The disposition of lidocaine during a 12-hour intravenous infusion to postoperative horses.

Lidocaine is administered as an intravenous infusion to horses for a variety of reasons, but no study has assessed plasma lidocaine concentrations during a 12-h infusion to horses. The purpose of this study was to evaluate the plasma concentrations and pharmacokinetics of lidocaine during a 12-h infusion to postoperative horses. A second purpose of the study was to evaluate the in vitro plasma protein binding of lidocaine in equine plasma. Lidocaine hydrochloride was administered as a loading dose, 1.3 mg/kg over 15 min, then by a constant rate IV infusion, 50 microg/kg/min to six postoperative horses. Lidocaine plasma concentrations were measured by a validated high-pressure liquid chromatography method. One horse experienced tremors and collapsed 5.5 h into the study. The range of plasma concentrations during the infusion was 1.21-3.13 microg/mL. Lidocaine plasma concentrations were significantly increased at 0.5, 4, 6, 8, 10 and 12 h compared with 1, 2 and 3 h. The in vitro protein binding of lidocaine in equine plasma at 2 microg/mL was 53.06+/-10.28% and decreased to 27.33+/-9.72% and 29.52+/-6.44% when in combination with ceftiofur or the combination of ceftiofur and flunixin, respectively. In conclusion, a lower lidocaine infusion rate may need to be administered to horses on long-term lidocaine infusions. The in vitro protein binding of lidocaine is moderate in equine plasma, but highly protein bound drugs may displace lidocaine increasing unbound concentrations and the risk of lidocaine toxicity.