A CHRONIC MODEL FOR EXPERIMENTAL INVESTIGATION OF EPIDURAL-ANESTHESIAIN THE RABBIT

Background and Objectives. A rabbit chronic model has been developed f or investigation of epidural anesthesia without surgery. Methods. A ca theter was implanted via a nontraumatic route in the lumbar epidural s pace between the apex coccis sacri and the first vertebra coccygea aft er a 1-cm incision was made in the skin at the root of the tail. The l igamentum was punctured to allow the catheter to slide gently 10 cm in to the epidural space and avoid subperiosteal soft tissue dissection. The motor block effects of epidural lidocaine and bupivacaine were stu died 1 week after implantation. Three injections of lidocaine 1% (grou p I) or 2% (group II) or bupivacaine 0.25% (group III) or 0.5% (group IV) were given at 48-hour intervals to four rabbits in each group. Inc reasing concentrations (0.5%, 1.0%, and 2.0%) of lidocaine and bupivac aine (0.125, 0.25, and 0.5%) were injected into eight rabbits in each of groups V and VI. The spinal cord and meninges were examined histopa thologically in eight other chronically implanted rabbits that receive d no drugs. Results. Catheters were kept in place during 2 months in 4 0 rabbits without neurologic disturbance, discomfort, infection or wei ght loss. Complete motor block was observed during a mean 27, 43, and 51 minutes with increasing doses of lidocaine. Administration of bupiv acaine 0.125% led to incomplete motor deficit in three rabbits, wherea s all other rabbits exhibited complete motor block, lasting a mean of 26, 53, and 93 minutes for increasing doses. Linear relationships were found between the concentration of each drug and the duration of moto r block (r = .902, P < .001 for lidocaine; and r = .857, P < .001 for bupivacaine). Repeated injections of local anesthetics produced consis tent durations of motor block (Bland and Altman test). No significant histologic changes were observed. Conclusion. This rabbit model appear s to be a suitable tool for evaluating motor block of epidural agents under standardized experimental conditions.