Background and Objectives This study was initiated to evaluate the antinociceptive and motor blocking capabilities of epidurally administered 0.5% and 0.75% ropivacaine and bupivacaine using a blinded, random crossover design in the dog. Additionally, serum drug concentrations and serum protein binding were determined.

Methods Twelve male beagles were prepared with chronic epidural and arterial catheters under sterile conditions. The epidural space was identified by a loss-of-resistance technique using an 18-gauge thin-wall Crawford needle via the midline approach. The catheter was attached to a Schraeder-type Teflon valve sutured subcutaneously. Local anesthetic drugs were administered into the lumbar epidural space in a constant volume of 3.0 mL. Evaluation of antinociceptive and motor blocking qualities were evaluated at regular intervals. Arterial blood samples were drawn during the 15 minutes following local anesthetic injection for drug concentration and pharmacokinetic analysis.

Results Onset time of motor block in the ropivacaine 0.5% group was longer than in other groups but not significantly different. Duration of motor and sensory block demonstrated a dose-dependent relationship of both drugs. Bupivacaine 0.5% and 0.75% produced motor block (81 ± 42 minutes and 198 ± 44 minutes [mean ± SD], respectively) of significantly longer duration than corresponding concentrations of ropivacaine (69 ± 35 minutes and 133 ± 32 minutes, respectively). The onset times for sensory block of the vertebral dermatomes were not different between groups. No difference was found in duration of dermatomal sensory block between the 0.5% solutions. However, the duration of dermatomal sensory block was significantly longer in the bupivacaine 0.75% group than in any other group. Peak arterial serum drug concentrations were not different between drugs at equal concentrations and were reached between 2 and 6 minutes in all animals. Both drugs were highly bound to serum proteins (>98%). No severe adverse effects or sequelae were observed.

Conclusions The 0.5% solutions produced similar sensory block of the vertebral dermatomes. Duration of dermatomal block with 0.75% bupivacaine was longer than with the corresponding ropivacaine concentration. Ropivacaine produced motor block of shorter duration as compared with bupivacaine. Serum concentrations of the two drugs were similar after injection of the same doses. In this animal model, ropivacaine produced shorter durations of sensory and motor block than corresponding concentrations of bupivacaine. These data are consistent with previously published data in animals and humans.