32 Axillary Block

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

Nerve	Characteristics
Axillary	Accompanies posterior circumflex humeral artery Takeoff is proximal to teres major and latissimus dorsi tendons
Musculocutaneous	Lateral course through coracobrachialis Flat shape when inside this muscle If not visible, rule out median-musculocutaneous nerve fusion
Radial	Accompanies profunda brachii artery Travels to spiral groove of humerus Takeoff is distal to teres major and latissimus dorsi tendons
Median	Can be displaced to side of axillary artery with compression Accompanies brachial artery
Ulnar	Lies between axillary artery and vein Acoustic window through axillary vein Most consistent position (at 2- to 3-o’clock position with respect to artery) Travels to cubital tunnel
Medial antebrachial cutaneous	Lies between median nerve and ulnar nerve
Intercostobrachial and medial brachial cutaneous	Lie within the subcutaneous tissue of the medial arm Medial to axillary artery

Table 32-2 Clinical Considerations for Axillary Block with Ultrasound

Advantages	Disadvantages
Shallow nerves Working room Compressible site Remote from the lung No risk for phrenic nerve block	Divergent plexus Anatomic variation Arm positioning Multiple veins Catheters problematic