Effect of Concentration on Median Effective Dose (ED50) for Motor Block of Intrathecal Plain Bupivacaine in Elderly Patients

BACKGROUND The aim of the study was to determine the median effective dose (ED50) for motor block of various concentrations of intrathecally administered plain bupivacaine. MATERIAL AND METHODS Between 2011 and 2013, 64 patients aged ≥70 years, undergoing transurethral, or lower limb surgery with combined spinal and epidural anesthesia in a single hospital were enrolled. The patients were randomized into 3 groups to receive intrathecal 0.75% bupivacaine (Group 1), 0.375% bupivacaine (Group 2) or 0.25% bupivacaine (Group 3). Spinal anesthesia was achieved using injections of up-and-down doses of 0.75%, 0.375%, or 0.25% plain bupivacaine. The first patient in each group received 7.5 mg bupivacaine, and the testing interval was set at 0.75 mg. The efficacy of motor block in both legs was determined using a modified Bromage and a hip motor function scale. The ED50 for motor block was estimated according to the Dixon's up-and-down method. RESULTS The ED50 for motor block of bupivacaine was 6.10 (95% CI 5.58-6.66) mg in Group 1, 6.04 (95% CI 5.82-6.28) mg in Group 2, and 5.43 (95% CI 5.19-5.67) mg in Group 3. There were significant differences in the ED50 for motor block among the groups (P=0.008). CONCLUSIONS The ED50 doses for motor block with 3 bupivacaine concentrations were significantly different in elderly patients; the ED50 dose of 0.75% bupivacaine being significantly higher than that of 0.25% bupivacaine.