Respiratory impact of local anaesthetic volume after an interscalene brachial plexus block with an extrafascial injection: a randomised controlled double-blinded trial.

Background: We have previously demonstrated that an extrafascial injection of 20 ml of local anaesthetic for interscalene brachial plexus block (ISB) 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 vs a volume of 20 ml, while providing similar analgesia.

Methods: Sixty ASA physical status 1-3 patients scheduled for elective shoulder surgery under general anaesthesia were randomised to receive ultrasound-guided extrafascial ISB 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 min after the procedure. Secondary outcomes included duration of analgesia and i.v. morphine consumption at 24 h after surgery.

Results: The 30-min 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). Participants in the low-volume vs control group had a shorter duration of analgesia (550 vs 873 min; P<0.01) and higher i.v. morphine consumption (20 vs 12 mg; P=0.03).

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.

Clinical Trial Registration: NCT04726280.