Ultrasound Techniques

Ultrasound-guided Femoral Nerve Block - Single Injection

Figure 1

The patient is supine position. The premedication of an adult, average size patient typically consists of 2-4 mg of midazolam; 250mg -500mg of alfentanyl is administered just before insertion of the needle.

TIP: The main landmark is the femoral artery pulse at the femoral crease (Figure 1).
Figure 2

Equipment:

Ultrasound machine with 8-12 MHz probe

Nerve block tray with following equipment items:

  1. Betadine soaked sponges
  2. 4”x4” gauze packs
  3. Sterile gel for the ultrasound probe
  4. Local anesthetic (30-40ml)
  5. Insulated nerve stimulator needle (50 mm)
  6. Injection pressure monitor (optional)
  7. Sterile gloves
  8. Surface electrode (optional)
  9. Nerve stimulator (optional)
  10. Ruler and skin marker
  11. Remote controller (foot pedal) for the nerve stimulator (optional)
Figure 3

Be certain to understand which part of the probe corresponds to medial, lateral, superior and inferior part of the screen. After applying a large, sterile “Tegaderm” and sterile gel to the probe, touch the end of the probe with the finger and observe the position of the shadow that occurs with the touch.

TIP: Make sure that there is no air trapping between the “Tegaderm” and the probe. Any trapped air will results in “shadowing” due to the poor acoustic impedance of the air.
Figure 4

The ultrasound probe (8-12MHz) is applied on the femoral crease (Figure 4) and angled to visualize femoral artery (Figure 5).
Figure 5

The femoral artery (FA) is easily identified as a hollow, hypoechoic, pulsating viscus. When in doubt, a color Doppler mode can be used to demonstrate the blood flow. Pulse-wave Doppler (PWD) mode can also be used to distinguish between the pulsatile (arterial flow) and the laminar (venous) flow. It is common to see several arteries in this area, the larges of which after femoral artery is profunda femoris. The profunda femoris is typically seen deeper to the femoral artery (not seen in this image). Femoral vein (FV) is seen slightly superficial and medial to the artery.

Femoral nerve (yellow arrow; yellow circle) is seen as a slightly hyperechoic oval structure at a depth of approximately 1.8 cm and underneath fascia lata (white arrow).
Figure 6

The needle is connected to the nerve stimulator (1.0 mA; optional) and inserted perpendicular to the long axis of the ultrasound probe   (Figure 6) and its path is visualized on the ultrasound image (Figure 7).

Figure 7

The path of the needle is seen as a shadow (blue arrows) extending from the upper left corner of the image towards the femoral nerve (yellow arrow). Quadriceps muscle twitch is elicited as the needle approaches the femoral nerve and serves as an additional confirmation of the needle placement.

TIP: As the needle enters the deeper compartments, fascia lata is often seen indented, followed by a distinct “pop” as the needle pierces the fascia lata and enters the appropriate tissue plane.
Figure 8

After the femoral nerve is identified and the needle placement is deemed as adequate, 15-20 ml of local anesthetic is injected.  Frequent aspirations are performed to rule out intravascular injection. Resistance to injection (>20 psi) may indicate intraneural needle placement and should be avoided.

TIP: Estimate the depth of the femoral nerve (typically 2-4 cm) before inserting the needle.
Figure 9

The local anesthetic is shown layering around the femoral artery. Additional injection can be made lateral or medial to the artery to complete the spread of the local anesthetic.After the injection of local anesthetic, nerve stimulation can not be relied upon for any consecutive injections; reliance on the resistance to injection is the only protection against injection into a potentially anesthetized nerve.

TIP: If the resistance to injection exceeds 20 psi, maintain the syringe pressure to 20 psi and slightly rotate the needle. This almost always leads to dislodgment of the needle bevel from the low-compliant space the allows the injection <20 psi.

 

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DISCLAIMER: The material presented on this Web page has not been peer-reviewed. The indications, techniques and dosages on this Web page have been recommended in the medical literature and/or conform to OUR clinical practice. The medications and equipment have not necessarily been approved by the Food and Drug Administration (FDA) for use in the techniques and dosages for which they are recommended. The package insert for each drug and/or equipment should be consulted for use and dosage as recommended by the FDA. Because standards, practices and recommendations change, it is advisable to keep abreast of revised recommendations, particularly those concerning new drugs and techniques. While the techniques and dosages described are successfully used in our practice, they should be followed with a discretion since their complications may be dependent on the operator, patient and/or other accompanying clinical circumstances. The development and maintenance of this web page has not been supported by any pharmaceutical or medical manufacturing industry. The medications and/or equipment discussed in the web page is shown solely for teaching purposes. Similar equipment or medications from other manufacturers may produce similar clinical results to ours.