Many µ-opioid agonists (eg, morphine, hydromorphone, oxymorphone, methadone, fentanyl, alfentanil, remifentanil, sufentanil) have been sporadically available. In addition to analgesia and variable degrees of sedation, they provide anesthetic-sparing effects while maintaining cardiovascular safety. For premedication and intraoperative use by infusion, these drugs are largely interchangeable, provided the clinician has knowledge of their relative potency, onset and duration of action, and side effect profile.6
Buprenorphine (20-30 µg/kg IV, IM, or buccal), a partial µ agonist, may be used alone or in combination with other medications as a substitute for other µ agonists in dogs and cats for mildly- to-moderately painful procedures.6,14 It may also be used with other drugs for more complex and painful surgical procedures to minimize pain. A dosing interval of approximately 6 to 8 hours has been suggested in the perioperative period.14 Salivation, bradycardia, and respiratory depression may be observed with use; drug effects are generally not thought to be reversible. Sustained-release or long-acting formulations of buprenorphine for subcutaneous administration are available and are reported to provide between 24 and 72 hours of analgesia.15,16
Butorphanol (0.1-0.5 mg/kg IV, IM, or SC), a κ agonist and µ antagonist, is best used as a sedative and analgesic for presumed mildly painful procedures (eg, gastroduodenoscopy, colonoscopy, subcutaneous mass removal) or with adjunct analgesic techniques (eg, as a nerve block).6
Premedication with dexmedetomidine (3-10 µg/kg IM) can be considered in healthy dogs and cats to provide sedation and analgesia. Cardiovascular side effects may occur and present challenges with monitoring. If these effects are significant, partial reversal with atipamezole can lessen them; however alternative analgesia should be provided prior to reversal. Dexmedetomidine may also be administered as a constant-rate infusion in healthy dogs and cats; an initial maintenance dose of 1 µg/kg/hr IV has been suggested to provide analgesia and anesthesia-sparing effects.17