BACKGROUND: Ultrasound-guided supraclavicular block (UGSCB) can provide upper limb regional anaesthesia and is an emerging technique in the emergency department (ED). A potential adverse event is phrenic nerve block causing hemi-diaphragmatic paresis, but its incidence in ED remains unclear. METHODS: This nested prospective cohort study evaluated adult patients (≥ 18 years) without lung disease who underwent UGSCB for closed reduction of distal radius fractures. UGSCB was performed on the injured side using a two-injection method with 20 mL of 0.75% ropivacaine. The primary outcome was ipsilateral diaphragmatic excursion assessed using bedside ultrasound dynamic imaging before and 30 min post-UGSCB during three respiratory manoeuvres: tidal volume breathing (TV), deep inspiration (DI) and sniff inspiration (SI). Diaphragmatic paralysis was defined as ≥ 75% reduction in diaphragmatic excursion, averaged across the three manoeuvres. Secondary outcomes included vital signs including respiratory rate, oxygen saturations, blood pressure and heart rate. RESULTS: Sixteen participants were evaluated. UGSCB was associated with reduced ipsilateral diaphragmatic excursion for TV (mean difference [MD] 1.21 cm, 95% confidence interval [95% CI] 0.69-1.75), DI (MD 3.37, 95% CI 2.15 to 4.59) and SI manoeuvres (MD 1.95, 95% CI 1.23 to 2.68). Diaphragmatic paralysis was observed in 10 participants (62.5%). No significant changes in vital signs were observed following UGSCB. CONCLUSIONS: ED physician-performed UGSCB using a two-injection technique was commonly associated with hemi-diaphragmatic paresis, likely due to concomitant phrenic nerve block. However, this was not associated with respiratory or haemodynamic compromise in adult patients without lung disease.