FRCA Notes

Axillary Brachial Plexus Block

Resources

  • Anaesthetises the terminal branches of the brachial plexus in the axilla
  • The posterior, lateral and medial cords descend from the infraclavicular fossa into the axilla, further dividing into the main terminal branches of the plexus:
    1. The radial nerve, arising from the posterior cord
      • Lies posterior (deep) to the axillary artery, typically on the lateral side in the 4-6 o'clock position

    2. The median nerve, arising from branches of both the lateral and medial cords
      • Supero-laterally and close to the artery i.e. in the 9 o'clock to 12 o'clock quadrant

    3. The ulnar nerve, arising from the medial cord
      • Most often in the 2 o'clock position relative to the artery

    4. The musculocutaneous nerve
      • Leaves the plexus more proximally such that at the axilla it tends to lie in the fascial plane between coracobrachialis and short head of biceps, or within the body of either muscle
      • In 16-18% of cases it is adherent to the median nerve, instead leaving the plexus distally
      • In a proportion of cases it lies close to the artery in the 7-8 o'clock position, distinct from the median nerve

  • The radial, median and ulnar nerves tend to lie in close proximity to the axillary artery, within the axillary sheath
  • The classic anatomical description of the nerves relative to the axillary artery is only the case in approximately two-thirds of cases, with an otherwise variable configuration

  • The axillary vein most commonly lies between the 12 and 2 o'clock position with respect to the axillary artery i.e. somewhere between the median and ulnar nerves
  • Its position is, however, variable and there may be additional venous vasculature present, particularly in the 6 o'clock position

Distribution

  • Provides anaesthesia from the mid-arm downwards

  • Typically misses:
    • The radial nerve due to poor visibility, and therefore fails to anaesthetise the lateral aspect of the forearm
    • The musculocutaneous nerve

  • Doesn't anaesthetise:
    • The axillary nerve, and thus the skin over the deltoid (axillary badge are)
    • The intercostobrachial nerve (T2 dermatome), and thus the skin over the medial upper arm

Indications

  • Suitable for elbow, forearm and hand surgery
  • Coverage of the medial upper arm may facilitate AV fistula formation

Contraindications

General Specific
Patient refusal Inability to abduct arm appropriately
Local anaesthetic allergy
Localised infection
Lack of appropriate resources
Pre-existing nerve injuries


Standard regional anaesthetic set-up

  • Patient consent obtained
  • Ensure appropriate equipment available
  • Ensure adequate staffing i.e. trained assistant
  • Access to resuscitation equipment including intralipid
  • IV access obtained
  • Monitoring in situ
  • Correct LA dosing calculated
  • Stop Before You Block moment

Positioning/equipment/ergonomics

  • Ultrasound (linear probe) ± peripheral nerve stimulator ± pressure monitoring
  • Ultrasound associated with shorter block performance time and reduced incidence of complications such as vascular puncture
  • 50-80mm echogenic short-bevel needle

  • Supine or semi-recumbent position
  • Ipsilateral arm is abducted to 90° and externally rotated, with the elbow flexed
  • Beware excessive abduction, which may obscure the vasculature, limit proximal spread of LA, cause pain for the patient or stretch the plexus and render it prone to injury
  • Operator either in front of the exposed axilla with ultrasound behind the patient's arm, or vice versa

Local anaesthetic

  • Total injection volume recommended varies widely, from 15-40ml
  • Minimum effective volume in 90% of patients is quoted as 5.5ml for the musculocutaneous nerve and 23.5ml for the remaining nerves i.e. a total of 29ml
  • However other evidence suggests only 1-2ml per nerve may be needed for a successful block

Performance

  • The classical landmark approach to the axillary block involved puncturing the axillary artery to ensure proximity of the nerves

  • Linear probe is placed transversely on the proximal humerus to obtain a short-axis view of the relevant structures
  • The landmark, as with other BPBs, is the axillary artery as well as the conjoint tendon of teres major and latissimus dorsi
  • Hydrodissection may be necessary to delineate needle tip position and confirm nerve positions around the artery, particularly the radial nerve
  • NB there is frequently an acoustic enhancement artefact deep to the artery which can be mistaken for the radial nerve

  • Often multiple injections are required, with repositioning of the needle as required, targeting:
    1. The radial nerve by injecting posterior (deep) to the artery
    2. The ulnar and median nerves
    3. The musculocutaneous nerve

  • Frequent aspiration and slow administration of LA are necessary, as only slight pressure may compress axillary veins and lead to inadvertent puncture ± intra-vascular injection

Specific

  • Benefits from the lowest rate of complications of the brachial plexus blocks, and in particular lacks significant complications of more proximal blocks

Generic