The anterior rami of the lower six thoracic nerves T7-T12 and the first lumbar nerve, L1, innervate the anterior abdominal wall. Terminal branches of these nerves travel to the abdominal wall through a plane between the internal oblique and transverse abdominal muscles. The plane between these muscles is named the transverse abdominal plane. Ultrasound imaging of the abdominal
wall at the anterior axillary line between the iliac crest and costal margin reveals 3 muscles. These muscles the external oblique, internal oblique and transverse abdominal muscles. The muscles are separated by fascia which is hyperechoic on ultrasound. The target nerves for the TAP block are located between the transverse abdominal muscle and the fascia above the muscle. These nerves are commonly not visualized during ultrasound exam. The transversalis fascia lies just beneath the transverse abdominal muscle followed by the peritoneum containing the bowels which can be identified by peristalsis.
The TAP block is performed with the patient in the supine position. The ultrasound probe is placed between the iliac crest and costal margin in the mid axillary line. The external oblique, internal oblique and transverse abdominal muscles are identified during the initial ultrasound exam. The transverse abdominal muscle is the thinnest of the muscles and located just superficial to the peritoneum. The block needle is inserted 1-3cm medial to the transducer using the in plane approach. A "pop" may be felt as the needle is inserted just beneath the fascial layer separating the internal oblique muscle and the transverse abdominal muscle. This plane is named the transverse abdominal plane. After the needle is carefully aspirated 1-2ml of local anesthetic is injected. Proper needle placement will show deposition and spread of local anesthetic between the transverse abdominal muscle and the fascia above. The transverse abdominal muscle will be pushed deep to the fascia above as the local anesthetic volume injected is increased. This will help confirm proper placement of the block needle. The block needle should be visualized by ultrasound during the block.