Transversus abdominis plane block (classic approach)

Relevant anatomy

The ventral rami of the lower six thoracic nerves (T7 to L1) emerge through the intervertebral foraminae to pass through the corresponding intercostal spaces, and enter a fascial plane between the transversus abdominis and the internal oblique muscles of the abdominal muscular wall (known as the TAP) accompanied by blood vessels. They follow the curvilinear course of this neurovascular plane to reach the anterior abdominal wall as far as the semilunar line at the lateral border of the rectus abdominis muscle ( Fig. 36.1 ).

The abdominal wall consists of three muscle layers: the external oblique, the internal oblique, and the transversus abdominis muscles and their associated fascial sheaths. The three muscles, as well as the parietal peritoneum, are innervated by the ipsilateral ventral rami of T7 to L1. The external oblique and the anterior lamella of the internal oblique aponeurosis pass anteriorly to the rectus muscle, forming the anterior rectus sheath. The aponeuroses from the posterior lamella of the internal oblique muscle and the transversus abdominis muscle pass posteriorly to the rectus muscle, forming the posterior layer of the sheath. At this point, the ventral rami of the lower thoracic nerves are located between the posterior rectus sheath and the rectus muscle. They run medially within the sheath before perforating the muscle anteriorly, forming the anterior cutaneous branches. Along their course through the TAP, the lower thoracic spinal nerves give origin to the lateral cutaneous branches posterior to the midaxillary line. Within the TAP, the nerves communicate with each other, forming neural plexuses in close proximity to the vessels in this neurovascular plane.