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 meg kg(-1)) were best described by a two-compartment model in five ponies. Total body clear ance was 4 (SD 0.60) 1 kg h,(-1) t(1/2 alpha) 7.6 (0.91) minutes and t(1/2 alpha) 51.3 (13.09) minutes. In one pony the one-compartmental model was be st fit, and total body clearance was 4.2 1 kg h(-1) and t(1/2) was 11 minut es. Medetomidine plasma levels had fallen below the limits of quantificatio n (0.05 ng ml(-1)) within 4 hours. Medetomidine 5 meg kg(-1) i.v. followed by an infusion of 3.5 meg kg h(-1) for two hours provided a constant level of sedation reaching steady state p lasma medetomidine levels of 1-1.5 ng ml(-1) within 30 minutes. Sedation wa s reversed effectively by atipamezole (60 meg kg(-1)) i.v. The pharmacokine tics of medetomidine make it suitable for prolonged use by infusion, such a s is required as part of a total intravenous anaesthetic technique in horse s.