Interscalene block (ISB) consistently demonstrates superior pain relief after shoulder surgery. However, patients can experience complications such as blurred vision, hoarseness and shortness of breath with the standard volumes of local anesthetic used in contemporary practice. Patients with respiratory diseases are precluded because of the potential effects on respiration. However, they are then exposed to the risks of general anesthesia and opioid medications which can also have detrimental effects on respiration. Ultrasound guided ISB can reduce the local anesthetic volume required to produce anesthesia for shoulder surgery; this may reduce the complications from ISB to allow patients previously unable to benefit to have this technique, such as patients with obesity and respiratory disease. In order to do so the investigators will use an up-down methodology to find the minimum volume of local anesthetic to provide sufficient anesthesia for shoulder surgery.

Inclusion Criteria:

• Patients undergoing elective arthroscopic shoulder surgery in the sitting position
• Patients aged >=18 and <=75 years
• ASA I-III

Exclusion Criteria:

• Patient refusal for surgery under ISBPB with sedation
• Lack of capacity to provide informed consent
• Pre-existing COPD, unstable asthma or diaphragmatic dysfunction
• Coagulopathy
• Infection at injection site
• Allergy to local anesthetics
• Opioid tolerance (>30mg oral morphine or equivalent/day for >1 week)
• BMI>40
• Inability to complete questionnaires and assessments (consent, follow-up phone call) due to language barrier
• Pregnancy or lactation