Methadone infusions matched fentanyl for pain control in 42 dogs after spinal surgery

A constant-rate infusion of methadone at 0.2 mg/kg/h provided the same postoperative analgesia as fentanyl in dogs recovering from spinal surgery, according to a prospective randomized trial published in Frontiers in Veterinary Science.

What the study found

Forty-two dogs with intervertebral disk disease undergoing spinal surgery at the University of Veterinary Medicine Hannover received either methadone or fentanyl infusions starting before surgery and continuing through the postoperative period. Pain scores measured by the Glasgow Composite Measure Pain Scale short form showed no significant difference between the two groups. Six dogs in the methadone group and four in the fentanyl group required rescue analgesia during the trial period. Tactile sensory thresholds measured with von Frey filaments were not different between groups.

How the study was built

This was a prospective, randomized, blinded clinical trial. The anesthetist, surgeons, imagers, and ward nurses evaluating patients did not know which opioid each dog received. Dogs were privately owned patients with thoracolumbar or cervical disk disease requiring surgical intervention. The methadone group received a preoperative bolus of 0.25 mg/kg followed by 0.2 mg/kg/h during surgery, then 0.1 mg/kg/h for 72 hours, then 0.05 mg/kg/h for the final 24 hours. The fentanyl group received 5 μg/kg preoperatively, 5 μg/kg/h during surgery, 2 μg/kg/h for 72 hours, then 1 μg/kg/h for the final 24 hours. Assessments occurred at 1, 2, 4, 18, 24, 42, 48, 66, 72, 90, and 96 hours after extubation. This is a controlled trial in surgical patients at a single teaching hospital, not an experimental pain model. The design can support causal claims about the infusion protocols tested but cannot tell you whether different dose ratios, different surgical populations, or different pain scales would produce the same result.

The number, and its error bars

The study does not report a p-value for the primary pain-score comparison. It states that CMPS-SF scores were not different between groups and did not change significantly over time within groups, analyzed by multiple Mann-Whitney tests with significance set at α = 5%. The rescue-analgesia rate was 6 of 21 dogs in the methadone group and 4 of 21 in the fentanyl group, evaluated by survival analysis. The abstract does not give confidence intervals, hazard ratios, or effect sizes for these comparisons.

What the authors say it cannot tell you

The abstract states no limitations. That itself is a limitation. A 42-dog trial at one hospital in a heterogeneous population of disk-disease cases, thoracolumbar and cervical, ambulatory and non-ambulatory, almost certainly has boundaries the full paper would name. The abstract also does not state whether the study was powered to detect non-inferiority, which is the clinical question when comparing two established analgesics.

Who paid for it

The abstract does not state a funding source and does not declare conflicts of interest.

What this means for your practice

VeterinaryPracticeNow's read, not the study's. Everything in this section is our editorial interpretation for practice owners. The study does not claim any of it.

If we stock both opioids and run spinal cases, this gives us evidence that methadone infusions at the dose rates tested perform as well as fentanyl for postoperative pain control. The practical angle is DEA scheduling and supply chain: both are controlled substances, and methadone may offer different handling considerations depending on our practice's controlled-substance protocols. The dose rates the study used are now a reference point we can cite when writing or revising spinal-surgery pain protocols.

The limitation that matters for practice is the heterogeneity the authors mention but do not break down. Thoracolumbar and cervical cases are different pain pictures, and ambulatory versus non-ambulatory dogs are different recovery trajectories. The study pooled them, so we cannot tell from the abstract whether methadone's performance held across all subgroups or whether one population drove the equivalence. If our case mix skews heavily to one type, the aggregate result may not predict our own outcomes.

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The other thing to watch: the study does not report sedation scores or adverse events in the abstract, only that profound sedation was a disqualifying criterion. The stepwise dose reductions the protocol used suggest the investigators anticipated accumulation over multi-day infusions. If we switch from fentanyl to methadone infusions, we'd track sedation and respiratory rate closely in the first few cases until we know how our patient population responds.

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