There are numerous descriptions and techniques for TAP blocks in the literature, with inconsistency in the definitions used and specific outcomes measured. In a systematic review by Abdallah et al this heterogeneity is highlighted as a major limitation to meaningful analysis of the literature, and has almost certainly contributed to the conflicting conclusions between the trials and reviews to date (1-5).
The following table provides reference to some of the commonly used TAP block terminology in the literature (6-9):
Rafi first described the TAP block in 2001 as an anatomic landmark guided ‘double pop’ technique to achieve a field block of the 7th-11th intercostal nerves, the subcostal, illioinguinal and iliohypogastric nerves (T7-L2) (11-13). The landmarks used defined the triangle of petit and the appropriate point of needle insertion, as shown in Figure 6. McDonnell et al conducted the first study using this technique and reported a sensory block from T7 to L1 in three healthy male volunteers. They also produced radiological evidence (CT and MRI) of the spread of 20mls of dye in the TAP 20-240 minutes after injection, from the superior margin of the iliac crest to the level of the costal margin, and posteriorly to the quadratus lumborum (13). These findings have since been reproduced by Carney et al (9).
Figure 6: The triangle of petit, bound posteriorly by the latissimus dorsi muscle, anteriorly by the external oblique, with the iliac crest forming the base. Courtesy of Carney et al (9)
Hebbard later described a technique using ultrasound at the level of the midaxillary line just superior to iliac crest for surgery of the lower abdominal wall (14). The most cephalad extent of the sensory block with this technique is T9 in the current literature (9, 15), and even with 30mL injectates there is no evidence of spread to the upper intercostal plexus on MRI up to six hours post block (16). Hebbard also went on to describe a technique for analgesia of the supra-umbilical abdomen, which he coined the ‘oblique subcostal’ technique (12), and describes it as being more suitable for incisions T7-T11 (9). In a cadaveric study comparing a single injection of 20ml of dye lateral to the linea semilunaris, with multiple injections of 5mLs along the costal margin, the single injection technique most commonly involved T10-11 and T9-12 maximally, while the multiple injection technique most commonly involved T9-11 and T7-T12 maximally (17). In his technical description of the block, Hebbard emphasises the need for injection along the entire subcostal margin to insure good coverage. This includes the medial border to cover the higher segmental nerves, which may require infiltration between the rectus muscle and posterior rectus sheath if there is no transversus abdominis muscle until more laterally; and extending down to the iliac crest to cover T12-L1 (18).
The contrast in spread and duration with the lateral and subcostal ultrasound techniques and the original landmark technique is likely related to the different site of injection. This has lead to investigation of more posterior ultrasound techniques, however the literature is inconsistent as to what constitutes a ‘posterior TAP’ as noted below in the table of terminology. One description involves positioning the needle at the intersection of the quadratus lumborum and the lateral abdominal muscles, superficial to the transversalis fascia (9, 19). With this approach there is evidence of posterior cranial pooling of infiltrate between the transversalis fascia, the quadratus lumborum muscle and the psoas muscles on MRI, with non-contiguous spread into the paravertebral space T5/6-T10 in three healthy male volunteers. The posterior approach resulted in a more extensive, predictable and posterior spread of contrast than the subcostal or lateral TAP (9). An anatomic explanation relates to the diversity and variability of the trajectory of the intercostal and subcostal nerves in the TAP and the deviation of the lateral cutaneous nerve. This variability is potentially less posteriorly with higher order branches of the T6-L1 segmental nerves being in closer proximity (1). In a meta-analysis of 12 randomised control trials, Abdallah et al showed that this may be associated with improved clinical outcomes when compared with a control group using systemic opioids. While in the posterior TAP trials there was superior pain control at rest and movement from 12-48 hours, and reduced morphine requirements at 12-24 hours (mean 9mg, 95%CI -16.83 to -1.45, p=0.02) and 24-48 hours (mean 5mg, 95%CI -9.54 to -0.52, p=0.03), in the lateral TAP trials there was no statistically significant difference. However it is noted that only three trials and less than 100 patients contributed to the posterior TAP cohort, with a high degree of heterogeneity between all studies. Some authors caution that variation in the extent of posterior abdominal wall muscle layers potentially makes this a more difficult ultrasound technique, which may consequently limit utility (9). Randomised control trials comparing the different TAP techniques are required before definite assessments can be made.
The main limitation of the landmark technique relates to the inability to confirm accurate needle and block placement (14). The location of the triangle of petit is also highly variable, small, difficult to identify in obese patients, absent in 17% of patients, and not consistently defined in the literature (3, 20). However inappropriate ultrasound and needling techniques can still lead to the incorrect placement of local anaesthesia with a failed block or serious intraperitoneal injury, as noted below in the discussion about complications. Currently there is no evidence base to support landmark or ultrasound techniques as being superior in terms of clinical outcome.
The placement of catheters for continuous local anaesthetic infusion for the subcostal TAP technique is commonly described, and comparable to epidural in some instances (21). Contrastingly lower TAP catheter techniques rarely feature in the literature. The later is possibly due to the relatively prolonged duration of effect often described with the lower TAP techniques, with some studies showing a clinically significant reduction in pain for up to 24-36 hours after a single bolus (1, 3, 13, 22). In a study by Stoving et al the sensory and motor block duration was rigorously monitored, and after 20mL of 0.75% ropivacaine bilaterally the duration of block was quoted to be approximately 10 hours with large variation (10). In an effort to replicate the quality and duration of analgesia of an epidural, two recent studies have described the use of lateral and posterior TAP catheters (7, 22). Currently there is a paucity of evidence to assess the true risks and benefits of the use of lower TAP catheters.
Figure 8: Ultrasound images of the anterio-lateral abdominal wall along the costal margin. Medially the rectus abdominis is supero-medial to the transversus abdominis. At the lateral edge of rectus the linea semilunaris forms an aponeurosis of the three muscles that define the lateral abdominal wall. Courtesy of Hebbard et al (18) KEY: SC=subcutaneous; R=rectus abdominis, T=transversus abdominis, A=aponeurosis or linea semilunaris, E=external oblique, I=internal oblique, X=xyphoid, CM=costal margin, IC=iliac crest, U=umbilicus.
Figure 9: A schematic of the antero-lateral abdominal wall along the costal margin, showing the passage of a needle for a subcostal TAP block. Courtesy of Hebbard et al (18). KEY: N=needle, RA=rectus abdominis, TA=transverse abdominis, LA=local anaesethic, SC=subcutaneous, A=aponeurosis or linea semilunaris, TA=transversus abdominis, IO=internal oblique, EO=external oblique, IC=iliac crest, X=xyphoid.
Figure 10: Ultrasound images of a classic lateral approach. Courtesy of Borglum (16) KEY: PC=peritoneal cavity, TA=transversus abdominis, IO=internal oblique, EO=external oblique
Figure 11: Ultrasound images of a posterior approach. Various techniques are described, with imaging and infiltration posterior to the mid axillary line, and at variable posterior limits of the transversus abdominis before it forms the thoacolumbar fascia (10) and intersects with quadratus lumborum (9, 19). Courtesy of Stoving et al and Carney et al (9, 10)
For a full description see the comprehensive technical report written by Hebbard et al in 2010 (18).
The patient is placed in the supine position and a linear transducer (6-15 MHz) is placed medial to the costal margin just under the xiphoid process. Start by identifying the rectus and transversus abdominis muscle, as well as the superior epigastric arteries with the help of colour doppler. Moving the ultrasound probe laterally identify the classic layering of the external oblique, internal oblique, transversus abdominis muscles of the lateral abdominal wall, as well as the peritoneal cavity (18). In some patients it may be easier to identify the peritoneum and identify the TAP plane as one muscle layer superficial. A 15-20cm 18ga tuohy needle is inserted in-plane to the transducer and advanced in a medial to lateral direction along the costal margin. The target end-point is in the neurovascular layer between the transversus abdominis muscle and rectus abdominis muscle medially, internal oblique laterally.
In his description of the technique, Hebbard advices bending the tuohy needle slightly, inserting it with the concave side initially away from the skin, rotating it in the opposite direction as the needle advances to follow the curve of the body, as shown in Figure 9. He also advocates for the use of generous hydrodissection, diluting a patient’s maximum subtoxic dose of local anaesthetic with saline up to 40-80mL, injecting 10-15mL at both the superior and inferior limit to extend the reach of the block beyond the needle tip (18). For maintenance of the block a catheter can be passed through the tuohy needle under strict aseptic technique. For the purposes of our departmental protocol, when placing a subcostal TAP catheter we recommend the use of saline for hydrodissection, and reservation of the first local anaesthetic bolus dose down the catheter once it is placed.
For the classic lateral TAP block the patient is placed in the supine position and a linear transducer (6-15 MHz) is placed transversely to the mid-axillary line. An 18ga tuohy needle is inserted in-plane to the transducer in a anterior to posterior direction. The target end-point is the fascial neurovascular plane between the internal oblique and transversus abdominis muscles. To find the correct fascial plane we recommend advancing the needle tip just into the transversus abdominis muscle and then incrementally pulling the needle tip back using regular aspiration and hydrodissection of 1mL aliquots until there is easy spread and evolution of an elliptical hypoechoic space – the eye sign.
Various techniques are described. Patients may be placed in the supine or lateral position. Carney et al describes placing a linear transducer (6-15 MHz) obliquely over the postero-lateral abdominal wall, posterior to the mid-axillary line between the costal margin and iliac crest. A tuohy needle is inserted and passed anterior to posterior in an in-plane technique. The target end-point is at the intersection of the quadratus lumborum and the lateral abdominal muscles, superficial to the transversalis fascia. Alternatively the transducer can be placed as it would be for a lateral TAP and then moved laterally to reveal the tapering of transversus abdominis and internal oblique into the thoracolumbar fascia, with needle placement 1cm anterior to this.
The current body of literature supports TAP blocks as a clinically effective and acceptable component of multimodal analgesia to limit opioid requirements and potentially substitute epidural analgesia. However the literature is disappointingly unable to define the requirements of optimal technique, and the specific clinical applications are yet to be determined. The current knowledge base suggests the following:
• Do not use TAP blocks for midline incisions (9, 10).
• The subcostal technique is most appropriate for incisions above the umbilicus, covering incisions in the T6-T10 dermatomes. Use is associated with occasional sparing of T12 or incisions extending laterally beyond the anterior axillary line (21), with definite sparing of the L1 dermatome (23).
• A lateral TAP technique is recommended for incisions below the umbilicus T10-T12, with up to a 50% failure of L1 blockade (3). A combination technique with a BD-TAP allows for more extensive coverage from T6-T12 (16).
• The role of a posterior TAP for more extensive sensory coverage is described inconsistently in the literature. Current evidence suggests it is similar if not superior to the lateral technique in terms of the extent and duration of effect (24).
In the systematic review by Abdallah et al, it was proposed that TAP block is unlikely to be equally effective for all types of abdominal surgery, and that efficacy of the block will depend on block technique, local anaesthetic dose and volume and timing of injection. In the review of 18 randomised controlled trials of intermediate to good quality published prior to 2011, the extent of heterogeneity limited meaningful meta-analysis of these important variables. The authors were able to make the following weak observations:
• The majority of trials suggest superior early pain control over a broad range of surgical procedures. There appears to be analgesic benefit with use in laparotomy for colorectal surgery, open appendectomy, and laparoscopic cholecystectomy surgery, with insufficient data to determine the efficacy in other surgeries.
• There was a trend towards superior analgesic outcomes when >15mL/side of local anaesthetic was used.
• Some analgesic advantage was noted in 5/5 trials where the block was performed in the triangle of Petit and 7/12 trials performed in the mid-axillary line.
• 8/9 trials using pre-incisional block and 4/9 with post-incisional block had better analgesic outcomes. McDonnell et al documented that the sensory deficit was maximal at 90 minutes (13). Sequential MRI imaging also shows that the calculated area of injectate coverage can almost double over six hours for lateral TAP blocks, using both 15 and 30mL injectate volumes, however this didn’t correlate with any discernable change in the area of sensory cutaneous block (16). This provides some explanation for the potential benefits of placing an abdominal block prior to incision, or the poor effect in the early post-operative period. In terms of catheter placement prior to incision, any benefit needs to be weighed against the logistics of securing a catheter and preventing its contamination during surgery.
There have been two randomised controlled trials comparing TAP technique with epidural analgesia (7, 21). The first compared bilateral ultrasound guided subcostal TAP catheters using 1mg/kg 0.375% Bupivacaine 8 hourly for the first 72 hours poster-operatively, and thoracic epidural, in 62 patients having elective open hepato-biliary or renal surgery (21). While the study was limited by the lack of blinding, with the added uncertain relevance of all patients receiving an epidural that was used intra-operatively, the study indicated that epidural provided no significant advantage over subcostal TAP catheters postoperatively. The visual analogue pain scores on coughing and at rest from 8-72 hours, nausea scores and patient satisfaction scores were not significantly different between both groups, although tramadol consumption over 72 hours was significantly higher in the TAP group. The therapeutic failure rate was also similar at 22% (7/31) in the epidural group and 30% (8/27) in the TAP group (21).
The second study found that single subcostal TAP blocks with continuous bilateral posterior TAP catheters, using 0.25% levobupivacaine at 8ml/hr for 48 hours were not inferior to a thoracic epidural (7). In 61 patients having laparoscopic colorectal surgery with an incision below T10, there was no significant difference in visual analogue pain scores on coughing 24 hours post-operatively, tramadol consumption in 48 hours, time out of bed, time to first flatus, nor length of hospital stay between each group. The therapeutic failure rate was also comparable, with 13% (4/31) in the epidural group and 7% (2/32) in the TAP group, the later two being in patients with severe inflammatory bowel disease having laparoscopic ileocaecal resection. One patient in the TAP group was also noted to develop an abdominal wall haematoma, with the cause not being clear as to whether the TAP catheter or the surgical port insertion contributed (7).
There are two cases of liver trauma following TAP block in the literature. The first was a 50kg woman having a hysterectomy, who received a landmark guided TAP block pre-incision and was noted to have a liver injury intra-operatively. Subsequently she was noted to have an enlarged liver (25). The second was a 61 year old male who presented with a strangulated inguinal hernia and received an ultrasound guided TAP block pre-incision, deteriorating with peritonitis in the subsequent hours post-operatively (26). There are also three cases of bowel haematoma in children following ilio-inguinal nerve block by landmark technique, raising this as a possible complication with TAP blocks (27). These cases highlight how essential it is to directly visualise the needle tip throughout placement of the block.
Accurate identification of muscle and fascial layers is also important due to the risk of leg weakness from femoral nerve palsy. There is a case report in the literature describing a patient who fell and fractured their ankle following TAP block for an inguinal hernia repair (28). Cadaver studies have illustrated that the femoral nerve is the same tissue plane as the space deep to transversus abdominis, with the transversalis fascia continuous posteriorly with the iliacus fascia, which is immediately deep to the femoral nerve.
While there is a potential risk of local anaesthetic toxicity, there have been nil reports of this in the literature. The other main complications reported in trials to date have been technical, with disconnection of the filter from the catheter, accidental removal of the catheter, leakage around the catheter insertion site, or pain associated with the catheter (22).
While some authors conclude the lack of block related complications reported in TAP trials endorses the safety of the technique, it should be pointed out that the number of cases published to date would in the order of 1500 (1). This is well below that required to assess for rare complications like those we are concerned about with epidural use, or those that may not have been assessed for in trials, such as delayed surgical wound infections following discharge from the pain team or hospital.
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28. Walker G. Transversus abdominis plane block: a note of caution! British journal of anaesthesia. 2010;104(2):265.
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