Sonographic Landmarks

The first step in performing TAP blocks with ultrasound guidance is to identify the muscles of the anterolateral abdominal wall (Table 54-1). The external oblique is usually the most echogenic muscle of the anterolateral abdominal wall. The external oblique and internal oblique muscles typically extend farther posteriorly than the transversus abdominis muscle. Retroperitoneal fat (hypoechoic appearance on ultrasound scans) lies under the posterior aspect of the transversus abdominis muscle. The layers underneath the transversus abdominis muscle are (in order) the transversalis fascia, extraperitoneal fat, and peritoneum. The quadratus lumborum muscle is hypoechoic and therefore difficult to visualize on ultrasound scans (as is the retroperitoneal fat).

Table 54-1 Ultrasound Anatomy of the Anterolateral Abdominal Wall and Related Structures

Rankin G, Stokes M, Newham DJ. Abdominal muscle size and symmetry in normal subjects. Muscle Nerve 2006;34:320–6; Hebbard PD, Barrington MJ, Vasey C. Ultrasound-guided continuous oblique subcostal transversus abdominis plane blockade: description of anatomy and clinical technique. Reg Anesth Pain Med 2010;35(5):436–41; Gore RM, Callen PW, Filly RA. Displaced retroperitoneal fat: sonographic guide to right upper quadrant mass localization. Radiology 1982;142(3):701–5; Callen PW, Filly RA, Marks WM. The quadratus lumborum muscle: a possible source of confusion in sonographic evaluation of the retroperitoneum. J Clin Ultrasound 1979;7(5):349–52; Hanbidge AE, Lynch D, Wilson SR. US of the peritoneum. Radiographics 2003;23:663–84; Thickman DI, Ziskin MC, Goldenberg NJ, et al. Clinical manifestations of the comet tail artifact. J Ultrasound Med 1983;2:225–30.

The nerves of the abdominal wall are most visible where they enter the TAP. In this location they are relatively large and shallow with the surrounding muscles providing contrast. The T11 and T12 (subcostal) nerves are often accompanied by intercostal arteries and can often be identified as they run within the TAP. The iliohypogastric and ilioinguinal nerves cross over the anterior surface of the quadratus lumborum muscle² but are difficult to visualize in this anatomic location.

Suggested Technique

TAP blocks can be performed in supine or lateral position. Bilateral TAP blocks are usually performed in supine position with the operator and ultrasound machine in one location (block across the midline and on the same side). The lateral approach is the best way to provide access beyond the posterior border of the transversus abdominis muscle. The quadratus lumborum is more directly behind the posterior corner of the transversus abdominis muscle in this position and the contents of the abdominal cavity fall away from the needle trajectory. The lateral position is more intuitive for the operator and retracts soft tissue away from the transducer by gravity. This soft tissue retraction makes the block easier in obese patients. Small rolls can be used to bump the hip and chest to facilitate posterior approach to TAP block in the supine position.

Perform TAP block near the posterior corner of the transversus abdominis muscle. To do this the transducer is placed between the costal margin and iliac crest in the midaxillary line at the level of the umbilicus. Adjust the transducer position between the costal margin and pelvic brim to identify nerves entering the TAP. These nerves are often seen as a neurovascular bundle (an intercostal AVN bundle from T11 or T12). Once the correct plane of injection is established it can be extended by slowly injecting while gently advancing the block needle within the TAP.

Place the needle tip through the TAP and into the quadratus lumborum muscle. Slowly inject as the needle is withdrawn so that local anesthetic layers over the surface of the muscle. Additional local anesthetic can be placed within the TAP and the plane between the external and internal oblique muscles as the needle is removed.

Because the success of this block depends on extensive distribution of local anesthetic to many nerves of the abdominal wall, most practitioners inject a high volume (20 mL per side) of dilute, long-acting local anesthetic. As expected for this anatomic location, the pharmacokinetics of TAP block are intermediate between intercostal block and ilioinguinal block.

The nerves of the TAP are tacked down to the surface of the transversus abdominis muscle by fascia.³ Some therefore consider the optimal plane for infiltration of anesthetic to be between this fascial layer and the transversus abdominis muscle.³ Injections within the transversus abdominis muscle itself often result in successful block of nerves of the lower abdominal wall.⁴ The nerves of TAP are not solely confined to the plane between the internal oblique and transversus abdominis, but rather have a long-running (albeit limited) course within this plane.

The abdominal wall receives motor branches in a segmental fashion from the intercostal nerves. The “flank bulge” sign can sometimes be appreciated in thin subjects following unilateral TAP block, indicating muscle relaxation from motor block (versus swelling from arterial puncture).

FIGURE 54-1 Anatomic dissection of the anterolateral abdominal wall showing the long running course of nerves within the transversus abdominis plane between internal oblique muscle (reflected away) and the underlying transversus abdominis muscle.

(From Jankovic ZB, du Feu FM, McConnell P. An anatomical study of the transversus abdominis plane block: location of the lumbar triangle of Petit and adjacent nerves. Anesth Analg 2009;109[3]:981–5.)

FIGURE 54-2 For classic posterior transversus abdominis plane block the transducer is placed near the midaxillary line between the costal margin and pelvic brim for a transverse imaging plane. Approaches in supine (A) and lateral (B) positions are shown.

FIGURE 54-3 Transverse image of the anterolateral abdominal wall clearly defining the borders of the muscle layers. Here two layers of subcutaneous tissue can be identified. The underlying muscles are the external oblique, internal oblique, and transversus abdominis. The transversus abdominis plane (TAP) lies between the internal oblique and transversus abdominis muscles. TAP injection should be performed near the posterior border of the transversus abdominis muscle so that the injection distributes where the nerves enter the TAP.

FIGURE 54-4 Sonogram showing a nerve entering the transversus abdominis plane (TAP). This is likely the 12th intercostal nerve (the subcostal nerve). Because of their inclined course, intercostal nerves are seen more posterior as the probe moves cephalad against the costal margin with a transverse plane of imaging. Nerves of the abdominal wall travel over the surface of the adjacent quadratus lumborum muscle to enter the TAP.

FIGURE 54-5 Transverse sonogram illustrating multiple nerves within the transversus abdominis plane (TAP). On rare occasions more than one nerve can be seen along its course within the TAP.

FIGURE 54-6 The peritoneum is hyperechoic and produces comet-tail artifact. The structure should be easily identified underneath transversus abdominis muscle and transversalis fascia.

FIGURE 54-7 Sonograms illustrating in-plane approach to transversus abdominis plane (TAP) block. Sonograms are shown before (A) and after (B) needle tip placement. The needle approaches the posterior corner of the transversus abdominis muscle.

FIGURE 54-8 Sonogram illustrating the injection for transversus abdominis plane (TAP) block. The injection for TAP block should be elliptical with well-defined margins for extensive distribution.