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Ultrasound-Guided Serratus Anterior Blocks
Introduction
Rib fractures are a relatively common injury in trauma patients presenting to the Emergency Department (ED), with studies estimating the incidence between 9% and 38%.[1][2][3] Rib fractures are associated with significant morbidity and mortality[1][2][4][5], especially in older adults.[6][7][8][9] Rib fracture pain can impair respiratory mechanics. This may lead to pulmonary complications such as atelectasis, hypoxia, pneumonia, and ultimately respiratory failure.[10] Therefore, effective analgesia is essential to rib fracture management for patient comfort and long-term outcomes.[11] Intravenous opioids and opiates have been a mainstay of treatment but have numerous potential negative side effects, including respiratory depression, depressed cough reflex, and delirium in older adults.[6]
Given the importance of pain management for rib fractures, there has been increased interest in exploring nerve blocks as potentially safer alternatives for analgesia. Intercostal nerve blocks are very effective at blocking rib fracture pain.[12] For patients with fractures at multiple rib levels, the population with the highest risk of complications, performing a separate intercostal block for each fractured rib might not be practical. While ultrasound guidance for intercostal blocks has helped reduce the incidence of iatrogenic pneumothorax, it is still a possible high-risk complication of intercostal blocks, especially for novice practitioners.[13] Additionally, intercostal blocks run the potential risk of increased local and systemic toxicity, given the amount of anesthetic required and the high rate of systemic absorption.[14] Regional anesthesia techniques for thoracic neuroblockade also include the thoracic paravertebral block (TPVB) and the erector spinae block (ESB). The TPVB involves injecting a local anesthetic into the paravertebral space, where it can spread to block spinal nerves as they exit the intervertebral foramina. This ultrasound-guided block is very effective for pain management, but like an epidural block, it requires a high level of procedural skill. The TPVB has potential side effects such as hypotension, urinary retention, respiratory compromise, and nausea and vomiting.[15][16][17] The ESB involves ultrasound guidance to identify the fascial plane between the erector spinae muscle and the vertebral transverse process. Then, the practitioner uses ultrasound guidance to inject local anesthetic in that fascial plane.[18] Numerous studies have compared the efficacy of the TPVB and the ESB, and both are effective at anesthesia for acute rib fractures, while the ESB appears to have fewer side effects.[19] Like the TVPB, the ESB block requires advanced procedural skills and comfort with the relevant anatomy.
An alternative method for pain management in acute anterolateral and lateral rib fracture pain is the serratus anterior plane block (SAPB). This block has become popular due to its efficacy, relative ease, and limited side-effect profile.[20][21][22][23]
Anatomy and Physiology
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Anatomy and Physiology
The serratus anterior muscle originates from the superolateral aspect of the first to eighth ribs or the first to ninth ribs and inserts on the medial border of the scapula. It consists of 7 to 10 serrated tendinous projections that originate on each rib and are innervated by the long thoracic nerve.[24] The serratus anterior muscle is bound by two potential spaces at the level of the fifth rib at the mid-axillary line: 1) a superficial plane that is formed between the anterior aspect of the serratus anterior and the posterior aspect of the overlying latissimus dorsi muscle, 2) a deep plane that is formed between the posterior aspect of the serratus anterior and the external intercostal muscles and ribs.[20] SAPB performed in both the superficial and deep planes has achieved excellent analgesia to the anterolateral and lateral chest wall with reportedly similar efficacy and an equivalent area of cutaneous sensory loss.[25][26] Although it is suggested that the superficial plane may achieve analgesia for a longer period.[20]
The SAPB targets the lateral cutaneous branches of the thoracic intercostal nerves, which arise from the anterior rami of the thoracic spinal nerves and run in a neurovascular bundle immediately inferior to each rib. At the midaxillary line, the lateral cutaneous branches of the thoracic intercostal nerve traverse through the internal intercostal, external intercostal, and serratus anterior muscles, innervating the musculature of the lateral thorax.[27] Therefore, these branches of the intercostal nerves travel through the two potential spaces described above. Local anesthetic inserted into these planes will spread throughout the lateral chest wall, resulting in paresthesia of the T2 through T9 dermatomes of the anterolateral thorax.[20][27]
Indications
The SAPB has been used effectively to manage pain in the context of anterolateral and lateral rib fractures, rib contusions, thoracoscopic surgery, thoracotomy, breast surgery, and post-mastectomy pain syndrome.[20][22][23][28] In the setting of rib fractures, the block effectively provides analgesia for anterolateral and lateral rib fractures but may be ineffective for anterior and posterior rib fractures.[27] It is indicated for trauma patients with two or more rib fractures that require opioid analgesia to maintain respiratory status and comfort.
Contraindications
Absolute contraindications for SAPB are an allergy to local anesthetics or a soft tissue infection at the intended injection site. Anatomical variations that impair the sonographic visibility of key landmarks, such as scars or fibrosis from previous thoracic surgeries or subcutaneous air from trauma, are considered relative contraindications. It's worth noting that due to the superficial placement of this block and the minimal risk of vascular injury for experienced providers, therapeutic anticoagulation doesn't generally pose a contraindication for this procedure.
Equipment
A high-frequency (eg, 5 to 10 MHz or 6 to 13 MHz) linear transducer should be used to allow detailed visualization of superficial structures, ideally with a larger footprint to allow for better in-line visualization of the length of the needle during the procedure. Given this is a relatively large volume plane block, the patient should have an intravenous line and be placed on cardiac monitoring to allow for monitoring for local anesthetic systemic toxicity (LAST) with easy access to lipid emulsion therapy as needed.
Personnel
The SAPB is performed by trained personnel who are familiar with the use of musculoskeletal and procedural sonography. Although it is not necessary depending on provider skill and preference, given that the SAPB is a relatively high-volume injection, it is recommended to have two providers involved in the procedure. The first provider should manipulate the ultrasound and guide the needle placement, while the other provider holds the syringe(s) and controls the injection of the block. In the two-provider technique, a typical setup involves connecting the needle to an IV extension tubing on one end. Then, on the other end of the IV tubing, a three-way stopcock is used, with one port attached to a saline flush and the other connected to the anesthetic syringe.
Preparation
Informed consent should be obtained from the patient. The ultrasound machine should be situated opposite the operator to allow the provider to see the screen in-line with the approach of the needle. Sterile technique should be employed, including an ultrasound transducer cover and sterile gel. The procedure can be performed with the patient in either the supine or lateral decubitus position, depending on patient and provider comfort. The chest wall should be cleaned with 2% chlorhexidine or a similar solution, as per local sterile technique protocols.
Technique or Treatment
The high-frequency linear transducer should be placed on the patient's midaxillary line in the transverse plane at the level of the fifth rib. Image depth should be set to allow visualization of the rib, pleural line, and overlying serratus anterior and latissimus dorsi muscles (usually 3-5 cm, depending on the patient's size). The overlying skin can be anesthetized with a small volume of local anesthetic. Then, using ultrasound guidance, the needle is advanced in-plane at approximately 45° toward the fifth rib. For the superficial SAPB, the local anesthetic is injected anteriorly to the serratus anterior in the plane created with the latissimus dorsi muscle. The thoracodorsal artery runs in this plane, and while care should be taken not to puncture this vessel, the spread of the injectate around the vessel confirms the appropriate injection plane. For the deep SAPB, the local anesthetic is injected anteriorly to the rib and deep to the serratus anterior.
The needle should be visualized in its entirety throughout the procedure, ensuring vessels are avoided while moving through the soft tissue. Once the needle tip is in the correct plane, inject a small volume (1 to 3 mL) of normal saline or local anesthetic to confirm placement and hydro-dissect the fascial layers, opening the potential space. After establishing the fascial plane, slowly inject a larger volume of dilute local anesthetic, such as 30 mL of 0.25% bupivacaine. This anesthetic should manifest as a growing anechoic stripe between the fascial layers. After the injection, withdraw the needle and cover the puncture site with the appropriate dressing.
Complications
Rebound pain is possible since the analgesia provided by bupivacaine typically lasts around six hours. Local anesthetic systemic toxicity (LAST) is a potential complication of regional anesthesia. For this reason, a dilute anesthetic is used, and a maximum dose of 2 mg/kg of bupivacaine is recommended.[29] Pneumothorax is a potential complication but would suggest significant provider error since the fascial planes targeted in this block are superficial to the ribs, and the pleural line can be visualized clearly on ultrasound. If a pneumothorax is suspected, ultrasound can help to confirm lung sliding immediately after the procedure. Nerve injury is unlikely, given the needle is not steered directly at nerves but instead toward the plane through which the nerves run.
Clinical Significance
The ultrasound-guided SAPB offers an effective approach to the management of anterolateral and lateral chest wall pain, particularly in the context of traumatic rib fractures, and is a desirable alternative to opioid analgesia, especially in older adults.
Enhancing Healthcare Team Outcomes
Opioid abuse represents a public health crisis in the United States. Because almost half of ED patients complain of pain, and approximately 17% of patients discharged from EDs receive opioids, expanding options for effective analgesia in the acute setting can assist emergency physicians (EPs) in prescribing opioids responsibly.[30] The addition of regional anesthesia, such as the SAPB to the EP’s armamentarium, is particularly valuable in treating opioid-naïve patients since initiation of short-course opioid therapy seems to confer increased risk for further opioid use and addiction, with a number needed to harm of 48.[31][32] [Level 4] Nerve blocks such as the SAPB can be part of a multi-modal approach to the management of pain in the acute setting that offers to reduce reliance on opioids and improve long-term outcomes[11][33].
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References
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