Effects of intravenous administration of lidocaine on the thermal threshold in cats

Objective - To determine the effects of IV administration of lidocaine on thermal antinociception in conscious cats. Animals - 6 cats. Procedure - 2 experiments were performed in each cat (interval of at least 2 months). In experiment 1, lidocaine pharmacokinetics were determined for each conscious cat following IV administration of a bolus of lidocaine (2 mg/kg). In experiment 2, data from experiment 1 were used to calculate appropriate doses of lidocaine that would achieve predetermined plasma lidocaine concentrations in the cats; lidocaine (or an equivalent volume of saline [0.9% NaCl] solution as the control treatment) was administered IV to target pseudo-steady-state plasma concentrations of 0, 0.5, 1, 2, 5, and 8 μg/mL. Skin temperature and thermal threshold were determined at the start of the experiment (baseline) and at each concentration. Samples of venous blood were obtained at each target concentration for plasma lidocaine concentration determination. Results - In experiment 2, actual plasma lidocaine concentrations were 0.00 ± 0.00 μg/mL, 0.25 ± 0.18 μg/mL, 0.57 ± 0.20 μg/mL, 1.39 ± 0.13 μg/mL, 2.33 ± 0.45 μg/mL, and 4.32 ± 0.66 μg/mL for target plasma concentrations of 0, 0.5, 1, 2, 5, and 8 μg/mL, respectively. Compared with baseline values, no significant change in skin temperature or thermal threshold was detected at any lidocaine plasma concentration (or saline solution equivalent). Skin temperature or thermal threshold values did not differ between lidocaine or control treatments. Conclusions and clinical relevance - Results indicated that these moderate plasma concentrations of lidocaine did not affect thermal antinociception in cats.