Ropivacaine in ultrasound-guided femoral nerve block: what is the minimal effective anaesthetic concentration (EC90)?

The objective of this study was to estimate the minimal effective anaesthetic concentrations of ropivacaine required to block the femoral nerve in 90% of patients. Forty-five patients who had knee surgery received ultrasound-guided femoral nerve block using 15ml ropivacaine. The ropivacaine concentration given to a patient relied on the efficacy of the block in the previous patient, using the biased-coin design up-down sequential method. In the event of successful block, the next patient was randomly assigned to receive either the same ropivacaine concentration or a concentration 0.02% w/v less. In the event of a failed block, the next patient received a concentration 0.02% w/v higher. Successful block was defined as complete sensory and motor block before surgery together with pain-free surgery. The minimal effective ropivacaine concentration was estimated to be 0.167% w/v (95% CI 0.14-0.184%). Perineural injection of 15ml ropivacaine 0.167% w/v under ultrasound guidance can provide successful femoral nerve block in 90% of patients.