Objective - To compare the disposition of lidocaine administered IV in awake and anesthetized horses. Animals - 16 horses. Procedure - After instrumentation and collection of baseline data, lidocaine (loading infusion, 1.3 mg/kg administered during 15 minutes (87 μg/kg/min); constant rate infusion, 50 μg/kg/min) was administered IV to awake or anesthetized horses for a total of 105 minutes. Blood samples were collected at fixed times during the loading and maintenance infusion periods and after the infusion period for analysis of serum lidocaine concentrations by use of liquid chromatography with mass spectral detection. Selected cardiopulmonary parameters including heart rate (HR), mean arterial pressure (MAP), arterial pH, PaCO2, and PaO2 were also recorded at fixed time points during lidocaine administration. Serum lidocaine concentrations were evaluated by use of standard noncompartmental analysis. Results - Serum lidocaine concentrations were higher in anesthetized than awake horses at all time points during lidocaine administration. Serum lidocaine concentrations reached peak values during the loading infusion in both groups (1,849 ± 385 ng/mL and 3,348 ± 602 ng/mL in awake and anesthetized horses, respectively). Most lidocaine pharmacokinetic variables also differed between groups. Differences in cardiopulmonary variables were predictable; for example, HR and MAP were lower and PaO2 was higher in anesthetized than awake horses but within reference ranges reported for horses under similar conditions. Conclusions and clinical relevance - Anesthesia has an influence on the disposition of lidocaine in horses, and a change in dosing during anesthesia should be considered.
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