Truncal Blocks

Ki J. Chin

Monica Liu

KEY POINTS

1. When used within the framework of a multimodal regimen, truncal blocks provide effective analgesia for a wide variety of surgical procedures involving the chest and abdomen.

2. Truncal blocks are generally safe to perform and result in fewer side effects than neuraxial blocks.

3. Except for thoracic paravertebral blocks (TPVB), truncal blocks provide somatic but not visceral analgesia.

4. Each specific truncal block anesthetizes only part of the chest or abdomen. Thus, the choice of technique must be carefully tailored to the surgical procedure.

5. Bilateral truncal blocks are required in surgical procedures involving the midline.

6. Most truncal blocks target fascial planes (where nerves travel) instead of the actual nerves. This confers safety and simplicity at the expense of some interindividual variability in intensity and extent of sensory blockade.

7. Transversus abdominis plane (TAP), rectus sheath, quadratus lumborum (QL), and ilioinguinal/iliohypogastric nerve blocks provide analgesia for the anterolateral abdominal wall.

8. Thoracic paravertebral, pectoral (PECS 1 and 2), and serratus plane blocks provide analgesia for the anterolateral chest wall and thoracic cage.

I. Anatomy. Detailed knowledge of the muscular and fascial layers of the chest and abdominal wall, and the path taken by nerves within these layers, is essential to perform ultrasound-guided truncal blocks successfully.

II. Anatomy of the anterolateral abdominal wall (Fig. 12.1)

A. The layers of the abdominal wall consist of skin, subcutaneous tissue, muscles and their associated fascia, and parietal peritoneum.

FIGURE 12.1 Surface anatomy, muscular layers, and nerves of the anterolateral abdominal wall. The external and internal oblique muscles and aponeuroses have been cut away on the right to show the transversus abdominis plane (TAP). The lateral cutaneous branches arise from their respective spinal nerves at or posterior to the mid-axillary line and supply the skin of the lateral abdominal wall up to the midclavicular line. The T7-T9 nerves enter the TAP at or medial to the midclavicular line. Communicating branches between the spinal nerves give rise to neural plexuses within the TAP and the rectus sheath. The rectus sheath is absent midway between the umbilicus and pubis. (Image Copyright 2017 American Society of Regional Anesthesia and Pain Medicine. Used with permission. All rights reserved.)

B. The three flat muscles have their origins on the ribs and on the dense thoracolumbar fascia of the back. They wrap around laterally to encase the abdominal contents (Fig. 12.1).

1. External oblique. The largest and most superficial flat muscle with fibers that run inferomedially.

2. Internal oblique. Runs deep to the external oblique with fibers that travel superomedially.

3. Transversus abdominis. The deepest of the three flat muscles with fibers that run transversely.

C. In the anterior part of the abdominal wall, these three muscles taper off into aponeuroses that blend together to form the rectus sheath, which encases the vertically oriented rectus abdominis muscle. The rectus abdominis is a paired muscle separated by the linea alba along the midline of the abdomen. The linea alba results from the aponeurotic fusion of all three flat muscles.

D. The anterior layer of the rectus sheath is formed from the aponeuroses of the external oblique and the internal oblique muscles. The posterior layer is formed from the aponeuroses of the internal oblique and transversus abdominis muscles. The posterior rectus sheath ends midway
between the umbilicus and pubic symphysis, at the arcuate line, where only the transversalis fascia separates the rectus abdominis muscle from the peritoneal cavity (Fig. 12.1).

E. The rectus abdominis has tendinous attachments to the anterior rectus sheath that gives rise to the appearance of a “six pack”; the posterior rectus sheath compartment is not segmented.

F. The transversalis fascia runs deep to the transversus abdominis muscle, separating it from the parietal peritoneum, and is continuous with the fascia iliaca inferiorly, the endothoracic fascia superiorly, and the anterior layer of the thoracolumbar fascia posteriorly.

G. The anterior abdominal wall is innervated by the T6-T12 intercostal nerves and the L1 nerve (Fig. 12.1). At the angle of each rib, the intercostal nerves give off lateral cutaneous branches, which subsequently emerge in the mid-axillary line to supply the lateral chest and abdominal wall.

H. The main nerve trunks continue anteriorly and run in the transversus abdominis plane (TAP), a neurovascular fascial plane between the internal oblique and transversus abdominis muscles. Within the TAP, the nerves form a plexus (1); this constitutes the target point for TAP blocks.

I. Upon reaching the edge of the rectus sheath (the linea semilunaris), the nerves pierce the latter and lie within the posterior rectus sheath where further branching and communication occur; this constitutes the target point for rectus sheath blocks. The nerves terminate in anterior cutaneous branches that ascend through the medial half of the rectus abdominis muscle to innervate the skin and subcutaneous tissue. One should note that the area adjacent to the abdominal midline is supplied by overlapping innervation from both sides.

J. It is also important to remember that the T6-T9 nerves supplying the upper abdomen only emerge from the costal margin and enter the TAP medial to the anterior axillary line. The T6 nerve emerges just lateral to the linea alba, whereas the T7-T9 nerves emerge at increasingly lateral locations (Fig. 12.1). The subcostal TAP block targets the T6-T9 nerves by injecting local anesthetic within the TAP medial to the anterior axillary line.

K. In contrast, the T10-T12 nerves enter the TAP more laterally, between the mid-axillary and anterior axillary lines. The lateral TAP block primarily targets these nerves by injecting local anesthetic in a more lateral location within the TAP at the level of the mid-axillary line.

L. The ilioinguinal and iliohypogastric nerves, which originate from the lumbar plexus, enter the TAP in the region of the anterior third of the iliac crest and exit the TAP through the internal oblique muscle at a variable location medial to the anterior superior iliac spine (ASIS) (Fig. 12.2).

M. Both the TAP and rectus sheath are highly vascularized. The main arteries within the TAP are descending branches of the lower thoracic intercostal arteries and ascending branches of the deep circumflex arteries. The rectus sheath contains the superior and deep epigastric arteries.

FIGURE 12.2 The anatomy of the posterolateral abdominal wall. The external oblique muscle tapers off into a free edge that abuts the latissimus dorsi muscle. The internal oblique and transversus abdominis muscles taper off into aponeuroses that blend into the thoracolumbar fascia. The thoracolumbar fascia divides into three layers (anterior, middle, and posterior) to encase the quadratus lumborum (QL) and erector spinae muscles. The ilioinguinal and iliohypogastric nerves are L1 branches of the lumbar plexus that emerge from the lateral border of psoas major muscle to run over the anterior surface of QL and transversus abdominis muscles. They ascend into the transversus abdominis plane along the anterior half of the iliac crest. Medial to the anterior superior iliac spine, the nerves continue to ascend and pierce the internal oblique muscle and external oblique aponeurosis. (Image Copyright 2017 American Society of Regional Anesthesia and Pain Medicine. Used with permission. All rights reserved.)

III. Anatomy of the posterior abdominal wall

A. The posterolateral abdominal wall is formed by the external oblique, internal oblique, and transversus abdominis muscles; their aponeuroses; as well as the psoas major and quadratus lumborum (QL) muscles (Fig. 12.2).

B. The QL muscle attaches inferiorly to the iliac crest and runs posterior to the lateral arcuate ligament of the diaphragm to attach to the 12th rib. It is covered on its ventral surface by the anterior layer of the thoracolumbar fascia and transversalis fascia and on its dorsal surface by the middle layer of the thoracolumbar fascia.

C. The thoracolumbar fascia is a tough membranous sheet that envelopes these muscles and consists of anterior, middle, and posterior layers (Fig. 12.2). The anterior layer blends with the transversalis fascia and separates the QL and psoas major muscles. The middle layer blends laterally with the aponeuroses of transversus abdominis and internal oblique muscles and separates the QL and erector spinae muscles. The posterior layer is formed from the aponeuroses of the latissimus dorsi and serratus posterior muscles. These layers and the intermuscular planes
provide a potential path for local anesthetic (injected around the QL muscle) to spread superiorly toward the thoracic paravertebral space and the spinal nerves.

IV. Anatomy of the paravertebral space

A. Spinal nerves exit the spinal canal through the intervertebral foramina, which lie approximately midway between the transverse processes of adjacent vertebrae.

B. The transverse processes constitute the critical bony landmarks for paravertebral blocks. They cannot be palpated but must be located in relation to the spinous processes. Their relationship to the latter varies along the length of the spine (Fig. 12.3).

1. For thoracic vertebrae, the steep angle and bulbous tip of the spinous process mean that its cephalad edge lies at the level of the transverse process of the inferior vertebra.

2. For lumbar vertebrae, the cephalad edge of the spinous process is at the level of the transverse process of the same vertebra.

3. The 11th and 12th thoracic vertebrae represent a transition point between thoracic and lumbar vertebrae. The spinous processes of the T11-T12 vertebrae are elongated like those belonging to lumbar vertebrae, but their cephalad edge does not quite extend to the lower edge of their own transverse processes.

C. The thoracic paravertebral space is a wedge-shaped anatomical compartment bound medially by the vertebral bodies, intervertebral discs and foramen; anterolaterally by the endothoracic fascia, parietal pleura (T2-T10/11), and diaphragm (T10/11-T12); and posteriorly by the transverse processes and superior costotransverse ligaments.

1. The paravertebral space contains fat, extrapleural fascia, segmental nerve roots dividing into ventral and dorsal rami, the sympathetic chain, rami communicantes, and radicular vessels (Fig. 12.4).

2. It communicates with the contiguous paravertebral spaces (superiorly and inferiorly), the epidural space (medially), and the intercostal space (laterally).

3. The superior costotransverse ligament joins the inferior aspect of the transverse process (above) with the superior aspect of the neck of the rib (below).

4. The endothoracic fascia (deep fascia of the thorax) divides the paravertebral space into anterior and posterior compartments. The sympathetic chain can be found in the anterior compartment (i.e., the “extrapleural” paravertebral space), whereas the intercostal nerves and vessels lie in the posterior compartment (i.e., the “subendothoracic” paravertebral space).

V. Anatomy of the anterolateral thoracic wall

A. The major muscles of the anterolateral chest wall are the pectoralis major, pectoralis minor, and serratus anterior muscles (Fig. 12.5).

FIGURE 12.3 Bony anatomy relevant to the paravertebral block. The steep thoracic spinous processes mean that a transverse line drawn from the cephalad edge of the spinous process of one vertebra (e.g., T8) will intersect the transverse process of the vertebra below (T9). The spinal nerve roots emerge inferior to the transverse processes of the vertebrae for which they are named.

1. The pectoralis major is a thick triangular-shaped muscle that originates from the medial half of the clavicle and the lateral sternum and inserts into the lateral lip of the bicipital groove of the humerus. It is innervated by the medial and lateral pectoral nerves.

2. The pectoralis minor is a smaller triangular-shaped muscle that lies deep to the pectoralis major. It originates from the 3rd to 5th ribs near the costal cartilages and inserts on the medial border and superior surface of the coracoid process of the scapula. It is innervated by the medial pectoral nerve.

3. The serratus anterior is a large digitated muscle covering the anterolateral wall of the thorax. It lies superficial to the ribs and intercostal muscles. It originates from the upper borders of the first eight ribs laterally and inserts on the ventral surface of the medial border of the scapula. Its motor innervation is provided by the long thoracic nerve, which arises from C5-C7 roots of the brachial plexus.

B. The lateral pectoral nerve (C5-C7) and medial pectoral nerve (C8-T1) arise from the lateral and medial cord of the brachial plexus, respectively. The lateral pectoral nerve enters the intermuscular plane between the pectoral muscles in close proximity to the pectoral branch of the thoracoacromial artery. The medial pectoral nerve lies deep to the pectoralis minor muscle and pierces the latter to innervate the inferior aspect of the pectoralis major.

C. The chest wall is also innervated by the lateral and anterior branches of the upper thoracic intercostal nerves (Fig. 12.4). The thoracic intercostal nerves arise from the ventral rami of spinal nerves and travel anteriorly between the intercostal muscles. They give off a lateral cutaneous branch at the angle of the rib, which emerges over the serratus anterior muscle to supply the lateral aspect of the chest wall and the axilla (Fig. 12.5). The main nerve trunks continue anteriorly and pierce the internal intercostal and pectoralis major muscles in the parasternal area before terminating in anterior cutaneous branches.

FIGURE 12.4 Cross-section of thorax and anatomy of a typical thoracic spinal nerve. As the spinal nerve emerges from the intervertebral foramen, it divides into a ventral and dorsal ramus. The branches of the dorsal ramus supply the muscles and skin of the back. The ventral ramus continues anteriorly as the intercostal nerve, giving off a lateral cutaneous branch (close to the angle of the rib), which emerges in the mid-axillary line to innervate the lateral thoracoabdominal wall. The terminal anterior cutaneous branch emerges close to the midline to innervate the anterior thoracoabdominal wall. (Image adapted and used with permission from Maria Fernanda Rojas Gomez.)

D. The skin overlying the clavicle and the periclavicular chest wall is innervated by the supraclavicular nerves, which originate from the superficial cervical plexus.

VI. Drugs. Local anesthetic volume constitutes the primary consideration in truncal blocks because analgesic efficacy inherently depends on local anesthetic spread within the fascial plane. Typical injectates range between 15 and 30 mL in adults. The choice of local anesthetic solution and concentration should respect maximum recommended doses, with calculations based on lean rather than actual bodyweight. Long-acting local anesthetics, such as ropivacaine and bupivacaine, are most commonly used for truncal blocks.

VII. Techniques

A. Ultrasound-guided TAP block

1. Indications

a. Subcostal TAP block. Analgesia for upper abdominal (T6-T9) wall surgery.

b. Lateral TAP block. Analgesia for lower abdominal (T10-T12) wall surgery.

2. Contraindications

a. Usual contraindications to peripheral nerve blocks (i.e., lack of consent, local infection at the injection site, and allergy to local anesthetic agent)

3. Single-injection technique

a. Subcostal TAP block. The patient is positioned supine. Place a linear ultrasound transducer parallel to the costal margin lateral to the rectus sheath (Fig. 12.6). Identify the transversus abdominis muscle, which lies deep to rectus abdominis muscle (medially) and deep to internal oblique muscle (laterally). Infiltrate the subcutaneous tissues with local anesthetic. Using an in-plane technique, direct an 80- or 100-mm short-beveled needle in a posterolateral direction away from the midline, injecting local anesthetic in the fascial plane between the rectus abdominis/internal oblique and transversus abdominis muscles. A local anesthetic volume of 15 to 20 mL is typically used.

b. Lateral TAP block. The patient is positioned supine or with a slight lateral tilt for a more posterior approach. Place a linear ultrasound transducer in a transverse orientation in the mid-axillary line between the iliac crest and costal margin to identify the external oblique, internal oblique, and transversus abdominis muscles (Fig. 12.6). Infiltrate the subcutaneous tissues with local anesthetic. Using an in-plane technique, direct an 80- or 100-mm short-beveled needle in an anterior-to-posterior direction through the external oblique and internal oblique muscles to reach the TAP. Inject local anesthetic to hydrodissect and expand the fascial plane between the internal oblique and transversus abdominis muscles. A local anesthetic volume of 15 to 20 mL is typically used.

CLINICAL PEARL

The location at which the T6-T9 nerves emerge into the TAP signifies that a lateral TAP block will not anesthetize the upper abdomen. In fact, the area supplied by the T6 and T7 nerves is probably best anesthetized using a rectus sheath block (because the T6-T7 nerves emerge very close to the midline).

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