Ultrasound Guided Serratus Anterior Plane Block

Context

Rib fractures are common in trauma, occurring in up to 10% of patients. They are a significant cause of morbidity and mortality.¹

Management of pain in the emergency department can be challenging, due to co-existing injuries and unclear medical history.

Serratus anterior plane block (SAPB) can be used as part of multimodal analgesia in the treatment of rib fractures. This module may be used for reference alongside formal practical training in performance of the block.

Definition

Ultrasound guided SAPB is a relatively new approach for providing regional analgesia to patients with anterior lateral rib fractures from ribs two to nine.

The block coverage is shown in the below diagram (hatched area).²

Rib fractures are associated with multiple complications including pneumonia, acute respiratory distress syndrome, atelectasis and lung collapse. Pain from rib fractures causes hypoventilation and increased pulmonary complications. Patients with multiple rib fractures often require critical care admission.¹

SAPB was first developed as an analgesic option in breast and thoracic surgery. The block has since been described in case reports3 for rib fracture and intercostal drain analgesia in the emergency department and in a recent feasibility study.⁴ There remains limited evidence for performing the block. The addition of a SAPB to standard rib fracture care is reported to improve pain scores and reduce in-hospital opioid requirements.¹²

Serratus anterior:

• Originates on the anterior surface of ribs 1-8 and inserts onto the medial border of the scapula.
• Lies between latissimus dorsi above, and the ribs and pleura below.
• The thoracic intercostal nerves pierce serratus anterior.
• A potential space exists superficial and deep to serratus anterior.
• Within this space run the intercostal nerves (lateral cutaneous branches), long thoracic nerve and thoracodorsal nerve. The thoracodorsal artery runs along with the thoracodorsal nerve.

These nerves can be blocked in the superficial or deep space to provide analgesia from ribs one to nine. They are ideal for anterior or lateral fractures. More posterior fractures may be better suited to a more posterior nerve block such as an erector spinae (ESP) block, paravertebral block, or neuraxial technique if suitable. Nevertheless, in those where the lateral or sitting position to allow more posterior nerve block techniques is not possible (pain, concomitant injury, spinal precautions), an SAPB, which is able to be performed in the supine position, will likely provide analgesia for more posterior fractures.¹²

Patients should be assessed with a validated risk score, for example Battle et al.s STUMBL (STUdy of the Management of BLunt chest wall trauma) score;⁶ this predicts the risk of complications secondary to the rib injury, using patient age, number of fractures, underlying lung disease, anti-coagulant use and SpO2. The patients pain score should also be assessed.

Management is decided by the risk score and pain score, according to local rib fracture pathways. For example, low risk scores may be considered for oral analgesia and discharge; moderate risk scores for admission, PCA, and consideration of a nerve block procedure / anaesthetic team review; and higher risk scores for critical care review. Physiotherapy, high-flow nasal oxygenation, and pain team involvement may also be helpful.

Patients with anterior lateral rib fractures with a moderate / higher risk score or high pain score, those with more posterior fractures in whom the lateral or sitting position to allow more posterior nerve block techniques is not possible or these techniques are contraindicated, or in those in whom a chest drain is inserted, may benefit from SAPB as part of multimodal analgesia.

Advantages of SAPB

• Technically easy and superficial block.
• Performed with patient supine.
• Suitable for patients with associated spinal trauma or head injuries where paravertebral and epidural blocks may be contraindicated.
• Can be inserted in patients with a coagulopathy.⁷
• Can form part of an opiate sparing analgesic strategy.⁸

Contraindications to SAPB

There are few contraindications to performing the block.

Absolute

• Patient refusal
• Local anaesthetic allergy
• Infection at needle insertion point

Relative

• Distorted anatomy e.g. surgical emphysema

Complications of SAPB⁹

• Pneumothorax due to puncture of the pleura
• Vascular puncture the thoracodorsal artery lies medially
• Nerve damage unlikely with a plane block however the thoracodorsal nerve does run with the thoracodorsal artery
• Failure/inadequate block
• Local anaesthetic systemic toxicity (LAST)
• Infection

As with all regional techniques: Consent should be obtained, resuscitation / emergency (e.g. Intralipid) equipment should be available, intravenous access secured, standard monitoring applied (including three lead ECG, blood pressure and oxygen saturations) and asepsis maintained including the use of a dedicated single-use sterile transducer cover covering the probe and cable.¹⁰

A prep, stop, block safety check should be performed prior to starting the block to prevent wrong site injection.¹¹

Local anaesthetic dose

Levobupivacaine is the local anaesthetic of choice due to its duration of action (10-12 hours). The maximum dose of Levobupivacaine is 2mg/kg (based on lean body weight).

SAPB is a volume block; to be effective, a reasonable volume must be injected.

40mls of 0.25% levobupivacaine (100mg) is a standard dose in patients over 50kg.

If using 0.5% levobupivacaine, 20ml of 0.5% levobupivacaine (100mg) can be diluted in 20ml of 0.9% saline to give a total volume of 40ml.

If patients are below 50kg, the dose must be adjusted.

Local anaesthetic can be provided to skin at the insertion point prior to performing the block with 1 2ml of 1% lidocaine (10-20mg). Note that the total local anaesthetic dose should not exceed the combined maximum dose.

Equipment

• High frequency (linear) ultrasound probe with probe cover and sterile gel
• 50mm block needle (may change dependent on body habitus)
• Suitable cleaning solution (e.g. hydrex pink 0.5% chlorhexidine)
• Local anaesthetic in 2 x 20ml syringes
• 2ml syringe with 25G (orange) needle for skin infiltration (if using)

The patient is positioned supine or semi-recumbent, with the arm abducted (Figure 1).

 

The 5th rib is identified by scanning down from the clavicle. The probe is then rotated and held in the transverse plane (Figure 2).

 

The block is performed at the level of the 5th rib in the mid-axillary line via an in-plane technique (Figure 3).

 

With the probe positioned as in Figure 3, the following image is obtained.

 

Prior to performing the block, colour Doppler should be used to identify any vascular structures which may lie in the path of the needle.

 

Superficial block

The superficial SAPD is performed with the needle placed as shown in Figure 3. The needle is inserted medial to the probe and directed laterally and inferiorly in the sagittal plane. The needle is placed into the fascial plane between latissimus dorsi and serratus anterior around 1-2cm below the skin. Local anaesthetic (around 40mls) is then injected to hydrodissect between the muscles. (Figure 6)

Deep Block

The deep SAPB is performed with the same needle placement (see Figure 3). Ribs are seen as rounded structures with a bright hyper-echoic outline and black-out below this. The pleura is usually seen also as a bright hyper-echoic line sliding with respiration deep to the ribs. The needle is directed down onto the rib, taking care to avoid the pleura. It is then moved back slightly and local anaesthetic injected between the rib and serratus anterior muscle. This approach may be favourable in those with distorted anatomy secondary to rib fractures, or in the elderly where muscle layers may be poorly defined.

 

• As with all local anaesthetic blocks, safe injection is ensured by keeping the needle tip visualised throughout and repeated negative aspiration (at least every 5mls) to detect blood and help prevent accidental intravascular injection.
• If the patient experiences any paraesthesia the needle should be repositioned.
• The deep block may be easier to perform in those in distorted anatomy however there is increased risk of pneumothorax due to the proximity of the pleura.
• The block will not provide immediate analgesia and has an approximate onset time of thirty minutes.
• If local anaesthetic is used in any other procedure, e.g. for chest drain insertion, beware the total local anaesthetic dose is not a toxic dose.
• Knowledge of the management of local anaesthetic toxicity and the location of intralipid is important when performing any block.¹²
• The block will only provide temporary analgesia so should trigger a referral for ongoing assessment of pain score and consideration of catheter regional techniques.
• A serratus anterior catheter can be inserted whilst performing the block, but this should be performed in a sterile area, for example operating theatres, with a fully sterile approach (gown, gloves, drape, hat, mask, equipment etc.) to reduce the risk of infection.
• This is a low risk technique that requires minimal repositioning of the patient, which is evidenced to be effective as part of multimodal analgesia in the management of patients with rib fractures.

1. Witt CE, Bulger EM. Comprehensive approach to the management of the patient with multiple rib fractures: a review and introduction of a bundled rib fracture management protocol. Trauma surgery & acute care open, 2(1), e000064. [Accessed 1/5/21].
2. Blanco R, Parras T, McDonnell J. G, Prats-Galino A. Serratus plane block: a novel ultrasound-guided thoracic wall nerve block. Anaesthesia, 68(11), 11071113. [Accessed 1/5/21]
3. Durant E, Dixon B, Luftig J, et al. Ultrasound- guided serratus plane block for ED rib fracture pain control. American Journal of Emergency Medicine 2017; 35: 197.e36.(Accessed 1/5/21].
4. Schnekenburger M, Mathew J, Fitzgerald M, et al. Regional anaesthesia for rib fractures: A pilot study of serratus anterior plane block. Emergency medicine Australasia : EMA, 2021, 33: 788-793. Advance online publication. [Accessed 1/5/21].
5. 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. Published online May 01, 2024.
6. Battle, C.E., Hutchings, H., Lovett, S. et al. Predicting outcomes after blunt chest wall trauma: development and external validation of a new prognostic model. Crit Care 18, R98 (2014). [Accessed 1/5/21].
7. El-Boghdadly, K, Wiles, M. D. Regional anaesthesia for rib fractures: too many choices, too little evidence. Anaesthesia, 74(5), 564568, 2019.[Accessed 1/5/21].
8. Rose, P., Ramlogan, R., Sullivan, T. et al. Serratus anterior plane blocks provide opioid-sparing analgesia in patients with isolated posterior rib fractures: a case series. Can J Anesth/J Can Anesth 66, 12631264, 2019.[Accessed 1/5/21].
9. L May, C Hillermann, S Patil. Rib fracture management, BJA Education, Volume 16, Issue 1, January 2016, Pages 2632, [Accessed 1/5/21]
10. Topor B, Oldman M, Nicholls B. Best practices for safety and quality in peripheral regional anaesthesia. BJA Educ. 2020;20(10):341-347. doi:10.1016/j.bjae.2020.04.007
11. Haslam, N., Bedforth, N. and Pandit, J.J. (2022), Prep, stop, block: refreshing stop before you block with new national guidance. Anaesthesia, 77: 372-375. https://doi.org/10.1111/anae.15647
12. Management of severe local anaesthetic toxicit. Association of Anaesthetic, 2010. [Accessed 1/5/21].
13. Image1 Cutaneous distribution of thoracic nerves. (Testut.). Fig.820. Henry Gray (18251861). Anatomy of the Human Body. 1918. Bartleby.
14. Image2 Diagram of the course and branches of a typica intercostal nerve. Fig.819. Henry Gray (18251861). Anatomy of the Human Body. 1918.

Additional Resources

• Critical Care. Northhampton.com
• Blanco R, Barrington MJ. Pectoralis and Serratus Plane Nerve Blocks. NYSORA.