Anatomy
ANATOMY
The anterior rami of T6-L1 nerve roots supply sensation to the anterior abdominal wall. The intercostal nerves exit the intercostal space and run in the fascial plane between the internal oblique and transversus abdominis muscles. The anterior divisions run anteriorly to pierce the posterior rectus sheath. They then traverse the rectus abdominis muscle to pierce the anterior rectus sheath and supply sensation to the anterior abdominal wall.
Positioning
POSITIONING
Supine with arms by the side to allow access to the anterior abdominal wall. Place the transducer transversely across the abdomen above the level of the umbilicus.
Figure 1: Patient, transducer and needle positioning for a) lateral to medial in-plane rectus sheath block and b) cephalad to caudal in-plane rectus sheath block, just superior to the umbilicus.
Preliminary Scan
PRELIMINARY SCAN
Start with the transducer in the midline and identify the linea alba and rectus muscle on either side. The rectus sheath is the hyperechoic fascial plane encircling the rectus abdominis muscle. The transversalis fascia lies deep to the rectus sheath, leading to the ‘double hyperechoic line’ deep to the rectus abdominis muscle on ultrasound. The transversalis fascia encircles the whole peritoneal cavity. The peritoneum lies immediately deep to the transversalis fascia. Superior to the umbilicus care must be taken to identify and avoid the superior epigastric artery, which can lie deep to or within the rectus sheath. Doppler can be used to aid visualisation. Figure 2: a) ultrasound image showing the left rectus abdominis muscle b) annotated image (Rectus=rectus abdominis muscle; LA=linea alba; PRS=posterior rectus sheath; TF=transversalis fascia. The yellow line, superficial to the PRS, highlights the desired plane for local infiltration).
Technique
TECHNIQUE
Using an in-plane technique from either lateral to medial or cephalad to caudal, the needle is passed through the rectus abdominal muscle. The aim is to ll the plane between the rectus abdominis muscle and the posterior rectus sheath. If in the correct plane then local anaesthetic can be seen neatly peeling the rectus muscle off the posterior rectus sheath (Figure 3).
Figure 3: Ultrasound images of the anterior abdominal wall a) before rectus sheath block and b) after injection of local anaesthetic for rectus sheath block. (Large white arrow indicating transversalis fascia; Large yellow arrow indicating posterior rectus sheath; LA=local anaesthetic; small white arrows highlighting the needle passing through the rectus abdominis muscle).
Other Resources
Plan B/C/D blocks for the abdomen include: Quadratus lumborum blocks
More information about the rectus sheath block can be found at the excellent 
article here
Text and images have been reproduced from the 2nd edition of the RA-UK Handbook, which can be purchased on Amazon, or is received on joining RA-UK. This excellent resource also contains practical descriptions of all of the advanced blocks referenced in the editorial.