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