Ultrasound-guided transversus abdominis plane block in the dog: an anatomical evaluation

Veterinary Anaesthesia and Analgesia, 2011, 38, 267 271 doi:10.1111/j.1467-2995.2011.00612.x RESEARCH PAPER Ultrasound-guided transversus abdominis plane block in the dog: an anatomical evaluation Carrie A Schroeder*, Lindsey B C Snyder*, Caitlin C Tearney, Tracy L Baker-Hermanà & Kristopher M Schroeder *Department of Surgical Sciences, University of Wisconsin School of Veterinary Medicine. Madison, WI, USA University of Wisconsin School of Veterinary Medicine, Madison, WI, USA àdepartment of Comparative Biosciences, University of Wisconsin School of Veterinary Medicine. Madison, WI, USA Department of Anesthesiology, University of Wisconsin School of Medicine and Public Health. Madison, WI, USA Correspondence: Dr. Carrie Schroeder; Department of Surgical Sciences; University of Wisconsin School of Veterinary Medicine; 2015 Linden Drive; Madison, WI 53706, USA. E-mail: schroederc@svm.vetmed.wisc.edu Abstract Objective To describe the ultrasound-guided technique to the transversus abdominis plane (TAP) block in the dog and evaluate the spread of a local anesthetic/methylene blue solution. Study design Prospective experimental trial. Animals Ten adult Beagle cadavers weighing 11.1 ± 1.1 kg (mean ± SD). Methods Transversus abdominis plane (TAP) blocks were performed bilaterally by a single trained individual on unpreserved cadaver dogs using 10 ml of methylene blue/bupivacaine solution per site. Dissection of the abdominal wall was performed within 15 55 minutes of block to determine distribution of injectate and nerve involvement in the transversus abdominis fascial plane. Results The transversus abdominis fascial plane was adequately visualized via ultrasound and injected in twenty hemi-abdominal walls. Segmental branches of T11, T12, T13, L1, L2, and L3 were adequately stained in 20%, 60%, 100%, 100%, 90%, and 30% of injections, respectively. Conclusions and clinical relevance This anatomical study suggests that the transversus abdominis plane (TAP) block would provide adequate regional anesthesia of the abdomen, potentially extending to the cranial and caudal limits of the abdomen. This supports the clinical potential of this block in veterinary medicine. Keywords bupivacaine, dogs, regional anesthesia, transversus abdominis plane block, ultrasoundguided techniques. Introduction The transversus abdominis plane (TAP) block is a regional anesthetic technique that is rapidly gaining popularity among physicians in order to provide local anesthesia to the abdominal wall. This block is performed within the fascial plane that overlies the transversus abdominis, thus providing sensory blockade to the abdominal wall and improving patient analgesia in the peri-operative period for those patients with abdominal incisions. Published studies of this block report improved visual analog scores, lowered morphine consumption, and improved patient satisfaction following abdominal surgery for up to 48 hours with ropivacaine and 24 hours with levobupivacaine as compared to control groups (O Donnell et al. 2006; McDonnell et al. 2007, 2008; Carney et al. 2008; Belavy et al. 2009). Human cadaveric dye studies confirm staining of as far cranially as T10 and caudally to L1 267

corresponding to blockade of these segments when local anesthetic is used (McDonnell et al. 2007; Tran et al. 2009). Although spread of dye was confined to those spinal segments in cadaveric studies, sensory blockade in humans has been reported to extend as far cranially as the T7 dermatome (McDonnell et al. 2007). The lateral abdominal wall in mammals consists of three major muscle layers: the external abdominal oblique, the internal abdominal oblique, and the transversus abdominis. The fascial plane between the internal abdominal oblique and transversus abdominis muscles contains afferent branches of thoracic and lumbar innervating the abdominal wall (Evans 1993). Innervation to the canine abdominal wall and peritoneum is provided by the branches of T11, T12, and T13 cranially and branches of L1, L2, and L3 caudally (Evans 1993). This is in contrast to human anatomy where the abdominal wall is innervated by branches of T7-L1, where a 13th thoracic segment is lacking. Previous studies of injectate spread following TAP block in human cadavers suggest that this block may be limited to use in lower abdominal surgeries as spread was isolated to segments T10-L1 (Tran et al. 2009). The TAP block is extensively utilized for intraand post-operative analgesia in human patients, but has not been well described in the veterinary literature. A single case report of a TAP block performed in a lynx reported questionable efficacy due to confounding factors (Schroeder et al. 2010). This block holds great promise in providing regional anesthesia to veterinary patients undergoing abdominal surgeries. However, it is unknown whether this block would be effective in veterinary patients. The objective of this study was to evaluate the distribution of injectate following TAP block in canine cadavers in an effort to define the potential efficacy of this block for canine abdominal surgeries in vivo. (13 6 MHz HFL38 38 mm probe, SonoSite Micromaxx; SonoSite, Inc, Bothell, WA, USA) in a transverse orientation midway between the iliac crest and the caudal aspect of the rib cage and approximately 5 cm lateral to midline on the left side of the abdomen. The three layers of the abdominal wall were identified as external abdominal oblique, internal abdominal oblique, and transversus abdominis (Fig. 1). A 22-gauge 2 Tuohy needle (Kimberly Clark/Ballard Medical Products, Roswell, GA, USA) attached to an extension set primed with a 1:1 solution of 1% methylene blue (American Regent; Shirley, NY, USA) and 0.25% bupivacaine solution (Hospira, Inc., Lake Forest, IL, USA) was passed through the external and internal oblique muscle layers perpendicular to the skin. The needle was inserted beneath the long axis of the ultrasound beam utilizing an in-plane approach so that the needle could be visualized penetrating the abdominal layers. When the fascial plane between the internal abdominal oblique and transversus abdominis muscles was thought to be reached, a test dose of 1 ml methylene blue/bupivacaine solution was injected to confirm placement. If the injection was visualized in the incorrect plane, the needle was adjusted to ensure positioning in the correct plane. When the methylene blue solution was visualized in the proper fascial plane and confirmed with the test dose, the remainder of the solution was injected for a total of 10 ml per hemiabdominal wall (Fig. 2). The procedure was repeated in an identical manner on the right side of the abdomen. Dissection was performed between 15 and 55 minutes (mean 30 minutes) following successful injections of both hemi-abdominal walls. Surgical staples were placed at the cranial and caudal aspects Materials and methods Ten freshly euthanized adult Beagle cadavers weighing 11.1 ± 1.1 kg (mean ± SD) were obtained and immediately frozen. These animals were thawed at a later date and placed in lateral recumbency. All imaging and injection was performed by a single individual trained in ultrasoundguided regional anesthesia. The abdominal layers were imaged with a 6 to 13 MHz linear array probe Figure 1 Ultrasonographic image obtained in a live patient demonstrating layers of abdominal wall. Small dots on right of image indicate 1 cm depth markers. EO, external abdominal oblique; IO, internal abdominal oblique; TA, transversus abdominis; P, peritoneal cavity. 268 Ó 2011 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesiologists, 38, 267 271

Figure 2 Ultrasonographic image obtained in a live patient post-injection of local anesthetic solution (5 ml of 0.125% bupivacaine) into the fascial plane overlying the transversus abdominis. Small dots on right of image indicate 1 cm depth markers. EO, external abdominal oblique; IO, internal abdominal oblique; LA, local anesthetic; TA, transversus abdominis; P, peritoneal cavity. of the pool of injectate. The branches of the spinal were then visualized and the cranial-most nerve involved was traced back to its origin by an anatomist. The anatomist then determined nerve involvement by counting nerve segments present in the caudal direction that were adequately stained by the injectate. Adequate staining was determined by a minimum of 1 cm of dye along the long axis of the nerve. Results The three layers of the abdominal wall were successfully visualized in all specimens and 20 hemiabdominal walls from 10 cadavers were successfully injected and dissected. The methylene blue solution was found to be pooled in the fascia overlying the transversus abdominis muscle in all cadavers (Fig. 3). A small amount of solution was found in more superficial muscle layers both through retrograde tracking of solution through the needle pathway and through test injection into incorrect muscle planes. Branches of T11-L3 were found on the lateral aspect of the transversus abdominis muscle and were successfully identified by tracing the cranial-most nerve branch back to its emergence from the intercostal space. The most cranial nerve involved in the spread was T11, accounting for 20% of injections and the most caudal nerve involved was L3, accounting for 30% of injections. Branches of T12, T13, L1 and L2 were most commonly involved in the spread with 60%, 100%, 100% and 90% demonstrating adequate staining, respectively. Spread of injectate observed in individual cadavers is listed in Table 1. No evidence of Figure 3 Dissection of the lateral abdominal wall showing T12-L3 spread of methylene blue solution following ultrasound-guided TAP block in a cadaver. Table 1 Spread of 10 ml methylene blue solution (1:1 solution of methylene blue and 0.25% bupivacaine) over ventral branches of spinal following TAP block among cadaver hemi-abdominal walls Spinal nerve trauma to the abdominal walls or abdominal viscera was noted and there was no intra-abdominal methylene blue solution. Discussion No. of identified No. of stained T11 20 4 20 T12 20 16 80 T13 20 20 100 L1 20 20 100 L2 20 18 90 L3 20 6 30 Frequency of stained (%) A number of approaches to the TAP block have been described. The block was first described using a blind technique that utilized anatomical landmarks in order to inject through the Triangle of Petit, bound by the latissimus dorsi muscle posteriorly, the external abdominal oblique muscle anteriorly and the iliac crest caudally (McDonnell et al. 2004). Since that initial report, ultrasound-guided techniques have been described that allow visualization Ó 2011 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesiologists, 38, 267 271 269

of the layers of the abdominal wall as well as visualization of the needle and deposition of local anesthetic (Hebbard et al. 2007). It has been suggested that ultrasound guidance of regional anesthetic techniques may afford greater safety, efficacy and efficiency while decreasing the incidence of local anesthetic toxicity due to visualization of local anesthetic spread (Griffin & Nicholls 2010). In veterinary medicine, ultrasound-guidance offers the distinct advantage of directly visualizing anatomic structures that may be poorly conserved among species or even breeds. For instance, the depth of the transversus abdominis plane was extremely shallow in the beagles utilized in this study while visualization of this plane by the authors in other breeds reveals much thicker layers of abdominal musculature. In the case of this block, ultrasound guidance has the potential to offer a decreased incidence of peritoneal puncture as well as a higher likelihood of depositing local anesthetic into the correct plane. A study describing the spread of stain in the transversus abdominis plane following ultrasoundguided TAP block in human cadavers revealed staining from T10 to L1. This corresponds to staining across four nerve segments as humans possess 12 thoracic spinal (Tran et al. 2009). Our study found similar results with staining of encompassing 4 ± 1 (mean ± SD) segments, most commonly T12-L2. The TAP block has been reported to offer sensory blockade as far cranially as the T7 dermatome extending to the L1 dermatome (McDonnell et al. 2007) thus providing analgesia for abdominal surgeries as the human abdominal wall is innervated by branches of spinal T7- L1. Due to the fact that the innervation of the canine abdominal wall involves more caudal spinal, T11-L3, we believe that the TAP block would provide sensory blockade to a large proportion of the abdominal wall and parietal peritoneum. However, the authors recognize that anesthesia of the abdominal wall may not be complete as branches of T11 and L3 were only involved in 20% and 30% of injections, respectively. Additional analgesics may be needed for incisions encompassing cranial and caudal aspects of the abdomen. A limitation of this study is that the injection of solution into cadavers may not adequately reflect that spread of local anesthetic in vivo. Many of the cadavers had abdominal wall layers that had been compressed post-mortem and had been frozen and thawed, potentially altering the spread of dye. Live animals would likely exhibit less contraction of fascial planes, possibly leading to a greater spread of local anesthetic. Furthermore, the innervation of the abdominal wall and peritoneum is complex and communication between adjacent thoracolumbar has been described (Rozen et al. 2008). The extent of sensory blockade of this technique, therefore, may be entirely different in vivo. Another potential limitation was the amount of injectate utilized for the block. In this study 10 ml, roughly 1 ml kg )1, of solution was utilized. If the initial 1 ml test injection was in the incorrect plane, the remaining 9 ml of solution was injected into the correct fascial plane. This occurred in roughly 20% of hemi-abdominal walls. No appreciable differences in spread were present when initial test injection was incorrect. Unlike many ultrasound-guided blocks that utilize direct visualization and electrolocation of to deliver a very small volume of injectate, as low as 0.05 ml kg )1 (Campoy et al. 2010), this block relies on a large volume of injectate to cover several spinal. In humans, post-mortem evaluations of the TAP block used 20 ml of staining solution per hemi-abdominal wall (McDonnell et al. 2007; Barrington et al. 2009; Tran et al. 2009), roughly corresponding to 0.28 ml kg )1 in an average-sized 70 kg human cadaver. Dissection into the transversus abdominis plane revealed a large amount of injectate that seemed to pool rather than spread. It is possible that in the live patient, this pool would either continue to spread within the plane or, more likely, be systemically absorbed. Further studies may elucidate the importance of volume on the spread and efficacy of this block. Time from block until dissection could also impact the spread and efficacy of the TAP block. All dissections were carried out between 15 and 55 minutes following completion of the block, 30 minutes on average. There seemed to be no difference in spread among hemi-abdominal walls related to duration between the block and dissection suggesting that the spread occurs within 15 minutes. However, the time required to achieve sensory blockade in clinical cases utilizing this block will depend on the adequacy of this spread and the concentration and individual characteristics of the local anesthetic used. In order to provide adequate sensory blockade, it has been described that three nodes of Ranvier must be exposed to local anesthetic, corresponding to roughly 3 4 mm (Raymond et al. 1989). In order to consider a branch of a spinal nerve covered by stain, at least 1 cm of the nerve needed to be stained 270 Ó 2011 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesiologists, 38, 267 271

in this study. In most cases, the majority of the branch coursing through the transversus abdominis was stained. If staining of a nerve was questionable, it was not considered to be blocked. Therefore, this study potentially underestimates clinically significant spread of injectate. In summary, we think that the transversus abdominis plane block has the potential to provide excellent analgesia of the abdominal wall in dogs. It has been utilized to great success in a variety of abdominal surgical procedures in human patients. The ultrasound-guided technique offers the advantage of direct visualization of abdominal muscle layers as well as deposition of local anesthetic. Further studies on clinical patients are warranted to verify the efficacy of this block in providing perioperative analgesia for patients undergoing abdominal surgery. References Barrington MJ, Ivanusic JJ, Rozen WM et al. (2009) Spread of injectate after ultrasound-guided subcostal transversus abdominis plane block: a cadaveric study. Anaesthesia 64, 745 750. Belavy D, Cowlishaw PJ, Howes M et al. (2009) Ultrasound-guided transversus abdominis plane block for analgesia after Caesarean delivery. Br J Anaesth 103, 726 730. Campoy L, Bezuidenhout AJ, Gleed RD et al. (2010) Ultrasound-guided approach for axillary brachial plexus, femoral nerve, and sciatic nerve blocks in dogs. Vet Anaesth Analg 37, 144 153. Carney MB, McDonnell JG, Ochana A et al. (2008) The transversus abdominis plane block provides effective postoperative analgesia in patients undergoing total abdominal hysterectomy. Anesth Analg 107, 2056 2060. Evans HE (1993) Miller s Anatomy of the Dog (3rd edn). Saunders, Philadelphia, PA. pp. 308 312. Griffin J, Nicholls B (2010) Ultrasound in regional anaesthesia. Anaesthesia 65, 1 12. Hebbard P, Fujiwara Y, Shibata Y et al. (2007) Ultrasound-guided transversus abdominis plane (TAP) block. Anaesth Intensive Care 35, 616 617. McDonnell JG, O Donnell BD, Tuite D et al. (2004) The regional abdominal field infiltration (R.A.F.I) technique: computerised tomographic and anatomical identification of a novel approach to the transversus abdominis neuro-vascular fascial plane. Anesthesiology 101, A899 (abstract). McDonnell JG, O Donnell BD, Farrell T et al. (2007) Transversus abdominis plane block: a cadaveric and radiological evaluation. Reg Anesth Pain Med 32, 399 404. McDonnell JG, Curley G, Carney J et al. (2008) The analgesic efficacy of transversus abdominis plane block after cesarean delivery: a randomized controlled trial. Anesth Analg 106, 186 191. O Donnell BD, McDonnell JG, McShane AJ (2006) The transversus abdominis plane (TAP) block in open retropubic prostatectomy. Reg Anesth Pain Med 31, 91. Raymond SA, Steffensen SC, Gugino LD et al. (1989) The role of length of nerve exposed to local anesthetics in impulse blocking action. Anesth Analg 68, 563 570. Rozen WM, Tran TM, Ashton MW et al. (2008) Refining the course of the thoracolumbar : a new understanding of the innervation of the anterior abdominal wall. Clin Anat 21, 325 333. Schroeder CA, Schroeder KM, Johnson RA (2010) Transversus abdominis plane block for exploratory laparotomy in a Canadian lynx (Lynx canadensis). Proceedings of the 35th Annual Meeting of the American Society of Regional Anesthesia & Pain Management, Toronto, ON, Canada (abstract). Tran TM, Ivanusic JJ, Hebbard P et al. (2009) Determination of spread of injectate after ultrasound-guided transversus abdominis plane block: a cadaveric study. Br J Anaesth 102, 123 127. Received 7 June 2010; accepted 19 July 2010. Ó 2011 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesiologists, 38, 267 271 271