Three terminal branches of the brachial plexus (the median, radial, and ulnar nerves) lie close to the axillary artery in the axilla (Table 32-1). This makes the axilla a convenient place to block the brachial plexus (Table 32-2). Axillary block is traditionally performed by transarterial injection of local anesthetic around the axillary artery or by use of nerve stimulation to evoke motor responses. Transarterial block necessitates puncturing the axillary artery. Another weakness is failure to anesthetize the musculocutaneous nerve, which leaves the neurovascular bundle proximally underneath the pectoralis minor muscle at the level of the coracoid process.

Table 32-1 Characteristics of Terminal Branches of the Brachial Plexus in the Axilla

Table 32-2 Clinical Considerations for Axillary Block with Ultrasound

Ultrasound imaging improves axillary block of the brachial plexus. Almost all institutions have reported advantages to using ultrasound to guide this procedure.^1,2 Ultrasound can be used to guide injections around the axillary artery. In addition, the musculocutaneous nerve can be directly imaged to complete the axillary block (see Chapter 33).

Suggested Technique

The transpectoral approach for proximal axillary block is performed with the needle tip just inside the chest, before the nerves of the brachial plexus diverge. With this lateral to medial approach, the needle enters through the pectoralis major muscle.

Axillary block is performed with the patient in supine position. The arm should be slightly hyperabducted to allow the needle placement to be as proximal as possible. Slightly more than 90 degrees of abduction is optimal for probe positioning. Because the pectoralis major inserts on the humerus, hyperabduction of the arm reduces the pectoral ridge by retracting the pectoralis major toward the midline. A Mayo stand can be used to support the arm in this position. The operator should stand at the head of the bed to view the ultrasound display across the patient’s arm.

The pectoral ridge separates the needle entry point and the transducer for this proximal axillary block. This can allow for coverless imaging because the needle entry site is remote from the transducer. The skin preparation is over the pectoralis major muscle. Tilting and rotating the angle of the transducer slightly into the chest torso allow for more proximal imaging. The axillary veins can be used as a manometer to measure the amount of probe compression. The correct amount of pressure for this procedure just coapts the walls of the veins.

Structures potentially in the needle path when approaching the axillary neurovascular bundle are the cephalic vein, the tendon of the pectoralis major, and the musculocutaneous nerve. The cephalic vein lies in the deltopectoral groove; therefore, there is a low potential for venous puncture as the needle enters the skin.

The conjoint tendon of the latissimus dorsi and teres major inserts on the humerus and is a valuable landmark for this procedure. The radial nerve takeoff from the axillary artery is always distal to this tendon. The needle should enter the skin away from the transducer so that it approaches slightly deep to the artery and can easily be placed between the artery and conjoint tendon for injection.

Because the axillary artery and wall-hugging branches of the brachial plexus (median, radial, and ulnar nerves) are surrounded by common connective tissue, the ideal place for block needle tip placement is between these nerves and the artery. If this is done, the perivascular injections will separate the nerves from the artery. Injections at the outside corner of the nerves (away from the axillary artery) also can be successful, but careful assessment must show that the injection actually encircles the nerves.

Local anesthetic injections are made in front and in back of the axillary artery. The injection in back of the artery is typically done first. This brings the neurovascular bundle even closer to the skin surface. The musculocutaneous nerve is blocked separately (see Chapter 33).

It is especially important for the front-wall injection to be between the median nerve and the axillary artery. The median nerve crosses the front surface of the axillary artery. If local anesthetic tracks proximally along the median nerve to its medial and lateral cord contributions, two thirds of the brachial plexus should be anesthetized. The median, ulnar, medial antebrachial cutaneous, and musculocutaneous nerves all derive from the medial and lateral cords.

The intercostobrachial nerves (from T1 and T2) are not part of the brachial plexus but contribute to sensory innervation of the medial arm. Intercostobrachial nerve block can be achieved by subcutaneous infiltration of the medial arm at the axillary crease. Imaging guidance is usually not necessary for this procedure, but is sometimes used in obese patients.

Key Points

Axillary Block	The Essential Points
Anatomy	There are three wall-hugging nerves in the axilla (MN, RN, UN).
	All terminal branches lie superficial to the CT in the axilla (MN, RN, UN, MCN).
	The nerves are about 3 mm in diameter.
Image orientation	The CT has a medial (superficial) to lateral (deep) inclination.
Positioning	Supine, with arm abducted
	A blue foam headrest provides working room.
Operator	Standing on the lateral (cephalad) side of armboard (for laptop system)
	At the side of the patient (for system with movable display)
Display	Across the armboard (for laptop system)
	Across the table (for system with movable display)
Transducer	High-frequency linear, 23- to 38-mm footprint
Initial depth setting	25 mm
Needle	20 to 21 gauge, 70 mm in length
Anatomic location	Begin by scanning the axilla to identify the AA in SAX view.
	Slide transducer to obtain view of CT underneath AA
Approach	SAX view of AA, in-plane from lateral to medial
	Place the needle tip between the nerves and AA.
Sonographic assessment	The injections should separate the nerves from artery.
Anatomic variation	MCN-median fusion (5%-10%). The MCN can be fused with the median nerve in the axilla. This is sometimes referred to as a low-lying lateral cord.
	Duplicate axillary artery can occasionally be seen.

AA, Axillary artery (third part); CT, conjoint tendon of teres major and latissimus dorsi muscles; MCN, musculocutaneous nerve; MN, median nerve; RN, radial nerve; SAX, short axis; UN, ulnar nerve.

Clinical Pearls

• With this proximal approach to axillary block, the needle is naturally channeled into the space between the axillary artery and conjoint tendon of the latissimus dorsi and teres major for the posterior injection. A slight upward trajectory is all that is needed to steer the needle along and above this tendon complex.

• With classic approaches to axillary block, the radial nerve and musculocutaneous nerves are often spared.³

FIGURE 32-1 External photograph showing an in-plane approach to axillary block from the lateral aspect of the arm. In this example the display is placed across the armboard and the operator stands at the head of the bed.

FIGURE 32-2 External photograph showing an in-plane approach to axillary block from the lateral aspect of the arm. In this example the display is placed across the table and the operator stands at the side of the bed.

FIGURE 32-3 Short-axis view of the axillary artery and surrounding branches of the brachial plexus in the axilla. The conjoint tendon of the teres major and latissimus dorsi lies under the neurovascular bundle. Because the radial nerve takeoff is distal to the conjoint tendon, visualization of this structure ensures a compact brachial plexus suitable for complete block.

FIGURE 32-4 Short-axis views of the brachial plexus from probe manipulation in the axilla. With some subjects the axillary nerve can be viewed posterior to the axillary artery by tilting the probe into the chest. Both distal (A) and proximal (B) views are shown.

FIGURE 32-5 The profunda brachii artery shown in short-axis view with power Doppler imaging. The profunda brachii artery exits the back wall of the axillary artery to accompany the radial nerve into the spiral groove of the humerus.

FIGURE 32-6 The profunda brachii artery shown in long-axis view with B-mode imaging.

FIGURE 32-7 Axillary scout view imaging to look for veins before needle placement. Images are shown with (A) and without (B) probe compression. During needle placement the vein walls are maintained coapted to reduce the chance of venous puncture.

FIGURE 32-8 In-plane approach to axillary block. Sonograms are shown before (A) and after (B) needle placement. With this approach, the needle is naturally channeled into the space between the axillary artery and conjoint tendon of the latissimus dorsi and teres major for the posterior injection. A slight upward trajectory is all that is needed to steer the needle along and above the tendon complex.

FIGURE 32-9 Axillary block image sequence showing back-wall injection (A), front-wall injection (B), and front-wall injection as the needle is removed (C).

FIGURE 32-10 Axillary block image sequence. The block needle approaches the gap between the median nerve and axillary artery (A). As the needle is advanced and local anesthetic is slowly injected, the nerve and artery are separated (B). The beveled needle can be used as a shovel to scoop up the peripheral nerve and displace it away from adjacent structures (e.g., arteries or other peripheral nerves).

FIGURE 32-11 Short-axis view of the axillary artery after axillary block. After successful injection, the local anesthetic separates the nerves from the axillary artery.

FIGURE 32-12 Short-axis view of the medial antebrachial cutaneous nerve after axillary block. After injection this small nerve can often be identified between the median and ulnar nerves toward the skin surface. Visible separation of the wall-hugging nerves from the axillary artery ensures successful axillary block.

FIGURE 32-13 Median nerve after axillary block in short-axis view from distal (A) and proximal (B and C). The median nerve derives from both the medal and lateral cords, and these contributions can often be identified after successful axillary block because the local anesthetic tracks proximally to provide reverse acoustic contrast to outline the nerve borders.

FIGURE 32-14 Short-axis view after axillary block in a young child. The axillary block is a versatile block applicable in many clinical circumstances.