Peter Hebbard

Wangaratta Anaesthetic Group, Wangaratta, Victoria, Australia

Although abdominal wall field block is an established technique, ultrasound allows precise targeting of the neurovascular plane.This produces selective or extensive blockade of the somatic nerves, using both single shot and catheter infusion techniques. Knowledge of the anatomy of the neurovascular plane underlies all the positions for blockade.The nerve supply to the abdominal wall is segmental, anterior branches supply laterally to the anterior superior iliac spine (ASIS) while lateral branches supply the wall lateral to the ASIS.The nerves fromT9 to T12 pass through the intercostal or subcostal space and then between the transversus abdominis and internal oblique muscles.The nerves remain in that plane until the lateral border of the rectus abdominis muscle, and then with inter-individual variation, either pass into the lateral border of rectus or between the posterior border of rectus and the rectus sheath.The nerves fromT6 to T8 emerge from the anterior end of the intercostal space between the overlying the rectus muscle and the deep transversus abdominis before penetrating through rectus to give the anterior branch to the skin. L1 follows a unique pattern, dividing into the ilio-hypogastric and ilio-inguinal nerves. These nerves run in parallel to the intercostals, however the ilio-inguinal nerve may pass into the pelvis descending over iliacus muscle before emerging laterally into the abdominal wall to penetrate through to the TAP plane.This occurs lateral to the ASIS, the nerve then passes through internal oblique to lay between internal oblique and external oblique, before entering the inguinal canal.The ilio-hypogastric has a more consistent course through the TAP layer although it also penetrates through the internal oblique muscle at some variable point lateral to the ASIS to run between internal and external oblique.

Ultrasound based studies have shown the poor accuracy of blind abdominal wall injections.The ilio-inguinal/iliohypogastric block and the rectus sheath block may be performed using ultrasound guidance to improve the accuracy of local anaesthetic deposition.More extensive block of the abdominal wall may be achieved by placing local anesthetic into the neurovascular plane between the transversus abdominis and internal oblique muscles, the Transversus Abdominis Plane (TAP) block.The TAP block can avoid many of the difficulties of epidural analgesia such as excessive sympathetic block and concerns regarding epidural hematoma and infection. Both single shot and infusion techniques are used. If the ultrasound probe is held perpendicular to the abdominal wall the transversus plane is visible between 2 to 6 cm depth, well highlighted by the orientation of the fibers.The needle is introduced “in plane” and several cm from the transducer to approach the transversus plane perpendicular to the beam which optimizes needle imaging. Sub-umbilical analgesia may be achieved by placing local anesthetic laterally, near the most posterior extent of the transversus muscle in the mid-axillary line between the 12th rib and iliac crest (Posterior TAP block). If local anaesthetic is also placed into the same plane along the costal margin the block extends further above the umbilicus (Oblique Subcostal TAP block).There is continuity between the rectus block in the upper abdomen, the TAP block and the ilioinguinal block as they all target the same neurovascular plane. For further information on the techniques and ultrasound courses please visit the website of the University of Melbourne Point of Care course www.heartweb.com.au