Regional blocks III: Serratus Anterior Plane Blocks

The serratus anterior plane block (SAPB) is probably one of the easiest regional blocks to start doing in the trauma setting, as it requires the least amount of training and potentially has the greatest impact. The recent SABRE trial [1] showed how the block can reduce pain and opioid requirements for patients with rib fractures without increasing the risk of complications like pneumonia. This adds to the already extensive body of literature that supports the early use of regional anaesthesia in the trauma bay.

So, with that said, let’s talk detail:

The SAPB is a single shot block (i.e., one pass of the needle) that aims to anaesthetise or analgese multiple dermatomes of the anterolateral chest wall. It can be done safely and quickly with minimal risk to neighbouring structures, which makes it a very attractive block - not only for the management of multiple rib fractures, but also for intercostal drain insertion. Note that posterior rib fractures are better anaesthetised with an erector spinae plane block.

Images courtesy of the POCUS Atlas [2]

Anatomy

The plane between serratus anterior and latissimus dorsi contains the lateral cutaneous branches of each intercostal nerve, which can be seen below.

The local anaesthetic then reaches the nerves of adjacent spaces by spreading up and down through that fascial plane. The extent of this spread will depend on the level of the block and the volume of local anaesthetic infiltrated. There must be some degree of tracking deeper to the intercostal nerves themselves, as these also supply the ribs.

Procedure

1. Consent the patient for risks: as always there is a risk of damage to neurovascular structures (though there are few), infection, bleeding, failure of the procedure, local anaesthetic systemic toxicity, anaphylaxis, and - if you're truly lost - pneumothorax. Put them in a place where these complications can be monitored for and managed if necessary.

2. Position the patient in the most practical way that is still safe and comfortable for them. If the patient is still in spinal precautions you don’t really have any other option but supine, but if they are able to turn on their side, it does make your life a little easier. You basically want access to their chest wall at the level of the fractures in the mid-axillary line. Find the area you can best visualise the sonoanatomy as labelled in the pictures below, then determine where to best placed your ultrasound and trolley. Good ergonomics makes all the difference.

3. Draw up your pre-calculated dose of local anaesthetic (typically ropivacaine or bupivacaine), with any adjuvants, and dilute with normal saline, aiming for a total volume of 40mL. The serratus anterior plane is a large potential space, and you need volume to spread up and down the multiple nerve levels, so use extension tubing and two 20mL syringes.

4. Clean the skin with iodine or chlorhexidine, drop on your sterile ultrasound gel, and cover your probe.

5. Orient the probe transversely and place it at the mid-axillary line, as in the diagrams below, with your probe marker on your left. Use the high-frequency linear probe, though if the patient has a large chest wall and the plane is more than a few centimetres deep then you might need to use the curvilinear probe for deeper penetration. Keep in mind that if this is the case your needle might not be sufficiently long, and you may need to reconsider your site and approach.

6. Look at the visible structures and orient yourself:

Diagrams courtesy of Highland Ultrasound [3]

7. Now the most satisfying part of all. Using an in-plane approach like the one seen above, you keep your probe completely still and insert the needle through cleaned skin in-line with it. Watch as your needle tracks across the screen into the plane above serratus. Always visualise the tip of your needle! This is the core of ultrasound guided regional anaesthesia. Aspirate to make sure you aren’t in a vessel, then inject a small test 1-2mL. You should see very pleasing hydrodissection in the plane you are targeting. If you see muscle belly splitting apart from the local anaesthetic then reposition your needle and repeat these steps until you see the muscles “unzip” from the local being injected into the potential space. Well done! Keep going until all the local is in, then you can withdraw.

That’s it. All of this can take less than 5 minutes, trust me. Document your procedure, wait for the 20-30 minutes for the block to start taking effect (bearing in mind it may take up until an hour sometimes to work fully). Monitor the patient for about 20 minutes and that a wrap. Off to your next patient.

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Dr Marcus Kruger is a senior emergency medicine registrar with a passion for ultrasound, trauma, pain management, and perfecting the ultimate smash burger.

He has worked in a number of Emergency Departments in the Gauteng Province, and before that spent his time as a junior doctor in Durban. He loves empowering junior colleagues with new and practical skills.

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References

[1] Partyka C, Asha S, Berry M, et al. Serratus anterior plane blocks for early rib fracture pain management: the SABRE randomized clinical trial. JAMA Surg. 2024;159(7):810-7.

[2] Thoracoabdominal blocks. The POCUS Atlas [internet]. Accessed 29th May 2025. Available from: https://www.thepocusatlas.com/thoracoabdominal-blocks.

[3] Serratus anterior plane (easy). Highland Ultrasound (internet). Accessed 28th May 2025. Available from: https://highlandultrasound.com/serratus-anterior-plane-easy.