At the site of axillary block the terminal nerves of the brachial plexus form a particular pattern with the axillary artery (Fig. 19.1). Around the second part of the artery – the divisions being produced by the pectoralis minor muscle – the median nerve lies anteriorly, the radial nerve posteriorly, and the ulnar nerve posteromedially (Fig. 19.2). The axillary vein lies more medial. The musculocutaneous nerve has left the fascial sheath at the level of the coracoid and is thus unlikely to be anesthetized with single-injection axillary technique. The medial cutaneous nerve of arm and the intercostobrachial nerve lie subcutaneously.

Figure 19.1 Cadaver structures illustrating characteristic location of nerves to axillary artery. 1: axillary artery; 2: radial nerve; 3: median nerve; 4: ulnar nerve; 5: musculocutaneous nerve; 6: axillary vein.

Figure 19.2 Sagittal T1-weighted MR image of the axilla after injection of 10 mL of contrast, highlighting the arrangement of the nerves around the axillary artery.

Surface anatomy

For all blind techniques of axillary brachial plexus block, palpation of the axillary pulse is paramount. The needle insertion point is located proximally along the line of the axillary pulse. Abduction and external rotation of the shoulder and flexion of the elbow bring the axilla into view. The anterior axillary fold is formed by the pectoralis major muscle, with the posterior axillary fold by latissimus dorsi and teres major muscles. The pulsation of the lower part of the axillary artery can be felt by palpating the medial side of the arm just in front of the posterior axillary fold. In very muscular individuals, it may be necessary to decrease the degree of abduction to palpate the artery. The pulsation is followed proximally and marked.

With single-injection techniques, it is important to identify the axillary pulse as far proximal in the axilla as possible. This will increase the success rate for musculocutaneous nerve block with the single-injection technique. The belly of the coracobrachialis muscle should be identified because its infiltration will block the musculocutaneous nerve.

Sonoanatomy

In ultrasound scanning of the axillary brachial plexus, the patient is positioned supine with the arm abducted 90° on an arm board. A linear 6–13 MHz ultrasound transducer is used. Begin the examination by scanning the upper arm in the axilla just distal to the border of the pectoralis muscle. A transverse or short-axis view is used. Perform a systematic anatomical survey from superficial to deep and above and below the axillary artery. Identify the humerus and the triceps, biceps and coracobrachialis muscles. Identify the pulsatile axillary artery. Decrease transducer pressure to allow axillary veins to expand and be identified. The Doppler options facilitate identification of the vascular structures, thereby contributing to minimizing vascular puncture. The nerves lie in a close relationship to the axillary artery near the apex of the axilla, before they start diverging. There is considerable variation in their relationship to the artery and they are very mobile; slight pressure of the transducer can displace the nerves. The nerves appear as hypoechoic structures with a hyperechoic circumference (Fig. 19.3). By sliding the ultrasound transducer proximally towards the apex of the axilla, the musculocutanous nerve can be traced to its origin from the lateral cord (Fig. 19.4). Distally, it diverges from the axillary artery to travel in the coracobrachialis muscle. The axillary nerve also originates from the posterior cord and can be seen going cephalad towards the surgical neck of the humerus. By moving the ultrasound transducer distally, the radial nerve can be traced winding around the humerus.

Figure 19.3 Transverse view of the axillary part of the brachial plexus using a 6–13 MHz linear transducer. The arrows indicate the brachial plexus reflected as hypoechoic structures. AxA: axillary artery; AxV: axillary vein; R: radial nerve; M: median nerve; U: ulnar nerve.

Figure 19.4 Transverse view of the musculocutaneous nerve using a 6–13 MHz linear transducer. AxA: axillary artery; BiM: biceps muscle; BM: brachialis muscle; H:humerus; MN: musculocutaneous nerve.

Technique

Landmark-based approach

As for all regional anesthetic procedures, after checking that the emergency equipment is complete and functional, intravenous access, ECG, pulse oximetry, and blood pressure monitoring are established. Asepsis is observed.

The patient lies supine with the arm abducted (80°) and the elbow flexed (90°; Fig. 19.5). Hyperabduction should be avoided because it can make palpation of the artery difficult and distort the distribution of local anesthetic. The axillary artery is palpated as far proximally as possible under the lateral edge of the pectoralis major muscle, and fixed with the index and middle fingers. A 35-mm 21-G insulated stimulating needle is used. The stimulating current is set at 1.0 mA, 2 Hz, and 0.1 ms. The needle is advanced proximally at an angle of 30° in the direction (Fig. 19.6) of the neurovascular sheath. Entry of the needle into the neurovascular sheath is confirmed by a ‘fascial click’. The needle is advanced slowly until the appropriate muscle response is obtained.