Abdominal field blocks, including rectus sheath blocks (RSBs), have evolved since their introduction by Schleich in 1899.1 In the 1990s, several techniques that required multiple injections into the abdominal wall to achieve adequate analgesia were described.2 More recently, ultrasound imaging has been used to guide RSB injections near the postero-lateral aspect of the rectus sheath (RS) where anterior cutaneous branches of the intercostal nerves exit the transversus abdominis plane.3,4 Local anesthetic injected into the posterior RS bilaterally has been shown to provide analgesia of the upper abdominal wall.

Bilateral RSBs have advantages over epidural catheters for select patients. RSBs are considered superficial and can therefore be used in patients receiving anticoagulants.5,6 Furthermore, RSBs can be placed in supine patients, under general anesthesia or heavy sedation, as rescue blocks, and have limited or no hemodynamic effects. However, because the duration of single-shot injections is limited, indwelling catheters are often recommended for postoperative pain control.7

The average overall success rate of RSBs (defined as no intraoperative local anesthetic supplementation at cutaneous incision) has been described in literature at approximately 53%.8 Several hypotheses have been postulated to explain this observation. Although uncommon, some anterior cutaneous branches of the intercostal nerves may bypass the RS. Furthermore, full-thickness tendinous intersections, which could limit distribution along the posterior aspect of the RS, are evident in up to 18% of patients.9 Given that the distance between the lateral edge of the RS and nerve-associated perforators is approximately 3 cm in adults, more medial injections could be less effective.10–12 Overall, needle tip and local placement into incorrect layers likely influences block efficacy.

There is no consensus regarding injection strategy for an ultrasound-guided RSB. Several approaches have been described. Seidel et al8 described both medio-lateral (transverse view) and cranio-caudal (parasagittal view) needle approaches to place local anesthetic near the lateral edge of the RS and spread it between the rectus abdominis muscle and underlying double layer of fascia (ie, the transversalis fascia and the parietal peritoneum).13 Here we describe a hypoechoic triangle, a space where the distance between the thin investing fascia of the rectus abdominis muscle and the underlying double layer of fascia is enlarged, which allows for easier needle and local anesthetic placement between the correct fascial layers. Furthermore, this space promotes longitudinal distribution for multi-segmental RSBs. The traditional approach (ie, transverse ultrasound-guided RSB) relies on hydrodissection to identify a potential space between the correct layers and expand with local anesthetic. Our report describes a real space to allow for easier needle placement into correct layers, to allow less experienced providers to perform the block, and to allow for catheters to be placed longitudinally, traversing multiple rectus abdominis muscle segments more easily. This article adheres to the applicable Enhancing the QUAlity and Transparency Of health Research (EQUATOR) guidelines.

METHODS

The hypoechoic triangle can be found with a longitudinal ultrasound view (parasagittal imaging), near the lateral corner of the rectus abdominis muscle where the anterior cutaneous branches of the intercostal nerves enter from the transversus abdominis plane. The preferred anatomic location is superior to the umbilicus (xiphi-umbilical or xiphoid tendinous intersections, see Ali14). This anatomic location lacks large epigastric vessels (ie, the watershed zone between the superior and inferior epigastric arterial supply). The hypoechoic triangle lies just underneath tendinous intersections that separate adjacent rectus abdominis segments on the same side. The roof of the triangle is formed by a thin layer of fascia that invests the rectus abdominis muscle segments (the epimysium). The base of the triangle is formed by a double layer of fascia (comprised of the transversalis fascia and parietal peritoneum). Local anesthetic should be deposited between these layers. Here we describe our stepwise approach to perform an ultrasound-guided RSB using our new proposed hypoechoic triangle target from the viewpoint of a right-handed operator.

Scanning Protocol (Stepwise Approach)

For scanning protocol (stepwise approach), see Videos.

,,]},,]},,]}The ultrasound machine should be placed on patient’s right side and the operator should stand on the left. With the ultrasound probe in transverse orientation, identify the lateral border of the rectus abdominis muscle by scanning superior to the umbilicus. Position the transducer so that the centerline is over a point 1 to 2 cm medial to the lateral border of the muscle. Rotate the transducer 90° on the centerline to obtain a longitudinal view of RS segments (parasagittal view). The displayed image should have anatomic orientation (ie, superior on the right-hand side of the image). The hypoechoic triangle below the tendinous intersection between adjacent rectus abdominis segments is now identified and positioned near the center of the field of view by moving the transducer superior or inferior and adjusting the depth setting (usual range 3–5 cm). We typically choose the triangle at the xiphi-umbilical intersection (just superior to the umbilicus). Some further adjustment may be needed to maximize triangle size by sliding the transducer medial or lateral. The hypoechoic triangles are usually 2 to 3 mm in height with 10 to 30 mm base. Insert the block needle in-plane via a superior-inferior approach through the rectus abdominis muscle into the hypoechoic triangle. Injection should demonstrate bidirectional distribution and the rectus abdominis muscle will lift and the double layer of fascia will show downward displacement. The same process is then repeated for the left side, typically without changing operator position.

Figure 1A shows a simulated block of the traditional transverse view with a medial-lateral needle approach. Figure 1B is the corresponding ultrasound image that shows the target via this approach (which is <1 mm). Hydrodissection, when done correctly, allows for local anesthetic deposition in the correct layer. Figure 1C shows the parasagittal approach, with a cranio-caudal needle path to target the hypoechoic triangle. Figure 1D is the corresponding ultrasound image to highlight the large space of the target.

Figure 1.:

A, Simulated block showing the transducer placement and needle entry point for transverse (traditional) approach to rectus sheath block from medial-lateral. The right-handed operator usually stands on the left-hand side of the patient with the ultrasound display placed across the table. The umbilicus is shown for reference. B, The corresponding ultrasound image for (A), highlighting the needle approach and the traditional potential space target (arrow). C, Simulated block showing longitudinal approach. D, Corresponding ultrasound image (for C) from the same patient to highlight the large novel target (hypoechoic triangle) for rectus sheath block.

Some loss of resistance can be observed when the needle tip enters the hypoechoic triangle. Dilute, long-acting local anesthetic is used (eg, bupivacaine 0.25%, sometimes in combination with preservative-free dexamethasone or dexmedetomidine). We typically inject about 20 mL per side to get appropriate distribution and spillover into adjacent rectus segments. This approach allows for catheter placement longitudinally across multiple rectus muscle segments.

RESULTS

Figure 2 highlights our ultrasound-guided RSB with hypoechoic target on a select patient. Figure 2A is a parasagittal/longitudinal view about 1 to 2 cm medial to the lateral edge of the rectus muscle with a hypoechoic triangle below the tendinous intersection. As described earlier, the borders of this space are the thin layer of investing fascia of rectus abdominis muscle (F) on top and the double layer forming the base (transversalis abdominus [TA] and peritoneum [P]). Figure 2B shows an in-plane approach of needle tip placement with expansion of the space after local anesthetic administration. The bidirectional injection distribution (crescent shaped spread) can be appreciated across 2 segments (sometimes 3) on the same side. Figure 3 shows a transverse view on the same patient confirming lateral local anesthetic distribution within the RS segment (lateral distribution of the injection where the anterior cutaneous branches of the intercostal nerves enter this plane).

Figure 2.:

A (left), Parasagittal/longitudinal (novel approach) view showing adjacent rectus sheath muscle segments on the same side (RA), the tendinous intersections (TI) between the 2 segments, and underlying hypoechoic triangle (HT). The borders of this real space (as opposed to potential space via the traditional approach) are the thin layer of investing fascia of rectus abdominis muscle (F) and the double layer forming the base (transversalis abdominus [TA] and peritoneum [P]). B (right), In-plane approach of needle tip placement. Bidirectional distribution pattern and crescent spread appearance from injection within the hypoechoic triangle.

Figure 3.:

Transverse view confirming distribution within the rectus sheath segment (lateral distribution of the injection).

Clinical efficacy was tested by assessing cutaneous distribution of numbness 5 to 15 minutes after bilateral RSBs on 3 patients. All patients reported cutaneous numbness on the anterior abdominal wall midline and superior to the umbilicus. The average area of numbness was about 13 cm × 13 cm.

DISCUSSION

An RSB is a viable alternative to other trunk blocks or epidurals in select patients (ie sedated, supine, hemodynamically unstable, require rescue abdominal blocks, or anticoagulated). Some limitations include the need for bilateral injections for midline analgesia, lack of ability to provide visceral pain relief, risk of RS hematomas, and possible local anesthetic systemic toxicity (LAST) given large volumes of local anesthetic. The potential cutaneous extent of the block is essentially the extent of the local anesthetic spread underneath the rectus muscle. Because the anterior cutaneous branches of the intercostal nerves that enter the RS are too small to be directly imaged with ultrasound (approximately 200 µmdiameter11), one must instead rely on sonographic assessment of fluid distribution. Injections into the incorrect layer (between the substance of the rectus abdominis muscle and its thin investing fascia) will instead remain confined to a single segment and thus not provide extensive clinical analgesia. The traditional/transverse approach is challenging given its correct target is less than 1 mm in size (shown in Figure 1B). Practitioners are required to first identify and then hydrodissect the correct layers apart. This challenge could be 1 reason for the poor reported success rates of this block.8

Here we describe a known approach with a new target for ultrasound-guided RSB for easier needle placement, allow novice practitioners to readily complete this block, possibly reduce intramuscular injections and time required, and improve overall success. A key element of this approach is our newly described hypoechoic triangle, a real space where the correct layers for this block spread apart, allowing for a 2 to 3 mm space for needle tip placement. Given that this has shifted our institutional approach, we believe this is a useful technique that provides analgesia of the abdominal wall.

DISCLOSURES

Name: Balkarn S. Thind, MD.

Contribution: This author helped in sonogram acquisition and review, literature review, preparation, and editing of the article.

Conflicts of Interest: None.

Name: Stefan G. Simon, MD.

Contribution: This author helped in sonogram acquisition and review, literature review, preparation, and editing of the article.

Conflicts of Interest: None.

Name: Andrew T. Gray, MD, PhD.

Contribution: This author helped in sonogram acquisition and review, literature review, preparation, and editing of the article.

Conflicts of Interest: A. T. Gray has received equipment support from Rivanna Medical.

This manuscript was handled by: Mark C. Phillips, MD.

ACKNOWLEDGMENTS

The authors would like to thank Nahda Harati, MD, for her comments on the injection sonography.

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