Effects of continuous intravenous infusion of morphine and morphine-tramadol on the minimum alveolar concentration of sevoflurane and electroencephalographic entropy indices in dogs

Abstract Objective To 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. Design Prospective study. Animals Eight young, healthy German shepherds, weighing 26.3 ± 3.1 kg (mean ± SD). Methods Anaesthesia 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 (MAC B ); then during morphine infusion (MAC M ), (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 ) (MAC MT ). 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. Results The MAC B 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 Conclusion and clinical relevance In 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.