Femoral nerve block

CHAPTER 26 Femoral nerve block








Sonoanatomy


The femoral nerve is found lateral to the femoral artery in the groin, lying outside the femoral sheath and beneath the fascia lata and iliaca. These fascial layers are seen as hyperechoic lines. Its appearance on ultrasound (Fig. 26.4) is similar to many peripheral nerves, and described as multiple round or oval hypoechoic (dark) areas encircled by hyperechoic (bright) rims. The hypoechoic structures are the nerve fascicles. The femoral nerve may be identified on ultrasound as an oval (95% oval beneath the inguinal ligament) or triangular-shaped structure measuring approximately 3 mm in anteroposterior diameter and 10 mm in mediolateral diameter. Correct transducer angulation is essential for adequate nerve visualization. Femoral nerve anisotropy occurs with transducer angulation of as little as 10° from the vertical. Anisotropy is a property of muscles, nerves, and tendons which relates to the change in ultrasound appearance of the target structure with the scanning angle used. Nerves are best visualized when the ultrasound beam hits them at 90°. In this case, the femoral nerve becomes isoechoic with surrounding structures, and disappears from view, when the transducer is angled 10–13° from the vertical.



To identify the femoral nerve, the transducer is placed over the femoral vessels on the anterior thigh at 90° to the expected orientation of the nerve and vessels (Fig. 26.5). The vessels may be identified as large round hypoechoic structures, and blood flow may be demonstrated using colorflow Doppler (Fig. 26.6). The femoral nerve is then visualized a variable distance lateral to the femoral artery.




Jul 28, 2016 | Posted by in ANESTHESIA | Comments Off on Femoral nerve block
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