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Please confirm that an ethics committee approval has been applied for or granted: Yes: I’m uploading the Ethics Committee Approval as a PDF file with this abstract submissionApplication for ESRA Abstract PrizesBackground and Aims Superior trunk block (STB) has emerged as a potential alternative to the interscalene block for reducing the risk of hemidiaphragmatic paralysis (HDP) while maintaining effective analgesia. However, the optimal dose of local anesthetic for STB remains unclear. This dose-finding study aimed to determine the maximum effective volume of 0.25% ropivacaine required to prevent HDP in 90% of pre-operative patients (MEV90) undergoing shoulder arthroscopy with ultrasound-guided STB.Methods The Institutional Ethics Committee of Changzheng Hospital approved this prospective trial (CZEC2022SL05) prior to patient enrollment. A biased coin design was used for volume assignment. The local anesthetic volume administered to each healthy volunteer was determined based on the response of the previous participant, except for the first patient. In cases 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 cases of ‘success’ (absence of HDP), the next patient was randomized to receive either a higher volume (2 mL more) with a probability of 0.11, or the same volume with a probability of 0.89.Results A total of 52 pre-operative patients undergoing shoulder arthroscopy were enrolled. Using isotonic regression and bootstrap confidence intervals (CIs), the MEV90 for ultrasound-guided STB was estimated to be 7.6 mL (95% CI: 6.5–8.5 mL).Abstract P333 Figure 1Flow chart of the studyAbstract P333 Figure 2Superior trunk blockAbstract P333 Figure 3Diaphragmatic excursion at 3 hours laterConclusions For ultrasound-guided superior trunk block, the MEV90 of 0.25% ropivacaine required to avoid hemidiaphragmatic paralysis is estimated at 7.6 mL.