Axillary Block

Using ultrasound guidance the musculocutaneous, radial, median, ulnar and medial cutaneous nerves of the forearm can be reliably anaesthetised. At this level, the terminal branches of the brachial plexus begin to take on the more fasciculated appearance typical of peripheral nerves. Ultrasound visualisation of the needle is enhanced by the relatively superficial path to the nerves. The axillary plexus and the more peripheral nerves are good first blocks for those new to ultrasound techniques. The needle insertion may be performed across plane or in-plane, a perpendicular approach is also possible. Care must be taken to locate each individual nerve.

Fig 2.27 Diagram of the cross-sectional anatomy of the axillary brachial plexus

A common technique is an in-plane approach with the axillary vessels, nerves and muscles in short axis using a nerve stimulator to confirm the identity of nerves. The probe is placed transversely in the axilla of the abducted arm as for a nonultrasound guided axillary block. Pie graphs have been constructed to approximate the usual positions of the nerves relative to the artery however variability is common. 

Fig 2.29 Sonogram of axillary brachial plexus. The nerves as identified were traced by short-axis sliding and are in typical positions. The presence of oflatissimus dorsi tendon means the radial nerve will still be close to the artery. Musculocutaneous nerve is only just starting to pierce brachioradialis. The anatomy will probably become clearer after some solution is put in. It is particularly important to avoid bubbles which will impair the view

 The radial nerve can be more challenging to see and often has a slower onset of anaesthesia than the other nerves. This may be due to the depth of the radial nerve and its variable position. Often there is a hyperechoic structure immediately deep to the axillary artery, which may be the radial nerve but is often artefact. The position of the radial nerve has the most variability in relation to both the pie chart and distance from the artery. It may be located between the artery and ulnar nerve but is usually deeper, anywhere from 3 to 7
O’clock.

Passing the ultrasound probe down the arm beyond the tendon of latissimus dorsi the nerve may be seen passing posteriorly into the spiral groove of the humerus with accompanying vessels. Using a stimulator direct stimulation of the triceps muscle may occur however to be sure that the main trunk is being stimulated aim to achieve a distal response. The volume of local anaesthetic injected should be 10–15ml.

Fig 2.31 Near the completion of an across plane axillary block. Note the numerous dark spaces of local anaesthetic with fibrous septae between There is a nerve visible in the centre of the picture towards the left side, the needle tip is contacting the nerve and is visible in the video image but not identifiable in the still. (the patient experienced a parasthesia)

Fig 2.32 Needle and probe position for axillary block, perpendicular in-plane approach.

Confirmation of a nerve can be achieved by tracing it along its known course.Along the arm (proximal to distal) themedian nerve maintains a close perivascular position and at the elbow isimmediately medial to the artery. Theradial nerve diverges from the arterytowards the humerus and the ulnar nerveremains in a more superficial positionfurther from the artery.