Single-injection ultrasound-guided nerve blocks should be performed only in patients who are awake, alert, and able to cooperate with a neurologic exam. Patients with preexisting neurologic deficits in the distribution of the nerve being blocked are not candidates because these deficits obscure the ability to screen for postprocedural peripheral nerve injury (PNI). Caution is recommended in injuries that are at high risk for compartment syndrome (e.g., distal tibia fractures, crush injuries). Since ultrasound-guided nerve blocks may mask an evolving compartment syndrome, we recommend consulting the primary surgical service (e.g., trauma surgery, orthopedics) to assess the potential risks and benefits before performing a nerve block in a patient at risk for compartment syndrome.

PNI is an uncommon event defined as a persistent motor or sensory deficit or pain after a nerve block. Its incidence ranges from 0% to 2.2%. The mechanism of injury is not well understood; direct needle trauma from the needle, increased intrafascicular pressures from the anesthetic, direct cytotoxic effects of the anesthetic, and nerve ischemia secondary to the metabolic stress of the anesthetic have been proposed as possible mechanisms. For these reasons, we recommend three intuitive steps to reduce the risk of PNI. First, place the needle tip close to, but not in, the nerve fascicle. Second, slow low-pressure injections should be performed, and the procedure should be stopped if the patient experiences any new pain or paresthesias. Finally, we recommend not performing an ultrasound-guided nerve block in patients with underlying peripheral neuropathy.

The ultrasound system is generally positioned in front of the provider, on the opposite side of the body ( Fig. 42.1 ). To improve ergonomics, the ultrasound screen should be positioned in the provider’s direct line of sight, allowing visualization of both the needle and ultrasound screen without significant head turning. All nerve blocks should be performed with the patient on a cardiac monitor with continuous pulse oximetry.

Positioning of the Patient, Operator, and Ultrasound Machine.

The procedure site is between the provider and ultrasound machine with the screen in the operator’s direct line of sight.

The skin overlying the injection site should be cleaned with chlorhexidine or an equivalent solution. A small-gauge needle (25–30G) is used to anesthetize the skin at the site of injection. The ultrasound transducer should be covered with a transparent adhesive dressing or a sterile transducer sheath. Selection of a needle gauge and length depends on the specific block. A control syringe is preferred ( Fig. 42.2 ). Block-specific needles with blunt tips can be used but are not necessary to perform peripheral nerve blocks.

Control Syringe.

A control syringe is filled with anesthetic and preferably attached to a 20G, 3.5-inch spinal needle.

Consider both the duration and safety profile when choosing an anesthetic agent. In general, 2-chloroprocaine and lidocaine are preferred for novice providers over the longer acting agents due to their favorable safety profile. These agents are ideal for short procedures such as joint reductions, simple incision and drainage, and laceration repairs. The concentration of the anesthetic agent determines the rate of diffusion to the nerves and the onset of action; therefore, 3% 2-chloroprocaine and 2% lidocaine would have a more rapid onset when needed.

When a longer duration of anesthesia is required for pain control, such as with femoral or humeral fractures, bupivacaine and ropivacaine are preferred. These agents can be toxic to the heart and central nervous system, and novice providers who are less facile with real-time needle tip visualization should only use the shorter acting agents. Local anesthetic systemic toxicity (LAST) can occur with administration of an excessive dose, rapid absorption, or accidental intravenous injection, even when meticulous injection techniques are used. The addition of a vasoconstrictor, most commonly epinephrine, to shorter acting anesthetic agents can delay vascular absorption and increase the duration of action, which is a reasonable alternative to using a longer acting anesthetic.

Regardless of which agent is used, providers should be familiar with the signs and symptoms of LAST. Classically, the patient complains of tongue numbness and lightheadedness, which then progresses to muscle twitching, unconsciousness, seizures, and cardiovascular depression. Signs and symptoms can occur in any order. In cases where bupivacaine has been inadvertently injected into the vascular system, the use of a hyperlipophilic solution (20% Intralipid; 1.5 mL/kg bolus with a continued infusion of 0.25 mL/kg/min) should be infused. Providers who perform ultrasound-guided nerve blocks should have ready access to 20% Intralipid (Baxter, Deerfield, IL). Standard safety precautions mandate that providers never inject without visualizing the needle tip and always aspirate to confirm lack of vascular puncture before injecting anesthetic.

Locating peripheral nerves with ultrasound requires knowledge of adjacent anatomy. Nerves are best visualized and targeted for blocks when oriented in cross-section. Distal peripheral nerves appear as bundles of hyperechoic circles or as a “cluster of grapes” ( Fig. 42.3A ). However, proximal peripheral nerves, such as the roots of the brachial plexus, appear as individual anechoic circles that can be mistaken for blood vessels (see Fig. 42.3B ). Subtle tilting of the transducer may be necessary to obtain the highest quality images and minimize effects of anisotropy. Ultrasound image quality of nerves is more sensitive than other structures to the angle of insonation. Novice providers should verify that the intended target is not a vessel using color flow Doppler ( Fig. 42.4 ). For single-injection ultrasound-guided nerve blocks discussed in this chapter, a high-frequency linear transducer (6–13 MHz) is needed. Providers must maintain image orientation during ultrasound-guided peripheral nerve blocks by aligning the transducer marker (“notch”) and screen marker, depending on the approach (transverse vs. longitudinal) and type of nerve block.

Appearance of Nerves.

(A) The median nerve is covered in connective tissue and has the classic “honeycomb” appearance by ultrasound. (B) The brachial plexus nerve in the interscalene groove appears as anechoic bundles that can be mistaken for vasculature.

Color Flow Doppler.

Color flow Doppler demonstrates a vascular bundle next to the posterior tibial nerve.

The longitudinal approach is also called an in-plane or long-axis approach. The needle is inserted parallel to the long axis of the transducer ( Fig. 42.5A ). As the needle passes under the transducer, the entire length of the needle is visualized (see Fig. 42.5B ). The needle must be kept exactly in the center of the long axis of the transducer in order to visualize the needle in its entirety. A longitudinal approach for needle tip visualization is recommended for novice providers performing nerve blocks.

Longitudinal Approach.

(A) A longitudinal or in-plane approach is shown for an ultrasound-guided posterior tibial nerve block. The needle (red arrows) is inserted parallel to the long axis of the transducer (B). Anechoic anesthetic agent is seen surrounding the needle tip on the ultrasound image.

The transverse approach is also called an out-of-plane or short-axis approach. The needle is inserted in the center of and perpendicular to the long axis of the transducer at a steep angle (>60–80 degrees) to the skin ( Fig. 42.6 ). The needle tip is visualized only as a hyperechoic dot as it passes through the ultrasound beam ( Fig. 42.7 ). Safe and successful execution of this technique relies on confident visualization of the needle tip, which requires proficient spatial-motor skills to manipulate both the transducer and needle. For this reason, a transverse approach may be considered more suitable for experienced providers.

Transverse Approach.

In a transverse or out-of-plane approach to ultrasound-guided nerve blocks, the needle is inserted in the center of and perpendicular to the long axis of the transducer at a steep angle.

Transverse Approach.

An ultrasound image of a transverse approach shows only the needle tip and anechoic anesthetic spread above the nerve.

An ultrasound-guided femoral nerve block is an excellent method to decrease pain from proximal lower extremity injuries. In particular, the femoral nerve block is ideal for pain reduction in patients with hip fractures (intertrochanteric and subtrochanteric), femur fractures, and patellar injuries. In patients with a hip fracture, the femoral nerve block gives moderate pain reduction but does not provide complete analgesia because the obturator nerve, one of the three nerves that innervate the hip, is not blocked by this approach. However, this method can significantly decrease the need for intravenous opioid medications, reducing the risk of adverse events such as respiratory depression, confusion, and hypotension, especially in elderly patients.

A well-performed femoral nerve block with anesthetic tracking laterally under the fascia iliaca can also anesthetize the lateral cutaneous nerve. Both nerves innervate the anterior and lateral thigh and this approach can be used for lacerations or abscesses of the thigh. Even though the risk of developing compartment syndrome of the thigh is very rare, we recommend consulting with the primary surgical service before performing a femoral nerve block.

The femoral nerve is one of the branches of the lumbar plexus arising from the L1 to L4 ventral rami before descending toward the lower extremity. At the level of the inguinal canal, the femoral nerve is located just lateral to the femoral artery, beneath the fascia iliaca and superficial to the iliopsoas muscle; together these structures constitute important landmarks for this nerve block ( Fig. 42.8 ).

Anatomy of the Femoral Triangle.

The femoral nerve and the lateral femoral cutaneous nerve are lateral to the femoral artery.

With the patient in a supine position, place the linear high-frequency transducer parallel to and just below the inguinal canal ( Fig. 42.9 ). The transducer marker should be pointing to the patient’s right side. After locating the femoral artery and vein in cross-section, slide the transducer laterally to locate the femoral nerve. The femoral nerve appears as a hyperechoic triangular region immediately below the fascia iliaca and above the iliopsoas muscle ( Fig. 42.10 and Video 42.1 ). Fanning the transducer superiorly and inferiorly can improve identification of the triangular femoral nerve and its characteristic “honeycomb” appearance.

Femoral Block Transducer Position.

The ultrasound transducer is placed just below the inguinal crease for a longitudinal (in-plane) approach for a femoral nerve block. Note the needle entering the skin lateral and underneath the probe with the orientation of structures superimposed: (N) femoral nerve, (A) femoral artery, (V) femoral vein.

Femoral Nerve.

The femoral nerve (FN) is a triangular structure lateral to the femoral artery (FA) and femoral vein (FV) and deep to the fascia iliaca, a key landmark.

We recommend using a longitudinal (in-plane) approach from lateral to medial for femoral nerve blocks and depositing the anesthetic agent lateral to the femoral nerve, far from the femoral artery. After making a small skin wheal with local anesthetic, draw 20 mL of 2% lidocaine with epinephrine using a 3.5-inch (9 cm) 20G spinal needle. With the syringe in your dominant hand and the ultrasound transducer in your nondominant hand, enter the skin approximately 0.5 to 1 cm lateral to the transducer using a longitudinal (in-plane) approach. Needle insertion angle depends on the target depth of the fascia iliaca ( Fig. 42.11 ).

Femoral Nerve Block.

A lateral to medial longitudinal (in-plane) approach for ultrasound-guided femoral nerve block. The needle tip is deep to the fascia iliaca, and an anechoic pocket of fluid is stretching the tissues with injection of anesthetic (red arrow) . FA , Femoral artery; FN , femoral nerve; FV , femoral vein.

Advance the needle toward the junction of the hyperechoic fascia iliaca and lateral corner of the femoral nerve. Once beneath the fascia iliaca, aspirate to confirm the needle tip has not entered a vessel and then slowly inject 1 to 2 mL of local anesthetic. Placing anesthetic just below the hyperechoic fascia iliaca is key to obtaining an effective femoral nerve block. During injection of the anesthetic, an anechoic fluid pocket appears and can be seen stretching the fascia iliaca (see Fig. 42.11 and Video 42.2 ). After confirming an optimal needle tip location, proceed to inject a total of 10 to 20 mL of local anesthetic in 3 to 5 mL aliquots. If at any point the spread of local anesthetic is not visualized, then intravascular injection should be suspected and the procedure should be halted. Targeting your needle tip 1 cm lateral to the femoral nerve and vessels, yet deep to the fascia iliaca, reduces the risk of vascular puncture and intraneural injection.

The distal sciatic nerve innervates the lower extremity, making it an ideal block for fractures of the ankle, distal tibia, and fibula, and for 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. Tibial fractures are high-energy fractures commonly associated with compartment syndrome. Since a distal sciatic block obscures many clinical findings of compartment syndrome (pain, most notably), this block should only be performed after consulting with the primary surgical service.