Transversus Abdominis Plane (TAP) and Rectus Sheath Blocks: a Technical Description and Evidence Review

This article aims to review the anatomy relevant to the ultrasound techniques of the transversus abdominis plane (TAP) block and rectus sheath block (RSB). We discuss their analgesic efficacy for various surgical procedures, both as single shot and as a continuous infusion via catheters. RSB provides superior analgesia to local infiltration and has an opioid-sparing effect for umbilical surgery, laparotomy, and laparoscopic surgery. There is no high-quality evidence comparing RSB with epidural analgesia. Intermittent bolus through catheters appears to be more effective than continuous infusion for prolonged analgesia. Similarly, ultrasound-guided TAP block provides postoperative analgesic benefit after abdominal laparotomy or laparoscopy and cesarean delivery when long-acting neuraxial opioids are not used or contraindicated. Adding adjuvant such as dexamethasone and dexmedetomidine to local anesthetic prolongs the duration of TAP and RSB blocks and improves their efficacy. Use of ultrasound guidance makes the RSB and TAP blocks more reliable. Single-shot infiltration is useful for less extensive surgical procedures, while catheters are a useful alternative when thoracic epidural analgesia is contraindicated.


Anatomical Concepts
The rectus abdominis muscle (RAM) is a paired vertical muscle separated in the midline by the linea alba, it is wide and thin superiorly, increasing in thickness inferiorly. This muscle is surrounded by the rectus sheath, which is comprised of the aponeurosis of the more lateral abdominal muscles. The posterior layer becomes thin at the arcuate line, one-third of the way between the umbilicus and the pubic crest. At this line, the aponeurosis of the internal oblique and transversus abdominis goes anterior to the muscle, rather than posterior, leaving only the thin transversalis fascia posterior. The deep inferior and superior epigastric arteries run deep to rectus abdominis and superficial to the fibrous layer that forms the posterior rectus sheath. Care should be taken to avoid puncturing these blood vessels during the needle insertion.
Lateral to the RAM are the external oblique, internal oblique, and transversus abdominis muscles (superficial to deep). The intercostal nerves run in a plane between the internal oblique and transversus abdominis muscles, then penetrate the posterolateral border of the rectus muscle, innervating the muscle (muscular branch) and then the subcutaneous tissue and skin (anterior cutaneous branch). The deep circumflex iliac artery can be seen in the posterior TAP plane, where needle puncture can lead to hematoma.
Instillation of local anesthetic bilaterally between the rectus muscle and the posterior sheath provides midline analgesia for several dermatomes around the injection site. Injection between the internal oblique and transversus abdominis muscles This article is part of the Topical Collection on Regional Anesthesia * Vishal Uppal v.uppal@dal.ca

Introduction
Rectus sheath block (RSB) was originally described in 1899 and was used to provide abdominal wall relaxation during laparotomy before the introduction of neuromuscular blocking agents [1]. Currently the block is generally performed for postoperative analgesia, either as a single injection or with a continuous catheter technique. While the block was traditionally used for procedures around the umbilicus to provide analgesia from T9-T11, good clinical results are also achieved for higher dermatomes, up to T6, when the injection is performed higher. This makes the block useful for midline laparotomy, regardless of its level in the abdomen.
Ultrasound-Guided Rectus Sheath Block ( Fig. 1) Real-time ultrasound guidance is our preferred method for placing rectus sheath blocks as it ensures the local anesthetic is efficiently placed anterior to the posterior rectus sheath without being within the rectus muscle [2]. A linear array ultrasound transducer is placed on the abdomen in a transverse position congruent with the surgical incision (generally between T8 and T11 dermatomes). In the middle of the abdomen, the linea alba is observed on the ultrasound screen. The probe is then moved laterally to identify the rectus muscle, along with the subcutaneous tissue and the peritoneum. At the lateral border of the rectus muscle, the external oblique, internal oblique, and transversus abdominus can be identified. The needle tip is placed just deep to the posterolateral border of the rectus muscle. Correct placement is confirmed when injection causes the rectus muscle to lift cleanly off the rectus sheath deep to it (Fig. 1). Continuous hydrodissection during needle advancement is invaluable for efficiently identifying the correct plane. Between 10 and 15 ml of local anesthetic per side or 0.25 ml/kg in pediatrics is the usual dose. Another approach is to use a smaller dose at two different levels on each side when a large incision must be covered. Dexamethasone or dexmedetomidine can be added to the local anesthetic to prolong the block duration [3].

Alternative Techniques for Rectus Sheath Block
Landmark techniques can be utilized by either the anesthesiologist or the surgeon. A blunt bevel needle is advanced perpendicular to the abdominal wall at the lateral edge of the rectus muscle. After the skin is penetrated, the needle is withdrawn back to the skin then advanced through the first "pop" that represents the anterior rectus sheath. Next, the needle is advanced until it is pressed against the firm resistance of the posterior sheath. If performed by the surgeon during surgery with an open abdomen, the blunt needle can be palpated from within the abdomen. A correctly placed needle tip will have a distinctive "close, but not too close" feeling on palpation.
Alternatively, there has also been a laparoscopically assisted approach described [4].

Catheter Technique
A catheter may be placed at the time of block performance to extend the duration of analgesia. Catheter regimens vary, but intermittent bolus seems to be more efficacious than continuous infusion [5]. For example, 10 ml of 0.2% ropivacaine in each catheter every 4 h by a programmable pump is an efficacious and safe dose in adults.

Umbilical Hernia
One recent randomized control trial (RCT) has compared RSB with placebo for umbilical hernia surgery in adults where they found lower 24-h opioid consumption (mean morphine consumption 3.73 mg vs. 8.76 mg) and lower pain scores [6]. A meta-analysis of ten trials for RSB in pediatric umbilical surgery found a reduction in opioid use for the first 6-8 h (-0.03 mg/kg) [7••].
Rectus sheath catheters have been compared with thoracic epidural analgesia (TEA). One retrospective observational study for pancreas transplant found there was no significant difference in rescue analgesia and the technique was preferred due to their anticoagulation regimen [17]. Another observational study found equivalent pain scores and faster time to mobilization in the rectus sheath group [18].

Laparoscopy
Some recent studies have investigated the use of RSB for analgesia after laparoscopic surgery with positive results (24-h fentanyl use, 189 vs. 286 μg [19]; and verbal and numeric pain score at 6 h, 3 vs. 5 [20•]). In gynecological procedures, 2 RCTs showed RSB to be superior to placebo [19, 20•]. In one RCT of laparoscopic cholecystectomy, RSB resulted in less rescue analgesia compared with placebo and higher patient satisfaction compared with local infiltration [21]. For pediatric appendectomy, pain scores were reduced compared with placebo [22] and compared with in local infiltration [23] with similar overall [22] or reduced overall [23] opioid use.

Complications
Given the large area available for absorption after RSB, care should be taken to use appropriate doses and concentrations of local anesthetic [24••]. Adding epinephrine to reduce peak plasma concentration is also helpful [25].
The superior and inferior epigastric arteries are at risk of puncture or injury during the performance of rectus sheath block. Using ultrasound to avoid the arteries (and their attendant veins) can reduce the chance of puncture or inadvertent intravascular injection. When using a landmark technique, a lateral insertion point helps to avoid these vessels.

Conclusion
RSB has a proven analgesic benefit for umbilical surgery, laparoscopy, and laparotomy. It can provide midline abdominal analgesia anywhere from the symphysis pubis to the xiphoid process for several dermatomes around the injection point. RSB has few complications, which can be reduced by using ultrasound guidance. Single injection provides good analgesia for the first 6 h, with a waning effect up to 24 h. Continuous catheters can provide effective analgesia for extended durations.

Introduction
Rafi first described a technique of transversus abdominis plane (TAP) block using a landmark approach in 2001. However, it was only after the introduction of the ultrasound-guided technique in 2007 that the TAP block was more widely adopted in clinical practice. In the past 20 years, it has been one of the most widely investigated regional anesthesia technique. Since the original description of ultrasound-guided technique, various technical modifications have been described including lateral [26], posterior [27], subcostal [28], and continuous catheter techniques [29] .
The exact technique used should be suited for a surgical incision, as each provides different dermatomal coverage. Lateral TAP block typically gives a T10-T12 dermatomal coverage while the posterior TAP approach covers T9-T12 and is shown to provide a longer duration of analgesia with possibly some visceral analgesia [30]. The subcostal TAP approach provides T6-T9 dermatomal coverage and is usually suitable for upper abdominal surgeries.

Ultrasound-Guided TAP Block
The ultrasound-guided techniques are recommended over the landmark technique to ensure precise deposition of Lateral and Posterior TAP Block (Fig. 2) A linear high-frequency probe is generally ideal for performing the block. However, a curved array lower frequency probe may be needed in morbidly obese patients. After wide skin preparation with antiseptic, we recommend starting the scanning similar to the rectus sheath block (described earlier) to identify the rectus muscle. On sliding the ultrasound probe laterally towards iliac crest parallel to Tuffier's line, three lateral abdominal wall muscles (external oblique, internal oblique, and transverse abdominus muscle) can be easily visualized. Scanning more posteriorly the transversus abdominis muscles can be observed to taper. Since posterior TAP block is shown to provide longer analgesia and more extensive spread than lateral TAP block, posterior approach should be the preferred approach in clinical practice. Subcostal TAP Block (Fig. 3) As the name suggests, the ultrasound probe position in the approach is parallel to the subcostal margin. For identification of the TAP plane, a similar scanning approach is followed with identification of the rectus muscle. On sliding the ultrasound probe laterally, the transversus abdominis muscle is usually seen underlying the lateral part of rectus abdominis muscle. On further lateral scan, the three lateral abdominal wall muscles can be visualized.
The exact point of local anesthetic injection depends on the dermatomal coverage required. A more medial injection (between rectus muscle and transversus abdominis muscle) will produce analgesia of T6-T8 dermatomes. On the other hand, a more lateral injection between transverse abdominus muscles and internal oblique will more consistently result in T8-T10 dermatomal analgesia. Both medial-to-lateral and lateral-tomedial in-plane needling techniques have been described [31]. We prefer a lateral-to-medial approach as it is more ergonomic to perform the technique and catheter insertion is away from the surgical field.
Oblique subcostal TAP block has been described as a modification of the subcostal technique. In this technique, an extralong (15-20 cm) needle is used to inject local anesthetic in TAP plane from subcostal region to iliac crest with a potential to provide T6-L1 dermatomal coverage. The technique is difficult to perform, and therefore, has not gained popularity [28].

Catheter Technique
Continuous catheters have been described for all of the above techniques. The technique of catheter insertion involves hydrodissection of the planes. The number and site of catheters needed depend on the site and extent of surgical incision. Four-quadrant TAP catheters (bilateral subcostal and bilateral posterior TAP) can be used for midline abdominal incision extending from xiphisternum to pubic symphysis [29]. It must be stated that four catheters can be tedious to insert and maintain with further issues of local anesthetic systemic absorption and failure/dislodgement of one of the four catheters is high.

Evidence Base
A literature search of key databases for systematic reviews on TAP block for postoperative analgesia revealed more than 30 systematic reviews in different surgical populations. These include colorectal surgery, cesarean section, cholecystectomy, hysterectomy, appendectomy, donor nephrectomy, retropubic prostatectomy, and bariatric surgery.

TAP Blocks Versus No Block/Placebo
All recent reviews [32, 33••, 34, 35] suggest that the ultrasound-guided TAP blocks reduce postoperative pain scores in the first 6-24 h postoperatively (mean difference of 1.4 to 2 points on a scale of 0-10) and reduce the morphine requirement in the first 24 h (mean difference 7-15 mg) compared with placebo [33••, 34]. Further, TAP block may delay the time to first analgesic request (mean difference 2 h) [32] and may reduce the incidence of nausea/vomiting [36,37]. However, heterogeneity of type of surgery, block approaches, drugs, doses, and volumes warrant that the findings should be interpreted with caution.
Multiple systematic reviews have confirmed the analgesic efficacy of TAP block for open and laparoscopic colorectal surgeries [38, 39, 40•, 41-43]. Similarly, ultrasound-guided TAP block has been found to be an effective analgesic intervention in patients undergoing laparoscopic cholecystectomy [44,45] and hernia surgery [46]. TAP blocks have also been shown to significantly lower intraoperative and cumulative postoperative 24-h opioid consumption, provide better pain control, and decrease the 24-h incidence of postoperative nausea vomiting for renal transplant recipients [47].

TAP Blocks Versus Intrathecal Morphine
TAP blocks do not appear to provide additional analgesic benefit in patients who also received intrathecal morphine [34]. Therefore, the TAP block may be particularly useful when neuraxial techniques or opioids are contraindicated. Some of the older reviews (conducted before 2012) found a limited benefit of TAP block in the context of multimodal analgesia [48][49][50].

TAP Blocks Versus Thoracic Epidural Analgesia
A recent review synthesized the data comparing TAP block with TEA. Pain scores at rest on postoperative day 1 were equivalent for TAP block and TEA groups in children and in adults. However, the TEA group experienced a higher rate of hypotension and longer hospital length of stay compared with the TAP block group, without a difference in functional outcomes [51•].

TAP Block Versus Local Anesthetic Wound Infiltration
Three different reviews have synthesized the evidence comparing TAP blocks with local anesthetic would infiltration [52][53][54]. The evidence from these reviews indicates that TAP blocks provide superior and longer lasting analgesia when compared with local anesthetic wound infiltration.

Cesarean Delivery
Three systematic reviews on TAP blocks in the cesarean delivery population show similar results to the general surgical population [55, 56, 57••]. The reviews consistently show analgesic benefit of TAP blocks in patients who did not receive intrathecal morphine. No analgesic benefit of TAP block was observed when intrathecal morphine was used. Intrathecal morphine was associated with longer duration of analgesia compared with TAP block alone at the expense of an increased incidence of side effects such as pruritus [57••].
A recent review concluded that low-dose TAP blocks (bupivacaine equivalents less than or equal to 50 mg per side) for cesarean delivery provide analgesia and opioid-sparing effects comparable with high-dose TAP blocks. Therefore, a lower dose TAP block should be used to reduce the risk of local anesthetic toxicity without compromising the analgesic efficacy [58]. Another systematic review comparing TAP blocks with wound infiltration for cesarean delivery demonstrated no statistically significant difference in analgesia or side effects for patients who received multimodal analgesia but no intrathecal morphine [59].

Hysterectomy
Three recent systematic reviews found that TAP blocks provide significant postoperative early and 24-h pain control compared with placebo or no block among women who undergo a hysterectomy. There was reduced morphine consumption among patients who underwent total abdominal hysterectomy but not for total laparoscopic hysterectomy [60][61][62].

Pediatric Population
A recent review synthesized the evidence on rectus sheath and transversus abdominis plane blocks in children. Data from ten RCTs (n = 599) suggests that patients receiving TAP and RSB have a small but significant reduction in pain score and opioid use 6-8 h after surgery. However, a high level of heterogeneity in the included studies warrant caution in the interpretation of results [7].

Dexamethasone as an Adjuvant
Two recent systematic reviews have shown that dexamethasone added to local anesthetics in ultrasoundguided TAP block significantly decreased visual analogue pain scores by 1 point on a scale of 0-10 at rest for first 12 h, prolonged the time to the first request for additional analgesics by 3 h, and reduced the incidence of postoperative nausea and vomiting significantly (risk ratios = 0.40) without any increase in reported complications compared with the control group [63, 64••].

Dexmedetomidine as an Adjuvant
Another recent review that included twenty published trials found that the addition of dexmedetomidine significantly reduced rest pain scores for 8 h postoperatively (mean difference 0.8/10) and 4 h postoperatively on movement. Adjuvant dexmedetomidine significantly prolonged the duration of the TAP block by 3.3 h and did not affect the incidence of postoperative nausea and vomiting, hypotension, bradycardia, somnolence, or pruritus [65].

Liposomal Bupivacaine
Currently, there is a lack of data to support or refute the use of liposomal bupivacaine in TAP blocks for the management of postoperative pain [66].

Continuous TAP Catheters
A recent systematic review on TAP catheters included twelve small randomized controlled trials. The studies were extremely heterogeneous regarding the type of surgery, control group, and technique of catheter insertion and infusion. The authors concluded that TAP catheters have the potential to provide effective and safe analgesia. A few studies have demonstrated effects comparable with TEA, while TAP catheters are shown to be consistently superior to systemic analgesia alone.

Complications
It has been shown that the local anesthetic systemic concentrations commonly exceed accepted thresholds after both TAP blocks and RSB [24••]. Although there are case reports of seizures and ventricular arrhythmias, the incidence of local anesthetic systemic toxicity is extremely low. However, efforts should be made to not exceed the maximum recommended milligrams per kilogram local anesthetic dose [68]. This may be accomplished by using a lower concentration of local anesthetic as 0.375% ropivacaine and 0.5% ropivacaine have been shown to be equally effective for TAP block [69]. The risk of hematoma or visceral damage is extremely rare with ultrasound guidance. Femoral nerve block has been reported after TAP block, leading to 6-8 h of quadriceps weakness especially with larger volume of injectate [70].

Conclusion
Ultrasound-guided TAP block provide 12-24 h of postoperative analgesic benefit after abdominal laparotomy, laparoscopy, and cesarean delivery when long-acting neuraxial opioids are not used or contraindicated. It appears to be a safe technique, especially with ultrasound guidance. Consideration should be given to adding an adjuvant to prolong its duration of action and improve efficacy of single-shot TAP blocks. Although continuous TAP catheters have been used, they require maintenance of multiple catheter infusions and risk of failure. The studies evaluating its impact on the long-term and functional outcomes are lacking.

Compliance with Ethical Standards
Conflict of Interest Vishal Uppal, Sushil Sancheti, and Hari Kalagara declare that he has no conflict of interest.
Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any of the authors.
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