Interscalene Block

Clinical Applications

The principal indication for interscalene block is surgery on the shoulder. Blockade occurs at the level of the upper and middle trunks. Although this approach can be used for forearm and hand surgery, blockade of the inferior trunk (C8 through T1) is often incomplete and requires supplementation at the ulnar nerve for adequate surgical anesthesia in that distribution.^[8]

Technique

The brachial plexus shares a close physical relationship with several structures that serve as important landmarks for the performance of interscalene block. In its course between the anterior and middle scalene muscles, the plexus is superior and posterior to the second and third parts of the subclavian artery. The dome of the pleura lies anteromedial to the inferior trunk.

This technique can be performed with the patient's arm in any position and is technically simple because of easy identification of necessary landmarks. ^[9] The patient should be in the supine position, with the head turned away from the side to be blocked. The posterior border of the sternocleidomastoid muscle is readily palpated by having the patient briefly lift the head. The interscalene groove may be palpated by rolling the fingers posterolaterally from this border over the belly of the anterior scalene muscle into the groove. A line is extended laterally from the cricoid cartilage to intersect the interscalene groove, indicating the level of the transverse process of C6.

1687

Figure 44-1 Roots, trunks, divisions, cords, and branches of the brachial plexus.

Figure 44-2 A, Cutaneous distribution of the cervical roots. B, Cutaneous distribution of the peripheral nerves.

1688

Figure 44-3 Interscalene block. The fingers palpate the interscalene groove, and the needle is inserted with a caudad and slightly posterior angle.

The use of a nerve stimulator or elicitation of paresthesias is recommended with this technique to place the local anesthetic solution accurately. After ordinary sterile precautions and injection of a skin wheal, a 22- to 25-gauge, 4-cm needle is inserted perpendicular to the skin with a 45-degree caudad and slightly posterior angle ( Fig. 44-3 ). The needle is then advanced until a paresthesia (usually C5 and C6 dermatomes) or nerve stimulator response is elicited. This usually occurs at a very superficial level. Paresthesia or motor response of the arm or shoulder is equally efficacious. ^[10] If a blunt needle bevel is used, a "click" may be detected as the needle passes through the prevertebral fascia. If bone is encountered within 2 cm of the skin, it is likely to be a transverse process, and the needle may be "walked" across this structure to locate the nerve. Likewise, contraction of the diaphragm indicates phrenic nerve stimulation and anterior needle placement; the needle should be redirected posteriorly to locate the brachial plexus.

After the appropriate paresthesia or motor response is obtained, the needle is stabilized. The use of flexible extension tubing facilitates the maintenance of the needle position while aspiration and injection occur. After negative aspiration, 10 to 40 mL of solution is injected incrementally, depending on the desired extent of blockade. Radiographic studies suggest a volume-to-anesthesia relationship, with 40 mL of solution associated with complete cervical and brachial plexus block.^[9] Clinical studies, however, indicate variable blockade of the lower trunk (i.e., ulnar nerve) even with large volumes of solution.^[8] Digital pressure above the injection site and downward massage along with a 45-degree head-up position may facilitate caudad spread and blockade of the lower trunk.

Side Effects and Complications

Ipsilateral phrenic nerve block resulting in diaphragmatic paresis occurs in 100% of patients undergoing interscalene blockade,^[11] even with dilute solutions of local anesthetics, and is associated with a 25% reduction in pulmonary function.^{[12] [13]} This effect probably results from anterior spread of the solution over the anterior scalene muscle and may cause subjective symptoms of dyspnea. Although rare, respiratory compromise can occur in patients with severe respiratory disease. Involvement of the vagus, recurrent laryngeal, and cervical sympathetic nerves is rarely significant, but the patient experiencing symptoms related to these side effects may require reassurance. The risk of pneumothorax is low when the needle is correctly placed at the C5 or C6 level because of the distance from the dome of the pleura.

Severe hypotension and bradycardia (i.e., Bezold-Jarisch reflex) have been reported in awake, sitting patients undergoing shoulder surgery under an interscalene block. The cause is presumed to be stimulation of intracardiac mechanoreceptors by decreased venous return, producing an abrupt withdrawal of sympathetic tone and enhanced parasympathetic output. This effect results in bradycardia, hypotension, and syncope. The frequency is decreased when prophylactic β-blockers are administered. ^[14]

Nerve damage or neuritis can occur in any peripheral nerve block, but it is uncommon and usually is self-limited. Some surgical approaches to the shoulder, such as total shoulder arthroplasty, are associated with neurologic risk to the brachial plexus.^[15] In such cases, an interscalene block should be placed postoperatively for pain relief after the surgical service has ascertained and documented that no neurologic damage has occurred. Epidural and intrathecal injections have been reported with this block, a finding emphasizing the importance of inserting the needle in a caudad direction. The proximity of significant neurovascular structures may increase the risk of serious neurologic complications when interscalene block is performed in heavily sedated or anesthetized patients. ^[16]

Several vascular structures are in proximity to a correctly placed needle. Local anesthetic toxicity as a result of intravascular injection should be guarded against by

1689