In the axilla and the upper arm, the neurovascular bundle is located in
the internal bicipital sulcus, which separates the flexor muscle compartment
of the arm (biceps and coracobrachialis muscles) from the extensor
compartment (triceps). At this level, terminal braches of the brachial
plexus, such as the musculocutaneous, median, ulnar, and radial nerves, are
located superficially, usually within 1 to 2 cm of the skin. A linear 10- to
15-MHz probe is therefore recommended. To obtain a transverse view of the
neurovascular bundle with the arm abducted at 90 degrees and the forearm
flexed, the probe is positioned perpendicular to the long axis of the arm,
as close to the axilla as possible (Figure 51–4A). The round
pulsatile axillary artery is easily identified in the bicipital sulcus and
is distinguished from the axillary veins that are readily compressed. Nerves
in the axilla are round to oval-shaped and hypoechoic with internal
hyperechoic areas, presumably the epineurium. In the axillary region, the
median and ulnar nerves are usually lateral and medial to the artery,
respectively (Figure 51–4B). The radial nerve is often posterior or
posteromedial to the artery, but nerve location is highly
variable.17 The musculocutaneous nerve often branches off
more proximally and can be seen as a hyperechoic structure. It can be found
between the biceps and coracobrachialis muscles for a short distance before
entering the body of the coracobrachialis muscle (see Figure 51–4B). When
performing an axillary block, it is best to inject local anesthetic around
each nerve individually to achieve consistent success. Local anesthetic
spread within the sheath compartment may be presumably restricted by the
septa when observed under ultrasound.18