Effective volumes of 1.5% mepivacaine with different sodium concentration for ultrasound guided popliteal block.

STUDY OBJECTIVE: To determine if a solution of 1.5% mepivacaine diluted with 5% dextrose, which decreases the sodium concentration by 30%, results in reduced volume requirements for a complete sensory block, in the case of an ultrasound guided popliteal nerve block.

DESIGN: A randomized controlled study.

SETTING: Operating room.

PATIENTS: We included seventy ASA 1-3 patients, undergoing unilateral "hallux valgus" repair under ultrasound guided popliteal nerve block.

INTERVENTIONS: An ultrasound guided popliteal nerve block was performed on all patients, with 1.5% mepivacaine using the normal dilution (ND group, thirty-five patients) or the 5% dextrose dilution (D5 group, thirty-five patients). Starting with 25ml in each group, increasing or decreasing it by 1ml on subsequent patients, depending on the success or failure in the previous one (Dixon's "up-and-down" sequential allocation).

MEASUREMENTS: Effective dose in 50, 90, and 95% of patients (ED50, ED90, and ED95) of 1.5% mepivacaine in both groups. Onset time and duration of the blocks, side effects, and neurological complications.

MAIN RESULTS: There were no statistically significant differences between ED50 in ND group (6.2ml; 95% confidence interval, 5.2-7.5), and D5 group (5.8ml; 95% CI, 5.1-7). Also no statistically significant differences in ED90 (7.7ml, 95% CI 6.9-8.1 in the D5 group; 7.8ml, 95% CI 7-8.1 in the ND) or in ED95 (7.9ml, 95% CI 7.1-8.2 in the D5 group; 8ml, 95% CI 7.2-8.2 in the ND) were found. Onset time for a complete sensory block in D5 group was 14min (95% CI, 12-17) and 15min in ND (95% CI, 13-18), p=0.66. Neither severe side effects, nor neurological complications were reported.

CONCLUSIONS: A dilution of 1.5% mepivacaine with 30% less sodium concentration does not decrease volume requirement for ultrasound guided sciatic nerve block at popliteal level.