Effect of transdermally administered fentanyl on the minimum alveolar concentration of isoflurane in cats

Objective: To determine the effect of two doses of fentanyl, administered transdermally, on the minimum alveolar concentration (MAC) of isoflurane in cats. Study design: Prospective, randomized study. Animals: Five healthy, spayed, female cats. Methods: Each cat was studied thrice with at least 2 weeks between each study. In study 1, the baseline isoflurane MAC was determined in triplicate for each cat. In studies 2 and 3, isoflurane MAC was determined 24 hours after placement of either a 25 or 50 μg hour^-1 fentanyl patch. In each MAC study, cats were instrumented to allow collection of arterial blood and measurement of arterial blood pressure. Twenty-four hours prior to studies 2 and 3, a catheter was placed and secured in the jugular vein and either a 25 or 50 μg hour^-1 fentanyl patch was placed in random order on the left thorax. Blood samples for plasma fentanyl determination were collected prior to patch placement and at regular intervals up to 144 hours. After determination of MAC in studies 2 and 3, naloxone was administered as a bolus dose (0.1 mg kg^-1) followed by an infusion (1 mg kg ^-1 hour^-1) and MAC redetermined. Results: The baseline isoflurane MAC was 1.51 ± 0.21% (mean ± SD). Fentanyl (25 and 50 μg hour^-1) administered transdermally significantly reduced MAC to 1.25 ± 0.26 and 1.22 ± 0.16%, respectively. These MAC reductions were not significantly different from each other. Isoflurane MAC determined during administration of fentanyl 25 μg hour^-1 and naloxone (1.44 ± 0.16%) and fentanyl 50 μg hour^-1 and naloxone (1.51 ± 0.19%) was not significantly different from baseline MAC (1.51 ± 0.21%). Conclusions and clinical relevance: Fentanyl patches are placed to provide long-lasting analgesia. In order to be effective postoperatively, fentanyl patches must be placed prior to surgery. Plasma fentanyl concentrations achieved intraoperatively decrease the need for potent inhalant anesthetics in cats.