Refining a great idea: the consolidation of PECS I, PECS II and serratus blocks into a single thoracic fascial plane block, the SAP block.

The popularity of ultrasound-guided nerve blocks has impacted the practice of regional anesthesia in profound ways, improving some techniques and introducing new ones. Some of these new nerve blocks are based on the concept of fascial plane blocks, in which the local anesthetic is injected into a plane instead of around a specific nerve. Pectoralis muscles (PECS) and serratus blocks, most commonly used for post op analgesia after breast surgery, are good examples. Among the nerves targeted by PECS/serratus blocks are different branches of the brachial plexus that traditionally have been considered purely motor nerves. This unsubstantiated claim is a departure from accepted anatomical knowledge and challenges our understanding of the sensory innervation of the chest wall. The objective of this Daring Discourse is to look beyond the ability of PECS/serratus blocks to provide analgesia/anesthesia of the chest wall, to concentrate instead on understanding the mechanism of action of these blocks and, in the process, test the veracity of the claim. After a comprehensive review of the evidence we have concluded that (1) the traditional model of sensory innervation of the chest wall, which derives from the lateral branches of the upper intercostal nerves and does not include branches of the brachial plexus, is correct. (2) PECS/serratus blocks share the same mechanism of action, blocking the lateral branches of the upper intercostal nerves, and so their varied success is tied to their ability to reach them. This common mechanism agrees with the traditional innervation model. (3) A common mechanism of action supports the consolidation of PECS/serratus blocks into a single thoracic fascial plane block with a point of injection closer to the effector site. In a nod to transversus abdominus plane block, the original inspiration for PECS blocks, we propose naming this modified block, the serratus anterior plane block.