Figure 35–1. Anatomic relationship in the femoral triangle.

Figure 35–2. Tissue sheaths and femoral nerve. Femoral artery and vein relationships.

Figure 35–3. Composition of the femoral nerve at the level of blockade.

Figure 35–4. Motor innervation of the femoral nerve.

Located under the fascia iliaca, the posterior branches innervate the quadriceps muscle and the knee joint and give off the saphenous nerve. The saphenous nerve supplies the skin of the medial aspect of the leg below the knee joint (Figure 35-5).

Clinical Pearls

It is useful to think of the mnemonic NAVEL (nerve, artery, vein) going from lateral to medial when recalling the relationship of the femoral nerve to the vessels in the inguinal crease.

Landmarks

The following landmarks are used to determine the site of needle insertion: inguinal ligament, inguinal crease, femoral artery (Figure 35-6).

Clinical Pearls

In obese patients, the identification of the inguinal crease can be facilitated by asking an assistant to retract the lower abdomen laterally (see Figure 36-7).

Equipment

A standard regional anesthesia tray is prepared with the following equipment:

• Sterile towers and gauze packs

• 20-mL Syringe with local anesthetic

• Sterile gloves, marking pen

• One 25-gauge, l½-in. needle for skin infiltration

• A 5-cm long, short-bevel, insulated stimulating needle

• A peripheral nerve stimulator and a surface electrode

BLOCK TECHNIQUE

Patient position. The patient lies in the supine position. The ipsilateral extremity is abducted 10–20 degrees.

Site of needle insertion. The site of needle insertion (see Figure 35-6) is located at the femoral crease but below the inguinal crease and immediately lateral ( 1 cm) to the pulse of the femoral artery.

Clinical Pearls

Note that the description of the femoral nerve block technique provided here varies from the common description of this block, where the needle is inserted at the level of the inguinal ligament.²⁹

In fact, the femoral nerve is approached at the femoral crease, well below the inguinal ligament. This more distal needle insertion site prevents the possibility of insertion of the needle into the pelvis and allows insertion of the needle more tangentially.³⁰

This approach also facilitates insertion of the catheter when a continuous technique is performed.

Single-Injection Technique

After skin disinfection, local anesthetic is infiltrated subcutaneously. In obese patients, the lower abdomen is retracted laterally to allow access to the inguinal area (Figure 35-7). The needle is connected to a nerve stimulator set at a current intensity of 1.0 mA (0.1 msec/2 Hz) and introduced at 45- degree angle to the skin in a cephalad direction (Figure 35-8).

Figure 35–5. Sensory innervation of the femoral nerve and distribution of anesthesia with femoral nerve block.

The needle is advanced through the fascia lata (a loss of resistance is often perceived, but not relied upon) until quadriceps muscle contractions (ie, patellar twitch) are obtained. The current output is then gradually decreased while the needle is advanced. The position of the needle is judged adequate when patellar twitches are elicited with current output between 0.2 and 0.5 mA. After a negative aspiration test for blood, 15-20 mL of local anesthetic is injected. Some common responses to nerve stimulation and appropriate action to troubleshoot are featured in Table 35-1.

Figure 35–6. Anatomic landmarks for femoral nerve block. IL, Inguinal ligament; IC, inguinal crease; FA, femoral arterial pulse. The needle insertion site (X) is located just below the inguinal crease, 1 -2 cm lateral to the pulse of the femoral artery.

Figure 35–7. Maneuver to facilitate exposure of the anatomy during femoral nerve blockade: The lower abdomen in obese patients may obstruct the access to the femora/inguinal region. This can be remedied by simple retraction of the lower abdomen laterally throughout the procedure.

In an attempt to fasten the onset time and increase the safety of such block, a multiple injection technique, that is, elicitation of a vastus medialis, intermedius, and medialis twitch and separate injection of local anesthetic on each nerve branch, has been recently suggested.^31,32 When compared with a single injection, the volume of local anesthetic required to block the nerve and the onset time of anesthesia were significantly reduced. However, 14% of patients reported paresthesia, and 28% reported discomfort during block performance.³³ Therefore, more data and better injection monitoring techniques are necessary before this approach to femoral block can be widely recommended.

Figure 35–8. Needle insertion in femoral nerve blockade. The needle is connected to a nerve stimulator set at 1.05 mA current intensity and introduced at a 30- to 45-degree angle to the skin in a cephalad direction.

Clinical Pearls

The sensation of “loss of resistance” may be better perceived when the bevel of the needle is oriented downward.

The needle tip should be positioned below fascia lata and iliaca to obtain a complete femoral nerve block.

Passage through the fascia iliaca may be difficult to perceive in some patients. In such circumstances, when patellar twitches are obtained, advance the needle deeper until it disappears. Increase the current output to 1 mA and withdraw the needle until the muscular twitches reappear. At this point, optimize the needle position.

A volume of local anesthetic larger than 20 mL is frequently suggested in various texts. However, a larger volume is not necessary because it does not lead to better success rate.³⁴

A small dose (eg, 0.1 mL per 20 mL) of epinephrine to the initial bolus dose of local anesthetic solution may be added to rule out intravascular injection.

Table 35–1.

Common Resp onses to Nerve Stimulation and Action to Obtain Femoral Nerve Twitch

Continuous Technique

The continuous technique is similar to the single-injection technique. After passage through the fascia lata, the needle is advanced to elicit a patellar twitch using a current output between 0.2 and 0.5 mA (0.1 msec) (Figure 35-9). The catheter is then inserted 5–10 cm beyond the tip of the needle or introducer. It is secured in place with a stitch tunnelling and/or a dressing. After a negative aspiration test for blood, a bolus dose of 20 mL of local anesthetic is injected and followed by a continuous infusion of dilute local anesthetic (Figure 35-10).

Figure 35–9. Continuous femoral nerve block: Needle insertion.

Figure 35–10. Testing the catheter for patency and accidental intravascular placement before initiation of continuous infusion of local anesthetic.

Clinical Pearls

Catheter insertion should be without resistance. When this is not the case, the needle should be withdrawn to the skin and reinserted.