The ultrasound-guided technique of interscalene brachial plexus block differs from nerve stimulator or landmark-based techniques in several important aspects. Most importantly, distribution of the local anesthetic is visualized to assure adequate spread around the brachial plexus. Ultrasound guidance allows multiple injections around the brachial plexus, therefore eliminating the reliance on a single large injection of local anesthetic for block success as is the case with non–ultrasound-guided techniques. Ability to inject multiple aliquots of local anesthetic also may allow for the reduction in the volume of local anesthetic required to accomplish the block. Repetition of the block in case of inadequate anesthesia is also possible, a management option that is unpredictable without ultrasound guidance. Finally, the risk of major vessel and nerve puncture during nerve block performance is reduced.
The brachial plexus at the interscalene level is seen lateral to the carotid artery, between the anterior and middle scalene muscles (Figures 2, 3, and 4). Prevertebral fascia, superficial cervical plexus and sternocleidomastoid muscle are seen superficial to the plexus. The transducer is moved in the superior-inferior direction until two or more of the brachial plexus trunks are seen in the space between the scalene muscles. Depending on the depth of field selected and the level at which the scanning is performed, first rib and/or apex of the lung may be seen. The brachial plexus is typically visualized at a depth of 1 to 3 cm.
The interscalene approach to brachial plexus blockade results in anesthesia of the shoulder and upper arm. Inferior trunk for more distal anesthesia can also be blocked by additional, selective injection, deeper in the plexus. this is accomplished either by controlled needle redirection inferiorly or by additional scanning to visualize the inferior trunk and another needle insertion and targeted injection.
• Ultrasound machine with linear transducer (8–14 MHz), sterile sleeve, and gel • Standard nerve block tray (described in the equipment section) • One 20-mL syringe containing local anesthetic • 5 -cm, 22-gauge short-bevel insulated stimulating needle • Peripheral nerve stimulator • Sterile gloves
Landmarks and Patient Positioning
Any position that allows comfortable placement of the ultrasound transducer and needle advancement is appropriate. The block is typically performed with the patient in supine, semisitting, or semilateral decubitus position, with the patient’s head facing away from the side to be blocked. The latter position may prove ergonomically more convenient, especially during an in-plane approach from the lateral side, in which the needle is entering the skin at the posterolateral aspect of the neck. A slight elevation of the head of the bed is often more comfortable for the patient, and it allows for better drainage and less prominence of the neck veins. Adherence to strict anatomic landmarks is of lesser importance for the ultrasound-guided interscalene block than it is the case for the surface anatomy-based techniques. Regardless, knowledge of the underlying anatomy and the position of the brachial plexus is important to facilitate recognition of the ultrasound anatomy. Scanning usually begins just below the level of the cricoid cartilage and medial to the sternocleidomastoid muscle with a goal to identify the carotid artery.
The goal is to place the needle in the tissue space between the anterior and middle scalene muscles and inject local anesthetic until the spread around the brachial plexus is documented by ultrasound. The volume of the local anesthetic and number of needle insertions are determined during the procedure and depend on the adequacy of the observed spread of the local anesthetic.
With the patient in the proper position, the skin is disinfected and the transducer is positioned in the transverse plane to identify the carotid artery (Figure 5). Once the artery is identified, the transducer is moved slightly laterally across the neck. The goal is to identify the scalene muscles and the brachial plexus that is sandwiched between the anterior and middle scalene muscles.
When the visualization of the brachial plexus between the scalene muscles proves difficult, the transducer is lowered to the supraclavicular fossa. At this position, the brachial plexus is identified lateral and superficial to the subclavian artery, (Figure 6). From here, the brachial plexus is traced cranially to the desired level.
The needle is then inserted in-plane toward the brachial plexus, typically in a lateral-to-medial direction (Figure 7 A), although medial-to-lateral needle orientation also can be chosen if more convenient. As the needle passes through the prevertebral fascia, a certain "give" is often appreciated (Figure 7 B). When nerve stimulation is used (0.5 mA, 0.1 msec), the entrance of the needle in the interscalene groove is often associated with a motor response of the shoulder, arm, or forearm as another confirmation of the proper needle placement. After a careful aspiration to rule out an intravascular needle placement, 1 to 2 mL of local anesthetic is injected to document the proper needle placement (Figure 8 A). Injection of several milliliters of local anesthetic often displaces the brachial plexus away from the needle(Figure 8 B).
• The presence of the motor response to nerve stimulation is useful but not necessary to elicit if the plexus, needle, and local anesthetic spread are well-visualized. • The neck is a very vascular area, and care must be exercised to avoid needle placement or injection into the vascular structures. Of particular importance is to avoid the vertebral artery, and branches of the thyrocervical trunk: inferior thyroid artery, suprascapular artery, and transverse cervical artery. • Never inject against high resistance (>15 psi) because this may indicate a needle-nerve contact or an intrafascicular injection. • Pro and con of multiple injections: • Pro: May increase the speed of onset and success rate of the interscalene block. • Pro: May allow for a reduction in the total volume and dose of local anesthetic required to accomplish block. • Con: May carry a higher risk of nerve injury because part of the plexus may be anesthetized by the time consecutive injections are made. • NOTE: Avoidance of high resistance to injection and needle–nerve contact is essential to avoid intrafascicular injection because reliance on nerve stimulation with multiple injections is diminished.
In an adult patient, 15 to 25 mL of local anesthetic is usually adequate for successful and rapid onset of blockade. Smaller volumes of local anesthetics can also be effective, however, their success rate in everyday clinical practice may be inferior to those reported in meticulously conducted clinical trials.