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To evaluate medetomidine as a continuous rate infusion (CRI) in horses in which anaesthesia is maintained with isoflurane and CRIs of ketamine and lidocaine.Prospective, randomized, blinded clinical trial.Forty horses undergoing elective surgery.After sedation and induction, anaesthesia was maintained with isoflurane. Mechanical ventilation was employed. All horses received lidocaine (1.5 mg kg−1 initially, then 2 mg kg−1 hour−1) and ketamine (2 mg kg−1 hour−1), both CRIs reducing to 1.5 mg kg−1 hour−1 after 50 minutes. Horses in group MILK received a medetomidine CRI of 3.6 μg kg−1 hour−1, reducing after 50 minutes to 2.75 μg kg−1 hour−1, and horses in group ILK an equal volume of saline. Mean arterial pressure (MAP) was maintained above 70 mmHg using dobutamine. End-tidal concentration of isoflurane (FE′ISO) was adjusted as necessary to maintain surgical anaesthesia. Group ILK received medetomidine (3 μg kg−1) at the end of the procedure. Recovery was evaluated. Differences between groups were analysed using Mann-Whitney, Chi-Square and anova tests as relevant. Significance was taken as p < 0.05.FE′ISO required to maintain surgical anaesthesia in group MILK decreased with time, becoming significantly less than that in group ILK by 45 minutes. After 60 minutes, median (IQR) FE′ISO in MILK was 0.65 (0.4–1.0) %, and in ILK was 1 (0.62–1.2) %. Physiological parameters did not differ between groups, but group MILK required less dobutamine to support MAP. Total recovery times were similar and recovery quality good in both groups.A CRI of medetomidine given to horses which were also receiving CRIs of lidocaine and ketamine reduced the concentration of isoflurane necessary to maintain satisfactory anaesthesia for surgery, and reduced the dobutamine required to maintain MAP. No further sedation was required to provide a calm recovery.