Introduction

Ultrasound-guided nerve blocks in emergency care are an expanding area of interest and research.

While traditional “blind” or landmark-based nerve blocks (such as radial, median, and ulnar blocks performed at the wrist using anatomic landmarks) are frequently performed in emergency care settings,^1,2 the use of ultrasound to perform real-time injection of an anesthetic agent around a target nerve is a newer application in the emergency department (ED). Ultrasound guidance for abdominal wall,³ sciatic,⁴ tibial,⁵ sural,⁶ intercostal,⁷ brachial plexus,⁸ and forearm nerve blocks has been shown to be efficacious in emergency care settings.⁹ Ultrasound guidance is also used with increasing frequency in the pediatric population.¹⁰ This article will focus on performing ultrasound-guided radial, median, and ulnar nerve blocks, together termed the “forearm nerve blocks.”

For background, a review of the basic motor and sensory functions of the radial, median, and ulnar nerves and the appropriate clinical examination to test their function is helpful (Table 1).

Indications in Emergency Practice

Forearm nerve blocks provide excellent anesthesia of the hand. Based on the sensory innervation described above, forearm nerve blocks are useful in cases of fractures, lacerations, or deep space infections of the hand. Unfortunately, the forearm nerve blocks described in this article do not provide anesthesia to the distal forearm or wrist and should not be performed for injuries affecting these areas.

Patient Selection

Appropriate ED patient selection for regional anesthesia procedures is important. The following is a brief list of exclusion criteria for this type of procedure:

• Patient unable to give consent (altered mental status, language barrier, seems unable to return to the ED if a complication from the nerve block occurred).
• Pre-existing nerve injury, numbness, tingling or weakness in the affected extremity.
• Injuries requiring urgent consultation or serial neurological examinations during admission (we recommend the ED provider communicate with the consultant before all blocks, since the sensory – and in some cases motor – exam will be altered).

Procedure

Position the patient with the affected extremity held palm-up and resting comfortably on a table or other hard surface. Place the ultrasound machine in line with the practitioner’s line of sight such that the practitioner has an unobstructed view of the ultrasound screen.

Forearm nerve blocks should be performed using the high-frequency linear (usually 10-5 MHz) transducer held in a transverse orientation. A soft-tissue or nerve preset if available is ideal to locate the nerves in the forearm. For consistency, we recommend pointing the probe marker toward the right side of the patient with the directional indicator on the left of the screen (as per standard emergency ultrasound convention).

Specifics for Locating Each Nerve

Systematically scan the forearm starting distally and moving proximally to find each nerve. Remember that the goal is to locate each nerve at a point where there is sufficient distance from vascular structures to avoid inadvertent vascular puncture. This must be balanced against selecting a site where nerve depth makes the procedure more difficult.

Median nerve. The median nerve does not have an associated vascular structure except in rare cases. To locate the median nerve, start at the wrist with the mid-point of the transducer over the middle of the wrist crease. Move the probe proximally and look for the nervous structure that exists at the junction of several fascial planes. The median nerve lies in the fascial plane between the flexor digitorum superficialis and profundus. As the probe moves more proximal, tendon structures will disappear, while the classic “honeycomb” bundle of the median nerve will persist (photo 1).

Radial nerve. The radial nerve is located radially to the radial artery. Starting at the wrist, locate the pulsatile radial artery. Immediately radial to the radial artery, a collection of nerve bundles is seen.

Often, the radial nerve is difficult to identify as a separate structure at the wrist because of its close proximity to the artery. Trace the radial artery proximally, and by approximately mid-forearm the radial nerve should be located at a safe distance from the radial artery (photo 2).

Ulnar nerve. The ulnar nerve is located at the ulnar aspect of the ulnar artery. Starting at the wrist, locate the pulsatile ulnar artery. Immediately ulnar to the ulnar artery, a small nerve bundle can be visualized. Like the radial nerve, the ulnar nerve lies close to the ulnar artery. As the provider moves the probe in a more proximal manner, the ulnar nerve will separate from the artery, allowing an ideal location for injection while reducing risk of inadvertent arterial puncture (photo 3).

The examiner should locate the desired nerve at a location sufficiently separate from vascular structures (if possible), but not so deep that it cannot be reached with a standard needle.

Procedure Details

Skin prep. The skin should be cleansed with alcohol or other cleansing agent (such as chlorhexidine).

Patient monitoring. We recommend placing the patient on continuous pulse oximeter monitoring to allow the operator to be aware of the patient’s heart rate and oxygen saturation at all times.

Local anesthetic. A skin wheal of local anesthetic (1% lidocaine with or without epinephrine) should be injected with a (30-gauge) tuberculin syringe.

Syringe preparation. While the local anesthetic is taking effect, fill a syringe with 10 cc of 1% lidocaine without epinephrine. Bupivacaine should not be used by novice providers in case of inadvertent vascular puncture. In our practice, we use a standard 25-gauge, 1.5-inch needle to perform the block, and our providers often use longer-acting anesthetics once they’ve become familiar with performing these procedures.

Ultrasound-guided injection. With the transducer held in a transverse orientation at the predetermined site, introduce the needle approximately 0.5 cm laterally to the base of the transducer. For blocking the median nerve, the needle can be introduced on either side of the probe, with the location of other neurovascular structures and examiner’s hand dominance guiding the decision. For the radial nerve, the needle should be introduced on the radial side of the probe (photo 4), while for the ulnar nerve the needle should be introduced on the ulnar side of the probe (photo 5). Note that this is a description of the in-plane technique. Although we also perform these blocks using an out-of-plane technique (similar to a short-axis approach to vascular access), we have found that novice users are more successful using an in-plane technique.

The needle should be visualized from the moment it passes under the ultrasound transducer (photo 6). If the needle is not identified, the operator should not advance the needle. The needle should be removed or the probe moved to allow clear visualization of the needle. The needle should be directed from the skin entry site to the most distal border (either superficial or deep) of the nerve (photo 7). The needle should not be allowed to penetrate the actual nerve structure at any point.

Prior to injecting any anesthetic agent, draw back on the syringe to confirm that the needle is not in a vascular structure. Gently inject 0.5-1 cc of anesthetic and observe a spread of anechoic (black) fluid around the superior border of the nerve (photo 8). If anechoic fluid is not seen exiting the needle tip, the operator should not deposit more anesthetic. The probe should be adjusted to confirm that the tip of the needle is clearly visualized before more anesthetic agent is deposited. The goal is to have the anechoic fluid surround the nerve completely. Withdraw the needle towards the skin and redirect it to the area where the fluid has not reached. Repeat the injection at this site (photo 9). When complete, the nerve should be circumferentially surrounded by a layer of anechoic fluid, resembling a donut (photo 10). With the transducer, trace the nerve both proximally and distally to confirm this “donut” appearance.

Inject only as much anesthetic as is needed to create this “donut” appearance. With experience, we have found that the three forearm nerves can be adequately blocked with a total of 10 mL of anesthetic agent (much lower than the maximum dosing guidelines). Withdraw the needle and apply a dressing to the skin site.

Duration and toxicity of anesthetic agents. Table 2 shows the duration and maximum dosages of the most commonly used anesthetic agents.

Complications

Ultrasound-guided nerve blocks are a useful technique to supply anesthesia to the hand, but like all procedures, may have associated complications.

Inadvertent vascular puncture and intraneural injections (two feared but uncommon complications) can be avoided by selecting an injection site where the nerve and artery are sufficiently separate and by visualizing the needle tip during the procedure. Inadvertent vascular puncture can also be avoided by drawing back on the syringe before injection, and ensuring that anechoic fluid is visualized on the ultrasound screen when anesthetic is deposited. To avoid intraneural injections, infuse anesthetic under low pressures. If the patient experiences any pain or paresthesias during the injection, stop injecting and pull back the needle until it is clear that the needle tip is not within the nerve bundle.

References

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