The specific anatomy of the femoral nerve compartment at the inguinal region makes the femoral nerve block ideal for ultrasound guidance. Specifically, the femoral nerve is large, superficial, and easily located adjacent to the femoral vessels (whose sonographic appearance is well known to many emergency physicians for the placement of central venous catheters and the evaluation for deep vein thrombosis). Dolan et al. found that ultrasound guidance for femoral nerve blocks resulted in a significantly higher success rate versus a blind technique.¹⁰

Fundamentals

The appearance of nerves on ultrasound depends on the relative amount of connective tissue and fat. Nerve roots close to the spinal cord are primarily composed of axons and cerebrospinal fluid, which are both hypoechoic. As the nerves move peripherally, they divide into individual fascicles sheathed by hyperechoic perineurium, fat, and connective tissue, creating a characteristic honeycomb appearance on ultrasound. The reflections necessary to produce a clear sonographic image of peripheral nerves are sensitive to the angulation of the ultrasound probe such that tilting the probe 5-15 degrees can lead to total loss of the nerve image. This is a property known as “anisotropy,” and can be used to distinguish nerves from tendons: Tendons exhibit significantly more anisotropy and their appearance is dramatically altered with much smaller changes in transducer angle tilt.^10,11

For safe and effective performance of emergency nerve blocks, the needle tip must be clearly maintained in view before advancing the needle. The needle tip is best visualized when it is in-plane and nearly parallel to the transducer; as the angle of insertion becomes more steep, the needle tip becomes progressively more difficult to visualize. Any deviations away from the transducer’s imaging plane will result in loss of the needle image. Reducing gain, using small-volume test injections, and repetitive small needle movements all help improve the visibility of the needle tip.¹¹

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Procedure

Patient Positioning and Probe Selection

Patients should be in the supine position on a cardiac monitor with continuous pulse oximetry. As with all ED procedures, advanced cardiac life support equipment should be readily available. A linear high-frequency probe with a large footprint provides optimal