Ultrasound reduces the minimum effective local anaesthetic volume compared with peripheral nerve stimulation for interscalene block

Previous studies have demonstrated that lower local anaesthetic (LA) volumes can be used for ultrasound (US)-guided interscalene brachial plexus block (ISB). However, no study has examined whether US can reduce the volume required when compared with nerve stimulation (NS) for ISB. Our aim was to do this by comparing the minimum effective analgesic volumes (MEAVs). After ethics approval and informed consent, patients undergoing shoulder surgery were recruited to this randomized, double-blind, up–down sequential allocation study. The volume used for both US and NS was dependent upon the success or failure of the previous block. Success was defined as a verbal rating score of 0/10, 30 min after surgery. Ten needle passes were allowed before defaulting to the opposite group. Patients received general anaesthesia. Pain scores and analgesic consumption were assessed by a blinded observer. Statistical comparisons of continuous variables were performed using Student’s t-test and Mann–Whitney U-test as appropriate. Categorical variables were analysed using χ2 test. MEAV values were estimated using log-transformed up–down independent pairs analysis and probit regression. Significance was assumed at P