Background <p>Compared to interscalene block (ISB), the superior trunk block (STB) lowers the incidence of hemidiaphragmatic paralysis (HDP) without compromising analgesic efficacy, making it a valuable alternative. However, the lack of consensus on the optimal local anesthetic (LA) dose for STB limits its standardized clinical application. This study employed a dose-finding protocol to determine the maximum effective volume of 0.25% ropivacaine required to prevent HDP in 90% of patients (MEV90) undergoing shoulder arthroscopy with ultrasound-guided STB.</p> Methods <p>A biased coin design was used for volume assignment. All patients except the first one received a LA volume determined by the response observed in the immediately preceding participant. In case of “failure” (defined as the presence of HDP after STB), the subsequent patient received a lower volume (2 mL less than the previous dose). In case of “success” (absence of HDP), subsequent patients were randomly allocated to received either: (1) an increased volume (2&#xa0;ml higher than the previous dose) with an 11% probability, or (2) the same volume with an 89% probability.</p> Results <p>The study cohort comprised 52 patients indicated for shoulder arthroscopy finally. Using isotonic regression with bootstrapped confidence intervals (CIs), the MEV90 of 0.25% ropivacaine for ultrasound-guided STB was estimated at 7.6 mL (95% CI: 6.5–8.5 mL).</p> Conclusions <p>The MEV90 of 0.25% ropivacaine for ultrasound-guided superior trunk block to prevent hemidiaphragmatic paralysis was 7.6 mL.</p> Trial registration <p>Chinese Clinical Trial Registry (ChiCTR2200059042); first registered 23/04/2022.</p>