Abdominal field block: a new approach via the lumbar triangle

I wish to describe what I believe to be a novel approach to abdominal field block. The technique, as originally described, entails multiple injections and administration of potentially toxic doses of local anaesthetic agent 1. This new approach involves identifying the neurovascular plane of the abdominal musculature and injecting a local anaesthetic agent therein. The only area of the abdominal wall where the internal oblique muscle can be localised directly is the ‘lumbar triangle of Petit’ where it forms the floor of this triangle. In most people, the lumbar triangle is situated just behind the highest point of the iliac crest. Local anaesthetic agent deposited in the area of the lumbar triangle will block the lower intercostal nerves, the iliohypogastric and the ilioinguinal nerves as they traverse between subcostal margin and the iliac crest. The iliac crest 2 has ventral and dorsal segments. The ventral segment has external and internal lips and a rough intermediate zone. The crest's summit is level with the L3−4 intervertebral space. The lower fibres of the external oblique and the latissimus dorsi muscles are attached to the external lip. A variable interval exists between the most posterior attachment of external oblique and the most anterior attachment of latissimus dorsi. Here, the crest forms the base of the ‘lumbar triangle of Petit’. The floor of the triangle is the internal oblique muscle, which is attached to the crest's intermediate area. The transversus abdominis muscle is attached to the anterior two-thirds of the crest's inner lip. The lumbar triangle is bounded anteriorly by the free posterior border of external oblique, posteriorly by the lower, lateral margin of latissimus dorsi and inferiorly by the iliac crest. The seventh to eleventh intercostals nerves, subcostal nerve, iliohypogastric and ilioinguinal nerves, all run a variable part of their courses between internal oblique and transversus abdominis muscles. With the patient in the supine position, a finger is walked posteriorly from the anterior superior iliac spine along the top of the iliac crest until it dips slightly inward. On further posterior movement, the finger-tip is felt to slip over the edge of a muscle. At this point, the finger is assumed to be abutting on the lateral border of latissimus dorsi where it is attached to the external lip of the iliac crest (Fig. 8). Without moving the hand, the skin is pierced anterior to the finger-tip with an 18G cutting needle at the level of the external lip. A 24G, blunt-tipped, 2-inch needle (1Plexufix®, Ref: 0489 1562, B. Braun.) is inserted perpendicular to the skin until it touches bone of the external lip (Fig. 9). Thereafter, the needle is slowly advanced over the intermediate zone of the iliac crest until a definite ‘pop’ or ‘sensation of giving way’ is felt. At this juncture, the needle has reached the plane between the internal oblique and the transversus abdominis muscles. After negative aspiration, 20 ml of a local anaesthetic agent is injected. Only one injection is required for a unilateral incision, e.g. in appendicectomy, while bilateral injections are administered for midline or transverse abdominal incisions. For a successful block, the injectate must disappear between the muscle layers without any apparent swelling of the abdominal wall. I have used this technique for more than 2 years and performed the abdominal field block via the lumbar triangle on more than 200 patients without any untoward sequelae. For bilateral injections, the maximum dose limit of the local anaesthetic agent is carefully observed. If the needle is advanced too far, a second ‘pop’ is felt. This indicates that the needle has passed through the transversus abdominis muscle attached to the internal lip of the iliac crest and must be withdrawn and re-inserted. Even after the second ‘pop’, the needle would have to pass through the transversalis fascia and parietal peritoneum before reaching the peritoneal cavity. The use of a fine-gauged, blunt-tipped needle helps to minimise the possibility of visceral damage if the needle is advanced too far inadvertently. The block may not be easy in obese patients because of difficulty in identifying landmarks. In these subjects, the point of needle insertion is chosen 2.5 cm behind the highest point of the iliac crest, a landmark easily palpable in most people. In elderly patients, the whole thickness of the iliac crest can be grasped between two fingers due to the loss of muscle mass and tone. It makes block easier to perform, bearing in mind that the needle tip must not be advanced beyond the inner lip of the crest. In small children, the lumbar triangle is felt like a tiny hole in the abdominal wall just behind the highest point of the iliac crest. I recommended a 24G 1-inch needle (1Plexufix®, Ref: 0489 152, B. Braun) for use in children. If extraordinary resistance is felt during injection, the needle may have lifted the periosteum in which case it should be withdrawn and re-sited. Finally, I would emphasise that this new approach of abdominal field block via the lumbar triangle should be avoided in patients with a lumbar hernia in which the hernial sac protrudes through the lumbar triangle.