There are 3 main muscle layers (from superficial to deep):
In the midline these are replaced by the paired rectus abdominis muscles. These have fascial coverings which form important planes.
The innervation of the anterior abdominal wall arises from the anterior rami of nerve roots T7-12 and the iliohypogastric and ilioinguinal nerves from L1. These nerves primarily travel in a neurovascular plane between the internal oblique and the transversus abdominis. They then penetrate the posterior rectus sheath to pass through to provide anterior cutaneous branches. The iliohypogastric nerve also initially travels in this plane but then penetrates the internal oblique to run between this and the external oblique before giving off cutaneous branches. The ilioinguinal nerve runs more inferiorly than this and actually passes deeper, perforating the transversus abdominis at the level of the iliac crest.
Roots T7-9 supply the dermatomes between the costal margin and umbilicus. The T10 root supplies the level of the umbilicus. Roots T11 and 12 supply the skin below the umbilicus. The iliohypogastric nerve supplies the skin over the inguinal region. The ilioinguinal nerve provides cutaneous innervation to the medial thigh, anterior scrotum/labia and innervates the inguinal hernial sac.
The rectus sheath block, particularly with catheter insertion, is a regional anaesthetic technique which can be used for abdominal surgery analgesia. The use of a catheter technique can be used as an alternative to epidural analgesia for patients undergoing a laparotomy, for example when a neuraxial technique is not possible.
Indications Midline and para-median abdominal incisions The block/infusion will only provide cutaneous blockade, so consideration of any visceral element of pain is also essential.
Contraindications These are similar to most regional anaesthetic techniques. Absolute
Local anaesthetic toxicity
Large abdominal wall defects (anatomy may be distorted)
Injury to adjacent structures - bowel, peritoneum, vessels
Local anaesthetic toxicity - intravascular placement, incorrect dosing
Catheter failure - blockage, migration
Local anaesthetic toxicity
Technique There are two main variations in technique:
Ultrasound guided insertion
Direct insertion by surgeon
Each approach has advantages and disadvantages. The goal is to insert the catheter into the plane between the rectus abdominis muscle and the posterior rectus sheath.
The US approach can be done before or after the surgical procedure, and US can be used to assist direct insertion. A significant advantage is that it can be performed after the surgical procedure, for example as a rescue in cases of epidural failure. It can also be used to provide intraoperative analgesia if inserted preoperatively. Insertion preoperatively has the risk of the catheters getting in the way of the surgical field.
Surgical insertion can be performed at the end of the procedure and is a fairly simple surgical skill to learn. It is usually quicker than the US guided technique.
US guided technique The equipment used is:
Linear US probe - high frequency, depth 4-6cm
Epidural catheter kit - catheter, filter
Full sterility is required.
Start with probe in transverse plane over mindline, halfway between xiphoid process and umbilicus
Identify key structures, moving laterally to the side that the block is being performed on - rectus abdominal muscle, posterior rectus sheath
Rotates probe to longitudinal direction, maintaining landmarks
Use in-plane approach to guide needle down to below rectus abdominis muscle
Use saline to hydrodissect plane between muscle and posterior rectus sheath
Insert catheter into fluid space under direct vision, allowing 8 cm of catheter to be in the space
Infusion Local anaesthetic can be delivered by a bolus or infusion technique. The administration has to be given bilaterally. A bolus regime may improve mobilisation due to fewer restricting connecting lines. However, this requires staff to administer and may have a risk if LA toxicity. Infusions can be given my electronic pumps or non electronic methods e.g. elastomeric devices.
Example regimes (for each side) include:
20 ml 0.25% leveobupivacaine 6 hourly
20ml 0.2% ropivacaine 6 hourly
0.2% ropivacaine 8ml/hr
Evidence Base This is currently still fairly limited. A large RCT is currently ongoing looking at the comparison of rectus sheath catheters with thoracic epidural analgesia in laparotomy patients (TERSC study).
Transverse Abdominis Plane (TAP) Block
The TAP block aims to block the cutaneous innervation of the abdominal wall by placing local anaesthetic within the transverse abdominis plane (between the internal oblique and transverse abdominis muscle) where the anterior rami of the lower thoracic (T7-12) and L1 roots run. The landmark technique has traditionally been done but an US technique had probably superseded it. This block is a volume block to ensure adequate spread within the fascial plane, needing about 20-30ml per side - this may impact on the concentration that can be used. Examples include 0.25% levobupivacaine or 0.25% ropivacaine.
Indications The block is useful for any lower abdominal surgery (dermatomes T10 and below) e.g. appendicectomy, c-section. If the incision crosses or approaches the midline then bilateral blocks would be needed.
Contraindications They are essential the general contraindications to regional blocks
Complications Very rare but the same as for rectus sheath blocks.
Ultrasound technique The equipment required:
Linear US probe, high frequency
Full ANTT - gloves, drape, probe cover, skin disinfectant
Lidocaine for skin (if done awake)
US block needle e.g. 50mm
Local anaesthetic agent - 20-30ml
Full sterility is achieved and maitained
The US probe is positioned in a transverse plane on the mid-axillary line between the iliac crest and the costal margin
An in plane technique, bringing the needle in from an anterior direction is commonly used
The anatomy is identified, specifically looking for the target plane between internal oblique and transverse abdominis muscle layers.
The needle is advanced in-plane to the position, usually feeling the ‘pops’ as the layers are passed through
Initial injection of small volumes of local anaesthetic or saline can help confirm location in the correct plane.
A large volume (20-30ml) of appropriate strength local anaesthetic is then injected, carefully aspirating beforehand and during, and visualizing the spread along the TAP.
Landmark technique The landmark here is the triangle of Petit, the features of which can be palpated in many patients (may be difficult in cases of obesity) This is found in the flank between the costal margin superiorly (forming the apec of the triangle) and the iliac crest inferiorly (forming the base of the triangle). The anterior side of the triangle is formed by the edge of the external oblique and the posterior side by the edge of the latissimus dorsi. Full ANTT and skin anaesthesia is needed as above. The correct plane is identified by a tactile feel of ‘pops’ as the needle is passed through fascial planes - a blunt needle helps with this After passing the needle through the skin, the needle is advanced deep. Two pops will be felt, the first after passing through the external oblique, the second after passing through the internal oblique. This should be the correct plane, and local anaesthetic can be injected as above.