In the emergency department, ultrasound-guided femoral nerve blocks can provide rapid and definitive control of acute pain resulting from traumatic injury to a lower extremity. Additional benefits include reduced risk of opioid-associated complications such as nausea, respiratory depression, and delirium.¹ Femoral nerve blocks are also an attractive alternative to procedural sedation, which requires additional monitoring and personnel (both of which may be limited in the ED setting). Peripheral nerve blockade in the acutely injured patient may blunt the systemic inflammatory stress response and reduce the associated risks of thromboembolism and immunosuppression. Additionally, there is increasing interest in the role of early peripheral nerve blocks in preventing the “wind-up” phenomenon of central sensitization that can lead to chronic pain syndromes and post-traumatic stress disorder.^2,3 Despite the strong evidence supporting their efficacy, safety, and ease of execution, femoral nerve blocks remain relatively underutilized in the ED.⁴

Clinical Indications

The ultrasound-guided femoral nerve block (USFNB) is typically used for fractures of the femur and injuries to the patella and its tendinous attachments. When combined with a lateral femoral cutaneous block, the femoral nerve block provides surgical anesthesia to the anterior and lateral thigh sufficient for deep wound exploration or incision and drainage of an abscess. In particular, several studies suggest that USFNB is superior to traditional intravenous or intramuscular analgesia. Advantages include more rapid onset of analgesia, reduced total pain scores, and reduced risk of oversedation and oxygen desaturation.^1,5,6 Recently, Beaudoin et al. demonstrated the ability of emergency physicians to learn and successfully use USFNB for pain control due to hip fractures in ED patients.⁷

Caution should be taken in patients clinically judged to be at high risk for compartment syndrome of the thigh, and a discussion with consulting orthopedic surgical and/or pain services should occur before performing the procedure. The compartments of the lower leg are innervated via the sciatic nerve, and blockade of the femoral nerve should not alter the clinical presentation of compartment syndrome of the lower leg.⁸

Anatomy

The femoral nerve is one of the three major branches of the lumbar plexus that arises from the first through fourth lumbar ventral rami (L1-L4). The lumbar plexus passes from the spinal roots through the psoas muscle, then descends in a groove between the iliacus and psoas muscles and branches into three major divisions before entering the thigh: the femoral, lateral femoral cutaneous, and obturator nerves. At the level of the inguinal ligament, the femoral nerve passes anterior to the psoas muscle and lateral to the femoral artery, then divides into its superficial and deep branches that ultimately supply sensation to the femur, hip joint, anteromedial thigh, knee, and the medial side of the leg from the knee to the foot.⁹

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

Choice of Local Anesthetic

We recommend bupivacaine 0.25%-0.5%, or lidocaine 1%-2% drawn into a 20- to 30-mL syringe. All providers using nerve blocks should be familiar with the standard recommended dosages and clinical signs of toxicity, and have established protocols for treatment of local anesthetic toxicity that incorporate use of lipid infusions. For novice users, we recommend lidocaine rather than bupivacaine because of its higher safety profile in cases of inadvertent vascular injection. Using epinephrine-containing anesthetics can prolong the duration of the block and also provides an additional safety benefit: Sudden tachycardia, hypertension, and characteristic electrocardiographic T-wave morphology changes indicate accidental intravascular injection, alerting providers before large volumes of local anesthetics have been injected. A midcaliber needle such as 21 or 22 gauge provides adequate needle visualization without significant tissue trauma. Standard cutting needles can be used, and the length should be based on the depth of the nerve. We recommend a standard 20- or 22-gauge 3.5-inch spinal needle for most cases.

Injection Technique

After ultrasound localization of the femoral nerve, raise a skin wheal with 3-5 mL of anesthetic at the anticipated point of needle entry. We suggest administering superficial anesthesia with a thin needle (25-30 gauge) after alcohol skin prep, prior to sterilizing the site and assembling the required material. This interval will allow for adequate local cutaneous anesthesia and reduce patient discomfort.

In the single-operator technique, the operator places the probe transversely across the femoral region of the upper thigh roughly parallel to the inguinal crease. The femoral vessels are then identified and centered on the screen. Gentle compression with the probe will collapse the femoral vein (medial) more easily than the artery (lateral). If desired, identification of the femoral vessels can be confirmed by using color Doppler imaging. With the femoral artery identified and centered on the screen, the operator should then follow the artery proximal to the inguinal ligament and distal to the takeoff of the profunda femoris artery. Proximal to this bifurcation, the femoral nerve will appear as a triangular or oval honeycomb structure 3-10 mm in diameter (image 3) covered anteriorly by the hyperechoic fascia iliaca.

We suggest a lateral to medial in-plane approach. After the femoral nerve and overlying fascia iliaca are identified, enter the skin with the needle bevel up about 1 cm lateral to the probe. The angle of entry will depend on the target depth of the fascia iliaca. More shallow angles of entry will improve needle visibility.

Advance the needle slowly, maintaining the shaft and tip in view at all times. Target the hyperechoic fascia iliaca overlying the iliopsoas muscle 1-3 cm lateral to the femoral nerve. Once beneath the fascia iliaca, aspirate to confirm the needle tip has not entered a vessel and then slowly inject 3-5 mL of local anesthetic. With the needle tip in view, the spread of hypoechoic injectate should be visualized in real time with superficial movement of the fascia iliaca toward the skin surface (image 4). After confirming optimal needle tip location, proceed to inject a total of 10-20 mL of local anesthetic in 3- to 5-mL aliquots. If at any point the spread of local anesthetic is not visualized, intravascular injection should be suspected and the procedure halted. After injection, examine the patient for any signs of anesthetic toxicity such as perioral numbness, dizziness, or convulsions.

Remaining at least 1 cm lateral to the femoral nerve and vessels reduces the risk of vascular puncture or intraneural injection. When larger volumes (20-40 mL) of local anesthetic are used, there is typically sufficient fascial spread of the local anesthetic to block the lateral femoral cutanteous nerve, resulting in a “fascia iliaca block.” Retrograde spread of local anesthetic sufficient to block the obturator nerve in a so-called “3-in-1 block” is highly unpredictable, and additional maneuvers such as distal compression and Trendelenberg positioning have questionable efficacy.

Evaluating Block Efficacy

Successful blocks are associated with direct visualization of hypoechoic local anesthetic displacing the femoral nerve anteriorly, and subsequent tracking of anesthetic distally resulting in a donut-like pattern circumferentially surrounding the nerve. Depending upon the local anesthetic used, block onset should occur within 15-25 minutes and last 3-8 hours.

Conclusion

Ultrasound-guided femoral nerve block is an ideal procedure for the emergency physician to incorporate into clinical practice. Controlling pain from hip, midshaft femur, and patella fractures is often difficult, and classically taught techniques (progressively increasing doses of intravenous opioids) can lead to respiratory depression and changes in sensorium. Ultrasound-guided femoral nerve blocks enable targeted deposition of anesthetic and are a crucial part of a multimodal approach to pain management for the acutely injured patient.

References

1. Mutty CE, Jensen EJ, Manka Jr MA, Anders MJ, Bone LB. Femoral nerve block for diaphyseal and distal femoral fractures in the emergency department. J. Bone Joint Surg. Am. 2007;89(12):2599.
2. Baker BC, Buckenmaier C, Narine N, et al. Battlefield anesthesia: Advances in patient care and pain management. Anesthesiol. Clinics 2007;25(1):131-45.
3. Malchow RJ, Black IH. The evolution of pain management in the critically ill trauma patient: Emerging concepts from the global war on terrorism. Crit. Care Med. 2008;36(7 Suppl):S346-57.
4. Grabinsky A, Sharar SR. Regional anesthesia for acute traumatic injuries in the emergency room. Expert Rev. Neurother. 2009;9(11):1677-90.
5. Fletcher AK, Rigby AS, Heyes FL. Three-in-one femoral nerve block as analgesia for fractured neck of femur in the emergency department: A randomized, controlled trial. Ann. Emerg. Med. 2003;41(2):227-33.
6. Foss NB, Kristensen BB, Bundgaard M, et al. Fascia iliaca compartment blockade for acute pain control in hip fracture patients: A randomized, placebo-controlled trial. Anesthesiology 2007;106(4):773.
7. Beaudoin FL, Nagdev A, Merchant RC, Becker BM. Ultrasound-guided femoral nerve blocks in elderly patients with hip fractures. Am. J. Emerg. Med. 2010;28(1):76-81.
8. Karagiannis G, Hardern R. Best evidence topic report: No evidence found that a femoral nerve block in cases of femoral shaft fractures can delay the diagnosis of compartment syndrome of the thigh. EMJ 2005;22(11):814.
9. Winnie AP, Ramamurthy S, Durrani Z. The inguinal paravascular technique of lumbar plexus anesthesia: The 3-in-1 block. Anesth. Analg. 1973;52(6):989-96.
10. Dolan J, Williams A, Murney E, Smith M, Kenny GN. Ultrasound guided fascia iliaca block: A comparison with the loss of resistance technique. Reg. Anesth. Pain Med. 2008;33(6):526.
11. Capdevila X. Ultrasound guidance for nerve blocks: Principles and practical implementation. Anesth. Analg. 2009;109(4):1355.