Postoperative analgesia of methadone in cats: epidural and intramuscular administration

The aim of this study was to investigate the analgesic, sedative and neuroendocrine effects of epidural (EP) and intramuscular (IM) methadone in cats submitted to ovariohysterectomy. The pre-anesthetic medication was acepromazine (0.1mg kg(-1), IM) followed by induction of anesthesia with intravenous thiopental, 12mg kg(-1), and anesthesia maintenance with halothane. After anesthesia stabilization the cats were randomly assigned to three groups of eight animals each and received EP or IM methadone, 0.2mg kg(-1) diluted with saline to 0.2mL kg(-1), EP and IM, respectively, or a saline placebo (S), 0.2mL kg(-1). Pain measurements, degree of sedation, rescue analgesia requirements, adverse effects and serum cortisol concentration were recorded. Cortisol, pain and sedation scores did not differ among the groups. Rescue analgesia was administered 4, 9 and 11 times in the EP, IM and S treatment, respectively. In the IM and S rescue analgesia treatments were required early (first hour postoperative), whereas in the EP treatment, additional analgesics were required after the third hour postoperative. In conclusion, methadone epidural reduced the postoperative analgesic requirements and produced longer analgesia when compared to intramuscular administration in cats undergoing ovariohysterectomy.