The relative motor blocking potencies of intrathecal ropivacaine: effects of concentration

This study established the median effective dose (ED(50)) for motor block of intrathecal 1% and 0.1% ropivacaine and determined the effects of the concentration of the solution injected on the motor block obtained. We enrolled into this prospective, randomized, double-blind, sequential allocation study 54 parturients undergoing elective Cesarean delivery under combined spinal-epidural technique. Parturients were randomized to receive intrathecal ropivacaine either 1% or 0.1%. The initial dose was chosen to be 4 mg, with subsequent doses being determined by the response of the previous patient (testing interval, 1 mg). The occurrence of any motor block in either lower limb within 5 min from the intrathecal injection of the study solution was considered effective. The motor block at 5 min was 6.1 mg for 1% ropivacaine (95% confidence interval [CI], 5.1-7.1) and was 9.1 mg (95% CI, 7.8-10.3) for 0.1% ropivacaine (P = 0.0013; 95% CI difference, 1.3-4.7). The relative efficacy ratio of the 2 concentrations was 1.5 (95% CI difference, 1.2-1.9) in favor of the larger concentration. The ED(50) of spinal ropivacaine to produce motor block in pregnant patients was significantly influenced by the concentration of the local anesthetic, with dose requirements being increased by 50% for the smaller concentration.

The minimum local anesthetic dose for motor block with 0.1% ropivacaine is 50% larger than the 1% concentration with a relative efficacy ratio of 1.5. Our findings suggest that more diluted local anesthetic solutions determine less motor block, and this may be considered in ambulant laboring parturients.