RESUMO
OBJECTIVE: To assess the effect of two intravenous (IV) doses of lidocaine on the minimum anesthetic concentration (MAC) of isoflurane in chickens. STUDY DESIGN: Blinded, prospective, randomized, experimental crossover study. ANIMALS: A total of six adult female chickens weighing 1.90 ± 0.15 kg. METHODS: Chickens were anesthetized with isoflurane and mechanically ventilated. Isoflurane MAC values were determined (T0) in duplicate using an electrical noxious stimulus and the bracketing method. After MAC determination, a low dose (LD; 3 mg kg-1 followed by 3 mg kg-1 hour-1) or high dose (HD; 6 mg kg-1 followed by 6 mg kg-1 hour-1) of lidocaine was administered IV. MAC determination was repeated at 1.5 (T1.5) and 3 (T3) hours of lidocaine administration and blood was collected for analysis of plasma lidocaine and monoethylglycinexylidide (MEGX) concentrations. Pulse rate, peripheral hemoglobin oxygen saturation, noninvasive systolic arterial pressure and cloacal temperature were recorded at T0, T1.5 and T3. Treatments were separated by 1 week. Data were analyzed using mixed-effects model for repeated measures. RESULTS: MAC of isoflurane (mean ± standard deviation) at T0 was 1.47 ± 0.18%. MAC at T1.5 and T3 was 1.32 ± 0.27% and 1.26 ± 0.09% (treatment LD); and 1.28 ± 0.06% and 1.30 ± 0.06% (treatment HD). There were no significant differences between treatments or times. Maximum plasma lidocaine concentrations at T3 were 496 ± 98 and 1200 ± 286 ng mL-1 for treatments LD and HD, respectively, and were not significantly different from T1.5. With treatment HD, plasma concentration of MEGX was significantly higher at T3 than at T1.5. Physiological variables were not significantly different among times with either treatment. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of lidocaine did not significantly change isoflurane MAC in chickens. Within treatments, plasma lidocaine concentrations were not significantly different at 1.5 and 3 hours.