The purpose of this study was to determine the effects of an infusion of dexmedetomidine, a co-infusion of morphine-lidocaine-ketamine (MLK), and a co-infusion of dexmedetomidine-morphine-lidocaine-ketamine (alpha-MLK) on minimum alveolar concentration (MAC) of isoflurane in dogs. The MAC of an inhalant anesthetic required to prohibit purposeful movement is a measure of anesthetic potency (Eger et al 1965). Isoflurane is the most commonly used inhalant anesthetic in veterinary practice (Lozano et al 2009), but has potent vasodilatory effects and causes a dose-dependent decrease in mean arterial pressure in anesthetized dogs (Steffey and Howland 1977). Additional drugs are used during anesthesia to decrease the inhalant anesthetic requirement, a concept referred to as balanced anesthesia. Each of the drugs we infused has a different central nervous system receptor mechanism of action. Co-infusion of MLK, as well as the infusion of each drug separately, has been shown to reduce MAC in isoflurane-anesthetized dogs (Muir et al 2003). Dexmedetomidine, an alpha-2 agonist, has been shown to reduce isoflurane MAC in dogs (Pascoe et al 2006). Dexmedetomidine is a commonly used sedative with analgesic and muscle relaxant properties. The effect of a co-infusion of alpha-MLK on isoflurane anesthesia has not been critically evaluated in the dog. Our hypothesis was that infusion of alpha-MLK would significantly decrease isoflurane MAC (MAC-Iso) compared to dexmedetomidine or MLK infusions.
For this study, six dogs were anesthetized to determine MAC-Iso (baseline). Following MAC-Iso determination, each dog was anesthetized on three separate occasions with isoflurane, with a minimum of 7 days between anesthetic episodes. On each occasion, and in random order, dexmedetomidine, MLK, or alpha-MLK infusions were administered, and MAC was determined. Additionally, hemodynamic and metabolic parameters as well as bispectral index (BIS) were measured during each experiment. The investigator was blinded to the infusion given. Data were analyzed using ANOVA for repeated measures. Dunnett and Tukey posttests were performed to identify differences within and among groups, respectively, when differences were detected.
The MAC-Iso was 1.3 ± 0.15%. Dexmedetomidine, MLK, and alpha-MLK significantly lowered MAC-Iso by 30, 55, and 90%, respectively. Heart rate was significantly decreased from baseline in all infusion groups. Heart rate did not differ significantly between dexmedetomidine and alpha-MLK infusion groups. Mean arterial pressure for alpha-MLK increased significantly from baseline. Systemic vascular resistance was significantly increased for the dexmedetomidine and alpha-MLK infusion groups. The BIS values increased significantly compared to baseline for the MLK and alpha-MLK infusion groups. There was no significant difference for time to extubation and sternal recumbency among groups.
Infusions of dexmedetomidine, MLK, and alpha-MLK reduced MAC-Iso in dogs. Changes in cardiovascular parameters were within clinically acceptable ranges for healthy dogs. BIS can be an effective monitoring tool for measuring the depth of anesthesia.