ANTINOCICEPTIVE AND MOTOR-BLOCKING EFFICACY OF ROPIVACAINE AND BUPIVACAINE AFTER EPIDURAL ADMINISTRATION IN THE DOG

Background and Objectives. This study was initiated to evaluate the an tinociceptive and motor blocking capabilities of epidurally administer ed 0.5% and 0.75% ropivacaine and bupivacaine using a blinded, random crossover design in the dog. Additionally, serum drug concentrations a nd serum protein binding were determined. Methods. Twelve male beagles were prepared with chronic epidural and arterial catheters under ster ile conditions. The epidural space was identified by a loss-of resista nce technique using an 18-gauge thin-wall Crawford needle via the midl ine approach. The catheter was attached to a Schraeder-type Teflon val ve sutured subcutaneously. local anesthetic drugs were administered in to the lumbar epidural space in a constant volume of 3.0 mL. Evaluatio n of antinociceptive and motor blocking qualities were evaluated at re gular intervals. Arterial blood samples were drawn during the 15 minut es following local anesthetic injection for drug concentration and pha rmacokinetic analysis. Results. Onset time of motor block in the ropiv acaine 0.5% group was longer than in other groups but not significantl y different. Duration of motor and sensory block demonstrated a dose-d ependent relationship of both drugs. Bupivacaine 0.5% and 0.75% produc ed motor block (81 +/- 42 minutes and 198 +/- 44 minutes [mean +/- SD] , respectively) of significantly longer duration than corresponding co ncentrations of ropivacaine (69 +/- 35 minutes and 133 +/- 32 minutes, respectively). The onset times for sensory block of the vertebral der matomes were not different between groups. No difference was found in duration of dermatomal sensory block between the 0.5% solutions. Howev er, 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 co ncentrations and were reached between 2 and 6 minutes in all animals. Both drugs wee highly bound in serum proteins (>98%). No severe advers e effects or sequelae were observed. Conclusions. The 0.5% solutions p roduced similar sensory block of the vertebral dermatomes. Duration of dermatomal block with 0.75% bupivacaine was longer than with the corr esponding 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 t his animal model, ropivacaine produced shorter durations of sensory an d motor block than corresponding concentrations of bupivacaine. These data are consistent with previously published data in animals and huma ns.