Often they are difficult to visualize because of

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Often they are difficult to visualize because of anisotropy. Anisotropy refers to the difference in echogenicity seen in tendons and nerves based on the angle of the ultrasound beam ( Figure 16.36 ). Thus, be sure to angle the probe back and forth if difficulty is encountered visualizing nerves in their expected locations. Technique Probe selection Peripheral nerve blocks are generally performed with a high-frequency (5–10 MHz) linear probe. Procedural ultrasound 327 Ultrasound for procedure guidance
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Special equipment A marking pen may be used to highlight surface anatomy. Often a stop- cock and flexible tubing is attached from the needle to the anesthetic-filled syringe. This allows a second provider to administer anesthetic while the Figure 16.35 Longitudinal appearance of the ulnar nerve (arrows). Note the dark, continuous fascicles and bright perineurium. (A) (B) Figure 16.36 ( A ) The radial nerve (arrow) appears dark when the ultrasound beam is held at a shallow angle. The radial artery (A) is visible, with veins on either side. ( B ) When the probe is held at 90 degrees to the nerve, it appears much more echogenic and is more easily visualized. 328 Procedural ultrasound Ultrasound for procedure guidance
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primary operator guides and stabilizes the needle in the optimal location. It is also less cumbersome when large volumes ( > 10 mL) of anesthetic are to be infiltrated. Blunt (non-cutting) needles are generally recommended for nerve blocks, to minimize tissue injury, and some authors advocate the use of ultrasound-optimized needles. These needles appear more echogenic (and often more so at the tip) and are easier to visualize, but are also more expensive. Setup Position the patient as usual for any nerve block procedure. Ensure that you can visualize the ultrasound screen easily while you are in your typical position. As with vascular access, you should be able to see the patient’s anatomy and the ultrasound image equally well and without putting yourself in an awkward position. Procedure Nerve blocks are performed using real-time dynamic guidance, typically in the long-axis (in-plane) view. Femoral nerve block The femoral nerve lies lateral to the femoral artery, superficial to the iliopsoas muscle. With the affected leg in neutral position, identify the inguinal crease. Scan the groin to reveal the femoral nerve, artery, and vein. The nerve is generally visible as an echogenic, roughly triangular structure in the short axis ( Figure 16.37 ). Occasionally the nerve itself is not well visualized, and the needle will instead be directed lateral to the artery toward the nerve’s Figure 16.37 Femoral vein (V), artery (A), and nerve (arrow) from medial to lateral in the left inguinal crease. Procedural ultrasound 329 Ultrasound for procedure guidance
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expected location. The appearance of local anesthetic infiltrated around the femoral nerve is demonstrated in Figure 16.38 .
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