The roots of the cervical plexus emerge from the transverse processes between the scalene muscles, and pass laterally. They are approached from behind the posterior belly of the sternomastoid muscle to avoid the carotid sheath (figure 33a–d).
The interscalene block is a useful approach to the brachial plexus: the anesthetic is placed deep to the prevertebral fascia between the scalenus anterior and scalenus medius. In thin subjects, the lateral border of the scalenus anterior can be seen and palpated (figure 34a–b). The needle is aimed at the transverse process of the sixth cervical vertebra, this is at the level of the cricoid cartilage. The entry point is usually just behind the external jugular vein.
A brachial plexus block is lower and more laterally placed than the scalene block (figure 35), and passes close to the dome of the pleura and lung; these structures may be damaged. The entry point is 2 cm above the middle of the clavicle, passing posteromedially and downwards at 45 degrees, towards the C7 transverse process. The anesthetic is again placed deep to the prevertebral fascia.
The prevertebral fascia follows the cords of the brachial plexus around the axillary artery as the axillary sheath, passing through the cervico- axillary canal into the upper arm. The cords are closely applied to the artery, and identification of the artery is the key to an axillary block. The anesthetic is placed within the axillary sheath but outside the artery (figure 36a,b). The needle is directed at the artery, entry into the sheath is felt by a slight loss of resistance. Great care is taken to ensure that no blood is withdrawn through the syringe, as this indicates that the needle is within the axillary artery or vein. Once an intravascular insertion has been excluded, the anesthetic is injected; this diffuses along and outside the artery to the nerves within the axillary sheath.