51 Intercostal Nerve Block
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
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
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