Phrenic Nerve Imaging

The phrenic nerve is a small (<1 mm in diameter), monofascicular nerve that arises from the third, fourth, and fifth cervical ventral rami. The most consistent and largest contribution is from the fourth cervical ventral ramus. At the level of the cricoid cartilage, the phrenic nerve is essentially coincident with the C5 ventral ramus.¹ As the nerve descends the neck, it travels from lateral to medial over the surface of the anterior scalene muscle. The nerve usually enters the chest between the subclavian artery and vein. Accessory phrenic nerves are observed in 60% of specimens and often derive from the fifth cervical ventral ramus.²

Phrenic nerve imaging is important for two reasons. First, it may be possible to reduce the incidence of transient pulmonary complications related to phrenic nerve block using ultrasound. Low volumes of local anesthetic (5 mL) administered for interscalene block using ultrasound guidance appear to reduce the incidence of concomitant phrenic nerve block.³ Ultrasound guidance allows more caudal approaches to brachial plexus block with an even lower chance of phrenic nerve block. Second, direct trauma to the nerve can potentially be avoided during regional anesthetic procedures in the cervical region.

FIGURE 28-1 External photograph showing the approximate transducer position for phrenic nerve imaging in the cervical region.

FIGURE 28-2 Phrenic nerve imaging with ultrasound. At the level of the cricoid cartilage, the phrenic nerve and C5 ventral ramus lie side by side. At more caudal positions in the neck, the phrenic nerve crosses medially over the surface of the anterior scalene muscle and appears monofascicular.

FIGURE 28-3 Sonogram showing the course of the phrenic nerve. The superficial (transverse) cervical artery lies superficial to the phrenic nerve in the midportion of the anterior scalene muscle.

FIGURE 28-4 Echotexture of the phrenic nerve. On occasion the phrenic nerve appears bifascicular on ultrasound scans, possibly from an accessory phrenic contribution from the fifth cervical ventral ramus.