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Background Diaphragmatic paresis is a known complication of the interscalene block used for postoperative analgesia in shoulder surgery. A technique involving the injection of normal saline through the interscalene catheter to alleviate this condition has shown promise. This method, termed the “washing-off” technique, dilutes the local anesthetic around the phrenic nerve, mitigating respiratory symptoms.Case presentation A 65-year-old male patient with multiple comorbidities (American Society of Anesthesiologists physical status classification 4) was scheduled for arteriovenous brachiocephalic fistula creation under regional anesthesia. Following an interscalene block with 32 mL of mepivacaine 1.5%, the patient experienced acute respiratory distress, with SpO 2 at 88% despite 6 L O2 via nasal cannula. To avoid intubation, a 20 mL normal saline injection was administered through single-shot interscalene injection under ultrasound guidance. Within 5 min, respiratory distress markedly improved, allowing the patient to converse. Surgery proceeded without complications, maintaining SpO2 at 99% with 6 L O2. Postoperatively, the patient remained stable, with SpO2 at 98% on 2 L O2, and was discharged from the recovery room without additional oxygen requirements.Conclusions The “washing-off” technique’s mechanism may involve dilutional effects, pH changes or local sodium concentration alterations affecting the phrenic nerve. This case demonstrates its effectiveness in an acute setting, enabling surgery under regional anesthesia without intubation or any additional analgesia. The previously considered placebo effect appears unlikely here.