BLOCK AT A GLANCE
Interfascial plane injection of local anesthetic (LA) either deep or superficial to the serratus anterior muscle at the level of the third-sixth ribs.
Indications: Analgesia after breast surgery, thoracoscopy, rib fractures, and procedures requiring lateral or anterior thoracic wall incisions
Goal: Spread of LA under the superficial or deep fascia of the serratus anterior muscle to block the lateral branches of the intercostal nerves IIl to VI
Local anesthetic volume: 15 to 20 mL
The ultrasound (US)-guided pectoralis and serratus plane blocks are interventional analgesia techniques used after surgeries on the hemithorax. They are considered potential alternatives to the thoracic epidural, paravertebral, intercostal, and intrapleural blocks. Compared to the paravertebral or thoracic epidural, the serratus anterior plane block offers benefits in terms of simplicity, safety, and ease of performance. For instance, needle insertion away from the neuraxis and critical anatomical structures may reduce the risk of spinal cord injury, epidural hematoma or infection, or pleural puncture. However, in terms of analgesia, the serratus anterior plane block does not provide equivalent results to the thoracic paravertebral or epidural blocks. Also, the resulting analgesic patterns may vary according to the distribution of the injectate through the fascial planes, which is determined primarily by the volume, injection site, and injection force. To date, studies have shown that injections, either superficial or deep to the serratus anterior muscle, appear to have similar analgesic effects. Nonetheless, the minimum effective volume, optimal injection site, and the number of injections have not been well-established.
The risk of local anesthetic systemic toxicity (LAST) should be considered due to the absorption of the medication across a large surface. Always keep the maximum dose of LAs in mind, consider using a pharmacologic marker (e.g., epinephrine) to detect intravascular injection, and be cautious with the dose especially in high-risk populations.
The thoracic wall is innervated by the intercostal nerves, originating from the ventral rami of the thoracic spinal nerves (T3-T10). The intercostal nerves travel under the inferior border of the ribs, between the innermost and internal intercostal muscles. At the midaxillary line, the lateral cutaneous branches arise from the intercostal nerves and pierce the intercostal and serratus anterior muscles. From there on, they divide into anterior and posterior branches to innervate the lateral aspect of the thoracic wall (Figure 35-1).
Anatomy course and branches of an intercostal nerve.
The serratus anterior muscle originates from the anterior surface of the first through eighth ribs, to insert on the medial aspect of the scapula. It is innervated by the long thoracic nerve (C5-C7), which is a branch of the brachial plexus. Myofascial planes are formed both superficial and deep to this muscle, which is pierced by the ...