Pharmacokinetics of intravenously administered ketamine in the horse

The metabolism and distribution of ketamine and its two major metabolites (norketamine and dehydronorketamine) was investigated in 10 horses undergoing airway surgery. Following premedication with xylazine (1.1 mg kg-1 intravenously) anaesthesia was induced by the rapid injection of ketamine at a dose of 2.2 mg kg-1 intravenously. Anaesthesia was maintained with halothane vaporized in oxygen and nitrous oxide (50:50). Serially collected blood samples were analysed by a sensitive gas liquid chromatographic technique. Plasma ketamine concentrations declined biexponentially with a rapid initial distribution phase (t1/2 2.89 +/- 0.25 minutes) being followed by a slower elimination phase (t1/2 65.84 +/- 3.46 minutes). Norketamine was found in the plasma of all the horses, but only at low concentrations while there was very little dehydronorketamine detected. It seems that recovery from ketamine anaesthesia in the horse depends mainly on rapid redistribution of the drug from the central compartment and this explains the abrupt recovery from ketamine anaesthesia often observed in the horse.