Axillary Block

Clinical Applications

The axillary approach to the brachial plexus is the most popular because of its ease, reliability, and safety.^[20] Blockade occurs at the level of the terminal nerves. Although blockade of the musculocutaneous nerve is not

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Technique

Anatomic concepts that should be considered before an axillary block include the following:

1. The neurovascular bundle is multicompartmental ( Fig. 44-6 ).^[24]
2. The axillary artery is the most important landmark; the nerves maintain a predictable orientation to the artery.
3. The median nerve is found superior to the artery, the ulnar nerve is inferior, and the radial nerve is posterior and somewhat lateral ( Fig. 44-7 ; see Plate 5 in the color atlas of this volume).
4. At this level, the musculocutaneous nerve has already left the sheath and lies in the substance of the coracobrachialis muscle.
5. The intercostobrachial nerve, a branch of the T2 intercostal nerve, is usually blocked by the skin wheal overlying the artery; however, adequate anesthesia for

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   the tourniquet can be ensured by extending the wheal 1 to 2 cm caudad and cephalad.

Figure 44-6 Axillary block. Computed tomogram after an axillary block with 0.5% bupivacaine combined with iodothalamate. Separate 10-mL injections of solution were made after obtaining median and radial nerve paresthesias transarterially. Contrast medium appears to remain in three separate compartments.

Figure 44-7 Axillary block. The arm is abducted at right angles to the body. Distal digital pressure is maintained during needle placement and injection of the local anesthetic (see Plate 5 in the color atlas of this volume).

The patient should be in the supine position with the arm to be blocked placed at a right angle to the body and the elbow flexed to 90 degrees. The dorsum of the hand rests on the bed or pillow; hyperabduction of the arm with placement of the hand beneath the patient's head is not recommended because this position frequently obliterates the pulse.

The axillary artery is palpated, and a line is drawn tracing its course from the lower axilla as far proximally as possible. The artery is then fixed against the patient's humerus by the index and middle fingers of the left hand, and a skin wheal is raised directly over the artery at a point in the axilla approximating the skin crease. Proximal needle placement and maintenance of distal pressure facilitate proximal spread of the solution.

Method of Needle Localization

Several methods of identifying the axillary sheath have been described, all with reportedly good results. Overall, paresthesias are unnecessary. However, multiple injections may shorten the onset and may improve the reliability of blockade.

1. Paresthesias can be sought with a 25-gauge, 2-cm needle, beginning deeply (i.e., radial nerve) or with the nerves supplying the surgical site. Needles longer than 2 cm are rarely needed to reach the neurovascular bundle; smaller needles and a short needle bevel may be associated with a lower risk of nerve damage.^{[22] [25]} Each paresthesia is injected with 10 mL of local anesthetic.
2. A nerve stimulator can also be employed with an insulated needle to locate the nerves. This technique obviates the need for paresthesias, and although unproven, it may lower the risk of nerve damage.^[26]
3. A short-bevel needle can be advanced until the axillary sheath is entered, as evidenced by a fascial click, whereupon 40 to 50 mL of solution is injected after negative aspiration.^{[20] [27]}
4. A transarterial technique can be employed, whereby the needle pierces the artery and 40 to 50 mL of solution is injected posterior to the artery; alternatively, one half of the solution is injected posterior and one half is injected anterior to the artery. Great care must be taken to avoid intravascular injection with this technique, particularly because the pressure of injection within the compartments of the axillary sheath may move anatomic structures in relation to the immobile needle. Some practitioners avoid intentional arterial puncture in the belief that it is unnecessarily traumatic.
5. Field block of the brachial plexus with a fanlike injection of 10 to 15 mL of local anesthetic solution on each side of the artery is a variation of the sheath technique. Paresthesias, although not sought, are often encountered in this technique and provide evidence of correct placement.

When the injection is completed, the arm should be adducted and returned to the patient's side. This prevents the humeral head from obstructing proximal flow of the solution; distal pressure and massage may also help. Vester-Andersen and colleagues^[28] were unable to consistently block the musculocutaneous nerve with volumes up to 80 mL. If the musculocutaneous nerve is not blocked by the axillary approach, it can be blocked by injection within the body of the coracobrachialis muscle or at the elbow superficially at the lateral aspect of the antecubital fossa just above the interepicondylar line.

Success Rate with Axillary Block Techniques

The success rate for an axillary block depends on the definition of a successful block (i.e., surgical anesthesia versus blockade of all four terminal nerves of the upper extremity), the technique used to localize the brachial plexus, and the number of injections. Success rates with single-injection techniques can vary.^{[29] [30]} Thompson and Rorie^[24] concluded that the presence of multiple compartments limits diffusion of the solution (and the success of single-shot techniques). Although Partridge and coworkers^[31] confirmed the presence of these compartments, they concluded that the “septa” dividing them were incomplete on the basis of injections of methylene blue and latex solutions into cadavers. The controversy surrounding single- versus multiple-injection techniques remains unresolved.

Eliciting a paresthesia is as efficacious as peripheral nerve stimulation (with a motor response of 0.5 to 0.8 mA). Most studies suggest that two-injection transarterial techniques are equivalent to single-paresthesia or single-nerve stimulation approaches. In general, the efficacy of paresthesia and peripheral nerve stimulator techniques increases when multiple injections are employed. Conversely, success rates with perivascular or fascial click approaches are variously reliable. ^[7] Familiarity with a variety of techniques for axillary block of the brachial plexus allows the anesthesiologist maximal flexibility in tailoring the anesthetic approach to the clinical situation.

Side Effects and Complications

Nerve injury and systemic toxicity are the most significant complications associated with the axillary approach. The assertion that neuropathies are more common with the paresthesia technique may be valid, but it is not supported by the available data. Even when paresthesias are not sought, they often occur unintentionally. ^[32] Injection of large volumes of local anesthetic, particularly with the transarterial approach, increases the risk of intravascular injection and systemic toxicity of local anesthetics. Hematoma and infection are rare complications. Central neural blockade and pneumothorax are not complications, as in other approaches to the brachial plexus.