Approximately 10% of injured patients presenting to the emergency department (ED) demonstrate rib fractures. Rib fractures are associated with significant morbidity and mortality, especially in the elderly. Pulmonary complications, including pneumonia, often become apparent 2 or 3 days after injury, when respiratory function is compromised, secondary to pain. Therefore, effective analgesia is an important component of rib fracture management. Intravenous opioids are a mainstay of treatment but have side effects, including respiratory depression, depressed cough reflex, and delirium in the elderly. The serratus anterior plane block (SAPB) is an alternative that has become popular due to its efficacy, relative ease, and limited side-effect profile.
The serratus anterior muscle originates from the anterior aspect of ribs 1 through 7-10 and inserts on the medial border of the scapula. It consists of 7 to 10 serrated tendinous projections that originate on each rib and is innervated by the long thoracic nerve. Deep and superficial potential spaces bound the serratus anterior. At the level of the fifth rib, the superficial plane forms from the anterior aspect of the serratus anterior and the posterior aspect of the latissimus dorsi muscle. The deep plane forms from the posterior aspect of the serratus anterior and the external intercostal muscles and ribs. Either plane will achieve analgesia to the anterolateral chest wall with reportedly similar efficacy and an equivalent area of cutaneous sensory loss.
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. These branches of the intercostal nerves, therefore, 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.
The SAPB has been used effectively for the management of pain in the context of rib fractures, rib contusions, thoracoscopic surgery, thoracotomy, breast surgery, and post-mastectomy pain syndrome. In the setting of rib fractures, the block is effective in providing analgesia for lateral rib fractures but may be ineffective for anterior and posterior rib fractures.
Absolute contraindications to the SAPB include allergy to local anesthetic medications or soft tissue infection in the area of the procedure. Relative contraindications include any anatomical variation that makes sonographic visualization of landmarks difficult. These include scarring and fibrosis due to prior thoracic surgery.
A high-frequency (5 to 10 MHz or 6 to 13 MHz) linear transducer should be used to allow detailed visualization of superficial structures. Cardiac monitoring and an intravenous line are recommendations, and access to lipid emulsion therapy should be confirmed.
The SAPB is performable by trained personnel who are familiar with the use of musculoskeletal and procedural sonography.
Informed consent should be obtained from the patient. The ultrasound machine should be situated opposite the operator, plugged in, and powered on. Sterile technique, including an ultrasound transducer cover and sterile gel, should be employed. The patient should be supine or in a lateral decubitus position.
A high-frequency linear transducer should be placed on the patient’s midaxillary line in the transverse plane, at the level of the fifth rib, with the indicator oriented toward the operator’s left. With the rib, pleural line, and overlying serratus anterior and latissimus dorsi muscles visualized, the skin can be anesthetized with a small volume of local anesthetic. Then, using ultrasound guidance, the needle is advanced in-plane at an angle of approximately 45 degrees towards the fifth rib. For the superficial SAPB, the local anesthetic is injected anteriorly to the serratus anterior.
For the deep SAPB, the local anesthetic is injected anteriorly to the rib and deep to the serratus anterior. The entirety of the needle should be visualized at all times throughout the procedure, and care should be taken to avoid any vessels while traversing the soft tissue. Once the needle tip is observed in the appropriate plane, a small aliquot of 1 to 3 mL of local anesthetic should be injected to confirm placement and to hydro-dissect the fascial layers and open the potential space. After opening the fascial plane, a larger volume of dilute local anesthetic, for example, 30 to 40 mL of 0.25% bupivacaine, should be gradually injected. The local anesthetic will appear as an enlarging anechoic stripe between fascial layers. Remove the needle and cover with sterile gauze or adhesive dressing. Reassess the patient’s pain level for improvement approximately five minutes after the block is complete.
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, dilute anesthetic is used, and a maximum dose of 2 mg/kg of bupivacaine is the recommendation. Pneumothorax is a potential complication but would entail catastrophic 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 towards the plane through which the nerves run.
The ultrasound-guided SAPB offers an effective approach to the management of lateral chest wall pain, particularly in the context of traumatic rib fractures, and is a desirable alternative to opiate analgesia, especially in the elderly.
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 opiates responsibly. 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. [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.
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