Serratus Anterior Block

Rationale.

WHY SERRATUS ANTERIOR BLOCK?

Around 15% of all trauma admissions are due to blunt chest wall injury. In the United Kingdom alone around 16,000 patients a year are registered in the Trauma Audit & Research Network with chest wall injury. Rib fractures do not only occur after high-speed RTC’s, but also after seemingly minor falls in the elderly population. They are associated with high mortality and morbidity (1, 2)

Achieving adequate analgesia is a cornerstone in rib fractures, not only for comfort but also to ensure adequate ventilation and oxygenation. In a patient with already injured lung tissue and V/Q mismatch, the inability to increase minute volume can have severe consequences. Further, pain impairs the patient’s ability to clear secretions by coughing, leading to atelectasis and possibly pneumonia.

As with other injuries, regional anaesthesia has the potential to provide fast, direct and prolonged analgesia, with fewer systemic side effects compared to procedural sedation or systemic analgesia. In advanced-age patients with multiple comorbidities, managing pain by blocking nerves can significantly reduce the need for opioid use (3).

The serratus anterior block is a safe and technically simple form of regional anaesthesia that can provide effective analgesia for up to 14 hours. It provides reliable anaesthesia from the 2nd until the 9th rib.

WHY USE ULTRASOUND-GUIDED NERVE BLOCKS?

Ultrasound guidance allows clinicians to visualize the target structures (nerves and fascial planes), monitoring the advancement of the needle and surrounding structures in real life to guide accurate placement of the anaesthetic agent adjacent to the nerve. It additionally shows structures to avoid, such as vascular structures, increasing the procedure safety.

WHO?

Often the STUMBLE Score (5) is used to risk stratify patients and decide who should be considered for a regional nerve block. In basis, any patient with 2+ acute rib fractures and without absolute contra-indications is a candidate for this block. Both the patient’s comfort and vital signs can be a stimulus to perform the block sooner rather than later. SAB is often only advised for anterolateral fractures. However, more research is emerging that shows that it also provides analgesia for more posterior fractures (6,7).

Anatomy.

The nervous supply of the chest is provided by the intercostal nerves, that originate from the ventral ramus of each thoracic nerves. The intercostal nerves run from posterior to anterior deep to the intercostal membrane, further in the costal groove. From the second rib on, around the angulation of the rib the lateral cutaneous branches arise and penetrate through the intercostal muscles and serratus anterior muscle. They divide into anterior and posterior branches. The intercostal nerve itself continues to run deep and pierces through the transverse thoracic muscle directly next to the sternum, where it becomes the anterior cutaneous branch. The SAB aims to block both the lateral cutaneous nerves as the anterior cutaneous nerve.

The serratus anterior muscle originates from the first to the eigth or nineth rib and lays as a fan over the chest towards posterior where it attaches to the scapula. Medial to it lies superficially the pectoralis minor. Lateral it is overlayed by the latissimus dorsi.

The serratus plane block aims to distribute local anaesthetic in the fascia surrounding the serratus anterior muscle. This can be done either superficially, by injecting in the fascia between the latissimus dorsi and serratus anterior, or deep, by injecting between the serratus anterior and the intercostal muscles and rib.

Considerations.

The only absolute contra-indication is an allergy to local anaesthesia. As with every invasive procedure, there is a risk of infection and bleeding post-procedure. Although the use of ultrasound increases safety in this procedure, local anaesthetic systemic toxicity is a rare but well-known complication of regional anaesthesia.

Anticoagulation use or a bleeding disorder is not an absolute contra-indication for this block. Isolated or multiple posterior fractures could trigger the clinician to consider the use of other regional anaesthesia, such as an erector spinae bloc, paravertebral block or epidural catheter. However, especially if not readily available, SAB can be used in posterior fractures as well.

The block might fail, especially when distortion of the anatomy by trauma or extensive subcutaneous emphysema limits the visibility of the anatomical structures.

The RCEM has alerted recently on a ‘side-effect’ of an effective block: by removal of the stimulus, patients that previously received opioids might become more prone to respiratory depression and sedation.

The patient should be informed of the procedure, the alternatives and possible complications, and should give consent.

PROBE

High-frequency, Linear

  • Pre-set: musculo-skeletal

  • Depth: depending on the patients habitus around 3-5 cm

  • Marker: probe marker pointing toward the patient’s anterior chest.

Procedure.

SETTING

The placement of a serratus anterior block should take place in an area where the patient can be monitored during and 30 minutes after the procedure. It should be performed by a clinician trained in US-guided nerve blocks and the equipment, along with emergency drugs and intralipid solution should be readily available.

Serratus anterior block is a volume block, with the minimum volume advised to use being 30-40 ml. Dilution of local anaesthetic using normal saline is almost always done, considering the maximum safe dose. Usually levobupivacaine is the preferred drug, giving the prolonged length of action (18-20 hrs) and its better safety profile compared to bupivacaine.

EQUIPMENT

  • Bedside monitor

  • Sterile gloves and gown for the operator

  • Skin prep (Chlorhexidine 2%)

  • US machine

  • Local anaesthetic of choice (dose calculated and checked with 2-person verification)

  • Sterile peripheral-nerve block pack:

    • Sterile US probe sleeve or tegaderm and sterile gel

    • 2x 20 mls specific nerve block syringe to draw up local anaesthetic

    • 20-22G Nerve Block needle with nerve block syringe extension

    • Small dressing

HANDS ON

The block can be performed either in the supine position, or with the patient laying on the uninjured, opposite site to facilitate the operator. The arm of the patient should be abducted.

  • See above for equipment prepared, including sterile preparation.

  • Consider use of Stop-before-you-block proforma to improve safety

  • Perform survey scan to help locate anatomical landmarks.

  • Use in-plane, US-guided technique to advance needle from anterior to posterior.

  • Maintain visualization of needle tip throughout the procedure; if needle tip is not correctly visualised: stop, withdraw, optimise view and reposition

  • When desired target structure is reached, ask the second operator to aspirate, preventing intravascular injection of local anaesthetic

  • Ask the second operator to inject 3-5mls of local anaesthetic.

  • The muscle layers should be seen to separate easily, peeling off or ‘unzipping’ the fascia. If needle fluid is injected against noticeable resistance, stop, and try repositioning the needle tip

  • When a satisfactory position is reached, ask the second operator to repeatedly aspirate and inject small aliquots (3-5mls) of local anaesthetic in the fascial plane, until all delivered.

  • Withdraw needle and dress skin.

Gallery.

Author: Ruth Sneep | Review: Tim Harris | Editor: Felipe Urriola

Reference.

 

  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. 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). https://doi.org/10.1186/cc13873

  3. Beaudoin FL, et al. A comparison of ultrasound-guided three-in-one femoral nerve block versus parenteral opioids alone for analgesia in emergency department patients with hip fractures: a randomized controlled trial. Acad Emerg Med. 2013;20(6):584-91.

  4. LITFL. https://litfl.com/serratus-anterior-plane-block/#:~:text=The%20Superficial%20Serratus%20Anterior%20Plane,with%20acute%20anterolateral%20rib%20fractures.

  5. Battle C, Hutchings HA, Driscoll T, O'Neill C, Groves S, Watkins A, Lecky FE, Jones S, Gagg J, Body R, Abbott Z, Evans PA. A multicentre randomised feasibility STUdy evaluating the impact of a prognostic model for Management of BLunt chest wall trauma patients: STUMBL Trial. BMJ Open. 2019 Jul 26;9(7):e029187. doi: 10.1136/bmjopen-2019-029187. PMID: 31350248; PMCID: PMC6661629.

  6. Almeida CR. Serratus anterior plane block for posterior rib fractures: why and when may it work? Regional Anesthesia & Pain Medicine 2021;46:835-836.

  7. Singh P, Sakharpe A, Kaur J, et al. Efficacy of serratus anterior plane block in pain control in traumatic posterior rib fractures: A case series. Trauma. 2022;24(4):346-349. doi:10.1177/14604086211046779

  8. Elwen F, et al. Serratus Plane Block. Regional anaesthesia, tutorial 427. World federation of societies of anestesiologists. 23 June 2020. https://resources.wfsahq.org/atotw/9317/ accessed 03 May 2023.