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The Axillary Brachial Plexus Block

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he axillary brachial plexus block is a time-honored favorite among blocks.      Many styles were developed over the years but the ultrasound guided technique has become a standard in facilities that have access to an ultrasound machine because of its simplicity and reproducible success.

The axillary brachial plexus is an ideal anatomical venue for the use of ultrasound guidance.  The target structures are invariably superficial and are predictably arranged among easily identifiable muscular and vascular landmarks.

The 3 primary nerves (median, ulnar, & radial) of the brachial plexus supplying the arm and hand are arranged around the brachial artery and the last important nerve (the musculocutaneous) is held immobile within the substance of the coracobrachialis muscle.

Placing a small amount of local anesthetic solution (3-8 ml) at each nerve location will cause almost immediate anesthesia over their respective areas of innervation.

Still the block has practical limitations or at least ideal applications.  Injecting local anesthetic around the brachial plexus at the axillary level produces anesthesia best from the level of the mid-forearm to the hand.  Procedures above mid-forearm to the shoulder seem better tackled with a supraclavicular level approach to the brachial plexus.  Procedures involving primarily the shoulder such as arthroplasties and clavicular repairs lend themselves more to the interscalene level approach. 

Likewise, in order to perform the axillary approach you must have access to the axilla.  If for some reason the patient cannot or will not raise their operative arm high enough to allow you to reach the axilla, you are left using an alternative.  In these cases, it is reasonable to use a supraclavicular, infraclavicular, or even interscalene technique.  If the operative procedure is to involve the hand, the higher the block, the longer the “soak time” to allow the local anesthetic molecules to reach those more central portions of the nerve that cover the hand.

 

The Block in Brief

The Axillary Brachial Plexus Block

 

Comments

Patient Position

Supine

 

Ultrasound Probe

10 – 14 mHz Linear

High range, Linear array

Needle

2”

 

Needle Approach

In-Plane or Out-of-Plane. Out-of-Plane may be preferable to place local behind artery for ulnar nerve contact.

If Out-of-Plane approach used may need 2nd entry point to reach musculocutaneous n.

Local Solution Volume

15 – 30 ml

 

Key Anatomy Landmark

 Brachial Artery at Axilla

Median, Ulnar, Radial nerves positioned around artery. Musculocutaneous nerve in nearby coracobrachialis muscle.

Miscellaneous

Slightly different patterns of nerve location exist. 

Danger of Patient Falling while block working

 

Patient Position & Probe Placement

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The patient for axillary block should be supine with the operative arm abducted, and if possible, elbow flexed and hand place near or behind the head for stability.  It's helpful to place a pillow behind the forearm and elbow to increase the comfort and stability of the position.  Move the pillow slightly away from the operative side to allow more room for the hands of the person doing the block.

The axilla and anterior portion of the shoulder should be prepped using your facility's protocols, and sterile gel should be placed just posterior to the insertion of the pectoralis major muscle on the proximal humerus, perpendicular to the axis of the brachial artery.

The first landmark to look for is the brachial artery which lies near the surface.  Alongside the artery, there will be at least 2 veins which will collapse with even moderate pressure on the ultrasound probe.  It is a good idea to collapse the veins with the probe to simplify the needle approaches to the nerves.

Image Interpretation

Adjust the probe so that the brachial artery profile is in the center of the picture and adjust the scan depth so that the artery and its surrounds take up most of the screen.

In the resulting picture the following rules are usually true;

  • The nerve in the most lateral position is the Median Nerve
  • The nerve in the most medial position is the Ulnar Nerve
  • The nerve behind the artery is the Radial Nerve

 

Needling

The nerve targets of the axillary brachial plexus are generally fairly superficial, between 1 – 2 cm in depth, so your block needle doesn't need to be more than 1 – 1 inches in length.

The nerves can be approached using either the In-Plane or the Out-of-Plane approach, but the Out-of-Plane crosses less tissue and is described here.

Create a skin wheal centered on the brachial artery and parallel to the axis of the ultrasound probe approximately 1 cm long to allow for 2 needle entry points to address the spread of nerves. 

Insert the block needle through the skin and aim tangentially either at the median or ulnar nerve.  Attempt to place the needle tip next to the target nerves.  This can be made easier by periodically aspirating and injecting a small amount of local solution and then using the space created by the injected volume to maneuver the needle tip into a more advantageous position.

Keep in mind that there are at least a couple veins being compressed by the ultrasound probe while you maneuver the needle and inject.  It is important that you see the volume accumulate in the tissues as you inject.  If you are injecting and there is no evidence of the volume on the ultrasound screen, it is possible you are inadvertently injecting intravascularly.  Aspirate on the syringe and adjust the ultrasound probe to include the needle tip in the picture.  Adjust the needle position if necessary.

Approach and inject around each of the 3 nerves surrounding the axillary artery using this method. 

After taking care of the 3 para-arterial nerves, adjust your ultrasound probe to view the musculocutaneous nerve in the coracobrachialis muscle, just lateral to the artery.  The musculocutaneous nerve usually lies at a depth of 3 – 4 cm within the substance of the coracobrachialis.  The nerve appears as a bright, echodense, usually linear, structure, arranged parallel to the skin surface.

Because of the position of the musculocutaneous nerve in relation to the other nerves of the brachial plexus at the axilla, it is sometimes necessary to make a separate needle entry point to address the musculocutaneous nerve if you are using the out-of-plane needle approach.  This is why we advise a longer skin wheal at the beginning of the procedure.  If necessary, just remove the needle from the original position and reinsert it slightly more laterally to get an unimpeded path to the musculocutaneous nerve. 

Whatever needle approach you favor, aim straight for the musculocutaneous nerve and watch the ultrasound screen for movement of the nerve indicating that the needle is pressing against it.  When this is noted, aspirate and inject the local anesthetic solution.

The amount to inject at each site varies according to your clinical judgment.  A good, solid block can be created with as little 2 - 3 mls per nerve, the duration of the block will vary directly with volume injected up to about 7 to 8 mls per nerve.  Duration will vary from 8 – 24 hours based on the amount and proximity of the local solution injected and whether epinephrine was added to the solution.

Miscellaneous

As with other brachial plexus block positions, local cooling at the site of the injection can extend the duration of the block without additional pharmaceutical adjuncts.  We prefer the use of refrigerated cans of soda pop (or anything else, like soup, or peaches or beans) over the use of ice bags, because of the risk of thermal injury when used on insensate areas.  Placement of a cold can over the injection site for 10 – 15 minutes every hour can slow redistribution of the local anesthetic and make the block last significantly longer. 

 

 

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