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Transversus abdominis plane block: what is its role in postoperative analgesia?

Over the past 3 yr, a series of studies have highlighted the value of the transversus abdominis plane (TAP) block, after the initial description of the technique by Rafi. The technique involves injection of local anaesthetic solution into a plane between internal oblique (IO) and transversus abdominis (TA) muscles. This plane contains the thoracolumbar nerves originating from T6 to L1 spinal roots which supply sensation to the anterolateral abdominal wall. These multiple mixed segmental nerves branch and communicate as they run through the lateral abdominal wall between IO and TA muscles, within the TA fascial plane. The TAP block is performed usually bilaterally, within the ilio-lumbar triangle of Petit, bounded inferiorly by the iliac crest, posteriorly by the latissimus dorsi, and anteriorly by the external oblique (EO) muscles. The blunt technique uses a double-loss of resistance as the needle is advanced through the EO and IO fascia layers. The aim is to place the tip of the needle between the IO and the TA muscles. It is not yet clear as to what the actual spread of the block is. Studies in cadavers and healthy volunteers suggest that a 20 ml solution spreads from the iliac crest to the costal margin and ensures a complete sensory blockade of the abdominal wall. However, other studies have shown a more limited distribution of a dye solution injected in cadavers, extending from L1 to T10. Although this technique is apparently safe, it may be difficult, especially in obese patients because of failure to identify the landmark of the triangle of Petit resulting in an incorrect location of the needle. In addition, damage to viscera (liver and bowel) may occasionally occur. Thus, despite the initial description of the block using the blind technique, it appears prudent to recommend the use of ultrasonography to make a more precise and safer approach. Although thin nerve structures are difficult to identify clearly with ultrasonography, it provides reliable images of the three parietal muscles, their fascia, the intraperitoneal cavity, and the digestive tract. A needle is usually advanced ‘in-plane’, parallel to the ultrasound probe that is itself placed along the axial plane above the iliac crest, caudal to the costal margin. Since the structures are quite superficial, a highfrequency (7.5–12 MHz) transducer probe can be used. The image can be optimized by sliding the probe anteriorly and posteriorly. After location of the needle tip, local anaesthetic solution 15–20 ml is injected on both sides, resulting in non-echo surface extending between IO and TA. The analgesic efficacy of the TAP block has been demonstrated in prospective randomized trials compared with placebo, in different surgical procedures such as abdominal surgery, hysterectomy, retropubic prostatectomy, Caesarean section, laparoscopic cholecystectomy, and, in this issue of the British Journal of Anaesthesia, appendicectomy. All the studies have reported superiority of the TAP block in terms of reduction in visual analogue scale scores and morphine consumption (Table 1). Careful examination of the results raises some questions. In two studies, authors suggest that the decrease in morphine consumption lasted for 2 days. 8 However, the figures describing morphine consumption show that although the difference in morphine consumption between patients receiving TAP block and those in the placebo group remained significant between 24 and 48 h, this difference was established during the first 24 h and remained stable thereafter. In addition, despite a statistically significant increase in the delay before the first analgesic request in TAP groups compared with placebo, the mean or median delay was about 2–3 h and may sometimes be even shorter (Table 1). These results could be interpreted in two ways. On one hand, it could be suggested that effective analgesia provided by a TAP block is of limited duration. On the other hand, one can argue that, though decreasing, the analgesic effect of the block persists for at least 24 h and that the block could be considered as an integral part of a multimodal analgesic strategy including systemic analgesic agents to control residual pain. The fact that block may contribute to decreasing the incidence of morphine side-effects such as nausea and vomiting is also beneficial to the patient’s rehabilitation. It is appropriate to consider the potential advantages and drawbacks of the TAP block. Potential advantages include that it is a simple and effective analgesic technique, appropriate for surgical procedures where parietal pain is a significant component of postoperative pain. It can be performed when neuroaxial blocks are