Evaluating the effects of continuous intravenous infusions of tramadol and tramadol-lidocaine on sevoflurane minimum alveolar concentration (MAC) and entropy values in dogs

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The sparing effects of tramadol and tramadol-lidocaine infusion on the minimum alveolar concentration (MAC) of sevoflurane in dogs as well as the entropy indices were investigated. Anesthesia was induced in eight young, healthy German shepherds weighing 27.6 ± 3.2 kg (mean ± SD) and maintained with sevoflurane. A standard tail-clamp technique was used to determine sevoflurane MAC during infusion with: sevoflurane alone to measure baseline MAC (MAC_B); tramadol (intravenous loading dose of 1.5 mg/kg and constant rate infusion [CRI] of 2.6 mg/kg/hr; MAC_T); and tramadol-lidocaine (tramadol CRI of 2.6 mg/kg/hr; and lidocaine intravenous loading dose of 1.0 mg/kg and CRI of 6 mg/kg/hr; MAC_TL). The state entropy (SE), response entropy (RE), and RE-SE difference were recorded 5 min prior to and during tail clamping. MAC_B was 2.4 ± 0.2%. Tramadol and tramadol-lidocaine CRI decreased MAC to 2.2 ± 0.3% and 1.7 ± 0.3%, respectively. The MAC-sparing effect of tramadol-lidocaine was greater than that of tramadol alone (8.2 ± 8.9% vs. 30.1 ± 10.7%; P<0.01). SE and RE in all subjects, and RE-SE difference in most subjects, were increased (all P<0.05) when they responded purposefully to noxious stimulation. A tramadol-lidocaine combination infusion can reduce anesthetic requirements to a higher degree than tramadol alone. Furthermore, MACentropy, MAC required to prevent increased entropy in response to a painful stimulation, and MAC of sevoflurane were similar in dogs.