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 ).

Ultrasound still of anatomy of TAP block.

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.

Technique

A linear, high-frequency probe (8–12 MHz) is usually used for optimal identification of the different muscle layers and their corresponding fascial sheaths. However, a curvilinear, lower-frequency probe (2–5 MHz) may be used in obese patients. The block can be performed in the supine or lateral position, with the side to be blocked upwards, and a wedge beneath the lower side in order to stretch the flank on the upper side. The lower costal margin and the iliac crest are identified, and the probe is placed in a transverse orientation between the two bony landmarks at the midaxillary line. The probe is moved both cephalad and caudad to get the best view of the three muscles. Scanning too medially may only show two muscle layers because the external oblique muscle forms an aponeurosis; also, scanning more posteriorly may encounter the large latissimus dorsi muscle, which may confuse the view of the muscles.

The fascial layers appear as hyperechoic structures under ultrasound, giving the muscles their characteristic multiple striations.

A blunt needle is introduced from the posterior edge of the probe with the in-plane technique (parallel to the ultrasound beam) and advanced in a medial anterior direction through the skin, subcutaneous fat, and external and internal oblique muscles to reach the interfascial layer between the internal oblique and transversus abdominis muscles (TAP). The endpoint of the needle should be superficial to the transversus abdominis muscle. Deeper to this muscle, there is a layer of preperitoneal fat separating it from the peritoneum and the bowels, which are often identified by their peristaltic movements. A blunt needle is preferred to appreciate the tactile “pop” when crossing each fascial layer. The intramuscular location of the needle within the internal oblique muscle is identified by retraction of the needle when it is released, as well as swelling of the muscle with injection instead of separation from the transversus abdominis.

Indications

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  The TAP block can potentially provide unilateral analgesia to the skin, muscles, and parietal peritoneum of the anterior abdominal wall, although the extent of the block has been reported to be variable in different studies.

  
  
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  Bilateral blocks have been used for midline and transverse incisions.

  
  
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  Classical TAP has been reported to provide adequate analgesia following caesarian section, hysterectomy, hernia repair, kidney transplant, colostomy closure, and multiple other lower abdominal surgeries.

  
  
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  Both single-shot and continuous catheters have been used successfully.

  
  
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  The TAP block has been used for patients with chronic abdominal pain to identify somatic pain originating from the abdominal muscular wall and the parietal peritoneum versus visceral pain, which is transmitted via sympathetic innervation instead.

Pearls

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  Placement of the needle as far posteriorly as possible (by the midaxillary line or behind) has the theoretical advantage of blocking the lateral cutaneous branches before they exit the TAP.

  
  
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  The internal oblique muscle is usually identified as the largest muscle among the three abdominal muscles.

  
  
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  The transversus abdominis muscle sometimes shows as a hypoechoic band that can be confused with the underlying preperitoneal fatty layer. The peristaltic movements of the bowels within the preperitoneal fatty layer can identify it from the muscular layer.

  
  
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  An out-of-plane technique can be more suitable in obese patients when the needle path is not easily seen.