The pharmacokinetics of ketamine after a continuous infusion under halothane anaesthesia in horses

Abstract

The pharmacodynamics and pharmacokinetics of ketamine, when administered by infusion as an adjunct to halothane anaesthesia in horses, were investigated in 5 equine patients presented for routine castration. Anaesthesia was induced with detomidine, 20 μg/kg, followed by ketamine, 2.2 mg/kg bwt, the trachea intubated and the horses allowed to breathe halothane in oxygen. Five minutes later, a constant rate infusion of ketamine, 40 μg/kg min, was commenced and the halothane vaporiser concentration adjusted to maintain a light plane of anaesthesia. The mean infusion duration was 62 min (range 40–103). The ketamine was switched off approximately 15 min before the halothane. Plasma ketamine and norketamine levels, determined by high performance liquid chromatography, ranged from 0.74–2.04 μg/ml and 0.15–0.75 μg/ml, respectively, during the infusion period.

The harmonic mean elimination half-life of ketamine was 46.1 min, mean volume of distribution at steady state (Vdss) was 1365 (271) ml/kg, mean body clearance (Cl) was 32.3 (9.1) ml/min.kg, and average mean residence time for the infusion (MRTinf) was 105.9 (20.4) min, respectively.

Following termination of halothane, mean times to sternal recumbency and standing were 21.1 (6.9) and 41.6 (17.0) min, respectively. Surgical conditions were considered highly satisfactory, and physiological parameters were well preserved in most animals.