Prior to beginning any nerve block, it is recommended that a detailed motor and sensory exam be documented for the affected extremity for multiple reasons. Injured nerves may be prone to increased complications of regional anesthesia, and a thorough exam will insure that any pre-existing deficits are not falsely attributed to the block post recovery.

Patient positioning depends on the type of block being performed. The ultrasound should be placed so that the screen is in the operator’s direct line of vision, which may necessitate placing the machine on the opposite side of the procedure site (Fig. 22.3). A survey scan both proximal and distal to the anticipated site of needle insertion will allow the operator to determine the optimal location for the procedure by helping to identify the site with the clearest image of the nerve without adjacent vasculature.

The next step in preparing for a nerve block includes selection of needles, anesthetic, and skin sterilizing solution. Equipment may vary depending on the type and indication of block.

Sizes may range from 21 to 27 gauge needles (7, 8, 9). The size and location of the nerve may affect the gauge selection. A larger needle may be beneficial for a deeper nerve block, as its size may improve visualization at steeper entry angles. A smaller peripheral nerve is often best anesthetized with a smaller needle as the entry angle is typically shallower and its size may facilitate fine adjustments in needle tip placement. Although larger needles are typically easier
to visualize under ultrasound guidance, histologic examination reveals that smaller needles produce less nerve injury in cases of nerve penetration (10). Needle tip has also shown to play a histologic role in direct nerve trauma. Fascicular trauma is minimized with shorter beveled cutting needles (11,12). However long tapered (pencil-tip) needles have also shown to minimize the number of transected nerve fibers compared to cutting needles (13). The last consideration in needle selection concerns the use of echogenic needles. While the literature supports increased sonographic visualization of echogenic needles, no study has yet identified a clear benefit in safety or efficacy of echogenic needles in peripheral nerve blockade (14, 15, 16).

Operators should choose an anesthetic agent based on the preferred duration of anesthesia. Bupivacaine is the most commonly used long-acting amide anesthetic, yet is significantly more cardiotoxic than lidocaine. As a result, if procedures can be adequately accomplished with lidocaine alone, it is our recommendation to avoid bupivacaine use until sufficiently experienced with these techniques.

Additional considerations with regard to anesthetic volume must be taken into account. Table 22.1 shows common anesthetics and maximum doses. In peripheral nerve blocks, volume is an important issue as it relates to block effect and duration, as well as patient safety. While multiple studies have examined the minimum volume necessary for a given block, it is difficult to generalize the results as the concentrations and anesthetic additives may vary (17, 18, 19, 20). The primary goal of the block should be to circumferentially surround the nerve with a volume of anesthetic, which does not exceed the mg/kg dose for the patient. One small study demonstrated that lower volumes of an equivalent concentration of anesthetic primarily decrease block duration without altering efficacy; therefore, increasing volume may help prolong block duration, but not change the degree of anesthesia (21).

While some anesthesia literature describes most peripheral nerve blocks as a sterile practice, this is often in the setting of an operative procedure and/or placement of a nerve block catheter. Alternative descriptions include skin prep with betadine or chlorohexadine with sterile gel and a sterile adhesive dressing covering over the ultrasound probe (Fig. 22.4) (7,8,22). Still others only describe skin prep without specific gel or probe covers (8). From a practical standpoint, if the needle is visualized throughout its entire track from skin to nerve, ideally, only penetration of subcutaneous and muscular tissue will occur. Therefore, while skin prep is still necessary, the procedure should not be significantly different than any other intramuscular or subcutaneous injection provided that the needle does not pass directly through nonsterile ultrasound gel prior to skin penetration.

Performance of the block is often done using an in-plane technique wherein the needle is advanced in parallel to the long axis of the probe, while the nerve is imaged in the short axis (Fig. 22.5). A short axis view of the nerve provides the best view for circumferential deposition of anesthetic while viewing the surrounding structures and layers of the nerve bundle. Some practitioners favor an out-of-plane technique in which the nerve remains imaged in the short axis but the needle is placed perpendicular to the long axis of the probe. There is no clear evidence suggesting a clear
advantage to either approach. The probe should be held in the nondominant hand, with the needle and syringe held in the dominant hand, allowing finer control of the anesthetic. Prior to inserting the needle into the skin, any air remaining in the syringe or needle tip should be expelled, as inadvertent injection of air into the tissue will cause scatter artifact, making it difficult to visualize both needle and tissue anatomy (Fig. 22.6; VIDEO 22.2). As the needle is advanced, anesthetizing the tract to the nerve can help visualize the needle by decreasing the attenuation coefficient of the tissue. It is imperative that anesthetic should not be injected around the nerve or into tissue unless the needle tip is visualized. This is an essential step to decrease the risk of intravascular and intrafascicular injection. Minimizing the angle of the needle entry in relation to the probe will increase needle visualization by bringing it into a more perpendicular orientation to the ultrasound beam (Fig. 22.7). Once the needle tip is clearly visualized with the nerve in short axis, anesthetic should be deposited circumferentially around the nerve to ensure a complete block. It is important to note that due to the depth and location of surrounding structures, circumferential deposition of anesthetic may require needle redirection or more than one needle pass.

Perineural (around the nerve), intraneural (within the epineurium), and intrafasicular (within the perineurium) are all sonographically visible sites and have anatomic relevance in nerve blocks. A perineural block wherein the anesthetic is placed completely outside of the epineurium is generally the most accepted location of block with minimal risk of nerve injury (Fig. 22.8; VIDEO 22.3). Although an intraneural injection is considered unfavorable, there is little evidence to support this opinion (23). Multiple studies have
demonstrated that intentional and unintentional intraneural injections do not result in neurologic dysfunction after a normal recovery period (24). In some cases, intraneural injection has actually shown to be favorable in block onset time (Fig. 22.9; VIDEO 22.4) (12). In contrast, intrafascicular blocks are considered harmful and must be avoided, as damage to fascicles may result in near immediate damage (25). Also, the site of peripheral nerve blockade may play a role in the risk of fascicular damage as it relates to the ratio of cross-sectional fascicle to epineurium (25). For example, the sciatic nerve at the popliteal fossa contains more non-neural tissue than fascicles, which may explain the relatively low incidence of post-block neuropathy (26). Alternatively, the brachial plexus trunks are characterized by a higher ratio of neural to connective tissue. Subsequently, an advancing needle is more likely to encounter fascicles, thereby potentially increasing the risk of injury (25). This again highlights the necessity to maintain visualization of the needle tip at all times during needle advancement.