• Chulabhorn Mahidol, Sirirat Niyom, Chaiyakorn Thitiyanaporn, Apinun Suprasert, and Naris Thengchaisri.
• Department of Companion Animal Clinical Sciences, Faculty of Veterinary Medicine, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom, Thailand.
• Vet Anaesth Analg. 2015 Mar 1;42(2):182-6.

ObjectiveTo compare the effects of continuous rate infusions (CRIs) of intravenous (IV) morphine and morphine-tramadol on the minimum alveolar concentration (MAC) of sevoflurane, and on electroencephalographic entropy indices in dogs.DesignProspective study.AnimalsEight young, healthy German shepherds, weighing 26.3 ± 3.1 kg (mean ± SD).MethodsAnaesthesia was induced and maintained with sevoflurane. A standard tail-clamp technique was used for MAC determination. Within one anaesthetic period, MAC was first determined during sevoflurane anaesthesia alone (MACB ); then during morphine infusion (MACM ), (loading dose 0.5 mg kg(-1) IM; CRI, 0.2 mg kg(-1 ) hour(-1)) then finally during morphine-tramadol infusion (tramadol loading dose 1.5 mg kg(-1) IV; CRI, 2.6 mg kg(-1)  hour(-1) ) (MACMT ). At each change, periods of 45 minutes were allowed for equilibration. Stated entropy (SE), response entropy (RE), and RE-SE differences were measured five minutes prior to and during tail clamping.ResultsThe MACB was 2.1 ± 0.3vol%. The morphine and morphine-tramadol infusions reduced MAC to 1.6 ± 0.3vol% and 1.3 ± 0.3vol%, respectively. MAC was decreased below baseline more during morphine-tramadol than during morphine alone (39 ± 9% versus 25 ± 6%, respectively; p = 0.003). All SE and RE and most RE-SE differences were increased significantly (p < 0.05) over pre-stimulation in all groups when the dogs responded purposefully to noxious stimulation. When no response to noxious stimulation occurred, the entropy indices did not change.Conclusion And Clinical RelevanceIn dogs, combined morphine-tramadol CRI decreased sevoflurane MAC more than morphine CRI alone. Entropy indices changed during nociceptive responses in anaesthetized animals, suggesting that entropy measurements may be useful in determining anaesthetic depth in dogs.© 2014 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia.

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