Axillary Block

Published on 06/02/2015 by admin

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Last modified 06/02/2015

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7 Axillary Block

Perspective

Axillary brachial plexus block is most effective for surgical procedures distal to the elbow. Some patients can undergo procedures on the elbow or lower humerus with an axillary technique, but strong consideration should be given to a supraclavicular block for those requiring more proximal procedures. It is discouraging to carry out a “successful” axillary block only to find that the surgical procedure extends outside the area of block. This block is appropriate for hand and forearm surgery; thus, it is often the most appropriate technique for outpatients in a busy hand surgery practice. Some anesthesiologists find axillary block suitable for elbow surgical procedures, and continuous axillary catheter techniques may be indicated for postoperative analgesia in these patients. Because this block is carried out distant from both the neuraxial structures and the lung, complications associated with those areas are avoided.

Traditional Block Technique

Placement

Anatomy

At the level of the distal axilla, where the axillary block is undertaken (Fig. 7-1), the axillary artery can be visualized as the center of a four-quadrant neurovascular bundle. I conceptualize these nerves in quadrants like a clock face because multiple injections during axillary block result in more acceptable clinical anesthesia than does injection at a single site. The musculocutaneous nerve is found in the 9- to 12-o’clock quadrant in the substance of the coracobrachialis muscle. The median nerve is most often found in the 12- to 3-o’clock quadrant; the ulnar nerve is “inferior” to the median nerve in the 3- to 6-o’clock quadrant; and the radial nerve is located in the 6- to 9-o’clock quadrant. The block does not need to be performed in the axilla; in fact, needle insertion in the middle to lower portion of the axillary hair patch or even more distal to this is effective. It is clear from radiographic and anatomic study of the brachial plexus and the axilla that separate and distinct sheaths are associated with the plexus at this point. Keeping this concept in mind will help to decrease the number of unacceptable blocks performed. Also, this more distal approach to axillary block is similar to the mid-humeral brachial plexus block.

Position

The patient is placed supine, with the arm forming a 90-degree angle with the trunk, and the forearm forming a 90-degree angle with the upper arm (Fig. 7-2). This position allows the anesthesiologist to stand at the level of the patient’s upper arm and palpate the axillary artery, as illustrated in Figure 7-2. A line should be drawn tracing the course of the artery from the mid-axilla to the lower axilla; overlying this line, the index and third fingers of the anesthesiologist’s left hand are used to identify the artery and minimize the amount of subcutaneous tissue overlying the neurovascular bundle. In this manner, the anesthesiologist can develop a sense of the longitudinal course of the artery, which is essential for performing an axillary block.

Needle Puncture

While the axillary artery is identified with two fingers, the needle and syringe are inserted as shown in Figure 7-3. Some local anesthetic should be deposited in each of the quadrants surrounding the axillary artery. If paresthesia is obtained it is beneficial, although undue time should not be expended or patient discomfort incurred from an attempt to elicit a paresthesia. As illustrated in Figure 7-4, effective axillary block is produced by using the axillary artery as an anatomic landmark and infiltrating in a fanlike manner around the artery. Anesthesia of the musculocutaneous nerve is best achieved by infiltrating into the mass of the coracobrachialis muscle. This maneuver can be carried out by identifying the coracobrachialis and injecting anesthetic into its substance, or by inserting a longer needle until it contacts the humerus and injecting in a fanlike manner near the humerus (see Fig. 7-4).

When using a continuous catheter technique for an axillary block, stimulating or nonstimulating catheter kits may be used; I prefer the stimulating catheter (Fig. 7-5). With the nonstimulating catheter, the epidural needle is positioned either with the assistance of a nerve stimulator or with elicitation of paresthesia as an end point. After the needle is positioned, 20 mL of preservative-free normal saline solution is injected through the needle, and then the appropriate-size catheter is inserted approximately 10 cm past the needle tip. Once the catheter has been secured with a plastic occlusive dressing, the initial bolus of drug is injected and the infusion is started.

Ultrasonography-Guided Technique

Ultrasonography-guided axillary block has fallen out of favor at my institution because of our success with ultrasonography-guided supraclavicular nerve block. I believe that the supraclavicular approach is more efficient and predictable in producing surgical anesthesia; however, the following is provided as a guide to those who may wish to carry out an axillary nerve block with ultrasonographic guidance.

The operator should place the ultrasound transducer in the axilla perpendicular to the long axis of the neurovascular bundle (Fig. 7-6). The goal is to obtain an image with the axillary artery in the center as a pulsatile hypoechoic circle. Before needle insertion, the operator should attempt to identify the four major nerves of the brachial plexus at this level (Fig. 7-7). The musculocutaneous nerve may not be visualized in the same image with the other three nerves because it is often several centimeters anterior and lateral to the axillary artery; there is a high degree of variability in the axillary neurovascular structures.

Once the four nerves have been identified, color-flow Doppler mode should be used over the projected individual needle paths to rule out unsuspected vascularity. If vascularity is present, the operator should slide the transducer either more distally or more proximally to provide a clearer path for needle insertion. The needle approach is an in-plane technique, from either edge of the transducer. My experience suggests that manipulating the needle to ensure local anesthetic spread around all target nerves increases the success of the blocks. If the appearance of a neural structure is equivocal, nerve stimulation can be used to confirm the impression of a nerve. I typically inject 5 to 8 mL of local anesthetic for each nerve blocked (see Video 7: Axillary Nerve Block on the Expert Consult Website). image

To visually locate the musculocutaneous nerve, the operator should slide the transducer anterolaterally and locate the coracobrachialis muscle. The musculocutaneous nerve appears as a hyperechoic oval or circle lying between the coracobrachialis and biceps muscles (see Fig. 7-7). A separate needle insertion site is needed to block this nerve. The needle is advanced through the biceps muscle using the in-plane technique. We also use 5 to 8 mL of local anesthetic to block the musculocutaneous nerve identified by ultrasonography.

Pearls

To perform axillary block effectively, the operator must understand the organization of the peripheral nerves at the level of the lower axilla. The axillary sheath at this level is discontinuous, and multiple injections may be required to allow the axillary block to reach full effectiveness. This does not mean that a single injection cannot produce acceptable surgical anesthesia; however, the most consistently effective axillary blocks result from depositing smaller amounts of local anesthetic in multiple sites.

Another reminder when using a paresthesia-seeking axillary technique is that a radial paresthesia is obtained infrequently. Thus, the anesthesiologist should not persist in attempting to produce one but instead should inject the anesthetic in its expected position and let the local anesthetic volume produce the block. Further, because the four-quadrant axillary approach uses a “field block” to anesthetize the musculocutaneous nerve, it also does not require elicitation of a paresthesia to be effective. Because the median and ulnar nerves are more superficial when the arm is in an axillary block position, they are the nerves that paresthesia is most likely to affect. Nevertheless, unnecessarily seeking a paresthesia for an extended time, even for median or ulnar sites, may result in anesthetic delays and patient discomfort and will discourage anesthesiologists from carrying out this block. If one keeps in mind the quadrant approach to axillary block, this technique should be accomplished in an efficient manner.

A mnemonic that is useful for remembering the position of the nerves at the level of the axillary block is “M&Ms are tops” (i.e., median and musculocutaneous nerves lie more cephalad in the abducted arm). Everyone can relate the “top-notch” candy M&Ms to the cephalad position of the two “m” nerves.

When using a continuous catheter technique for postoperative analgesia or treatment of chronic pain, carefully securing the catheter will help prevent its unintentional removal. Also, the ability to place an infraclavicular catheter minimizes the number of axillary catheters needed in the anesthesiologist’s practice and is likely to improve her or his confidence in performing continuous brachial techniques.

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