Avoiding the Internal Mammary Artery During Parasternal Blocks: Ultrasound Identification and Technique Considerations.

Fascial plane chest wall blocks are an integral component to optimal multimodal postoperative analgesia in breast and cardiothoracic surgery, facilitating faster functional recovery and earlier discharge. Pectoral nerves block and serratus plane block have been used to treat postsurgical pain after breast and cardiothoracic surgeries; however, they cannot be used to anesthetize the anterior chest wall. Ultrasound parasternal block, or pectointercostal fascial block and transversus thoracis muscle plane block are two novel ultrasound-guided anesthetic and analgesic techniques that block the anterior cutaneous branches T2 to T6 intercostal nerves, providing anesthesia and analgesia to the anterior chest wall. Ultrasound parasternal block/pectointercostal fascial block and transversus thoracis muscle plane block are performed in the region of the internal mammary artery and could be considered to treat post-thoracotomy pain. This anatomic region is innervated by the anterior cutaneous branches T2-to-T6 intercostal nerves, which are obliterated during cardiac surgery artery harvesting. At the level of the fourth parasternal rib interspace, the internal mammary artery can be identified between the internal intercostal muscle and transversus thoracis muscle as a longitudinal pulsatile structure approximately 1.5 cm from the lateral border of the sternum. The transversus thoracis muscle is variable in many people and, thus, is an unreliable target and is difficult to visualize with ultrasound. Moreover, patients with a history of coronary artery bypass grafting could have tissue disruption in the transversus thoracis plane because of the internal mammary artery harvest, making transversus thoracis muscle identification more difficult. Despite ultrasound parasternal block and transversus thoracis muscle plane block having good safety profiles and reduced risk of complications, pneumothorax, local anesthetic systemic toxicity, and internal mammary artery injury or hematoma should be considered. If the block is performed before cardiac surgery, both the right and left internal mammary arteries could be damaged. The injury could render the internal mammary artery unusable for bypass grafting. If the block is performed after left internal mammary artery harvesting at the end of coronary artery bypass grafting, only the right internal mammary artery could be damaged. In patients in whom the internal mammary artery has been surgically used and the transversus thoracis muscle is difficult to visualize, ultrasound parasternal block should be considered. In patients in whom the internal mammary artery could be difficult to visualize or considering that it is in the vicinity of the transversus thoracis muscle plane block target and that the transversus thoracis muscle is difficult to visualize with ultrasound after internal mammary artery harvesting, then ultrasound parasternal block should be considered. The authors believe that ultrasound parasternal block is the safer regional technique for protecting the internal mammary artery and the pleura because it is more superficial. For this reason, ultrasound parasternal block also could be performed by inexperienced anesthesiologists. Although ultrasound parasternal block is more superficial, its superiority in terms of safety is yet to be proven. Additional studies are warranted to validate the authors' hypothesis.