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Background and Aims We have previously demonstrated that an extrafascial injection of 20 mL of local anaesthetic for interscalene brachial plexus block reduces the rate of hemidiaphragmatic paralysis by 70% compared with an intrafascial injection, with similar efficacy. In this double-blind trial, we tested the hypothesis that a local anaesthetic volume of 10 mL injected extrafascially would reduce the rate of hemidiaphragmatic paralysis versus a volume of 20 mL, while providing similar analgesia.Methods Sixty ASA I-III patients scheduled for elective shoulder surgery under general anaesthesia were randomised to receive ultrasound-guided extrafascial interscalene brachial plexus block using ropivacaine 0.75% 20 mL (control group) or 10 mL (low volume group) injected lateral to the brachial plexus sheath. The primary outcome was incidence of hemidiaphragmatic paralysis (diaphragmatic excursion reduction of >75%), measured by M-mode ultrasonography, at 30 minutes after the procedure. Secondary outcomes included duration of analgesia and intravenous morphine consumption at 24 postoperative hours.Results The 30-minute hemidiaphragmatic paralysis rate was 80% (95% confidence interval [CI] 61–91%) in the control group and 19% (95% CI 8–40%) in the low volume group (p<0.001). Patients in the low volume versus control group had a shorter duration of analgesia (550 vs. 873 min; p<0.01) and higher intravenous morphine consumption (20 vs. 12 mg; p=0.03).Abstract EP003 Figure 1Kaplan-Meier curve showing the duration of analgesia based on the volume of local anaesthetic injected (20 mL [control] versus 10 mL [low volume])Conclusions A low volume of local anaesthetic injected extrafascially reduced the rate of hemidiaphragmatic paralysis, but at the expense of a shorter duration of analgesia compared with standard dose extrafascial anaesthetic injection.