Comparison of the effects of epidural or intravenous methadone on the minimum alveolar concentration of isoflurane in dogs

The effects of epidural and intravenous (IV) methadone (0.5 mg/kg) on the minimum alveolar concentration of isoflurane (ISOMAC) were compared in dogs. Six dogs (16.5 ± 2.5 kg bodyweight) received three treatments in random order during isoflurane anaesthesia, with a 7 day washout interval between each study. Methadone was injected via a lumbosacral epidural catheter introduced 10 cm cranially into the epidural canal and the electrical stimulation for ISOMAC determination was applied either to the thoracic (EPT treatment) or to the pelvic limb (EPP treatment) during separate study days. In the IV treatment, ISOMAC was determined via electrical stimulation of the pelvic limb. Variables were recorded before (baseline), 2.5 and 5 h after drug injection.The ISOMAC decreased significantly (P < 0.05) from baseline at 2.5 and 5 h after methadone in all treatments. At 2.5 h, the magnitude of ISOMAC reduction did not differ between treatments (mean decreases from baseline: 30-33%). The ISOMAC reduction lasted longer following epidural methadone in the thoracic limb (decreases from baseline: 30% at 5 h in the EPT treatment vs. 19% and 16% in the EPP and IV treatments, respectively). Although the isoflurane sparing effect provided by epidural methadone was not significantly greater than IV methadone during the initial stage (2.5 h), it was more prolonged than the IV route in specific dermatomes (5 h in the thoracic limb) with the epidural technique employed. Methadone may therefore provide a greater isoflurane sparing effect when administered epidurally, compared to IV, when noxious stimulation occurs in specific dermatomes.