Axillary blk: U/S anatomy

An axillary block is often used for forearm or hand surgery. Local anesthetic is injected around the axillary artery, as the nerves of interest lie in close proximity. The patient is positioned in the supin position with the arm abducted 90°. The ultra sound probe is placed just distal to the bicipital grove, where the pectoralis major muscle inserts. The probe is placed in the short-axis orientation.

The axillary artery (AA) is identified as pulsatile and generally, non-compressible, with the axillary vein often running medial to it. The median nerve (MN) lies superficial and lateral to the AA. The ulnar nerve (UN) lies posterior to the AA. The radial nerve (RN) lies posterior to the AA and either medial or lateral. A second injection is made to block the musculocutaneous nerve located between the biceps muscle and coracobrachialis muscles, identified as an elongated oval.

The radial nerve innervates the dorsal surface of the first 3 ½ digits and this portion of the dorsal hand leading to the first 3 ½ digits, the triceps muscle, and the extensor muscles of the hand.

The ulnar nerve innervates the superficial, anterior and posterior portion of the last 1 ½ digits and the portion of the dorsal and palmar surface proximal to the last 1 ½ digits.

The median nerve innervates the anterior portion of the first 3 ½ digits and the palm leading to these 3 ½ digits, most of the sensory innervations of the palm.

The musculocutaneous nerve innervates the biceps, coracobrachialis and brachialis muscle.