Infraclavicular block of the brachial plexus was developed as a means to achieve complete brachial plexus anesthesia and was one of the first blocks to be described using ultrasound imaging.^1,2 This block is performed at the level of the pectoralis minor muscle. In this location the brachial plexus consists of three cords that compactly hug the walls of the second part of the axillary artery.³ The infraclavicular region has the advantage of being a secure place for a catheter that provides complete brachial plexus anesthesia. Another advantage is that anatomic variation is relatively uncommon in this region. In the infraclavicular region, the serratus anterior and subscapularis muscles form the margin of safety between the neurovascular bundle and the chest. Because proximal arm muscles are anesthetized (including the pectoralis and deltoid), infraclavicular blocks produce excellent tourniquet tolerance and conditions for upper extremity surgery.⁴ The block can be performed with the arm abducted or at the patient’s side (Table 31-1).

Table 31-1 Clinical Considerations for Infraclavicular Block

The major disadvantage of infraclavicular blocks is that the plexus is deep, with the neurovascular bundle being covered by both the pectoralis major and pectoralis minor muscles. Therefore, low-frequency, small, curved transducers are generally favored. Although the block can be performed with the arm at the side, it is still best to abduct the arm if possible to straighten the neurovascular bundle and retract the clavicle to provide more working room. Because the block is deep, an echogenic or large-bore needle is recommended (17 or 18 gauge) to improve needle tip visibility.

The three cords of the brachial plexus (medial, lateral, and posterior) are closely adherent to the second part of the axillary artery that lies beneath the pectoralis minor muscle. This arrangement of cords around the second part of the axillary artery is maintained across these spaces like a “three-point” star with the artery centered in the middle. The subscapularis muscle separates the infraclavicular brachial plexus from the lung, with a larger margin of safety laterally than medially. The pectoral nerves are sometimes visualized between the pectoralis major and pectoralis minor muscles.

Cord topography changes along the length of the axillary artery in the infraclavicular region.⁵ Proximally, the cords of the brachial plexus have more of a “supraclavicular” configuration in which all the cords are on the lateral side of the axillary artery. Following distally, the true “infraclavicular” configuration is when all cords surround the axillary artery before the takeoff of the axillary and musculocutaneous nerves. When the transducer is too proximal, the curved inclination of the ribs and pleura can be recognized. When the transducer is too distal, the coracobrachialis muscle and musculocutaneous nerve can be appreciated on the lateral side of artery.

The axillary vein lies on the medial (caudal) side of the axillary artery. Occasionally a duplicate axillary vein is observed. The cephalic vein enters the axillary vein from the lateral side by traveling through the deltopectoral groove. The cephalic vein can lie over the axillary artery in the infraclavicular region and can be recognized by its “tadpole” shape. Otherwise, there are few veins within the field of imaging for infraclavicular block.

Suggested Technique

Infraclavicular block is performed in supine position with the arm abducted. The transducer is placed about halfway between the supraclavicular and axillary locations for brachial plexus block. A short-axis view of the axillary artery under the pectoralis minor muscle is obtained. The needle tip is usually placed in-plane between the lateral cord and artery to inject local anesthetic that results in a U-shaped distribution under the posterior aspect of the artery. Some retraction of the lateral cord with the needle may be necessary to pull the cord farther laterally away from the axillary artery.

The fascial layers under the pectoralis minor and over the subscapularis muscle are effective for containing local anesthetic. The injection(s) should ideally separate the cords from the artery by placing the needle tip between each of the three cords and the artery. The anatomic location immediately posterior to the axillary artery appears to result in the most consistent brachial plexus anesthesia for both single-shot injections and catheters.

It is possible to use a linear transducer for infraclavicular block. However, this can be difficult because the working room is limited by the location of the clavicle and coracoid process. If a linear transducer is chosen, it is especially important to retract the clavicle away from the infraclavicular site by abducting the arm. Another difficulty with linear transducers is that it is difficult to rock the transducer back against the inclination of the chest wall. This can cause problems in patients with a large chest wall. Finally, most linear transducers produce a rectangular formatted image. Even for an average-sized patient, this results in a keyhole view (more depth than footprint) for infraclavicular block and can be frustrating for in-plane guidance.

Key Points

Infraclavicular Block	The Essentials
Anatomy	The PMi defines the second part of the AA.
	The cords of the BP hug the walls of the second part of the AA.
	The BP cords (lateral, posterior, and medial) are named with respect to the second part of the AA
	The cephalic vein (“tadpole sign”) crosses over the AA to enter the AV.
Image orientation	The AV lies medial (caudad) to the AA.
Positioning	Supine, with arm abducted if possible
	A blue foam headrest provides working room.
Operator	Standing on the lateral (cephalad) side of the armboard (for laptop system)
	At the side of patient (for system with movable display)
Display	Across the armboard (for laptop system)
	Across the table (for system with movable display)
Transducer	Medium-frequency curved, small footprint
Initial depth setting	40 to 50 mm
Needle	20 to 21 gauge, 70 to 90 mm in length
Anatomic location	Begin by scanning halfway between supraclavicular region and axilla. Slide transducer to obtain SAX view of AA underneath PMa and PMi. Perform infraclavicular block at the level of the PMi.
	The CB (and therefore MCN takeoff) should not be in view.
Approach	SAX view of second part of AA (under PMi), in-plane.
	Place the needle tip between the cords and AA.
	A U-shaped distribution under the AA is desired.
Sonographic assessment	The injection should separate the cords of the BP from the AA.
Anatomic variation	Duplicate axillary vein on lateral side of AA is common.
	“Supraclavicular” configuration to the BP cords is possible. This means all three cords of BP lie on the lateral side of the AA. If this condition is recognized, slide the probe more distally.

AA, Axillary artery; AV, axillary vein; BP, brachial plexus; CB, coracobrachialis muscle; MCN, musculocutaneous nerve; PMa, pectoralis major muscle; PMi, pectoralis minor muscle; SAX, short axis.

Clinical Pearls

• Infraclavicular block is best performed at the level of the pectoralis minor muscle where the cords of the brachial plexus surround the second part of the axillary artery. If the coracobrachialis muscle is within the field of imaging, there is some potential for takeoff of the musculocutaneous nerve and therefore incomplete block.

• Abducting the arm retracts the clavicle, thereby allowing more space to place the block needle for a shallower angle of approach to infraclavicular block.^5,6 Furthermore, the neurovascular bundle is straightened, thereby making imaging easier. In this position the brachial plexus lies closer to the skin surface and farther from the pleura compared with other arm positions.

• In the infraclavicular region the axillary artery and vein are separated when the medial cord of the brachial plexus lies between them. This gap can be identified on ultrasound imaging and indicates wall-hugging cords of the brachial plexus (rather than cords that lie on the lateral side of the axillary artery). Performing infraclavicular block where the cords surround the axillary artery is desired for complete brachial plexus anesthesia.

• Although debate continues, most clinical studies have suggested that brachial plexus anesthesia can be achieved with a single injection of local anesthetic under the second part of the axillary artery for infraclavicular block.⁷

FIGURE 31-1 External photograph showing an in-plane approach to infraclavicular block. The operator can stand at the head or side of the table. A curved probe is usually chosen because of the working room limitation imposed by the clavicle. If possible, the arm should be abducted to straighten the neurovascular bundle for infraclavicular block.

FIGURE 31-2 Short-axis view of the second part of the axillary artery under the pectoralis major and pectoralis minor muscles. The cords of the brachial plexus are named for their position with respect to the artery. The three cords of the brachial plexus have a triangular arrangement around the second part of the axillary artery, which lies underneath the pectoralis minor muscle. The clavipectoral fascia creates a favorable space for injection of local anesthetic and catheter placement in the infraclavicular region.

FIGURE 31-3 Sonogram showing in-plane approach to infraclavicular block. An echogenic needle has been used to place local anesthetic underneath the axillary artery deep to the pectoralis minor muscle.

FIGURE 31-4 Sonographic assessment of the distribution after infraclavicular block. The local anesthetic surrounds the cords and separates them from the walls of the axillary artery.

FIGURE 31-5 Short-axis (A) and long-axis (B) view of the axillary artery under the pectoralis minor muscle. The course of the posterior cord of the brachial plexus can be seen under the axillary artery.

FIGURE 31-6 Sonograms illustrating aberrant transducer positions for infraclavicular block. A, The transducer is placed too medial (proximal or central). In this sonogram all cords of the brachial plexus lie on the lateral side of the axillary artery. B, The transducer is placed too lateral (distal or peripheral). In this sonogram the coracobrachialis muscle is seen on the lateral aspect of the axillary artery.

FIGURE 31-7 The “tadpole” sign of the cephalic vein entering the axillary vein. Because the cephalic vein enters the axillary vein at 90 degrees, it is seen in long axis, whereas the axillary vein is seen in short axis.

FIGURE 31-8 Separation of the axillary artery and vein by cords of the brachial plexus. When the walls of the axillary artery and vein are not touching in the infraclavicular region the medial cord of the brachial plexus usually separates them. In this example, both the medial cord and posterior cord lie between the two vessels. Sonograms are shown before (A) and after (B) injection for infraclavicular block.

FIGURE 31-9 Sonograms illustrating intraneural injection during infraclavicular block. To access the posterior aspect of the axillary artery, the block needle was inadvertently placed through the lateral cord of the brachial plexus (A). Intraneural injection in a fracture pattern is noted after the block (B). Sliding the transducer away from the site verified nerve integrity was intact (C) and there were no neurologic sequelae.

FIGURE 31-10 Sonogram illustrating artifacts during imaging for infraclavicular block. In this example, refractile shadowing from the sides of the axillary artery is seen. Refractile shadows are the linear hypoechoic artifacts that emanate down from the sides of vessels and are formed by refraction of ultrasound waves. Posterior acoustic enhancement (amplification of echoes deep to the artery) typically occurs when this is noted. This enhancement artifact can resemble nerves (although the cords themselves frequently occupy this position).