during anesthesia significantly affects the serum concentration of transdermally delivered fentanyl and whether halothane or isoflurane affect these concentrations.Study DesignRandomized cross-over experimental trial.AnimalsSix mature, healthy Beagles (three males, three females) weighing 10.6 +/- 0.43 kg.MethodsA 50-microg hour(-1) fentanyl patch was applied 36 hours prior to anesthesia. Anesthesia was induced at time 0 (t = 0). Each dog received four treatments: isoflurane + normothermia (ISO-NORM), isoflurane + hypothermia (ISO-HYPO), halothane + normothermia (HAL-NORM), and halothane + hypothermia (HAL-HYPO). Dogs were intubated and maintained at 1.5 times MAC. Animals in the hypothermia treatments were cooled to 35 degrees C during anesthesia. Serum fentanyl analysis was performed at -36, -24, -12, 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 18, and 26 hours. Direct arterial blood pressures and arterial blood gases were monitored.ResultsThe mean body temperatures (+/-SEM) during the anesthetic period for the four treatments were: ISO-NORM = 37.7 +/- 0.07 degrees C, ISO-HYPO = 35.8 +/- 0.1 degrees C, HAL-NORM = 37.7 +/- 0.06 degrees C, and HAL-HYPO = 35.8 +/- 0.13 degrees C. The mean (+/-SEM) serum fentanyl concentrations (SFC) for both hypothermia treatments were significantly lower than baseline concentrations at t = 1 hour and persisted for the duration of anesthesia for the ISO-HYPO treatment but only from t = 1 to 2 hours for the HAL-HYPO treatment. Serum fentanyl concentrations returned to baseline within one hour of the end of anesthesia, regardless of body temperature. There were no significant differences between treatments for systolic or diastolic blood pressure but mean blood pressures were higher during normothermia versus hypothermia during the last hour of anesthesia.Conclusions And Clinical RelevanceHypothermia during inhalation anesthesia produced a significant reduction in SFC using transdermal administration and was more protracted with isoflurane than halothane anesthesia. While significant reductions in SFC occurred, the SFC were still within the range believed to confer analgesia.

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