Axillary Brachial Plexus Block USRA Below the Clavicle

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Axillary block of the brachial plexus remains a commonly performed means of anesthetizing the plexus, despite the requirement of placing multiple injections in order to facilitate complete anesthesia of all four nerves supplying the forearm and hand (musculocutaneous, median, radial, and ulnar nerves). Blockade of the axillary brachial plexus was successful by elicitation of parasthesia, transvascular (vascular landmark-guided) approach, or peripheral nerve stimulator-guided techniques. With the introduction of the ultrasound, brachial plexus block in the axilla has gained popularity.

In order to best visualize the sonoanatomy of the brachial plexus at the level of the axilla, the ultrasound probe is placed at the junction of the arm and the chest wall in the horizontal plane perpendicular to the axis of the humerus and the axillary vessels (Fig. 21.1a). With mild-to-moderate probe pressure, the axillary veins are collapsed and the pulsatile AA can be readily identified. With relaxation of probe pressure, one or often several axillary veins are identified superficial and deep to the AA. With some probe position variability, terminal branches (median, ulnar, radial) ofthe brachial plexus are identified as hyperechoic fascicular structures with the median nerve generally superficial and lateral to the AA, the ulnar nerve superficial and medial to the AA, and the radial nerve usually deep to the AA around the 5 o'clock position to the AA (the radial can be traced distally between the long head of triceps and humerus before passing around the spiral groove). The typical appearance of muscle tissue is evident in the biceps and coracobrachialis muscles lateral to the artery and the long head of the triceps medially (Fig. 21.8a). With small changes in ultrasound probe orientation/inclination caudad or cephalad, the hyperechoic musculocutaneous nerve can be seen within the coracobrachialis muscle (Fig. 21.8b). Typically the hyperechoic bony edge of the



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Figure 21.8a Ultrasound anatomical view in the transverse plane demonstrating the anatomical relations of the axillary brachial plexus. H humerus, AA axillary artery.

Brachial Complex Ultrasound
Figure 21.8b Ultrasound demonstrating the orientation of the musculocutaneous nerve in the axilla. Arrow pointing to the musculocutaneous nerve in cross section, AA axillary artery.

humerus can be seen deep to the axillary sheath, but probe penetration is usually set at a shallower depth than needed to see the humerus in order to magnify the view of the vessels and nerves in the axilla.

Ultrasound use can greatly facilitate performance of axillary brachial plexus block because all four nerves are readily visible around and in proximity to the AA - the median nerve (anterior/lateral to AA): when at the apex of the axilla the contributions from the medial and lateral cords can be seen uniting; the radial nerve (typically deep and medial to AA): a branch to the triceps can occasionally be identified separate from the radial nerve arising from its deep and medial side; the ulnar nerve (typically medial/superficial to AA): can be located medial to the axillary vein, but can also lie on the lateral aspect of the vein adjacent to the AA; and the musculocutaneous nerve: traced from the lateral cord high in the axilla and followed into the belly of the coracobrachialis muscle as it passes into the upper arm. Using ultrasound, axillary plexus nerves can be easily identified by confirming with nerve stimulation (appropriate muscular twitch) and/or tracing the nerves into the arm to follow their anatomical path to the elbow.

Ultrasound-guided axillary plexus block typically positions the probe in a transverse plane, and needle orientation is an in-plane technique (Fig. 21.1a). The block needle is passed from lateral to medial across the axilla (identifying the needle shaft and tip along its trajectory) in order to identify and surround each nerve with local anesthetic (Figs. 21.8c and 21.8d). Both ultrasound and nerve stimulation can be used to place the needle tip close to the nerve and confirm by electrical stimulation. As confidence increases with identification of individual nerves, there may be less need for nerve stimulation. The goal is to surround each nerve with local anesthetic by a dynamic process of real-time needle tip movement. For each nerve of the plexus, up to 10 ml of local anesthetic solution is used, but less (4 ml of local anesthetic) if ideal circumferential local anesthetic spread is visualized around each nerve.

The radial nerve may be difficult to locate or be obscured by ultrasound reflections from the AA because of its position posterior to the axillary vessels. However, it is helpful to define


Figure 21.8c Ultrasound of axillary brachial plexus block. Image demonstrating suggested block needle position for ultrasound-guided axillary block. H humerus, AA axillary artery, M median nerve, R radial nerve, U ulnar nerve. Note the location of local anesthetic (hypoechoic) spread within the axillary sheath.

Figure 21.8c Ultrasound of axillary brachial plexus block. Image demonstrating suggested block needle position for ultrasound-guided axillary block. H humerus, AA axillary artery, M median nerve, R radial nerve, U ulnar nerve. Note the location of local anesthetic (hypoechoic) spread within the axillary sheath.







Figure 21.8d Ultrasound of axillary brachial plexus block. Sonogram demonstrating anatomy of the axillary brachial plexus with needle approaching the brachial plexus in-line with the ultrasound probe. H humerus, A axillary vessels, U ulnar nerve, R radial nerve, M median nerve.

the muscular and vascular anatomy initially and then look for a hyperechoic structure posterior to the artery or vein, especially upon injection of local anesthetic. The nerves may also be followed into the upper and lower arm and confirmed by their typical anatomic paths. The musculocutaneous nerve usually can be followed high in the axilla as it divides from the lateral cord and followed in the muscular septa between biceps and coracobrachialis muscles.

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