The axillary artery is identified by scanning the anteromedial aspect of the proximal arm. The axillary veins are identified just medial and often slightly superficial to the axillary artery. The radial nerve is identified posterior and lateral to the axillary artery. The ulnar nerve is normally located superficial and medial to the axillary artery. The median nerve is located superficial and lateral to the axillary artery. These nerve bundles are normally hyperechoic by ultrasound exam. The musculocutaneous nerve is located lateral to the neuromuscular bundle in or near the fascia between the biceps and coracobrachialis muscles. The musculocutaneous nerve is often oval in shape and presents on ultrasound with a hypoechoic body and hyper echoic border (image 1).
The patient is positioned supine with the back slightly elevated for patient comfort. The arm is abducted 90 degrees and should be supported for patient comfort. The ultrasound probe should be placed in the transverse orientation on the medial aspect of the arm just distal to the pectorals major muscle (image 2). Once the neuromuscular bundle is identified by ultrasound an appropriate needle path should be chosen to avoid inadvertent puncture of the axillary veins. The block needle is inserted lateral to the ultrasound probe using the in plane approach. Keep in mind it is not necessary to apply local anesthetic individually to the ulnar, median and radial nerves. It is common that a single injection of local anesthetic near the axillary artery will spread circumferentially around the artery and cover all three nerves. If adequate spread is not visualized by ultrasound guidance the block needle may be easily repositioned to ensure adequate coverage. With the ultrasound probe positioned as previously described the musculocutaneous nerve is located lateral to the neuromuscular bundle between the biceps and coracobrachialis muscles. Again, the musculocutaneous nerve is described as an oval structure with a hypo echoic body and bright hyperechoic border. Using the same inplane entry point the block needle is positioned adjacent to the musculocutaneous nerve and local anesthetic is injected making sure to visualize adequate spread of local anesthetic under ultrasound guidance.
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