Axillary Block of the Brachial Plexus

Fig. 22.1

Patient positioning and surface landmarks for axillary block of the brachial plexus

The patient is positioned supine with the arm abducted 70–80° and externally rotated, the elbow flexed at 90°, and the dorsum of the hand facing the table. Surface landmarks include:

Anterior axillary fold
- Formed by the pectoralis major muscle
- Palpating the axillary artery just deep to the insertion of this muscle will identify the optimal site of needle insertion if attempting to block the musculocutaneous nerve.
Posterior axillary fold
- Formed by the latissimus dorsi and teres major muscles.
- The lower part of the axillary artery can be palpated anterior to this fold on the medial side of the arm; trace this pulse to a proximal location for an optimal block for targeting all the nerves.
Bicipital sulcus or groove
- A groove between the tendons of the biceps and triceps brachii muscles

22.3 Nerve Stimulation Technique (Figs. 22.2 and 22.3; Tables 14.1 and 22.1)

Fig. 22.2

Flowchart of procedures and needle insertion site for axillary brachial plexus block

Fig. 22.3

Flowchart of the procedure for employing nerve stimulation techniques for axillary brachial plexus block

Table 22.1

Responses and recommended needle adjustments for nerve stimulation-guided axillary brachial plexus block

Correct response from nerve stimulation

Hand twitch with approximately 0.4 mA (median, radial or ulnar nerve)

For a higher success rate, multiple injections at each nerve are recommended:

Median nerve (C5–C8, T1; flexion of middle, index fingers, and thumb and pronation and flexion of wrist), ulnar nerve (C7–8,T1; flexion of ring and little fingers and ulnar deviation of wrist), radial nerve (C5–8, T1; extension of fingers and wrist)

Other common responses and needle adjustments

Muscle twitches from electrical stimulation

Upper arm (local twitches from biceps or triceps)

Explanation: needle angle is too superior or inferior

Needle adjustment: withdraw completely and redirect accordingly

Vascular puncture

Axillary artery with arterial blood in tubing

Explanation: needle in lumen of axillary artery

Needle adjustment: inject 2/3 of the local anesthetic posterior to the artery and 1/3 anterior to the artery

Axillary venous blood in tubing

Explanation: needle in lumen of axillary vein

Needle adjustment: redirect slightly more laterally or superiorly

Paresthesia without motor response

Needle has contacted brachial plexus

Explanation: stimulator, needle, or electrode malfunctioning

Needle adjustment: none if typical distribution of paresthesia (inject); reinsert if atypical

Bone contact

Humerus (2–3 cm deep)

Explanation: needle has advanced beyond plexus, too deep

Needle adjustment: withdraw to subcutaneous tissue and reinsert with an angle 20–30° more superior or inferior

A 30–50 mm, 22–24G insulated needle is typically introduced at the upper border of the axillary artery in a direction towards the midpoint of the clavicle using approximately a 45° angle to the skin and directing the needle medially, dorsally, and caudally. A perpendicular needle direction may also be used, with the needle directed towards the upper border of the artery and the humerus.
A “pop,” indicating loss of resistance, is felt as the needle penetrates the axillary sheath surrounding the plexus.
Applying an initial current of 0.8–1.0 mA (2 Hz, 0.1 ms) is sufficient for stimulation of the plexus. The current is reduced to aim for a threshold current of 0.4 mA (0.1 ms) while obtaining the appropriate motor response.
A distal motor response in the hand is ideal, although forearm twitches may be used.
Bone contact indicates that the needle has advanced to the humerus and that the needle should be withdrawn.
Arterial blood in the tubing indicates axillary artery puncture, and the needle should be withdrawn slightly; venous blood (axillary vein) necessitates withdrawing and redirecting the needle more laterally or superiorly.
For the separate block of the musculocutaneous nerve, elbow flexion is the desired motor response.
The musculocutaneous nerve is usually found between the biceps and coracobrachialis muscles (Fig. 22.6a). In order to block the nerve, a separate needle puncture within the belly of the coracobrachialis muscle is performed. After grasping the belly of the muscle, use the same needle insertion site as for the axillary block, and direct the needle towards the interior of the muscle.
If a tourniquet is required for the surgery, the intercostobrachial nerve should be anesthetized using a small ring of local anesthetic at the upper aspect of the arm. Alternatively, a small amount of local anesthetic solution can be injected during the withdrawal of the needle.