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

Peripheral nerve block and local anaesthetic dose, how much is enough?

“…It is probably too soon to provide an answer to the question: 'how much is enough?' Both the reports in this issue of Anaesthesia [18,19] confirm that a successful block can be achieved with comparatively small doses of local anaesthetic in patients undergoing major surgery, albeit in injectate volumes that are large in relation to the surface area of the target nerve. Undoubtedly, additional work will follow to aid clinicians in protecting patients from the unwanted effects of local anaesthetics.…”

“…In this issue of Anaesthesia, the topic of minimal effective dose and ultrasound-guided femoral nerve block is further explored. Taha and Abd-Elmaksoud [18] report a study to identify the 90% effective concentration of 15 ml ropivacaine injectate in ultrasound-guided femoral nerve block for major knee surgery. This study is impressive in that it reports the results of blocks on patients undergoing major knee joint surgery using exclusively regional anaesthesia.…”

“…Watson et al [19] employed this strategy to measure the ED 50 of levobupivacaine, using probit regression to calculate the ED 95 . Taha and Abd-Elmaksoud [18], in contrast, employed a probability-based sequential dosing strategy. Using this methodology, a successful block resulted in subsequent patients' receiving either: (i) the same ropivacaine concentration, the probability of which was 0.89; or (ii) a concentration of 0.02% w/v less, with a probability of 0.11.…”

“…While this adds to the complexity of the study design, it allows the direct calculation of the local anaesthetic's ED 90 , which is arguably a clinically more relevant estimate of minimal effective dose than ED 50 . Of note, Taha and Abd-Elmaksoud's study gives us a figure of 25 mg for the ED 90 of ropivacaine (calculated from the measured minimal effective anaesthetic concentration 90 with a fixed 15-ml injection) [18], considerably less than Casati et al's figure for ED 95 of 75 mg (derived from measurement of minimal effective anaesthetic volume 50 with a 0.5% solution) [17]. This may be accounted for by differences in both the study methodologies and, perhaps importantly, the peripheral nerve block techniques used.…”

“…In the reports by Taha and Abd-Elmaksoud [18] and Watson et al [19], the technical proficiency of the regional anaesthetist in both ultrasonography and peripheral nerve block may have directly influenced the dose of local anaesthetic required to achieve ultrasound guided femoral nerve. Expert practitioners will confidently identify target nerves, place needles in an appropriate place and ensure appropriate spread of local anaesthetic around the nerve.…”