Minimum effective volume of ropivacaine 7.5mg/ml for an ultrasound-guided infraclavicular brachial plexus block

Background Ultrasound guidance has been shown to reduce the minimum effective volume (MEV) of local anaesthetics for several peripheral nerve blocks. Although the lateral sagittal infraclavicular block (LSIB) is a well-established anaesthesia method, MEV for this technique has not been established. Our aim with this study was to determine the MEV using ropivacaine 7.5mg/ml for the LSIB method. Methods Twenty-five adult American Society of Anesthesiologists physical status I-II patients scheduled for hand surgery received an ultrasound-guided LSIB with ropivacaine 7.5mg/ml. A successful block was defined as anaesthesia or analgesia for all five sensory nerves distal to the elbow, 30min after local anaesthetic injection. The MEV for a successful block in 50% of the patients was determined by using the staircase up-and-down method introduced by Dixon and Massey. Logistic regression and probit transformation were applied to estimate the MEV for a successful block in 95% of the patients. Results The patients received ropivacaine 7.5mg/ml volumes in the range of 12.530ml. The MEVs in 50% and 95% of the patients were 19ml [95% confidence interval (CI), 1427] and 31ml (95% CI, 1845), respectively. Conclusions For surgery distal to the elbow, the MEV in 95% of patients for an ultrasound-guided LSIB with ropivacaine 7.5mg/ml was estimated to be 31ml (95% CI, 1845ml). Further studies should determine the factors that influence the volume of local anaesthetic required for a successful infraclavicular block.