Objective To investigate the safety, sedative and analgesic properties of methadone in combination with acepromazine prior to neutering in cats.
Study design Controlled clinical, block randomized, prospective, blinded study designed for regulatory purposes.
Animals 24 female and 21 male healthy cats.
Methods Cats received one of three opioids combined with acepromazine (0.05 mg kg−1) intramuscularly (IM) for premedication: Group 1: buprenorphine (0.02 mg kg−1), group 2: methadone (0.5 mg kg−1), group 3 butorphanol (0.4 mg kg−1). Sedation was assessed 30 minutes after premedication using a visual analogue scale (VAS) and simple descriptive scale. Anaesthesia was induced with alfaxalone and maintained with isoflurane in oxygen. Surgical ovariohysterectomy or castration was performed. Pain was assessed using an interactive VAS (IVAS) and mechanical nociceptive threshold (MNT) with a pressure rate onset device. Methadone (0.5 mg kg−1 IM) and meloxicam (0.2 mg kg−1 subcutaneously) were provided 6 and 8 hours after premedication respectively, or together as rescue analgesia (IVAS above 50).
Results Sedation scores, induction agent dose, pain scores at all time points and rescue analgesia were not statistically different between groups. In methadone treated cats there was no significant variation in MNT over time, suggesting a possible anti-hyperalgesic action, whereas in the other two groups lower thresholds were recorded at various time points after surgery compared to baseline. No cats required rescue analgesia after the second dose of methadone. No perioperative adverse effects occurred.
Conclusion and clinical relevance Methadone provided comparable sedation and analgesia to both buprenorphine and butorphanol when combined with acepromazine. Differences in analgesic efficacy between opioids might have been undetectable because of the surgical model and surgeon competency. Nevertheless, methadone is an effective analgesic in cats and its administration prior to feline neutering may be advantageous.