The effects of ultrasound guidance and neurostimulation on the minimum effective anesthetic volume of mepivacaine 1.5% required to block the sciatic nerve using the subgluteal approach.

BACKGROUND: We tested the hypothesis that ultrasound (US) guidance may reduce the minimum effective anesthetic volume (MEAV(50)) of 1.5% mepivacaine required to block the sciatic nerve with a subgluteal approach compared with neurostimulation (NS).

METHODS: After premedication and single-injection femoral nerve block, 60 patients undergoing knee arthroscopy were randomly allocated to receive a sciatic nerve block with either NS (n = 30) or US (n = 30). In the US group, the sciatic nerve was localized between the ischial tuberosity and the greater trochanter. In the NS group, the appropriate muscular response (foot plantar flexion or inversion) was elicited (1.5 mA, 2 Hz, 0.1 ms) and maintained to
RESULTS: The mean MEAV(50) for sciatic nerve block was 12 mL (95% confidence interval [CI], 10-23 mL) in Group US and 19 mL (95% CI, 15-23 mL) in Group NS (P < 0.001). The effective dose in 95% of cases was 14 mL (95% CI, 12-17 mL) in Group US and 29 mL (95% CI, 25-40 mL) in Group NS (P = 0.008).

CONCLUSIONS: US provided a 37% reduction in the MEAV(50) of 1.5% mepivacaine required to block the sciatic nerve compared with NS.