The effect of age on the median effective dose (ED50) of intrathecally administered plain bupivacaine for motor block.

In this study, we sought to determine the median effective dose (ED50) for motor block of intrathecally administered plain bupivacaine in adults (20-80 years) and to assess the effect of age on ED50 required for motor block. This study was performed in 129 adult patients undergoing transurethral, urological, or lower limb surgery under combined spinal and epidural anesthesia. Patients were stratified according to age as follows: 20 to 30, 31 to 40, 41 to 50, 51 to 60, 61 to 70, and 71 to 80 years. The spinal component of the anesthetic was established by bolus administration of up-and-down doses of 0.75% plain bupivacaine, determined by Dixon's method. The degree of motor block after intrathecal administration of each dose was evaluated by the modified Bromage and hip motor function score. The ED50 values were estimated from the up-and-down sequences using the method of Dixon and Massey and logistic regression. Other end points were included on the basis of sensory block level, duration of motor blockade, hypotension, and vasopressor requirements. ED50 for motor block using intrathecal bupivacaine was 10.22 mg (95% confidence interval [CI], 9.96-10.49 mg) in 20- to 30-, 9.52 mg (95% CI, 9.02-10.07 mg) in 31- to 40-, 8.37 mg (95% CI, 7.56-9.26 mg) in 41- to 50-, 7.30 mg (95% CI, 6.84-7.79 mg) in 51 to 60, 6.55 mg (95% CI, 6.01-7.13 mg) in 61- to 70-, and 5.78 mg (95% CI, 5.01-6.67 mg) in 71- to 80-year-old patients. The maximum cephalic analgesic level was L1-L2 level at 5 minutes and T10-L1 at 10 minutes after administration of intrathecal plain bupivacaine in the 6 age groups. There was a significant difference in the duration of motor blockade among groups. The ED50 for motor block of intrathecally administered plain bupivacaine decreased steeply with advancing age.