Morphine or lidocaine infusion as a pre-emptive analgesic for intraocular surgery in dogs

Abstract

Systemic administration of lidocaine has been shown to provide analgesia in acute and chronic pain models in rodents. Pre-emptive administration of lidocaine increases pain threshold following sciatic ligation in rats even when plasma concentrations of lidocaine are undetectable. This study investigated the postoperative analgesic effect of lidocaine administered intraoperatively to normal dogs undergoing OD phacoemulsification and lens extraction. Twelve healthy beagle or spaniel dogs (15.5 ± 1.7 kg; 2.5 ± 0.6 years) of either sex were premedicated with acepromazine (0.05 mg kg−1 IM), induced with propofol (4–6 mg kg−1 IV) and maintained on isoflurane in oxygen with IPPV. Treatment groups (n = 4) were: lidocaine (L), 1 mg kg−1 bolus followed by constant rate infusion (CRI) at 0.025 mg kg−1 minute−1; morphine (M), 0.15 mg kg−1 bolus followed by CRI at 0.1 mg kg−1 h−1; and saline (S), 0.05 mL kg−1 bolus followed by CRI at 0.075 mL kg−1 h−1. Drug concentrations were adjusted such that equal volumes kg−1 were delivered regardless of treatment. HR, direct mean pressure (MAP), and end-tidal isoflurane concentration (ETISO) were recorded every 5 minutes following drug bolus. Fifteen minutes following drug bolus, atracurium (0.2 mg kg−1 IV) was administered and lens extraction surgery was begun. Beginning at extubation (t = 0), dogs were monitored for pain using a standardized subjective pain-scoring system by a blinded observer at t = 0, 0.5, 1, 2, 3, 4, 6, 8, 16 and 24 hours. If dogs exhibited mild–moderate discomfort, morphine (1.0 mg kg−1 IM) was given and ‘time to treatment failure’ recorded. Incidence of treatment failure was analyzed using Fisher's Exact test; time to treatment failure was analyzed using a Wilcoxon Rank Sum test. Differences between treatments and effects of treatment over time for HR, MAP, and ETISO were analyzed using a repeated-measures ANOVA. Incidence of treatment failure was 100% in the S group, and was significantly lower in the M and L groups (50%). Time to treatment failure (mean ± SD hours) was significantly lower in the S group (0.25 ± 0.14; p = 0.03) than in the M (1.0 ± 0.25) or L (1.75 ± 1.25) groups. There was no significant difference between treatments in HR, MAP or ETISO at any time point. Results of this study suggest that lidocaine infusion provides pre-emptive analgesia comparable to low dose morphine infusion, without adverse effects on HR or MAP, in healthy dogs. Further investigation into postoperative analgesic effects of systemic lidocaine with a larger number of dogs is warranted based on results of this pilot study.