VOLUME 31, ISSUE 2

Thoracic trauma and surgery are frequently associated with moderate to severe pain. Poorly controlled acute pain is associated with detrimental physiologic consequences, multi-organ dysfunction, delayed recovery, and increased risk of developing chronic pain.1 Thoracic epidural or paravertebral catheters have traditionally been used to provide superior analgesia, improve pulmonary function, and reduce adverse effects associated with systemic opioid analgesia.2 However, epidural catheters are associated with increased risk of hypotension, urinary retention, pruritis, and motor weakness.3 Rarely, more serious complications occur, including epidural hematoma, epidural abscess, and persistent neurological injury.4 Although paravertebral techniques are associated with fewer adverse effects, catheter tip entry into the epidural space occurs in up to ten percent of paravertebral catheters placed with an anatomic approach, and the incidence of local anesthetic spread to the epidural space may be even higher when ultrasound guidance is used.5,6 Thus, lower risk peripheral techniques to treat thoracic wall pain have been proposed.7

The serratus plane block was first described by Blanco and colleagues for the treatment of unilateral chest wall pain associated with mastectomy.8 Further studies have demonstrated efficacy for the management of pain associated with thoracotomy and traumatic rib fractures, with fewer adverse effects than thoracic epidural analgesia.9,10,11 Benefits of the block include avoidance of risks associated with neuraxial catheter placement, ease of patient positioning, and potential for use in outpatient procedures. The serratus plane block targets the lateral cutaneous branches of the intercostal nerves that innervate the hemithorax and is performed at the level of the fifth rib. Spread of local anesthetic in the plane superficial or deep to the serratus anterior muscle reliably produces analgesia between the T2-T9 dermatomes, but spares the anterior cutaneous branches innervating the anterior chest wall and sternum, the posterior primary rami innervating the posterior wall and spine, and the supraclavicular nerves innervating the clavicle.8 It also spares the autonomic nerve fibers, which are responsible for hemodynamic variability with neuraxial and paravertebral techniques.

The block could theoretically be performed using an anatomic approach targeting the fifth rib, but ultrasound guidance provides visual confirmation of local anesthetic spread in the targeted tissue plane. The patient may be positioned either supine or lateral with the ipsilateral upper extremity abducted away from the body. Optimal imaging for most patients can be achieved with the wide linear ultrasound transducer positioned in the sagittal plane at the mid-axillary line overlying fifth rib. Muscular landmarks described by Blanco include the latissimus dorsi (superficial and posterior), teres major (superior), and serratus muscles (deep and inferior).8, The needle is advanced superior-to-inferior using an in-plane approach until just deep to the latissimus dorsi muscle and superficial to the serratus anterior muscle (superficial approach) or just deep to the serratus anterior muscle and superficial to the rib (deep approach). As with other tissue plane blocks, the suggested volume of local anesthetic for an effective block is 30-40 mL. Duration of blockade may be increased when the superficial approach is used.8 For prolonged blockade, catheter placement at the serratus plane has been described.10

The figures below depict sequential imaging of a deep approach to the serratus plane block in a thin woman with multiple rib fractures. The wide linear transducer was used, oriented cephalad to caudad. The depth shown is 3 cm. As discussed above, the muscular landmarks were identified prior to needle entry. The needle was advanced in-plane to the cephalad surface of the fifth rib. After negative aspiration, 10 mL of bupivacaine 0.25% was injected (Figure 1). Thereafter, the needle was advanced caudad over the fifth rib while the remaining 20 mL volume was injected The growing volume of literature demonstrating simplicity, analgesic efficacy, and safety of the serratus plane block for the treatment of unilateral chest wall pain warrants its consideration as a lower risk alternative to neuraxial or paravertebral techniques in certain patient populations.