Date Published: January 19, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Mark J. Young, Andrew W. Gorlin, Vicki E. Modest, Sadeq A. Quraishi.
The transversus abdominis plane (TAP) block is a relatively new regional anesthesia technique that provides analgesia to the parietal peritoneum as well as the skin and muscles of the anterior abdominal wall. It has a high margin of safety and is technically simple to perform, especially under ultrasound guidance. A growing body of evidence supports the use of TAP blocks for a variety of abdominal procedures, yet, widespread adoption of this therapeutic adjunct has been slow. In part, this may be related to the limited sources for anesthesiologists to develop an appreciation for its sound anatomical basis and the versatility of its clinical application. As such, we provide a brief historical perspective on the TAP block, describe relevant anatomy, review current techniques, discuss pharmacologic considerations, and summarize the existing literature regarding its clinical utility with an emphasis on recently published studies that have not been included in other systematic reviews or meta-analyses.
The transversus abdominis plane (TAP) block is a regional anesthesia technique that provides analgesia to the parietal peritoneum as well as the skin and muscles of the anterior abdominal wall . First described just a decade ago, it has undergone several modifications, which have highlighted its potential utility for an increasing array of surgical procedures . Despite a relatively low risk of complications and a high success rate using modern techniques, TAP blocks remain overwhelmingly underutilized . Although the block is technically straightforward, there is inertia regarding its adoption into clinical practice. In part, this may be related to limited sources for anesthesiologists to develop a comprehensive understanding of the transversus abdominis plane. As such, we provide a brief historical perspective on the TAP block, describe relevant anatomy, review current techniques, discuss pharmacologic considerations, and summarize the existing literature regarding its clinical utility.
Rafi first described the TAP block in 2001 . He portrayed it as a refined abdominal field block, with a targeted single shot anesthetic delivery into the TAP, a site traversed by relevant nerve branches. This was a significant advance from earlier strategies that required multiple injections . In this approach, utilizing surface anatomical landmarks, the TAP was reached by first identifying the lumbar triangle of Petit (Figure 1), an area enclosed medially by the external oblique, posteriorly by the latissimus dorsi, and inferiorly by the iliac crest . A 24-gauge, blunt-tipped, 2-inch needle was then advanced perpendicular to the skin through a preceding skin incision until a single confirmatory “pop” was appreciated. This sensation was thought to indicate proper needle depth for anesthetic delivery. In 2004, McDonnell et al. presented preliminary work on TAP blocks in cadavers and in healthy volunteers at the scientific meeting of the American Society of Anesthesiologists . Although referred to as the regional abdominal field infiltration (RAFI) technique, the authors brought forward preliminary evidence to support the anatomical basis for TAP blocks and demonstrated sensory loss spanning the xiphoid to the pubic symphysis following delivery of local anesthetic to the TAP via the triangle of Petit. By the time the study was completed and published in 2007, McDonnell and his colleagues had already adopted the term TAP block and had demonstrated its analgesic utility in patients undergoing open retropubic prostatectomy [6–8].
The musculature of the lateral abdomen has three layers (Figure 2). From superficial to deep, they are the external oblique, the internal oblique, and the transversus abdominis muscles. On its course from medial to lateral, the internal oblique muscle slopes upward and creates a small gap above the iliac crest. It is this sloping edge, above the iliac crest, that defines the medial aspect of the lumbar triangle of Petit (Figure 1). Based on cadaveric dissections, Jankovic et al. noted that the location of the medial edge of the triangle varies significantly between individuals, but is always located at a point posterior to the midaxillary line . The posterior edge of the triangle is the latissimus dorsi muscle. It is not uncommon for the triangle to be quite small or poorly defined. Often, the external oblique may overlap the medial edge of the latissimus dorsi muscle. The inferior aspect of the triangle is the iliac crest, and the peritoneum rests directly deep to the innermost muscle. The TAP is the fascial layer between the internal oblique and the transversus abdominis muscles. It exists as a continuous plane located at any point on the abdomen where the two innermost muscle layers exist. Anterior rami of thoracolumbar nerves that innervate the anterior abdominal wall pass through this plane as small, but well-defined neurovascular bundles. Furthermore, Rozen et al. described an extensive fascial layer, nonadherent to the deep surface of the internal oblique that bind down the nerves on its deep surface, superficial to the transversus abdominis muscle . They also observed that, while nerve segments from T6-L1 reliably innervate the abdominal wall, individual nerve segments branch and communicate extensively with other nerve segments as they travel in the TAP. Moreover, they noted that nerve segments entered the TAP from the costal margin in an inferolateral distribution such that segments from T6 entered adjacent to the linea alba whereas segments from T9 entered near the anterior axillary line (Figure 3). Along the anterior axillary line, between the costal margin and the ileum, near the triangle of Petit, nerves running in the TAP originate strictly from T9-L1.
In his original report, Rafi described the use of 20 mL of “a local anaesthetic agent” for each side requiring analgesia . Subsequently, McDonnell et al. reported the use of 20 mL of 0.5% lidocaine for each side in healthy volunteers . Table 1 provides a current summary of the various agents and related doses used in published clinical studies.
TAP blocks have been described as an effective component of multimodal postoperative analgesia for a wide variety of abdominal procedures including large bowel resection, open/laparoscopic appendectomy, cesarean section, total abdominal hysterectomy, laparoscopic cholecystectomy, open prostatectomy, renal transplant surgery, abdominoplasty with/without flank liposuction, and iliac crest bone graft [8, 15–19, 21–48]. Most reports demonstrate the efficacy of TAP blocks by highlighting some combination of reduced postoperative opioid requirement, lower pain scores, and/or reduction in opioid-related side effects.
Complications of the TAP block are rare. To date, there are no published reports in the English language of local anesthetic toxicity following TAP blocks. Griffiths et al. reported a mean peak plasma ropivacaine level of 2.54 ± 0.75 mcg/mL using a total dose of 3 mg/kg to perform bilateral TAP blocks . While this level is above previously established minimum toxic plasma levels of 2.2 mcg/mL, it is similar to levels achieved in other commonly utilized peripheral nerve blocks (e.g., 2.58 mcg/mL for axillary blocks). Kato et al. also suggested that toxic plasma levels maybe achieved when using 40 mL of 1% lidocaine . Though direct intravascular injection of local anesthetics is very unlikely with TAP blocks, these studies do suggest that systemic toxicity is possible, and, as such, caution should be exercised throughout drug delivery.
The TAP block is an effective and safe adjunct to multimodal postoperative analgesia for abdominal surgery. Multiple studies have demonstrated its superiority over standard medical therapy for postoperative pain control. Limited data also suggest, that in select patient populations, TAP blocks/catheters may provide comparable analgesia as well as patient satisfaction to epidural therapy. However, the data is less encouraging for patients who receive intrathecal morphine during c-section, where the addition of TAP blocks does not appear to improve postoperative pain control. Nonetheless, it may be a good alternative strategy for patients who are highly sensitive to opioids.