Several challenges are apparent when using abdominal wall blocks:
1. Varying ultrasound appearance of abdominal wall structures.
2. Variable anatomy of nerves.
3. Complexity of innervation.
4. Effects of obesity, air in tissues, dressings, wounds.
1. Extensive wounds.
2. Off-midline wounds.
3. Surgery under blocks alone.
Understanding the location and distribution of abdominal wall nerves is critical to successful abdominal wall blockade. The abdominal wall is innervated by the anterior divisions of spinal nerves T6 to L1, which run laterally between the transversus abdominis and internal oblique muscle layers of the abdominal wall in a potential space termed the transversus abdominis plane (TAP)(Fig. 1 and 2). Each nerve gives rise to a lateral and anterior branch. The lateral branch leaves the main nerve at the angle of the rib and passes obliquely through the chest (or abdominal) wall to emerge subcutaneously in the mid-axillary line (Fig. 1). This lateral branch supplies the external oblique muscle and overlying skin of the lateral abdominal wall. The anterior branches continue anteriorly in the TAP to supply the transversus abdominis and internal oblique muscles before penetrating the rectus sheath to supply the rectus abdominis muscle and skin of the anterior abdominal wall (from the midline to approximately the anterior superior iliac spine) (Fig. 1 and 2).
Fig. 1. Anatomy of the thoracoabdominal nerves. AB=Anterior branch (motor and sensory innervation shaded purple). LB=lateral branch (motor and sensory innervation shaded green).
Fig. 2. Terminal anterior (yellow) and lateral (green) branches of the thoracoabdominal nerves on the transversus abdominis muscle surface. Note the lateral branches above T11 emerging through the intercostal space.
The anterior branches are reliably found in the TAP at the level at which the 3 muscle layers (external oblique, internal oblique and transversus abdominis) are most readily imaged: approx. 45 degrees from the midline and between the costal margin and the iliac crest. All 3 muscles become aponeurotic as they pass towards the midline with the external oblique and internal oblique aponeuroses forming the rectus sheath around the medially situated rectus abdominis muscle (Fig. 3).The aponeurosis of the transversus abdominis muscle fuses with the rectus sheath, however, near the costal margin, the transversus abdominis muscle has a fleshy portion that extends deep to the rectus muscle (Fig. 3).
The rectus muscles are readily identified near the midline, where with transverse scanning, they have a symmetrical appearance. The posterior rectus sheath is characteristically imaged as 2 distinct layers almost in contact with each other. This sheath forms the basis for the rectus sheath block.
Fig. 3. Abdominal wall from anterior with the transversus muscle exposed. The thoraco-abdominal nerves, muscle edges and main arteries are shown.
The nerves of the abdominal wall may be thought of as an extensive plexus with many communications between adjacent nerves. The nerves also communicate in the lateral abdominal wall via a plexus accompanying the deep circumflex iliac artery, and in the rectus abdominis muscle via the inferior and superior epigastric arteries (Fig. 3). These anastomotic connections and the possible entry of sympathetic nerves at the origin of these vessels may explain the relative difficulty in obtaining complete abdominal wall blockade with the TAP block alone. In addition, unilateral TAP block often does not extend fully to the midline, suggesting some midline cross-over of nerves (unpublished observation by the author).
The abdominal wall muscles, particularly the rectus abdominis may be affected by denervation from previous surgery and age. In both instances, the muscle may become thinner and fibrotic (sonographically thinner/whiter). Obesity increases the thickness of the adipose layers found both external and internal to the muscles although there is no adipose tissue between the muscle layers themselves. The superficial scarpa’s fascial layer may become thickened with adipose tissue, thus resembling a muscle layer.
Midline incisions are readily anesthetised as only the anterior branches require blocking: bilateral TAP injections extending to the segments supplied by the wound area will therefore produce adequate midline analgesia. Intra-abdominal structures are usually not blocked by this technique, necessitating multi-modal and/or opioid analgesia.
Ensuring adequate LA spread within the TAP is important for successful abdominal wall blockade. There is considerable inter-individual difference in the ease of spread of solution within the TAP. It is not known whether this can be predicted based on patient factors, or is more dependent on injection technique. Impressions are that spread is better in young patients, particularly children, and in the upper abdomen compared to near the iliac crest. In adults, a single point injection of 20 ml LA will generally cover 10 to 15 cm in the midline or 3 vertebral segments.
A line extending along the sub-costal oblique line (caudad of the costal margin) to near the anterior superior iliac spine represents the neural plane for the anterior branches of the segmental nerves. Blockade along this line ('Subcostal TAP block') is adequate for midline incisions. Extensive incisions require extensive spread of LA within the TAP, which is best produced by hydro-dissection of LA across the direction of the nerves.
Catheters may be successfully placed along the sub-costal oblique line for subsequent infusion or bolus regimes. For incisions lateral to the rectus abdominis muscle, the muscular and cutaneous lateral branches also contribute to wound pain, and therefore need to be blocked for successful anesthesia/analgesia.
Surgery under peripheral block alone
Successful surgical anesthesia with abdominal wall blocks alone can be challenging, possibly because of the extensive anastomosis between nerves, the ascending and descending plexuses, and possible sympathetic inflow along vessels. A useful approach involves TAP block (ensuring extensive LA spread using hydrodissection) supplemented with subcutaneous LA over the area of the incision. An alternative approach is to widely infiltrate the tissues with dilute LA using ultrasound imaging to ensure spread within the target area. Although LA is known to cause dose dependent myotoxicity, animal studies have shown the effect is reversible. There is also a paucity of reported adverse effects from intramuscular LA despite over 100 years of accidental and deliberate injection.
Surgical site pain may arise from an intra-abdominal organ or omentum protruding through a hernia, which will not be covered by peripheral block.