The purpose of this study is to determine the minimum effective volume of local anesthetic for interscalene brachial plexus block using ultrasound guidance and nerve stimulator guidance.

This will be a single group assignment, double-blind, up-down sequential allocation study. Interscalene brachial plexus block (ISBPB) is a common nerve block given to patients who are undergoing shoulder surgery. This block has a low, but still significant, rate of complications. Reducing the volume of local anesthetic given during the block may allow some of these complications to be avoided. Participants will undergo ISBPB under ultrasound and nerve stimulator guidance. Patients will initially receive 15mL of local anesthetic (bupivacaine 0.5% with epinephrine). The volume used for each subsequent patient depends upon the success or failure of the previous patient's block - a failure will cause the volume given to increase by 1.0 mL, while a success will cause the volume to decrease by the same amount.

The goal of this study is to determine the minimum effective anesthetic volume of bupivacaine 0.5% providing anesthesia in patients when using ultrasound and nerve stimulator to guide placement of the block. The secondary outcome will be the evaluation of diaphragmatic function and post-surgery analgesia.

Inclusion Criteria:

• Patients receiving interscalene brachial plexus block for shoulder surgery
• Age between 21 and 65 years
• ASA physical status I/II
• Body mass index < 35 kg/m2

Exclusion Criteria:

• Chronic obstructive pulmonary disease
• Psychiatric history
• Allergy to bupivacaine
• Infection in the site of block
• Coagulation disorders