Determination of the median effective dose for motor block of intrathecally administered different concentrations of bupivacaine in younger patients. The influence of solution concentration

OBJECTIVE: To determine the median effective dose (ED50) for motor block of intrathecally administered different concentrations of bupivacaine in younger patients.

METHODS: This study was conducted at the Department of Anesthesiology, The First College of Clinical Medical Science, China Three Gorges University, China, on 40 American Society of Anaesthesiologists (ASA) I-II younger patients (18-40 years) undergoing lower limb surgery. These patients were classified into 2 groups according to a computer-generated random number table (Group A - 0.375% bupivacaine, and Group B - 0.75% bupivacaine). Spinal anesthesia was established by bolus administration of various up-and-down doses of 0.375%, or 0.75% plain bupivacaine. These up-and-down doses were determined by Dixon's up-and-down method. The first patient of each group was given 7.5 mg bupivacaine, and the testing interval was set at 0.75 mg. The dose was up, or down, 0.75 mg increments according to the failure or success of the preceding patient's motor block. The degree of motor block after intrathecal administration of bupivacaine was evaluated by the modified Bromage and Hip motor function score. The ED50 were estimated from the up-down sequences using the method of Dixon and Massey.

RESULTS: Our results showed that ED50 for motor block of intrathecal bupivacaine was 8.890 mg in Group A, and 9.998 mg in Group B, and the relative motor blocking potency ratio was 1.12.

CONCLUSION: The ED50 for motor block of intrathecally administered bupivacaine with higher concentration was slightly higher than that of a lower concentration.