Hadzic's Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd

42. Common Ultrasound-Guided Truncal and Cutaneous Blocks


General Considerations

The ultrasound-guided transversus abdominis plane block, or TAP has become a commonly used regional anesthesia technique for a variety of indications. It is largely devoid of complications and can be performed time-efficiently, either at the beginning or the end of surgery for use as postoperative analgesia. Similar to ilioinguinal and iliohypogastric nerve blocks, the method relies on guiding the needle with ultrasound to the plane between the transversus abdominis and internal oblique muscles, to block the anterior rami of the lower six thoracic nerves (T7-T12) and the first lumbar nerve (L1). Injection of local anesthetic within the TAP potentially can provide unilateral analgesia to the skin, muscles, and parietal peritoneum of the anterior abdominal wall from T7 to L1, although in clinical practice, the extent of the block is variable.

Ultrasound Anatomy

The anterior abdominal wall (skin, muscles, and parietal peritoneum) is innervated by the anterior rami of the lower six thoracic nerves (T7-T12) and the first lumbar nerve (L1). Terminal branches of these somatic nerves course through the lateral abdominal wall within a plane between the internal oblique and transversus abdominis muscles. This intermuscular plane is called the transversus abdominis plane. Injection of local anesthetic within the TAP can result in unilateral analgesia to the skin, muscles, and parietal peritoneum of the anterior abdominal wall. The exact cephalad-caudad spread and extent of anesthesia and analgesia obtained with the TAP block is variable. This subject is not well researched; the actual coverage is likely dependent on the technique details, place of needle insertion (lateral-medial) and volume of local anesthetic injected. Additionally, the patient’s anatomical characteristics may also influence the spread of the injected solutions.

Imaging of the abdominal wall between the costal margin and the iliac crest reveals three muscle layers, separated by a hyperechoic fascia: the outermost external oblique (EOM), the internal oblique (IOM), and the transversus abdominis muscles (TAM) (Figures 42.1-2 and 42.1-3). Immediately below this last muscle is the transversalis fascia, followed by the peritoneum and the intestines below, which can be recognized as moving structures because of peristalsis. The nerves of the abdominal wall are not visualized consistently, although this is not necessary to accomplish a block.


FIGURE 42.1-2. Innervation of the anterior and lateral abdominal wall. IH, iliohypogastric nerve; IL, ilioinguinal nerve.


FIGURE 42.1-3. Schematic representation of the abdominal wall muscles.


• Obese patients have a large subcutaneous layer of fat that can make positive identification of the three muscle layers challenging. A rule of thumb is that the internal oblique muscle is always the “thickest” layer, and the transversus abdominis is the “thinnest.”

Distribution of Blockade

The exact distribution of abdominal wall anesthesia following a TAP block has not been well documented or entirely agreed on by practitioners. The most fervent proponents of TAP technique maintain that reliable blockade of dermatomes T10-L1 can be achieved with moderate volumes of local anesthetic (e.g., 20–25 mL). Claims of blockade up to T7 after single injection of large volume have been made, but these results are not consistently reproduced in clinical practice. In our practice, some TAP blocks have resulted in complete anesthesia for inguinal herniorrhaphy; at other times, the results have been less consistent. Additional research is indicated to clarify the spread of anesthesia and factors that influence it.


Equipment needed is as follows:

• Ultrasound machine with linear transducer (6–18 MHz), sterile sleeve, and gel (in very obese patients, and when a more posterior approach is used, a curved transducer might be needed)

• Standard nerve block tray

• Two 20-mL syringes containing local anesthetic

• A 50- to 100-mm, 20- to 21-gauge needle

• Sterile gloves

Landmarks and Patient Positioning

This block typically is performed with the patient in the supine position. The iliac crest and costal margin should be palpated and the space between them in the mid-axillary line (usually 8–10 cm) identified as the initial transducer location. The block is almost always performed under general anesthesia in pediatric patients; a common option for adults as well.


The goal is to place the needle tip in the plane between the IOM and the TAM, to deposit local anesthetic between the muscle layers, and confirm the proper spread of the injectate under ultrasound guidance.


With the patient supine, the skin is disinfected and the transducer placed on the skin (Figure 42.1-4). The three muscle layers should be identified (Figures 42.1-5A and B). Sliding the transducer slightly cephalad or caudad will aid the identification. Once the transverse abdominal plane is identified, a skin wheal is made 2 to 3 cm medial to the medial aspect of the transducer, and the needle is inserted in-plane in a medial to lateral orientation (Figures 42.1-1Aand 42.1-6). The needle is guided through the subcutaneous tissue, EOM, and IOM. A “pop” may be felt as the needle tip enters the plane between the two muscles. After gentle aspiration, 1 to 2 mL of local anesthetic is injected to verify the location of the needle tip (Figure 42.1-6). When injection of the local anesthetic appears to be intramuscular, the needle is advanced or withdrawn carefully 1 to 2 mm and another bolus is administered. This gesture is repeated until the correct plane is achieved.


FIGURE 42.1-4. Transducer position in the transverse abdominal, at the anterior axillary line, between the costal margin and the iliac crest.



FIGURE 42.1-5. (A) Ultrasound anatomy of the abdominal wall layers. (B) Labeled ultrasound anatomy of the abdominal wall layers, EOM, external oblique muscle; IOM, internal oblique muscle; TAM, transverse abdominis muscle.


FIGURE 42.1-6. Simulated needle insertion (1) and distribution of LA (blue shaded area) to accomplish transversus abdominis plane (TAP) block. Shown are the external oblique muscle (EOM), internal oblique muscle (IOM), and the transverse abdominal muscle (TAM). Needle tip is positioned in the tissue sheath between IOM and TAM.


• An out-of-plane technique is more useful in obese patients. Because the needle tip may not be seen throughout the procedure, we recommend administering intermittent small boluses (0.5–1 mL) as the needle is advanced through the internal oblique muscle to confirm the position of the needle tip.

In an adult patient, 20 mL of local anesthetic per side is usually sufficient for successful blockade. We most commonly use ropivacaine 0.25%. In children, a volume of 0.4 mL/kg per side is adequate for effective analgesia when using ultrasound guidance.


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