the cricoid cartilage in the direction toward and past the posterior border of the sternocleidomastoid (SCM) muscle demarcates the C6 transverse process. This line will serve as the path to take when searching for the appropriate muscle twitch. The bony tubercle of the transverse process of C6 often can be palpated along this line. The posterior border of the SCM muscle is also marked and bisects the previously drawn line. The posterior border of the SCM can more easily be palpated by instructing the patient to raise their head off the table by flexing at the neck. Then the interscalene groove is marked (this will later serve as the nerve block needle insertion site) by palpating for the groove immediately behind and deep to the posterior border of the SCM muscle at a point along the C6 transverse process line drawn previously. A 22-gauge 3-5 cm b-bevel needle is connected to a nerve stimulator set at 1.0 mA (activate nerve stimulator subsequent to subcutaneous needle placement) and inserted at the mark of the interscalene groove. The needle is then directed perpendicular to the skin following shallow local infiltration and skin cleansing. The perpendicular orientation of the needle to the skin will mimic a caudal, posterior, and medial direction. The needle is inserted until an appropriate motor twitch of the deltoid or biceps muscle is obtained at a stimulation of between 0.2 and 0.5 mA or until a paresthesia to the arm or thumb is elicited. The muscle twitch typically occurs superficially at a depth of 1-2 cm (up to 3 cm in obese patients). Approximately 20-40 ml of local anesthetic is injected following a negative aspiration for blood/CSF. It is important to aspirate frequently during delivery of the anesthetic in small aliquots. A continuous single orifice catheter may be inserted to provide continuous infusion of local anesthetic, although securing such a catheter into the interscalene groove may prove to be difficult with this conventional approach.
Several side effects from an interscalene block are possible, including infection. The phrenic nerve and the sympathetic chain are located in the region of the cervical nerve roots. Patients may complain of dyspnea because the phrenic nerve is affected in 90-100% of interscalene blocks, resulting in an ipsilateral diaphragmatic paralysis. Therefore, in patients with respiratory compromise (such as severe chronic obstructive pulmonary disease (COPD)), blocking the diaphragm may not be a tolerable side effect. In addition, Horner's syndrome is common from blockade of the stellate ganglion sympathetic chain, which can result in ipsilateral myosis, ptosis, and anhidrosis. Nasal stuffiness and blockade of the recurrent laryngeal nerve may occur, causing hoarseness as a result of an interscalene block.
A complication that may occur is a pneumothorax, as the cupola of the lung may be located in the vicinity of the C6 tubercle (rare). This complication must be considered if the patient develops chest pain or cough, even hours after performing an interscalene block. One severe complication would be an intravascular or intraarterial injection (the external jugular vein often transverses the interscalene groove, and the vertebral artery is just anterior to the cervical roots) as inadvertent injection of as little as 1-3 ml of local anesthetic into the vertebral artery may result in seizure activity. Another possible complication is injection of local anesthetic into the intervertebral foramina that may result in a high epidural or spinal anesthesia that requires immediate intervention.
Twitches from the following muscles provide a similar success rate: pectoralis, deltoid, triceps, biceps, and any twitch of hand or forearm. The external jugular vein crosses close to the insertion site for this classical approach to the interscalene block. Shoulder surgery entails massive nociceptive input, and an interscalene block may provide relief of the deep somatic pain and reflex muscle spasm.
Clinical Uses Surgical
• Arthroscopic shoulder surgery and arthroplasty of the shoulder
• Rotator cuff repair, arthrolysis, and acromioplasty of the shoulder
• Proximal humerus surgery, humerus open reduction and internal fixation (ORIF)
For all the surgeries indicated above.
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