Pharmacokinetics and adverse effects of butorphanol administered by singleintravenous injection or continuous intravenous infusion in horses

Objective-To determine an infusion rate of butorphanol tartrate in horses t hat would maintain therapeutic plasma drug concentrations while minimizing development of adverse behavioral and gastrointestinal tract effects. Animals-10 healthy adult horses. Procedure-Plasma butorphanol concentrations were determined by use of high- performance liquid chromatography following administration of butorphanol b y single IV injection (0.1 to 0.13 mg/kg of body weight) or continuous IV i nfusion (loading dose, 17.8 mug/kg; infusion dosage, 23.7 mug/kg/h for 24 h ours), Pharmacokinetic variables were calculated, and changes in physical e xamination data, gastrointestinal tract transit time, and behavior were det ermined over time. Results-A single IV injection of butorphanol was associated with adverse be havioral and gastrointestinal tract effects including ataxia, decreased bor borygmi, and decreased defecation. Elimination half-life of butorphanol was brief (44.37 minutes). Adverse gastrointestinal tract effects were less ap parent during continuous 24-hour infusion of butorphanol at a dosage that r esulted in a mean plasma concentration of 29 ng/ml, compared with effects a fter a single IV injection. No adverse behavioral effects were observed dur ing or after continuous infusion. Conclusions and Clinical Relevance-Continuous IV infusion of butorphanol fo r 24 hours maintained plasma butorphanol concentrations within a range asso ciated with analgesia. Adverse behavioral and gastrointestinal tract effect s were minimized during infusion, compared with a single injection of butor phanol, Continuous infusion of butorphanol may be a useful treatment to ind uce analgesia in horses.