Analgesic effect of methadone constant rate infusion is not superior to fentanyl constant rate infusion in dogs after spinal surgery: a prospective, randomized, blinded, clinical trial

Abstract

This study aimed to compare the analgesic efficacy of methadone and fentanyl infusions at a 50:1 dose rate in dogs after spinal surgery. In a prospective, randomized, blinded clinical trial, 42 dogs with intervertebral disk disease were evaluated at 1, 2, 4, 18, 24, 42, 48, 66, 72, 90, and 96 h after extubation. Based on the treatment groups, the dogs received a presurgical bolus of 0.25 mg/kg methadone or 5 μg/kg fentanyl, followed by an intraoperative and postoperative opioid infusion. In the methadone (M) treatment group, the dogs received 0.2 mg/kg/h methadone, which was reduced stepwise to 0.05 mg/kg/h. In the fentanyl (F) treatment group, the initial dose of 5 μg/kg/h was reduced stepwise to 1 μg/kg/h. Assessments at each time point included a simple sedation score, vital parameters, pain scoring using the Glasgow Composite Measure Pain Scale (CMPS-SF), and sensory testing with von Frey filaments. Multiple Mann–Whitney tests were used for the analysis of CMPS-SF scores and sedation scores. Vital parameters, between and within the groups, were analyzed using mixed-effects analysis and Dunnett’s multiple comparison test. The need for rescue analgesia was assessed via survival analysis. The level of significance was set to α = 5%. In the postoperative period, six dogs in group M and four dogs in group F required rescue analgesia. CMPS-SF scores were not different between the groups M and F and did not change significantly over time within the treatment groups. Tactile sensory thresholds were not different between the groups M and F. Under the dosing regimens investigated, methadone did not provide superior postoperative analgesia compared to fentanyl. From a clinical perspective, both drugs provided effective postoperative analgesia in this heterogeneous cohort of dogs following spinal surgery.