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Please confirm that an ethics committee approval has been applied for or granted: Yes: I’m uploading the Ethics Committee Approval as a PDF file with this abstract submissionBackground and Aims The exact mechanism of action of local anesthetics in ESPB is not fully undersdut. A detailed study of the spread of local anesthetics during ESPB in published experiments on volunteers and deceased patients shows that a large part of the applied anesthetic or dye accumulates in the fascial space between the iliocostalis muscle and the serratus posterior superior muscle, i.e. outside the fascial space of the erectores trunci muscle group. We tried to apply a local anesthetic to the site of its accumulation in a direct way.Methods In a cadaver fixed by the F4L method, we applied 15 ml of blue water solution on the right side at the Th6 level and 12 ml of Latex green on the contralateral side into the space between the iliocostalis muscle and serratus posterior superior. We monitored the extent of distribution and the content of the depot under the fascia of the serratus posterior superior muscle. After obtaining the approval of the ethics committee, we performed the application of 20 ml of 1% xylocaine + 1 ml of Iomerone using the ESPB method and the SPPB method on one volunteer under US control. After application, we performed a CT + 3D reconstruction. We also determined the extent of analgesia using the pin prick method.Results The extent of analgesia was identical between ESPB and SPPB. The distribution of contrast medium propagated more laterally in SPPB and craniocaudally to a lesser extent than in ESPB. Neither method demonstrated the spread of contrast to the paravertebral space.Abstract P208 Figure 1Depo of contrast dayAbstract P208 Figure 2CT 3D of SPPBConclusions The analgesic effect of ESPB is not due to the penetration of LA into the paravertebral space. Our results explain the analgesic effect of ESPB and offer a new technically simpler method of analgesia for postoperative conditions and for the treatment of chronic pain.