Use of ultrasound-guided, peripheral-nerve blockade has become more popular in the last 5 years. Historically, peripheral-nerve blockade was facilitated by contacting the nerve with the block needle to elicit a parasthesia or by feeling a “pop” with fascial perforation (or both). In the 1970s, nerve stimulation allowed for more precise location of peripheral nerves based on motorstimulation patterns. Despite the growing popularity of regional anesthesia, it remains burdened with failures and occasional complications, including nerve injury and parasthesia, local anesthetic toxicity, pneumothorax, painful muscle stimulation, and neuroaxial anesthesia. With the decrease in cost and improved portability of high-resolution ultrasound devices, we now have access to devices that visualize peripheral nerves and surrounding structures to guide needle and catheter placement for peripheral-nerve blockade. Using ultrasound-guided, peripheral-nerve block decreases the length of procedures and the period during which the block takes effect; improves the quality of the block; improves the success rate; and reduces the risk for complications.

Understanding the basic physics of ultrasound is necessary to safely master its clinical uses. The echo is a sound wave generated by the vibration of piezoelectric crystals aligned at the tip of a probe. The received sound wave vibrates the piezoelectric crystals to generate an electric current. Structures are visualized depending on their specific acoustic impedance or echogenicity. Depth is measured by the delay of the echo wave’s return to the probe, assuming a speed of sound of 1540 m/sec in human soft tissue. The echo pulse can be absorbed, reflected, refracted, or scattered, creating artifacts. Image resolution and echo penetration relates to the frequency of the sent signal. High frequency signals (from 8 MHz to 15 MHz) produces high-resolution images at the expense of penetration and is suited for superficial structures only. As the frequency decreases (from 2MHz to 8 MHz), penetration increases, but resolution decreases. To maintain proper orientation, each probe has a mark on its side that corresponds to a mark on the screen.

Simultaneous use of a nerve stimulator is valuable to those learning to use ultrasound guidance. Attention to sterility is crucial as adding a probe
to the field complicates sterile technique. The identification of nerves requires detailed knowledge of trans-sectional anatomy and the characteristic echogenicity of the adjacent structures and nerves in question. Scanning a nerve in a transverse plane, as opposed to a longitudinal plan, facilitates its identification. Neurons appear to be hypoechoic or black, and connective tissue appears to be hyperechoic or grey or white. Each nerve has its own characteristic pattern of echogenicity, depending on its location: The trunks of the brachial plexus appear to be hypoechoic, whereas distal nerves are relatively hyperechoic. Nerves are anisotropic meaning their echogenicity changes depending on the angle of incidence of the echo beam, a property that can be utilized to differentiate peripheral nerves from other structures less anisotropic (tendons) or not anisotropic (muscle, fat, bone), which do not change echogenicity. Standardizing the approach by developing a specific pattern of recognition for each peripheral nerve is useful. First, specific surrounding structures are identified (bones, vessels, muscles), and then nerves are located. When learning the technique, the nerve, the needle shaft, and the tip of the needle should be visualized using the long axis of the probe to maintain constant visualization of the needle. A motor pattern can be elicited and the practitioner can observe appropriate local anesthetic spread in the tissue or the advancement of a catheter for continuous perineural infusion. Some authors have identified preferable patterns of local anesthetic spread along the nerve. Ideally, the local anesthetic should be seen bathing the entire nerve as the local anesthetic is injected to ensure reliable anesthesia of the nerve (donut sign). When dealing with separated nerves, as in the axillary approach of the brachial plexus, local anesthetic should be seen bathing all desirable nerves or the needle can be moved appropriately if only partial local anesthetic spread is visualized.