The use of ultrasound in regional anesthesia enables reduction in the local anesthetic volume. The present study aimed to determine the minimum effective volume of 0.375% bupivacaine with epinephrine for interscalene brachial plexus block for shoulder surgery. Following approval by the Research Ethics Committee, patients with a physical condition of I or II according to the American Society of Anesthesiologists, between 21 and 65 years old and subjected to elective surgery of the shoulder and interscalene brachial plexus block will be recruited. The volume of the anesthetic will be determined using a step-up/step-down method and based on the outcome of the preceding block. Positive or negative block results in a 1mL reduction or increase in volume, respectively. The success of the block is defined as the presence of motor block in two muscle groups and the absence of thermal and pain sensations in the necessary dermatomes within 30 minutes of the injection. Diaphragmatic paralysis, pulmonary function and analgesia will be quantified at 30 minutes, 4 and 6 hours. Data will undergo statistical analysis in order to determine minimum effective volume of 0.375% bupivacaine and, secondarily, the maximum volume that maintains the diaphragmatic function, evaluate diaphragm paralysis and its influence in pulmonary function.

Inclusion Criteria:

• Age between 21 and 65 years
• Candidates for an elective surgical intervention on the shoulder with indication for anesthesia via brachial plexus block
• Physical condition of I or II according to the American Society of Anesthesiologists
• Body mass index up to 35 kg.m-2

Exclusion Criteria:

• Chronic obstructive pulmonary disease
• Cognitive impairment or active psychiatric condition
• Infection at the site of the puncture for the block
• Coagulopathy
• Bupivacaine allergy