Antinociceptive, cardiopulmonary, and sedative effects of five intravenous infusion rates of lidocaine in conscious dogs.

Objective: To observe antinociceptive, cardiopulmonary and sedative effects of five different 12-hour lidocaine infusions in conscious dogs, and measure plasma lidocaine concentrations.

Study Design: Two-part randomized, prospective, blinded, cross-over experimental study.

Animals: Six neutered male, crossbred dogs approximately 1-2 years of age and weighing 29.1 +/- 4.0 kg.

Methods: Dogs received lidocaine [2 mg kg(-1), intravenous (i.v.)] or equivalent volume of 0.9% saline followed by infusion of either lidocaine at 10 (L10), 25 (L25), 50 (L50), 75 (L75), or 100 (L100) microg kg(-1) minute(-1), or equivalent rate of saline (Control). The study was conducted in two parts comparing L10, L25 and L50 to control, and then L75 and L100 to another control. Heart and respiratory rates, and indirect arterial blood pressure were measured for 12 hours; before (baseline), during and after infusion. Sedation was scored using descriptive categories, and nociceptive threshold determined using electrical cutaneous stimulation. Plasma lidocaine concentrations were measured using ELISA. Nonparametric and parametric tests for repeated measures were used and p < 0.05.

Results: Nociceptive thresholds were not different from the saline control treatment at any time. Respiratory rate decreased 2-12 hours in treatments L25, L75 and L100. Blood pressure increased after 4 hours in treatment L100 compared to baseline. Sedation scores increased compared to baseline (L10: 30 minutes-8 hours; L25: 30 minutes-2 hours, 8 hours; L50: 30 minutes, 8 hours, 12 hours; L75: 4-12 hours; L100: 15 minutes, 8-12 hours), and to Control. Treatment L75 had higher plasma lidocaine concentrations than L10; and L100 had higher concentrations than L10 and L25. Occasional vomiting was observed in dogs receiving lidocaine when plasma lidocaine concentrations exceeded 4 microg mL(-1).

Conclusions And Clinical Relevance: High lidocaine infusion rates did not have antinociceptive effects compared with saline and were associated with mild-moderate sedation and some signs of toxicity in awake dogs.