EMERGENCY ULTRASOUND SECTION
Give Me a Break: Ultrasound Guided Serratus Anterior Plane Block Shawn Sethi, DO*
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ou 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
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COMMON SENSE NOVEMBER/DECEMBER 2021
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.
