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.

Figure 19.5 The axillary block. The patient is placed in the supine position, with the arm abducted and forearm flexed to 80° and parallel to the long axis of the body.

Figure 19.6 Axillary block technique. The needle is first inserted anterior to the palpable axillary pulse. The needle is oriented to follow the course of the neurovascular bundle.

Stimulation of the median nerve produces flexion of the middle and index fingers, thumb, and pronation and flexion of the wrist. Stimulation of the ulnar nerve produces flexion of the ring and little fingers with ulnar deviation of the wrist, while stimulation of the radial nerve will produce dorsiflexion of the fingers and wrist. Finger flexion alone in the case of the ring and little fingers could represent either ulnar or median stimulation. The needle position is adjusted while decreasing the current to 0.30 mA with maintenance of the muscle response (Fig. 19.7). A muscle response in the upper arm should not be accepted. Individual nerves can be targeted and located with ease. Ideally, the nerve(s) supplying the area of surgery should be sought.

Figure 19.7 Axial oblique T1-weighted MR image showing spread of contrast outside and away from the axillary sheath. This followed excellent median nerve stimulation down to 0.5 mA and insertion of catheter. This suggests that perhaps 0.5 mA is too high a current to be sure that the needle is inside the axillary sheath. 1: brachial plexus and axillary vessels; 2: nerve sheath; 3: contrast spread.

Incremental injection of the local anesthetic is made with repeated aspiration. To block terminal nerves of the plexus, 40 mL of local anesthetic is sufficient. If a multiple-injection technique is used, 10 mL at each site is adequate. The stimulating current needs to be increased for the second and subsequent nerves because the previously injected local anesthetic will increase the electrical resistance in the area. To increase the likelihood of blocking the musculocutaneous nerve, digital pressure is maintained distal to the site of injection to encourage proximal spread of the local anesthetic within the axillary sheath; a tourniquet may be used for a similar effect (Figs 19.8 and 19.9). The arm is then placed across the chest.

Figure 19.8 Coronal T1-weighted MR image after injection of 40 mL of contrast. Injection was made without distal pressure being applied. 1: profunda brachii artery; 2: radial nerve; 3: median nerve.

Figure 19.9 Coronal T1-weighted MR image 30 min post injection of 40 mL of contrast. Note the spread is almost entirely away from the axilla, two-thirds of the length of the upper arm. 1: biceps muscle; 2: deltoid muscle.

If an upper arm tourniquet is to be used, additional block of the intercostobrachial nerve and medial cutaneous nerve of the arm is of value to reduce the cutaneous element of the pain due to the tourniquet. The intercostobrachial nerve is the lateral cutaneous branch of the second intercostal nerve. The medial cutaneous nerve of the arm originates from the medial cord of the brachial plexus. Block of both nerves can be achieved with a subcutaneous injection of local anesthetic in the medial aspect of the upper arm (Fig. 19.10). Injection should be made from the biceps muscle to the triceps muscle.

Figure 19.10 Intercostobrachial block technique. A subcutaneous injection (5–6 ml) in the axilla will anesthetize the intercostobrachial and medial cutaneous nerve of arm.

Musculocutaneous nerve block

The musculocutaneous nerve (C5, 6) is a branch of the lateral cord of the brachial plexus. It leaves the brachial plexus fascial sheath at the level of the coracoid process, pierces the coracobrachialis muscle, and then runs between the biceps and brachialis muscles in the arm. It passes through the antecubital fossa deep to the biceps tendon and continues as the lateral cutaneous nerve of the forearm. It supplies motor fibers to the coracobrachialis muscle, the biceps muscle, and brachialis muscles. It is the sensory supply of the skin covering the radial aspect of the lower two-thirds of the forearm.

The musculocutaneous nerve can be blocked as part of an infraclavicular technique using the vertical infraclavicular or coracoid approaches (see Ch. 18). An injection into the body of the coracobrachialis will also anesthetize it without the need to elicit paresthesia or a motor response. It can be blocked as part of the midhumeral approach (see Ch. 20). The lateral cutaneous nerve of the arm can be blocked at the elbow by an injection just lateral to the tendon of the biceps at the elbow crease. A 23-G needle is inserted to a depth of 2–3 cm and injection of 4 mL of local anesthetic is made.

Ultrasound-guided approach

Intravenous access, ECG, pulse oximetry and blood pressure monitoring are established. For the ultrasound-guided axillary block, the patient is placed in the supine position with the upper limb to be blocked abducted to 90° and flexed at the elbow, with the hand resting adjacent to the head. The operator stands or sits adjacent to the side to be blocked. For the axillary block, the ultrasound screen is placed below the shoulder on the side to be blocked (Fig. 19.11).

Figure 19.11 Global view of the block field for the ultrasound guided axillary block.

The ultrasound transducer is placed on the skin perpendicular to the long axis of the axillary artery, vein, and the elements of the brachial plexus (transverse plane) (Fig. 19.12). The round, pulsatile axillary artery is identified, and then minor adjustments are made to obtain a view of accompanying neuronal structures. The axillary vein will be more compressible than the axillary artery and generally possesses a thinner vessel wall. Minimal pressure should be exerted on the ultrasound tranducer to identify veins.

Figure 19.12 Orientation of the ultrasound transducer when performing the ultrasound-guided axillary block.

Typically, only one or sometimes two branches of the axillary brachial plexus may be seen on any particular view. However, a systematic examination of the entire axillary brachial plexus may be made by sliding the transducer slowly from medial to lateral, and up and down the arm, while the ultrasound transducer is oriented perpendicular to the axillary artery, and as a result, sequentially viewing the median, radial, ulnar and musculocutaneous nerves.

The skin is disinfected with antiseptic solution and draped. A sterile sheath (CIVCO Medical Instruments, Kalona, IA, USA) is applied over the ultrasound transducer with sterile ultrasound gel (Aquasonic, Parker Laboratories, Fairfield, NJ, USA). Another layer of sterile gel is placed between the sterile sheath and the skin. A skin wheal of local anesthetic is raised at a distance from the transducer to facilitate sterility and allow a shallow angle of approach to improve needle visualization.

Long axis approach

A 21-GA × 50-mm insulated needle or 2-inch Tuohy needle (18-G) is inserted parallel to the axis of the beam of the ultrasound transducer (Fig. 19.13). The needle is introduced vertically down, either through the lateral part of the pectoralis major, or biceps brachii, or coracobrachialis muscles. The operator can slide and tilt the transducer to maintain the needle tip within the plane of imaging as much as possible. The needle tip is slowly advanced under ‘real-time’ imaging to bring the needle tip to rest just deep to the axillary artery and adjacent to the radial nerve. A dual injection technique in which local anesthetic solution is injected both deep (radial nerve) and superficial (median and ulnar nerves) to the axillary artery should be employed.

Figure 19.13 Ultrasound transducer and needle positioning during the ultrasound-guided axillary block. Note the needle orientation parallel to the plane of the ultrasound beam.

Once the needle tip has been confirmed by ultrasonography to lie in close proximity to the median, ulnar or radial nerves, this can be confirmed by using a nerve stimulator (Fig 19.14). Characteristic motor activity in the hand is seen. Test injections for assessment of local anesthetic spread should be small (0.5 to 2 mL). If the local anesthetic spread is not seen on the ultrasound screen, the injection should be stopped. The needle is readjusted to allow complete encirclement of the nerves with local anesthetic. Local anesthetic appears as a hypoechoic image (Fig 19.15). Typically, a decreased volume of local anesthetic is required compared to non-ultrasound-guided axillary blocks. Lidocaine, 2%, with epinephrine 1 : 200 000 and sodium bicarbonate 8.5% solution (1/10 mL of solution) 5–10 mL per nerve to a maximum of 40 mL may be used. Other longer acting local anesthetic agents may be substituted for longer lasting blocks if a catheter technique is not used.

Figure 19.14 Real-time imaging of needle insertion for the long axis approach of ultrasound-guided axillary block. Notice the needle shaft marked with arrows and the needle tip (N) in close proximity to the brachial plexus. N: needle tip; AxA = axillary artery.

Figure 19.15 Axillary brachial plexus with surrounding local anesthetic (LA). N: needle; AxA: axillary artery; AxV: axillary vein; MN: median nerve; UN: ulnar nerve.

The needle is withdrawn and the musculocutaneous nerve is identified in the corocobrachialis muscle or between the biceps and brachoradialis. Once the needle tip has been confirmed by ultrasonography to lie in close proximity to the musculocutaneous nerve, this can be confirmed by using a nerve stimulator. Characteristic motor activity is seen. A deposit of local anesthetic is made around the musculocutaneous nerve (Fig. 19.16). If the musculocutaneous nerve is not seen clearly, 10 mL of local anesthetic may be injected into the belly of the coracobrachialis muscle. A subcutaneous wheal is raised on the medial side of the arm to block the intercostobrachial nerve.

Figure 19.16 Transverse ultrasound image in the axillary location showing the circumferential local anesthetic and the musculocutaneous nerve using a 6–13 MHz linear transducer. LA: local anesthetic; AxA: axillary artery; MN: musculocutaneous nerve.

Short axis approach

A transverse image of axillary brachial plexus is obtained and then the needle is introduced in short axis of beam to reach the nerves individually (Figs 19.17 and 19.18). The disadvantage is that the needle is not clearly seen at all times and the needle tip position is indirectly ascertained by tissue motion and spread of local anesthetic. Between 4 and 10 mL of 2% lidocaine is deposited around each of the identified nerves.