• A Casati, M Baciarello, S Di Cianni, G Danelli, G De Marco, S Leone, M Rossi, and G Fanelli.
• Department of Anaesthesia and Pain Therapy, University of Parma, Ospedale Maggiore di Parma, via Gramsci 14, 43100 Parma, Italy.
• Br J Anaesth. 2007 Jun 1;98(6):823-7.

BackgroundWe tested the hypothesis that ultrasound guidance may reduce the minimum effective anaesthetic volume (MEAV50) of ropivacaine 0.5% required to block the femoral nerve compared with nerve stimulation guidance.MethodsAfter standard premedication and sciatic nerve block were given, 60 patients undergoing knee arthroscopy were randomly allocated to receive a femoral nerve block with ropivacaine 0.5% using either nerve stimulation (group NS, n = 30) or ultrasound (group US, n = 30) guidance. The volume of the injected solution was varied for consecutive patients based on an up-and-down staircase method according to the response of the previous patient. The initial volume was 12 ml. A double-blinded observer evaluated the occurrence of complete loss of pinprick sensation in the femoral nerve distribution, with concomitant block of the quadriceps muscle: positive or negative responses within 30 min after the injection determined a 3 ml decrease or increase for the next patient, respectively.ResultsThe 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, 19-33 ml) in group NS (P = 0.002). The effective dose in 95% of cases (ED95) calculated with probit transformation and logistic regression analysis was 22 ml (95% CI, 13-36 ml) in group US, and 41 ml (95% CI, fs 24-66 ml) in group NS.ConclusionsUltrasound guidance provided a 42% reduction in the MEAV of ropivacaine 0.5% required to block the femoral nerve as compared with the nerve stimulation guidance.

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