Ultrasound-Guided Nerve Blocks




Background and Indications for Examination



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Regional anesthesia provides an alternative to general or local anesthesia, procedural sedation, and parenteral pain management. Conventional anatomic approaches to nerve blocks are imprecise, and success rates vary greatly from one practitioner to another. Electronic nerve stimulation is often used for improved nerve localization, but may not be available outside of surgical suites. In recent years, ultrasound-guided regional anesthesia has gained popularity as an adjunct or alternative to anatomic and nerve stimulation techniques. While simple blocks (eg, a digital block) have long been performed by non-anesthesiologists using the anatomic approach, the more widespread availability of point-of-care ultrasound is providing a safe and effective method to expand the use of nerve blocks in the emergency and critical care setting.




Ultrasound-guided nerve blocks consist of the identification of the target nerve(s), visualization of the surrounding anatomy (such as blood vessels, lymph nodes, and other important structures), and real-time observation of the local anesthetic spread. Direct visualization of the target nerve and deposition of local anesthetic has been shown to improve block success and to decrease some common complications associated with the procedure.




Bedside ultrasound-guided nerve blocks should be performed in:





  • The patient undergoing a painful procedure where a regional nerve block will be effective
  • Control of pain when parenteral analgesics are not desirable (eg, the elderly patient with hip fracture)
  • The patient who has chronic pain due to an underlying medical condition who will gain relief from a regional nerve block




Probe Selection and Technical Considerations for Nerve Blocks



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Linear Array Probe with a Frequency of 10–15 MHz



For evaluation of the superficial nerves, such as those in the forearm, the brachial plexus, and femoral nerves, a high-frequency (10–15 MHz) linear transducer is required which provides better resolution. A lower-frequency curvilinear probe (4–7 MHz), which provides better penetration, is recommended for deeper targets such as the sciatic nerve, or for more obese patients. A small footprint “hockey stick” transducer is preferred in pediatric patients where a smaller surface area is being explored.




Focus



The focus can be adjusted to the level of the nerve once found. This will further improve the resolution of the image.




Depth



The sonographer should initially startoff with a deep field and fully interrogate the area surrounding the nerve. This will help to identify nearby structures that the physician must avoid when inserting the needle and injecting anesthesia. Once the area has been fully explored, the depth can be decreased so that the nerve takes up most of the screen and is better visualized.




Gain or Time-Gain Compensation



The total gain or far-field gain (TGC) may be increased in order to brighten the signal returning from deeper structures. Most of the nerve blocks done in the acute care setting are superficial; therefore, the gain usually only needs to be increased with deeper targets or in obese patients.




Color-Flow Doppler



The use of color-flow Doppler can be helpful when attempting to properly identify the nerve and to avoid surrounding structures, such as lymph nodes and blood vessels. Nerves will not pick up color flow, but nodes and vessels will, which helps to avoid inadvertent injury to these other structures.




Probe Orientation for Nerve Visualization



Ultrasound-guided nerve blocks are most often done with the nerve visualized in the short axis. The resultant image is the nerve appearing as a circular or elliptical structure with a honeycomb appearance due to hypoechoic fascicles surrounded by a hyperechoic epineurium, consisting of connective tissue (Fig. 23-1a). This technique is the one used most often by physicians and the easiest for the novice to learn, allowing for visualization of vascular structures adjacent to the nerve.




Figure 23-1(a)



Median nerve (MN) as shown in its short-axis (transverse) at the mid-forearm. The nerve has a “honeycomb” appearance with hypoechoic nerve fascicles surrounded by hyperechoic epineurium. Unlike the ulnar and radial nerve, the median nerve in the forearm is not associated with an artery.




The nerve may also be visualized in a longitudinal (sagittal) plane by orienting the probe along the course of the nerve, again showing a characteristic fibrillar appearance (Fig. 23-1b). This may be a more challenging view, as the plane of the ultrasound needs to be directly in-line with the nerve.




Figure 23-1(b)



Median nerve (MN) as shown in its long-axis (sagittal) at the mid-forearm with a characteristic fibrillar appearance.





Probe Orientation for Needle Entry



Once the nerve is located in either plane, the needle may then be inserted and visualized using an “in-plane” or “out-of-plane” technique. In the in-plane technique, the needle enters from the edge of the probe (at the shorter side of the probe footprint) and is visualized entirely in the plane of the ultrasound beam (Fig. 23-2a). For ultrasound-guided anesthesia, the in-plane technique is recommended so that the entire length of the needle, not just the tip, is visualized, which helps to ensure precise deposition of anesthetics (Fig. 23-2b).




Figure 23-2(a)



Appropriate needle and probe positioning for the “in-plane” technique. The needle is inserted at the shorter side of the probe footprint. Note the use of a spinal needle attached to extension tubing and a syringe.





Figure 23-2(b)



Radial nerve block with the needle visualized in a long axis using an in-plane technique. Note the hyperechoic needle (N) visualized in its full length approaching the radial nerve (RN).




In general, we will be describing the nerve block procedure using short-axis visualization of the nerve with in-plane needle entry. Unlike other ultrasound-guided procedures, the probe is not withdrawn until the termination of the nerve block.




Dynamic Ultrasound Guidance



Ultrasound-guided nerve block procedures are done using real-time imaging. This technique allows the sonographer to directly visualize the needle approaching the nerve and the injection of anesthesia surrounding it in order to ensure an adequate block. This dynamic imaging also decreases the risks of injuring other surrounding anatomy, such as lymph nodes, blood vessels, or other vital structures. When using dynamic guidance, the sonographer must use sterile precautions including the use of probe covers and sterile gel.




Needle Choice



Standard injection needles (found in most emergency departments) tend to have long bevels (steep angle). While there is some controversy over the role of bevel angle in neurovascular injury, most authorities recommend a short bevel needle for more proximal nerve blocks. An alternative that is unlikely to cause injury is a pencil-point needle, with a side port for installation of anesthesia. Spinal needles are typically medium bevel or pencil point and may be appropriate for use in nerve blocks. Needle length may vary between 1.5 and 3 in, depending on the depth of the nerve, and gauge is typically between 22 and 25.




Anesthetic Choice



Choice of anesthetic may be guided by onset, duration of action, and toxicity. The addition of epinephrine will lengthen action and decrease toxicity. A benefit of ultrasound-guided anesthesia is that typically lower amounts of anesthetic are needed than if the procedure is done blind.



Lidocaine (Xylocaine) is a quick onset, short-acting anesthetic that is appropriate for brief procedures that are unlikely to have significant post-procedure pain. Toxicity occurs at ∼4.5 mg/kg without epinephrine and ∼7 mg/kg with epinephrine. Lidocaine is typically 1% or 2% (10 or 20 mg/mL). Thus, for a 70-kg adult, 30 cc or less of 1% lidocaine should be used.



Bupivacaine (Marcaine) is a long-acting anesthetic with slow onset of action. It may be mixed with lidocaine for longer-acting post-procedure control. Toxicity occurs at over 2.5 mg/kg, but concentrations of the solution are typically lower (0.25%). Bupivacaine is much more cardiotoxic than lidocaine if injected intra-vascularly and care should be taken with this agent. Some authorities recommend that a lipid emulsion injection be available for the therapy of the rare complication of cardiac arrest with intraarterial bupivacaine injection.



Mepivacaine (Polocaine, Carbocaine) is a medium-onset, medium-acting anesthetic with a toxicity at 7 mg/kg that is often recommended for regional anesthesia.




Performing Ultrasound-Guided Nerve Blocks



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Specific nerve blocks are discussed later. As in any ultrasound-guided procedure, optimal patient positioning is needed in order to gain adequate exposure to the desired anatomical area. The ultrasound machine is positioned so that the monitor faces the operator during the procedure. Sterile precautions are observed, including appropriately cleansing the skin, using a sterile probe cover and gel, and the use of sterile attire by the person performing the procedure.




Once the appropriate nerve(s) are identified, the adjacent area should be interrogated in order to identify the surrounding anatomy. The sonographer should take note of the soft tissue and musculature that the needle may need to puncture through, the location of the vasculature, the position of the rib(s) and lung tissue if performing a brachial plexus block, and the direction that the target nerve travels proximally and distally. This helps to plan the trajectory of the needle and to avoid complications. Once the surrounding area is examined, the nerve should be imaged in a transverse or short-axis plane and centered in the middle of the screen.




The superficial skin may be anesthetized by depositing local anesthetic adjacent to the footprint of the probe, and forming a wheal at the site of anticipated needle puncture, typically using a short 25G or higher needle. In an in-plane approach, the wheal will be adjacent to the shorter side of the footprint. A syringe should be filled with the desired anesthetic medication and connected to short-extension tubing. The amount of anesthetic used will depend on the type and accuracy of block, type of anesthetic, and patient size, but typically ranges from 5 to 30 cc total for an adult patient. Similarly, needle choice will vary in terms of length and gauge, as discussed earlier. A short bevel or non-cutting needle (spinal needle may be used) is preferred (see Fig. 23-2a).




Next, the needle should puncture the skin through the anesthetic wheal. The sonographer needs to focus on the ultrasound monitor while inserting the needle in order to track its course. The entire needle length will be visualized in an in-plane approach (see Fig. 23-2b). As the needle tip enters the desired location adjacent to the target nerve, about 1–2 mL of anesthetic should be slowly delivered as a test dose. The anesthetic spread should be observed as a band of hypoechoic ribbon dissecting out the target nerve from its surrounding soft tissue. The goal is to form a circular spread, or a “donut” around each nerve or nerve bundle, which will improve the visualization of the target nerve. If the spread is too distant from the target, the needle should be pulled back and repositioned and the test dose repeated. If the desired spread is achieved, then the rest of the anesthetic can be slowly injected. If the patient experiences pain during the injection or intraneuronal anesthetic spread is visualized, then the needle should be withdrawn by a few millimeters before further injection. Once the desired amount of medication is given, the needle can be removed completely and the nerve distribution(s) can be checked to ensure complete anesthetic effect.




The choice of anesthetics will depend on the goal of the nerve block. A short-acting medication, such as 1%–2% lidocaine (1.5–3 hours), is adequate for short procedures such as an incision and drainage, or an uncomplicated shoulder dislocation. A long-acting medication, such as 0.25%–0.5% bupivacaine, (10–14 hours) provides longer analgesia for more complex procedures. Also, commonly available is the intermediate-acting 1%–1.5% mepivacaine (4–5 hours).




Specific Blocks



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Brachial Plexus



The brachial plexus is made of the fifth to eighth cervical nerves, and the major part of the first thoracic nerve. As the nerve roots traverse distally, they form the superior, middle, and inferior trunks that lie between the anterior and middle scalene muscles. The trunks then organize themselves into the lateral, posterior, and medial cords behind the clavicle before becoming terminal nerves that innervate the upper extremity in the axillary crease.




Interscalene Block



The interscalene approach is the most proximal brachial plexus block and is performed on the nerve trunks situated in the groove between the anterior and middle scalene muscles. This approach mostly blocks C5-C7 and is best suited for providing analgesia to the shoulder, clavicle, and upper arm. In the acute care setting, this block is most useful for shoulder reductions or fractures and for injuries of the upper humerus.



It can also block the cervical plexus (C3-C4) in varying degrees. Due to the proximity of the phrenic nerve to the interscalene groove, this approach may cause a transient hemidiaphragmatic paralysis, although the incidence may be lower with ultrasound guidance and smaller amounts of anesthetic. However, respiratory compromise is a contraindication to interscalene block.



The patient should be placed in the supine position with their head turned to the contralateral side. This view can be obtained by scanning the lateral aspect of the neck at the level of the C6 cricoid cartilage. Once the cricoid cartilage is identified, the lateral border of the sternocleidomastoid muscle should be palpated and just lateral to that, the interscalene groove can be identified. The probe should be placed in a transverse oblique orientation over the groove at this level (Fig. 23-3a). The resultant image will show the sternocleidomastoid muscle located superficially and the internal jugular vein and the carotid artery medially. The anterior and middle scalene muscles are visualized laterally. The brachial plexus is lined up vertically within the interscalene groove and is seen as three nerves together in cross section, often described as a “traffic light” appearance (Fig. 23-3b).

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Dec 23, 2019 | Posted by in EMERGENCY MEDICINE | Comments Off on Ultrasound-Guided Nerve Blocks
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