Determining the Minimal Effective Volume (MEAV95) for Interscalene Brachial Plexus Block for Surgical Anesthesia (MEAV 95)

Determining the Minimal Effective Volume (MEAV95) for Interscalene Brachial Plexus Block for Surgical Anesthesia

Determining the Minimal Effective Volume (MEAV95) for Interscalene Brachial Plexus Block for Surgical Anesthesia

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.

to determine the MEAV95 of ropivacaine 0.75% at the level of root C5-6 using the dose-finding protocol. The success of surgical anesthesia is defined by the presence of sensory blockade (no sensation/score of 0 to pinprick testing) 30 minutes post injection of local anesthetic

The C5 myotome covers the deltoid muscle (the most powerful motion being shoulder abduction) and the biceps brachii muscle. Biceps brachii is also innervated by C6. Therefore, motor blockade will be assessed by testing shoulder abduction and forearm flexion. Motor function will be classified as present (any movement) or absent (no movement)

Bedside spirometry (FEV1, FVC, PEFR) and Pulse oximetry on air: at baseline, 30 minutes post block, 30min post-surgery, and 2 hours post surgery

Diaphragmatic excursion: ultrasound examination at baseline, 30 minutes post block, and 2 hours post surgery

Visual analog scale (VAS) - at baseline, after surgery at 30 and 60 minutes in the PACU, and at 1 hour intervals until discharge

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