Emergency Ultrasound Section: Give Me a Break: Ultrasound Guided Serratus Anterior Plane Block

Shawn Sethi, DO *

You are treating an elderly man who slipped and fell down several steps just prior to his arrival in the emergency department. He is in severe pain, gripping his right chest wall and flank. After a chest x-ray, you note fractures of ribs five through eight on the right. An attempt of multimodal pain control with oral analgesics, a lidocaine patch, and IV opioids is made but the patient continues to have pain. You are worried about respiratory splinting and are hesitant to continue with high doses of opioids. Is there an alternative?

You roll over the ultrasound machine to perform a serratus anterior muscle (SAM) plane block for more effective (and longer lasting) analgesia. This block is ideal for anterior or lateral rib fractures given the area of anesthetic coverage (T2 through T9 dermatomes, see figure 1). It is less likely to be effective for posterior rib injury. This plane block can be used in other scenarios, not just rib fractures, including chest wall abscess drainage, large burns, or peri-procedural anesthesia for chest tube placement.

To perform this block, you ask the patient to lay on their left side (with the affected side facing upwards, towards you). Next, the T4 to T5 space in their mid-axillary line is identified. This marks the approximate area of injection for maximal coverage. You place the high frequency linear probe, in the transverse plane, on the patient’s right chest wall with your probe marker facing to the patient’s right (figure 2). You see the two major anatomical landmarks, the latissimus dorsi muscle superficially and just deep to that, the SAM (figure 3). Above these muscle groups you can visualize soft tissue, and below, the ribs (with posterior shadowing) and the pleural line. The SAM block is a type of regional anesthesia known as a “plane” block. Plane blocks don’t target a single nerve, but rather a group of nerves traversing the same fascial plane. In this case, the fascial plane allows our anesthetic to bathe the lateral intercostal nerves which traverse this area. As seen in figure 3, the fascial plane surrounding the SAM is the target. Either the superficial (yellow dotted line) or deep fascial plane (blue dotted line) can be chosen for placement of the anesthetic. There is no consensus in the literature on whether performing the block superficial or deep to the SAM is superior. 1,2 Our practice is to block superficial to the SAM due to it being technically easier to perform and to avoid traversing the SAM, thereby possibly decreasing complications and pain related to the procedure.

Using an in-plane approach, you first guide your needle to the fascial plane and inject a small aliquot of saline just below the fascia. The will allow separation of the fascial plane from the muscle, also known as hydro-dissection, to ensure the proper placement of the anesthetic. Once the potential space between the fascia and muscle layer has been opened, injection of the anesthetic of choice can occur. An 18 to 22 gauge spinal needle with 20 to 40 cc of total volume will be required for this block— enough to adequately cover the fascial plane. As it is less cumbersome, you have the nurse hold the syringe with IV tubing and inject while you hold the needle and probe. To avoid local anesthetic systemic toxicity (LAST), you have mixed normal saline with the anesthetic to achieve the desired volume. As with all high volume regional anesthesia techniques, lipid emulsion therapy is nearby and the patient is on the cardiac monitor in case the patient develops LAST. You take care to avoid the thoracodorsal artery (of which color doppler can aid in visualization) and the pleura, just deep to the serratus muscle body. Following a successful block the patient feels relief, is spared further opioids and is able to take deep breaths with minimal pain.

Figure 1: Serratus anterior muscle plane block area of anesthetic coverage (T2 through T9 dermatomes).

Figure 2: Patient positioning for right sided SAM block.

Figure 3: Anatomical landmarks for SAM block.

This block should not be performed if there is evidence of soft tissue infection in the immediate area of injection. There is a low risk of complication with this block as the pleural line can be visualized the entire time, ensuring avoidance. And there is little risk of direct nerve injury as the needle is being directed to the fascial plane, instead of a specific nerve bundle.

Multiple studies from the anesthesiology literature have found decreased pain reduction, reduced need for opioids, and low rate of complication. 3,4 Additionally, teaching this technique to new learners is feasible. A 2020 study found that EM residents were able to achieve mastery and increased confidence in this block after a combination of simulated model training and instructional videos. 5

So, the next time you encounter a patient with multiple rib fractures, large burns, soft tissue infection of the chest wall, or requiring a chest tube, consider the ultrasound guided serratus anterior plane block for effective analgesia.

References

1. Clinical Ultrasound Fellow, Emory University

2. Bhoi D, Selvam V, Yadav P, Talawar P. Comparison of two different techniques of serratus anterior plane block: A clinical experience. J Anaesthesiol Clin Pharmacol. 2018 Apr-Jun;34(2):251-253. doi: 10.4103/ joacp.JOACP_294_16. PMID: 30104841; PMCID: PMC6066876.

3. Piracha MM, Thorp SL, Puttanniah V, Gulati A. “A Tale of Two Planes”: Deep Versus Superficial Serratus Plane Block for Postmastectomy Pain Syndrome. Reg Anesth Pain Med. 2017 Mar/Apr;42(2):259-262. doi: 10.1097/AAP.0000000000000555. PMID: 28079733.

4. Park MH, Kim JA, Ahn HJ, Yang MK, Son HJ, Seong BG. A randomised trial of serratus anterior plane block for analgesia after thoracoscopic surgery. Anaesthesia. 2018 Oct;73(10):1260-1264. doi: 10.1111/ anae.14424. Epub 2018 Aug 18. PMID: 30120832.

5. Khalil AE, Abdallah NM, Bashandy GM, Kaddah TA. Ultrasound-Guided Serratus Anterior Plane Block Versus Thoracic Epidural Analgesia for Thoracotomy Pain. J Cardiothorac Vasc Anesth. 2017 Feb;31(1):152-158. doi: 10.1053/j.jvca.2016.08.023. Epub 2016 Aug 21. PMID: 27939192.

6. Rider AC, et al. Using a Simulated Model and Mastery Learning Approach to Teach the Ultrasound-guided Serratus Anterior Plane Block to Emergency Medicine Residents: A Pilot Study. AEM Educ Train. 2020 Sep 27;5(3):e10525. doi: 10.1002/aet2.10525. PMID: 34041432; PMCID: PMC8138100.