CHAPTER 34 Transversus abdominis plane block
Clinical anatomy
The sensory innervation of the abdominal wall is derived from the sixth thoracic nerve to the first lumbar nerve (Fig. 34.1). The nerves arise from the ventral rami of their respective nerve roots and travel anteriorly in a groove beneath the respective ribs to lie between the internal intercostal muscle and the innermost intercostal muscle. The intercostal nerve gives a sensory branch at the mid-axillary line, which provides variable innervation from the posterior axillary line anteriorly. The nerve continues forward as the anterior cutaneous branch, which enters the transversus abdominis plane of the anterior abdominal wall and supplies the skin, muscles and parietal peritoneum in the abdomen (Fig. 34.2). The transversus abdominis plane (TAP) lies between the internal oblique and transversus abdominis muscles, and forms a connective tissue conduit through which the ventral rami of T6 to L1 travel. Deposition of local anesthetic solution in the TAP permits abdominal wall sensory blockade.
Surface anatomy
The surface anatomical landmarks for this block are: the iliac crest, the mid-axillary line and the costal margin. The needle insertion point is at the apex of the Lumbar Triangle of Petit, which is formed by the external oblique muscle anteriorly, the latissimus dorsi muscle posteriorly, and the iliac crest inferiorly (Fig. 34.3).
Sonoanatomy
The muscles of the anterior abdominal wall are readily visible as three distinct layers (external oblique, internal oblique, transversus abdominis) (Fig. 34.4). The muscle layers are seen as marbled transverse structures surrounded by bright hyperechoic fascial coverings. The target for this block is the space between the internal oblique muscle and the transversus abdominis muscle at the level of the apex of the Lumbar Triangle of Petit.
