Systemic lidocaine infusion as an analgesic for intraocular surgery in dogs: a pilot study

Objective To determine if systemic administration of lidocaine during intraocular surgery reduces postoperative ocular pain. Study design Randomized, masked, controlled experimental trial. Animals Twelve dogs weighing 15.5 +/- 1.7 kg (mean SD) and aged 2.5 +/- 0.6 years. Methods All dogs underwent a baseline ophthalmic examination and subjective pain score. Anesthesia consisted of acepromazine (0.1 mg kg(-1), IM), propofol (4-6 mg kg(-1), IV), and isoflurane in oxygen. There were three groups each receiving a bolus followed by an infusion (n = 4): saline (0.3 mL kg(-1) IV+ 0.2 mL kg(-1) hour(-1) IV): morphine (0.15 mg kg(-1) IV + 0.1 mg kg(-1) hour-(1) IV); and lidocaine (1.0 mg kg(-1) IV + 0.025 mg kg(-1) minute(-1) IV). All treatments began 15 minutes prior to starting of phacoemulsification and lens removal from the right eye. Pain scores were recorded at 0.5,1, 2, 3,4,6, 8,16, and 24 hours after t = 0 (extubation). Rescue morphine was administered (1.0 mg kg-1 IM) if the subjective pain score >9 (maximum = 24), and the dog was excluded from further data analysis. Differences in pain scores and time-to-treatment failure (TTF) were analyzed using the Wilcoxon's rank sum test. Differences in incidence of treatment failure were analyzed using Fisher's exact test. Physiologic data were analyzed using repeated measures ANOVA. Significance was defined as P < 0.05. Results Incidence of treatment failure was 100% in saline-treated dogs and 50% in morphine- or lidocaine-treated dogs. There was no difference in intraocular pressure, aqueous flare, cell count (or protein) between groups in the operated eye at any time following extubation. Conclusion and clinical relevance This pilot study suggests that intraoperative lidocaine may provide analgesic benefits similar to morphine for intraocular surgery in dogs, but more definitive research is needed. This model appears to be appropriate for pain assessment studies as the negative control group demonstrated 100% failure rate.