This study aims to determine the minimum effective volume (MEV90) of local anesthetic required to achieve sensory block of the supraclavicular nerves when performing a single-shot ultrasound-guided interscalene brachial plexus block (ISB). These nerves provide sensation to the skin over the clavicle and shoulder and are often involved in surgeries such as shoulder arthroscopy and clavicle repair.

This prospective dose-finding study seeks to determine the minimum effective volume (MEV90) of local anesthetic required to achieve sensory block of the supraclavicular nerves following a single-shot ultrasound-guided interscalene brachial plexus block (ISB). These nerves, branches of the superficial cervical plexus, provide cutaneous innervation over the clavicle and shoulder and may contribute to postoperative pain after shoulder and clavicular surgery. Establishing MEV90 is clinically relevant to optimize analgesia while minimizing the risk of local anesthetic systemic toxicity.

The study employs a biased coin up-and-down sequential design, commonly used in regional anesthesia dose-finding trials. The initial volume is 12 mL of bupivacaine 0.5% with epinephrine (5 µg/mL), with subsequent volumes adjusted in 2 mL increments based on the prior patient's response. The maximum permitted volume is 30 mL.

All blocks are performed under ultrasound guidance by experienced anesthesiologists (≥60 ISBs), following standard safety protocols.

This intervention does not introduce additional risk beyond routine care, as ISB is the gold standard for shoulder surgery analgesia. Results will inform whether a single interscalene injection provides adequate coverage of supraclavicular nerves or if a separate cervical plexus block is necessary for procedures involving both plexuses, such as clavicle surgery. Findings may refine volume recommendations, improve analgesic strategies, and reduce unnecessary drug exposure.

Inclusion Criteria:

• Age ≥ 18 years
• ASA physical status I to III
• Body Mass Index (BMI) between 18 and 35 kg/m²
• Weight ≥ 50 kg
• Scheduled for shoulder arthroscopy requiring an interscalene brachial plexus block for analgesia
• Able to provide informed consent

Exclusion Criteria:

• Inability to provide informed consent
• Coagulopathy (platelet count ≤100,000; INR ≥1.4)
• Significant pulmonary disease (restrictive or obstructive)
• Renal insufficiency (creatinine ≥1.2 mg/dL)
• Hepatic insufficiency (transaminases ≥100 U/L)
• Allergy to local anesthetics
• Pregnancy
• Pre-existing neuropathy
• Previous surgery in ipsilateral cervical or supraclavicular region
• Contralateral hemidiaphragmatic paralysis or vocal cord paralysis