Twelve pairs of intercostal nerves lie within or near the inferior groove of each corresponding rib. These nerves supply the skin and chest wall skeletal muscles. An intercostal artery and vein accompany each nerve and lie superior to it. Intercostal nerves are difficult to image with ultrasound because they are small and often covered by the caudal edge of the corresponding rib.¹ Proximal intercostal nerves are found in the classic subcostal position in 17%, in the midzone in 73%, and in the inferior supracostal position in 10% of anatomic specimens. The intercostal nerves migrate away from the ribs near the midaxillary line. Doppler ultrasound has been used to locate intercostal arteries for intercostal block.² Intercostal arteries are 3 to 4 mm in diameter and can be detected in an acoustic window 4 cm from the midline.³ Doppler measurements of the intercostal arteries are possible from T4 and lower. Ultrasound-guided intercostal nerve block has been used for acute and chronic pain management.⁴ Intercostal nerve blocks can be used for breast surgery and are best placed at T3, T4, and T5 for this procedure. Another common application is for thoracic trauma and chest tube placement.⁵

The subcostal nerve is the anterior ramus of the spinal nerve T12. This nerve supplies the lower abdominal wall and is not closely associated with the 12th rib. The subcostal nerve is about 3 mm in diameter and passes over the iliac crest.

Suggested Technique

Intercostal nerve imaging can be performed in the sitting, lateral, or prone position. The arms are forward to retract the scapulae laterally. This is particularly important when imaging the intercostal nerves above the fifth rib because of the overlying scapula and paraspinous muscles. When intercostal blocks are performed in sitting position, the right-handed operator stands and turns to the patient’s right to view the imaging display regardless of the side of the block.

The transducer is placed slightly medial to the posterior angulation of the ribs. In this location the nerves are shallow and relatively centrally located before branching. This also gives the block needle room to clear the inferior rib for in-plane approach. Because of the caudal angulation of the ribs, the transducer has a slight oblique orientation, with the transducer and block needle directed slightly away from the midline. Hand-on-needle hub approach provides optimal needle control for intercostal blocks.

Sonograms can sometimes demonstrate three layers of the intercostal muscles (external, internal, and innermost) covering the pleural line.⁶ The neurovascular bundle lies between the internal and innermost intercostal muscles. Intercostal interspaces have a flying-bat appearance on sagittal ultrasound scans because of acoustic shadowing of the ribs.⁷

For intercostal block the needle tip is placed near the caudal edge of the rib so that the needle tip can be identified between the acoustic shadows from the bone. If the needle tip is placed in the correct layer, the local anesthetic will distribute under the rib. To do this, advance the needle tip near the edge of the muscle and inject as the needle is withdrawn (similar to rectus sheath blocks). Needle tip visibility is essential for this procedure.

With intercostal nerve blocks, rapid and high peak plasma levels of local anesthetic are expected. Therefore, careful attention to drug dosing is essential. One of the potential benefits of ultrasound guidance is reduction of the risk for pneumothorax. The chance of developing a pneumothorax depends on the amount of aerated lung tissue traversed by the needle. The lung is particularly fragile in patients with chronic obstructive lung disease and emphysema. Demonstration of postinterventional lung sliding and comet-tail artifact from the pleura rule out pneumothorax. This examination is best performed over the nondependent portion of the lung (the anterior chest in supine position).

Another potential benefit of ultrasound guidance for intercostal block is the avoidance of arterial puncture. This complication can result in hemothorax. This is particularly noteworthy because the tracking between the lower border of the ribs and the neurovascular bundle is not always precise. There is variability of the relationship between the caudal edge of the ribs and the neurovascular bundle, especially at the lower rib levels and farther from the paravertebral region.

Clinical Pearls

• The easiest patient position to perform the block is prone, but lateral or sitting is also possible.

• Intercostal blocks are best performed near the posterior angulation of the ribs. In this location the nerves are shallow and relatively centrally located before branching.

• Both transverse and longitudinal in-plane approaches to intercostal injection result in similar distributions toward the paravertebral space.⁸

• With intercostal nerve blocks, rapid and high peak plasma levels of local anesthetic are expected. Therefore, careful attention to drug dosing is essential.

• Even with needle puncture of the pleura, the chance of pneumothorax is about 50%. This chance depends on the amount of aerated lung tissue traversed by the needle. The lung is particularly fragile in patients with chronic obstructive lung disease and emphysema.

• The tracking between the lower border of the ribs and the neurovascular bundle is not always perfect. Discrepancies are sometimes seen along the course of the rib.

• One of the benefits of ultrasound guidance for intercostal block is the avoidance of arterial puncture. This complication can result in hemothorax.

FIGURE 51-1 External photograph showing the approach to intercostal nerve block in sitting position (A). The corresponding sonogram of the intercostal interspace before needle placement is shown (B).

FIGURE 51-2 In-plane approach to intercostal nerve block. The needle tip advances between the ribs to place local anesthetic underneath the caudal edge of the superior rib.