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Interscalene and Supraclavicular Blocks (of the Brachial Plexus)
The first reported use of ultrasound to guide regional block was for brachial plexus block above the clavicle. These pioneers used an offline Doppler technique to mark the position of the subclavian artery as a surrogate landmark of the brachial plexus. Although today there may be many criticisms of this technique, their results were impressive: a 98% success rate with no complications.
Ultrasound imaging blurs the distinction between interscalene and supraclavicular blocks. If the brachial plexus is seen stacked between the anterior and middle scalene muscles, the block is generally referred to as an interscalene block. If the brachial plexus is seen as a compact group of nerves lying superior and lateral to the subclavian artery, the approach is generally referred to as a supraclavicular block. Because this distinction can be subtle, both blocks are treated together in this chapter.
Sonoanatomy of the Posterior Cervical Triangle
The brachial plexus typically forms from the ventral rami of C5, C6, C7, C8, and T1. The monofascicular ventral rami of the brachial plexus are hypoechoic and can sometimes be difficult to identify between the scalene muscles (the “stoplight sign”). The brachial plexus lies deep to the tapering posterolateral edge of the sternocleidomastoid in the neck. The ventral rami can be similar to blood vessels in their ultrasound appearance. The seventh cervical ventral ramus is usually the largest, with progressively smaller cephalad and caudad ventral rami. These size differences tend to equalize the size of the three trunks that form from the ventral rami of the brachial plexus.
The anterior tubercle of C7 is rudimentary (the longus capitis and anterior scalene muscles do not insert on it, in contrast to the anterior tubercle of C6). This sonographic landmark can be used to trace the level of blocks in the posterior cervical triangle. The vertebral artery lies in close proximity to the ventral ramus of C7 as it emerges from the sulcus. The thoracic (T1) contributions rise to join the remainder of the brachial plexus over the first rib (ascending nerves of the brachial plexus). These contributions lie adjacent to the tendinous insertions of the middle scalene on the first rib.
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