Objective - To characterize the antinociceptive action of IM-administered butorphanol, buprenorphine, or a combination of both by use of a thermal threshold method in cats. Animals - 2 male and 4 female domestic cats. Procedures - In a controlled, masked, randomized, crossover study design, thermal thresholds were measured by use of a thermal threshold-testing device developed for cats. Each cat received 4 treatments 1 week apart, consisting of 2 simultaneous IM injections in a random order (butorphanol-saline [0.9% NaCl] solution, buprenorphine-saline solution, butorphanol-buprenorphine, and saline solution-saline solution). The tester was unaware of the treatment given. Thermal thresholds were measured prior to injection, at intervals up to 12 hours, and at 22 hours after injection. Results - There was no significant change in threshold over time after saline solution administration. All 3 opioid treatment groups had significant increases in thermal threshold, compared with pretreatment values (butorphanol, from 50 minutes to 8 hours; buprenorphine, from 35 minutes to 5 hours; and butorphanol-buprenorphine, from 50 minutes to 8 hours). Thermal thresholds did not differ significantly among opioid treatments at any time points, and thermal thresholds of only 2 opioid treatments (butorphanol at 50 minutes and butorphanol-buprenorphine at 8 hours) were significantly different from that of saline solution. Conclusions and Clinical Relevance - All 3 opioid treatments provided similar antinociception, although there was considerable intercat variability in the response to the different opioid treatments. This emphasizes the importance of assessing each patient individually and applying the treatment that works best for that patient.
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