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  • Author or Editor: Glenn R. Pettifer x
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in Journal of the American Veterinary Medical Association


Objective—To determine whether moderate hypothermia during 4 hours of anesthesia with isoflurane substantially affects serum concentrations of transdermally administered fentanyl in the perianesthetic period in cats.

Animals—7 healthy mature cats.

Procedure—A fentanyl patch (25 µg/h) was applied to the shaved thorax 24 hours before induction of anesthesia. Anesthesia was induced at time 0. Each cat received 2 treatments in a random order. Treatments were isoflurane anesthesia with normothermia and isoflurane anesthesia with hypothermia. Cats were intubated, connected to a nonrebreathing circuit, and maintained at 1.3X minimum alveolar concentration for 4 hours. Cats in the hypothermia treatment groups were actively cooled to 35°C following the induction of anesthesia. Serum fentanyl analysis was performed at –24, –12, 0, 1, 2, 3, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, and 24 hours.

Results—Mean ± SEM serum fentanyl concentration (SFC) for the hypothermia treatment group (0.598 ± 0.3048 ng/mL) was significantly lower than the baseline concentration (1.834 ± 0.6393 ng/mL) at 1 hour. This significant reduction persisted for the duration of anesthesia for the hypothermia treatment group. Serum fentanyl concentrations returned to baseline values within 1 hour of the end of anesthesia, regardless of body temperature.

Conclusions and Clinical Relevance—Hypothermia during inhalant anesthesia induced a significant reduction in SFC obtained with transdermal administration. The impact of this reduction in SFC on the contribution of transdermally administered fentanyl to any reduction in the need for inhalant anesthesia remains to be determined. (Am J Vet Res 2003;64:1557–1561)

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in American Journal of Veterinary Research


Objective—To determine whether the minimum alveolar concentration (MAC) of isoflurane was altered by transdermal administration of fentanyl in normothermic and hypothermic dogs.

Design—Randomized complete block crossover design.

Animals—6 mature healthy dogs.

Procedure—Dogs received each of 4 treatments in random order. Following induction of anesthesia, normothermia was maintained in dogs that were treated with a fentanyl patch (F-NORM) or sham patch (C-NORM), or hypothermia was maintained in dogs that were treated with a fentanyl patch (F-HYPO) or sham patch (C-HYPO). The appropriate patch was applied 24 hours prior to induction of anesthesia. Anesthesia was induced with isoflurane in oxygen; the dogs were intubated and mechanically ventilated. Target esophageal temperatures were maintained within 1°C of baseline values (normothermia) or at 34.5°C (94.1°F; hypothermia) for 1 hour prior to starting MAC determinations. Supramaximal stimulation was achieved with an electrical stimulator attached to needle electrodes placed in the buccal mucosa of the lower jaw of the dog.

Results—Mean MAC ± SEM of isoflurane during C-NORM, C-HYPO, F-NORM, and F-HYPO treatments were 1.20 ± 0.17, 0.89 ± 0.18, 0.76 ± 0.10, and 0.81 ± 0.17, respectively. The mean MAC during C-NORM was significantly higher than values for the other treatments. There was no significant difference in mean MAC among the C-HYPO, F-NORM, and F-HYPO treatments.

Conclusions and Clinical Relevance—Data suggest that transdermal administration of fentanyl significantly reduces isoflurane requirements in normothermic dogs. The isoflurane MAC-sparing effects of transdermal fentanyl are not apparent in hypothermic dogs.

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in Journal of the American Veterinary Medical Association
in Journal of the American Veterinary Medical Association


Objective—To compare plasma fentanyl concentrations and analgesic efficacy during full or partial exposure to 25-μg/h transdermal fentanyl patches (TFPs) in cats undergoing ovariohysterectomy.

Design—Randomized controlled clinical trial.

Animals—16 client-owned cats.

Procedure—Cats were randomly assigned to receive full or partial exposure to a TFP; patches were applied approximately 24 hours prior to ovariohysterectomy. Rectal temperature, heart rate, respiratory rate, blood glucose concentration, and blood pressure were measured and pain severity was assessed periodically for 72 hours after patch application. Venous blood samples were collected for determination of plasma fentanyl concentration 0, 6, 12, 18, 24, 36, 48, 60, and 72 hours after patch application.

Results—Mean ± SD steady state plasma fentanyl concentration in cats in the full TFP exposure group (1.78 ± 0.92 ng/mL) was significantly greater than concentration in cats in the partial exposure group (1.14 ± 0.86 ng/mL). Steady state plasma fentanyl concentrations were evident between 18 and 72 hours after patch application. Subjective scores used to evaluate analgesic efficacy were not significantly different between treatment groups.

Conclusions and Clinical Relevance—Results suggest that delivery of fentanyl from TFPs can be reduced by decreasing the amount of exposed surface area. In cats weighing < 4 kg (9 lb), exposure to half a 25-μg/h TFP appears to provide adequate analgesia following ovariohysterectomy. (J Am Vet Med Assoc 2004:224:700–705)

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in Journal of the American Veterinary Medical Association