The Serratus Anterior Plane (SAP) Block: An Anatomical Investigation of the Regional Spread of Anesthetic Using Ultrasound‐guided Injection
Authors: Varghese Laura , Johnson Marjorie , Barbeau Michele , Rakesh Soendekoppam Vijayashankar , Magsaysay Pauline , Ganapathy Sugantha 
Published: November 2023

The Serratus Anterior Plane (SAP) Block is a relatively novel approach to providing regional anesthetic to the lateral hemithorax. However, the spread of the block, which determines its utility in thoracic and upper abdominal surgeries, is unclear.

To investigate its distribution and make recommendations for its clinical use, ultrasound‐guided SAP blocks were performed bilaterally on three fresh frozen cadavers (n=6) by an experienced anesthesiologist using a Sonosite M‐Turbo ultrasound machine (HFL38X/13‐6 MHz linear transducer). The ultrasound was used to count the ribs inferiorly from the clavicle until the level of rib 4, where the Serratus anterior muscle and Gerdy's ligament could be visualized. The insulated echogenic peripheral nerve block needle (21 gauge × 90 mm) was then inserted in‐plane along the midaxillary line, at the level of rib 4, just lateral to Gerdy's ligament and superficial to Serratus anterior. A total volume of 20 mL (methylene blue, 5% dextrose, 0.5% cellulose) was injected, with the methylene blue dye included to visualize the spread of injectate. Each cadaver was then dissected, allowing 15 minutes for the spread of the dye. The injection approach, viscosity and volume of fluid injected, and post‐injection wait period were designed to be comparable to clinical procedures.

Of 6 completed SAP blocks, 1 block failed as the injection occurred into the axilla and Gerdy's ligament, restricting the spread of the dye to the axillary fat. The remaining 5 blocks were successful. Of these blocks, dissection results showed a trend in spread from the lateral cutaneous branches of T2 to T5 intercostal nerves, with variable spread to T6. The long thoracic nerve was stained in all successful blocks, and the thoracodorsal nerve was stained in 4 of the successful blocks. These nerves are especially important for the pain management of patients undergoing breast surgeries.

The results of our study indicate that the SAP block's utility is limited to surgeries performed within T2–T5(T6) dermatomes and thus the upper lateral thoracic wall. This finding is in contrast to that of Blanco et. al1, who reported attaining anesthesia from T2–T9 in their volunteer study, which would allow the SAP block to be used in upper abdominal surgeries as well. A pin prick study will be completed to compare cadaver study findings with those attained from patients in the clinical setting.

In addition, contrary to Blanco et. al, the authors of this study consider the SAP block to be a difficult block to achieve consistent wide spread anesthesia. It is recommended that anesthetists are provided with specific training including regional ultrasound anatomy and approach technique before attempting the block. The fascial plane superficial to the Serratus anterior muscle should also be hydrodissected using saline or local anesthetic before the block is delivered to ensure proper needle placement.