Ultrasound Guided Nerve Blocks

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Leonard V. Bunting, M.D., Emilie J.B. Calvello, M.D., M.P.H.

Femoral Nerve Block, 3-in-1 Block Variation

The femoral nerve block provides anesthesia for the entire anterior thigh, the knee, and the femur. In addition, skin anesthesia is conferred over the medial aspect of the distal lower extremity via the terminal branches of the femoral nerve. The relative ease of identifying the femoral nerve and familiarity of the emergency medicine physician with the femoral triangle makes this block an excellent option for orthopedic injuries to the lower extremity. The femoral nerve block is generally well tolerated with infrequent
complications. [1-4]

A variation of the femoral nerve block, termed the 3 in 1 block, is described below. It provides greater anesthesia to the thigh and can be helpful for more extensive proximal injuries.

Illustration 1: Overview of femoral nerve sensory innervation.
(Green - femoral , Blue - lateral cutaneous femoral nerve)

II. Anatomy

The lumbar plexus gives rise to the femoral nerve (L2 – L4).  The femoral nerve enters the thigh passing under the inguinal ligament, located lateral to the femoral artery surrounded by the fascia iliaca which separates it from the femoral artery and vein.  Distal to the inguinal ligament, the femoral nerve splits into the anterior branch, providing skin sensation, and the posterior branches, supplying the quadriceps muscle, medial knee and skin sensation to the medial calf. The femoral nerve block must be performed just distal to the inguinal ligament so as to not miss anesthesia to one of the branches.  Medial and lateral skin sensation is supplied by the obturator and lateral cutaneous nerve respectively.

III. Scanning Technique

A high frequency (6 – 18 MHz) linear array probe is preferred and used with depth initially set between 4 and 6 cm and adjusted as needed. A curvilinear probe can be used if more depth is warranted. The probe is placed in the inguinal crease, parallel to the inguinal ligament and transverse to femoral vein and artery with the indicator towards the patient’s right. The leg is slightly externally rotated if possible. The probe is slid medial to lateral until the femoral vessels are seen. The nerve lies about 1-2 cm lateral to the artery, positioned below fascia iliaca and lata and above the ilieopsoas muscle and contained within a triangular-shaped sheath of fascia by the ligamentum ileopectineus. The nerve itself can have a triangular or oval shape and is often not clearly visualized. Because of this, the triangle created by the femoral artery medially, fascial planes anteriorly and the iliopsoas muscle posterioly is used as the target for the block. The nerve becomes visualized after injection.

Figure 1: Probe position and location of femoral nerve (yellow),
femoral artery (red) and femoral vein (blue).

Figure 2: Ultrasound of the femoral vein, artery and nerve.

IV. Nerve Block

The needle is preferably inserted in-plane. However, an out of plane approach is possible, and can aid in separating the nerve from the artery.  For the in-plane technique, the needle is inserted at the lateral border of the probe and advanced to the deep border of the femoral nerve. If the nerve is not well visualized, the deep border of the triangle described above is targeted. Advance the needle until the fascia lata and iliaca are punctured. Resistance and a ‘pop’ may be felt when passing through these planes if a blunt or short bevel needle is used. A small bolus of local anesthetic is injected, approximately 1cc. The bolus should surround the nerve and the nerve should appear more defined with a properly placed bolus. If this occurs, the injection is continued. If the bolus is not seen, or if the nerve becomes less defined, injection is stopped and the needle repositioned.  Reposition of the needle may also be necessary if the bolus fails to surround the anterior or medial aspect of the nerve. About 15 - 25 cc of local anesthetic is injected and paresthesia should occur within 15 – 30 minutes (Video 1 and 2).

Video 1:
Femoral Nerve Block

Video 2:
Femoral nerve with guidance

3 in 1 Block Variation

If more extensive anesthesia is required for the entire anterior thigh, the 3 in 1 block should be used.  The 3 in 1 block refers to blocking three nerves with one injection: the lateral cutaneous nerve of the thigh, the femoral nerve and the obturator nerve.  All three nerves are derived from the lumbar plexus and are covered by the same fascia sheath that extends to cover the femoral nerve.  If local anesthetic injected at the femoral nerve can be made to spread proximally, the other two nerves may also be blocked.

The technique for the 3 in 1 block is similar to that described above for the femoral nerve block with two differences. The first is that a larger volume of local anesthetic is used to aid spread. The minimum volume of 20 mL is recommended but up to 30 mL may be necessary.[5] Care should be taken with larger volumes of anesthetic to limit the total dose by dilution if necessary. The second difference is that during injection of the local anesthetic, pressure is applied 2 – 4 cm distal to the needle site and held for 30 seconds by an assistant. This encourages the necessary spread of anesthetic agent.

V. Pearls and Pitfalls

The needle angle should be as shallow as possible, as it greatly improves needle visualization.

Given the proximity to the femoral vessels, intravascular injection of LA is a risk with this block. The needle tip should be seen throughout the procedure to ensure its location lateral to the vessels. Also, any injection of LA should result in an anechoic bolus seen on ultrasound. If no bolus is seen, the needle should be repositioned.

If the nerve fails to dissect away from the femoral artery, consider an out of plane approach to position the needle and a bolus directly between the two.

VI. References

1. Williams R, Saha B.
Best evidence topic report. Ultrasound placement of needle in three-in-one nerve block.  Emerg Med J. 2006;23:401-03.

2. Casati A, Baciarello M, Di Cianni S, Danelli G, De Marco G, Leone S, Rossi M, Fanelli G.
Effects of ultrasound guidance on the minimal effective anaesthetic volume required to block the femoral nerve. Br J Anaesth. 2007;98:823-827.

3. Marhofer P, Schrögendorfer K, Koinig H, Kapral S, Weinstabl C, Mayer N. 
Ultrasonographic guidance improves sensory block and onset time of three-in-one blocks. Anesth Analg 1997/ 85: 854 – 857.

4. Oberndorfer U, Marhofer P, Bösenberg A, Willschke H, Felfernig M, Weintraud M, Kapral S, Kettner SC.
Ultrasonographic guidance for sciatic and femoral nerve blocks in children.  Br J Anaesth 2007; 98: 797 – 801.

5. Marhofer P, Kapral S, Sala-Blanch X.
Three-in-One Block. In Hadzic A (ed): Textbook of Regional Anesthesia. McGraw-Hill, 2007, pp 513.



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