The interscalene approach targets the brachial plexus at the level of the roots or trunks as they pass lateral to the great vessels and through the interscalene groove between the anterior and medial scalene muscles. The interscalene groove is located deep to the clavicular head of the sternocleidomastoid muscle and is bordered medially by the anterior scalene muscle and laterally by the middle scalene. Important landmarks that lie medial to the anterior scalene muscle include the carotid artery, internal jugular vein, and phrenic nerve. The dome of the pleura is located caudad and should not be considered at risk if the block is performed correctly at the level of the larynx.
Explore This IssueACEP News: Vol 31 – No 02 – February 2012
Occasionally, the interscalene block will fail to anesthetize the C8 dermatome that provides sensation to the fifth digit and distal medial forearm. This is because of a described anatomic variation in which the C8 nerve root lies deep to the C5-7 plexus and is separated by an additional fascial layer or muscle bridge that may limit the spread of local anesthetic.6 For emergency physicians who are using this block for proximal upper-extremity injuries, the lack of complete anesthesia of the C8 root is not clinically significant. For patients who require complete anesthesia below the elbow, a supraclavicular or infraclavicular approach may be more desirable.
The Procedure: Ultrasound-Guided Interscalene Brachial Plexus Nerve Blocks Patient Positioning and Probe Selection
The patient is placed on a cardiac monitor and positioned supine in a semireclining position with the head rotated 30 degrees away from the side of the injury. The ultrasound screen and cardiac monitor should be directly opposite the patient so that the physician can easily view the display screen while performing the block, permitting the visualization of the neck and ultrasound screen in the same visual axis (Fig. 1). A high-frequency linear transducer probe with a large footprint is ideal for the procedure. Always clean the ultrasound probe in a standard fashion and place a clear adherent dressing over the probe (Fig. 2), using sterile surgical lubricant as a coupling agent.
Choice of Local Anesthetic
Lidocaine 1%-2% drawn into a 20- to 30-mL syringe is recommended. All providers using nerve blocks should be familiar with the standard recommended dosages and clinical signs of toxicity, and have established protocols for treatment of local anesthetic toxicity that incorporate use of lipid infusions. For novice users, lidocaine rather than bupivacaine is recommended because of its better safety profile in cases of inadvertent vascular injection. Using epinephrine-containing anesthetics can prolong the duration of the block, and also provides an additional safety benefit: Sudden tachycardia, hypertension, and characteristic electrocardiographic T-wave morphology changes indicate accidental intravascular injection, alerting providers before large volumes of local anesthetics have been injected. A standard 21- to 23-gauge, 1.5-inch needle provides adequate visualization without significant tissue trauma but may not have sufficient length to reach the interscalene groove. For this reason, a 3.5-inch spinal needle is recommended in cases in which the depth of the relevant structures appears significantly deep to the skin surface as it interfaces with the ultrasound probe (Fig. 3).
Place a high-frequency linear transducer in a transverse orientation (probe marker facing the right of the patient) at the level of the larynx, identifying the internal jugular vein (IJV) and carotid artery. Clinicians comfortable with ultrasound-guided cannulation of the IJV should be able to locate the vascular anatomy of the neck at this level. From this position, slowly move the probe laterally past the great vessels until the border of the clavicular head of the sternocleidomastoid muscle (SCM) comes into view midscreen (Fig. 4).