Pharmacokinetics of medetomidine in ponies and elaboration of a medetomidine infusion regime which provides a constant level of sedation.

The pharmacokinetics of intravenous (i.v.) medetomidine (7 mcg kg(-1)) were best described by a two-compartment model in five ponies. Total body clearance was 4 (SD 0.60) 1 kg h,(-1)t(1/2alpha)7. 6 (0.91) minutes and t(1/2beta)51.3 (13.09) minutes. In one pony the one-compartmental model was best fit, and total body clearance was 4. 2 l kg h(-1)and t(1/2)was 11 minutes. Medetomidine plasma levels had fallen below the limits of quantification (0.05 ng ml(-1)) within 4 hours. Medetomidine 5 mcg kg(-1)i.v. followed by an infusion of 3.5 mcg kg h(-1)for two hours provided a constant level of sedation reaching steady state plasma medetomidine levels of 1-1.5 ng ml(-1)within 30 minutes. Sedation was reversed effectively by atipamezole (60 mcg kg(-1)) i.v. The pharmacokinetics of medetomidine make it suitable for prolonged use by infusion, such as is required as part of a total intravenous anaesthetic technique in horses.