Effects of ultrasound guidance on the minimum effective anaesthetic volume required to block the femoral nerve

Background: We tested the hypothesis that ultrasound guidance may reducethe minimum effective anaesthetic volume (MEAV₅₀) of ropivacaine0.5% required to block the femoral nerve compared with nervestimulation guidance. Methods: After standard premedication and sciatic nerve block were given,60 patients undergoing knee arthroscopy were randomly allocatedto receive a femoral nerve block with ropivacaine 0.5% usingeither nerve stimulation (group NS, n = 30) or ultrasound (groupUS, n = 30) guidance. The volume of the injected solution wasvaried for consecutive patients based on an up-and-down staircasemethod according to the response of the previous patient. Theinitial volume was 12 ml. A double-blinded observer evaluatedthe occurrence of complete loss of pinprick sensation in thefemoral nerve distribution, with concomitant block of the quadricepsmuscle: positive or negative responses within 30 min after theinjection determined a 3 ml decrease or increase for the nextpatient, respectively. Results: The mean (SD) MEAV₅₀ for femoral nerve block was 15 (4) ml (95%CI, 7–23 ml) in group US and 26 (4) ml (95% CI, 19–33ml) in group NS (P = 0.002). The effective dose in 95% of cases(ED₉₅) calculated with probit transformation and logistic regressionanalysis was 22 ml (95% CI, 13–36 ml) in group US, and41 ml (95% CI, fs 24–66 ml) in group NS. Conclusions: Ultrasound guidance provided a 42% reduction in the MEAV ofropivacaine 0.5% required to block the femoral nerve as comparedwith the nerve stimulation guidance.