EP081 A cadaveric study investigating the mechanism of action of the external oblique intercostal block

<h3>Background and Aims</h3> The external oblique intercostal (EOI) blocks have been reported to provide analgesia to both the anterior and lateral upper abdominal wall. However, the spread of injectate and the mechanisms of action remain to be fully elucidated. This cadaveric study aimed to evaluate the injectate spread following ultrasound-guided EOI block in human cadaver models. <h3>Methods</h3> Ultrasound-guided EOI block injections were performed bilaterally (n=7) on four soft embalmed Thiel cadavers. Thirty milliliters of saline-soluble dye were injected between the external oblique and external intercostal muscles at the sixth intercostal space. Anatomical dissection was subsequently conducted to assess the extent and depth of injectate spread. <h3>Results</h3> In all specimens, dye spread did not extend beyond the costal arch, and the anterior branches of the thoracoabdominal nerves were not stained. The dye infiltrated within the external oblique muscle, reaching its superficial layer across a median (IQR) of 3 (3–4) intercostal levels, but did not reach the site where the lateral cutaneous branches arise from the intercostal nerves along the mid-axillary line. Limited dye spread into the intercostal space was observed in 3 of 7 specimens. <h3>Conclusions</h3> In our cadaveric study, EOI block did not involve the anterior branches of the thoracoabdominal nerves. Our findings suggest that the EOI block may affect the lateral cutaneous branches of the intercostal nerves through the spread of injectate within the external oblique muscle. Future research is warranted to clarify the precise mechanisms of action and anesthetic coverage of the EOI block.