Nerve Blocks

Indications

The femoral nerve provides innervation to the entire anterior thigh and most of the femur and knee joint, making it an ideal nerve block for hip fractures, proximal and midshaft femur fractures, large anterior thigh wound repairs, and patellar injuries. In patients with a hip fracture, a femoral nerve block gives moderate pain reduction but does not provide complete analgesia to the hip because the obturator nerve, one of the three nerves that innervate the hip, is not blocked by this approach. However, the femoral nerve block can significantly decrease the need for intravenous opioid medications, thereby reducing the risk of adverse effects, including respiratory depression, confusion, and hypotension, especially in the elderly. Although the risk of developing compartment syndrome of the thigh is very rare, we recommend the block be performed after achieving consensus with the consultative service in situations that require it.

Anatomy

The femoral nerve is one of the major branches of the lumbar plexus arising from the first through fourth lumbar ventral rami (L1–L4) before descending toward the lower extremity. The femoral nerve can be identified lateral to the femoral artery, beneath the fascia iliaca and superficial to the iliopsoas muscle at the level of the inguinal canal ( Fig. 31.1 ).

Technique

Place the patient in a supine position. Attempt to externally rotate and abduct the hip, realizing that it is often not possible in the acutely injured patient and may not be needed. Place the high-frequency linear transducer just below and parallel to the inguinal crease. Locate the femoral artery and vein in cross-section, and slowly slide the transducer laterally to identify the femoral nerve. The femoral nerve appears as a hyperechoic, triangular structure below the hyperechoic fascia iliaca and above the iliopsoas muscle (see Fig. 31.1 ). Slightly fanning the probe in a caudal direction, perpendicular to the trajectory of the nerve, will often allow for better visualization of the nerve. The operator should also locate the fascia iliaca—the fascial covering that covers the femoral nerve. Placing the anesthetic just below the hyperechoic fascia iliaca is key to obtaining an effective femoral nerve block.

Once the femoral nerve and surrounding anatomic structures are identified, a small skin wheal should be made approximately 0.5 to 1 cm lateral to the probe, which will represent the site of needle entry. Perform the preparatory steps discussed earlier. A 20-gauge, 3.5-inch (approximately 8.9 cm) block needle is inserted with direct ultrasound visualization. Advance the needle slowly at an angle parallel to the transducer using an in-plane approach, maintaining the needle shaft and tip in view at all times. The angle of entry will depend on the depth of the target depth of the fascia iliaca. More shallow angles of entry will improve needle visualization. Target the hyperechoic fascia iliaca overlying the iliopsoas muscle 1 to 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 to 5 mL of local anesthetic in a dose that is appropriate for the concentration of the anesthetic and the weight of the patient. With the needle tip in view, the spread of hypoechoic fluid should be visualized hydrodissecting the potential space below the fascia iliaca and around the nerve ( Fig. 31.2 ). After confirming optimal needle tip location, proceed to inject a total of 20 mL of local anesthetic in a dose that is appropriate for the concentration of the anesthetic and the weight of the patient in 3- to 5-mL aliquots. The most common pitfall is failure to get under the fascia iliaca; therefore anesthetic should be visualized under the fascial plane with the spread of anechoic fluid. The risk of intravascular and intraneural injection can be reduced by targeting the needle tip to be 1 cm lateral to the femoral nerve and vessels, yet deep to the fascia iliaca. If at any point the spread of local anesthetic is not visualized, intravascular injection should be suspected and the procedure halted.

Indications

The distal sciatic nerve innervates most of the lower extremity, making it an ideal block for fractures of the ankle, distal tibia, fibula, and injuries of the foot. A distal sciatic nerve block does not anesthetize the medial aspect of the lower leg, which is innervated by the saphenous nerve, a distal branch of the femoral nerve. Thus this block is not ideal for superficial injuries of the distal, medial leg. It should also be noted that, although this block is ideal for pain relief for tibial fractures, tibial injuries are typically high-energy and commonly associated with compartment syndrome. As the distal sciatic block may obscure clinical findings of compartment syndrome, it should be performed only after speaking in consensus with consulting services.

Anatomy

The lumbosacral plexus forms the sciatic nerve in the posterior pelvis. As the nerve tracks inferiorly through the posterior thigh, it enters the popliteal fossa bordered by the long head of the biceps femoris superolaterally and by the semimembranosus and semitendinosus tendons superomedially. At the most proximal point in the popliteal fossa, the large sciatic nerve bifurcates into a tibial nerve medially and a common peroneal nerve laterally. The sciatic nerve can be blocked at any location from the proximal, deep gluteal region to the distal, superficial popliteal fossa. The distal location is generally preferred for USGNB, given the ease with which adequate patient positioning can be obtained and anatomic landmarks can be sonographically identified ( Fig. 31.3 ).

Technique

The distal sciatic nerve block at the popliteal fossa is most easily accomplished with the patient in a prone position. This allows easy access to the popliteal fossa and facilitates needle movements in the anteroposterior plane that correlate with images on the ultrasound screen. In patients who are unable to lie prone (e.g., cervical spine immobilization), the affected extremity must be elevated and supported with mild flexion of the knee to fit the transducer between the popliteal fossa and the patient’s bed. Needle movements made under visualization of a transducer facing upward will appear reversed in the anteroposterior plane on the ultrasound screen.

When appropriate patient positioning has been achieved, use a high-frequency linear transducer to locate the popliteal artery and vein at the popliteal crease (this may be more easily done with slight flexion at the knee). The tibial nerve is commonly located superficial to the popliteal vein and appears as a hyperechoic “honeycomb.” Once the tibial nerve is optimally visualized, slowly slide the transducer proximally. At the cephalad border of the popliteal fossa, the common peroneal nerve should be visualized joining the tibial nerve, forming the distal sciatic nerve ( Fig. 31.4 ). The operator should mark this location and note the depth of the nerve.

In contrast to a femoral nerve block in which the needle is inserted adjacent to the transducer, for a distal sciatic nerve block, we recommend inserting the needle farther away from the transducer on the lateral aspect of the leg. The distal sciatic nerve is often 2 to 4 cm deep to the skin surface, and a steep needle angle from a point of entry just adjacent to the ultrasound probe would prevent clear needle tip visualization. To improve needle visualization, we recommend measuring the nerve depth and entering the lateral leg at a similar depth with a fairly flat angle (parallel to the surface of a linear probe).

After placing a skin wheal at the site of needle entry, advance a 20-gauge, 3.5-inch needle slowly at an angle parallel to the transducer using an in-plane approach. Maintain the needle shaft and tip in view at all times. Target the honeycomb appearance of the sciatic nerve without placing the needle directly into the nerve. Place the needle tip just superficial to the nerve (anywhere from 2–3 mm) to reduce the risk of nerve injury. Gently aspirate to confirm there has been no inadvertent vascular puncture and then slowly inject 3 to 5 mL of local anesthetic in a dose that is appropriate for the concentration of the anesthetic and the weight of the patient. With the needle tip in view, the spread of anechoic fluid should be visualized in real time ( Fig. 31.5 ). For novice sonographers, we do not recommend attempting to inject on the deep aspect of the nerve because of the close proximity to the popliteal vasculature and the increased possibility of inadvertent intravascular injection. If at any point the spread of local anesthetic is not visualized, intravascular injection should be suspected and the procedure halted.