REctus Sheath Trial

Regional anesthesia techniques (or nerve blocks) are used to provide sensory blockade over the abdominal wall. The most established technique is thoracic epidural. This provides bilateral spread but is also associated with neuraxial complications, which has led to a decrease in clinical usage. The rectus sheath block (RSB) is an established regional anesthesia technique used to provide somatic analgesia to the midline anterior abdominal wall by depositing local anesthetic in the posterior rectus sheath, thereby targeting the terminal anterior branches of the lower thoracic intercostal nerves. The conventional approach involves placing the ultrasound (US) probe transversely across the rectus abdominis muscle and advancing the needle in-plane either lateral-to-medial or medial-to-lateral. While this technique achieves spread within the sheath, the cephalocaudal distribution of local anesthetic may be suboptimal, potentially limiting the extent of dermatomal coverage. A longitudinal probe orientation, with needle insertion from cephalad to caudad, may theoretically facilitate a more extensive cranio-caudal spread by aligning the injection axis with the anatomical fascial plane of the posterior rectus sheath. However, the relative efficacy of these two approaches has not been investigated in a controlled, comparative setting. Thus, this randomized, single-blinded trial aims to evaluate the dermatomal sensory distribution and ultrasound-assessed local anesthetic spread achieved by the transverse versus longitudinal in-plane approaches to ultrasound-guided RSB in healthy adult volunteers. To compare the dermatomal sensory block distribution, investigators will use dermatomal mapping with pinprick, and cold.