ocation, location, location! The success of regional anesthesia follows a simple principle. Accurate placement of local anesthetic in sufficient amounts around the target nerve will result in successful blockade. In practice many anesthesiologists consider regional anesthesia a second choice and perform only if absolutely necessary because they find the procedure challenging to do and the result unreliable. Because we cannot see nerves that we wish to anesthetize, regional anesthesiology remains an art that is difficult to master. Even for the experienced, nerve block is an exercise of seeking the nerve by trial and error and relying on indirect cues (e.g., nerve stimulation) for guidance. If regional anesthesia is to be as popular as general anesthesia, it must approach the same degree of simplicity and ease of success. One would hope that the art of placing a needle perineurally in regional anesthesia can be as simple as putting a needle intravenously for general anesthesia.

Turning Art Into Science
Ultrasound imaging may transform the art of regional anesthesiology into a science.¹ Ultrasound-assisted nerve block has been described for localization of the brachial plexus,^2-4 femoral nerve,⁵ lumbar plexus6 and sciatic nerve.^7-9 While ultrasound application for regional anesthesia is a relatively new and evolving concept, its use to accurately locate target lesion for tissue biopsy has been standard medical practice for many years. Although ultrasound-assisted nerve block was described more than two decades ago, it did not receive worthy attention until recent technological advancement showing nerves in high resolution. Ultrasound is a preferred imaging modality over others (e.g., computed tomography, magnetic resonance imaging and fluoroscopy) because it is portable, affordable and accessible in the operating room without radiation risk. This exciting technology offers several distinct potential clinical benefits over conventional nerve-seeking techniques. Not only can it visualize peripheral nerves and their neighboring structures, it also can visually track needle movement in real time and assess adequacy of local anesthetic spread at the time of injection. The ability to see the nerves, the needle and local anesthetic spread may be the key to a consistently successful block.

Ultrasound for musculoskeletal and nerve imaging is commonly in the 2-15 MHz frequency range depending on the depth of penetration required. Learning to recognize nerves on ultrasound is not difficult. On transverse (cross-sectional) view, a nerve is round or oval shaped, and this view is generally the best for nerve block. Peripheral nerves appear hypoechoic (dark) in the interscalene and supraclavicular regions but hyperechoic (bright) in most other upper- and lower-limb locations. A scan prior to needle insertion will show the exact nerve location, its size and depth from the skin, and is thus helpful in defining the desired site, angle and path of needle penetration. It can clearly demonstrate inter-individual variability in nerve location that cannot be otherwise detected by conventional block techniques. Moving the probe along the course of the nerve will show the location where the nerve divides. It is likely that a planned injection proximal to this point will result in a complete block. Ultrasound also shows structures surrounding the nerve and their locations, which helps to minimize the chance of inadvertent vascular and pleural puncture. There is no doubt that anatomical data captured on ultrasound examination at the time of nerve block is more valuable than atlas illustrations.

Real-Time Navigation
Another benefit of ultrasound is real-time visual navigation guidance at the time of needle advancement. When the needle and the ultrasound beam are aligned with (parallel to) each other, the needle shows up as a hyperechoic (bright) line on the screen. Needle movement toward the target can be tracked in real time, thus random needle movement and the number of needle attempts is minimized. One can then observe needle-nerve interaction when they make contact. It is interesting to see how the nerve is being pushed away by the block needle or it simply rolls around the needle. Tracking needle movement in real-time also can prevent the risk of inadvertent needle entry into the spinal canal that cannot be recognized by the nerve stimulator technique. Good hand-eye coordination is required for tracking the needle on ultrasound, and the necessary skill of aligning the block needle with the ultrasound beam can be acquired through hands-on practice.¹⁰ Echogenic needles also can provide improved visibility,¹¹ but there are no dedicated echogenic block needles at this time.

Visualizing Pattern Spread
Another attractive feature of ultrasound is its ability to show the pattern of local anesthetic spread at the time of injection. This is valuable because an incomplete block may result from an asymmetric or partial spread around the nerve. Under ultrasound guidance, it is now possible to adjust needle position half-way during local anesthetic injection to ensure complete circumferential spread. Ultrasound is superior to conventional nerve stimulation technique because motor response typically disappears following 1-2 mL of local anesthetic injection. Whether sufficient local anesthetic is spread longitudinally within the fascial compartment can now be assessed by scanning the nerve along its long axis. With proper scanning technique, ultrasound also can visually detect an intravascular injection, which is indicated by the absence of tissue expansion upon injection. Ultrasound also may distinguish an extraneural from an intraneural injection (tissue expansion versus increased nerve diameter), but such conclusions await further study.

Ultrasound imaging is an attractive tool for regional anesthesiology because of the many potential clinical benefits mentioned above. Both cart-based and portable, compact ultrasound machines are now available and suited for nerve imaging. In theory, visual guidance can impart confidence to anesthesiologists, safety to patients and efficient time utilization in the operating room. Outcomes data to demonstrate convincingly the clinical benefits of ultrasound are pending. While skeptics may doubt ultrasound will become a standard practice in the future, there is no doubt that this imaging technology will be a valuable and enduring part of practice in regional anesthesia.

References:

1. Marhofer P, Greher M, Kapral S. Ultrasound guidance in regional anaesthesia. Br J Anaesth. 2005; 94:7-17.

2. Perlas A, Chan VW, Simons M. Brachial plexus examination and localization using ultrasound and electrical stimulation: A volunteer study. Anesthesiology. 2003; 99:429-435.

3. Williams SR, Chouinard P, Arcand G, et al. Ultrasound guidance speeds execution and improves the quality of supraclavicular block. Anesth Analg. 2003; 97:1518-1523.

4. Sandhu NS, Capan LM. Ultrasound-guided infraclavicular brachial plexus block. Br J Anaesth. 2002; 89:254-259.

5. Marhofer P, Schrogendorfer K, Wallner T, et al. Ultrasonographic guidance reduces the amount of local anesthetic for 3-in-1 blocks. Reg Anesth Pain Med. 1998; 23:584-588.

6. Kirchmair L, Enna B, Mitterschiffthaler G, et al. Lumbar plexus in children. A sonographic study and its relevance to pediatric regional anesthesia. Anesthesiology. 2004; 101:445-450.

7. Gray AT, Huczko EL, Schafhalter-Zoppoth I. Lateral popliteal nerve block with ultrasound guidance. Reg Anesth Pain Med. 2004; 29:507-509.

8. Sinha A, Chan VW. Ultrasound imaging for popliteal sciatic nerve block. Reg Anesth Pain Med. 2004; 29:130-134.

9. Sites BD, Gallagher J, Sparks M. Ultrasound-guided popliteal block demonstrates an atypical motor response to nerve stimulation in 2 patients with diabetes mellitus. Reg Anesth Pain Med. 2003; 28:479-482.

10. Sites BD, Gallagher JD, Cravero J, Lundberg J, Blike G. The learning curve associated with a simulated ultrasound-guided interventional task by inexperienced anesthesia residents. Reg Anesth Pain Med. 2004; 29:544-548.

11. Schafhalter-Zoppoth I, McCulloch CE, Gray AT. Ultrasound visibility of needles used for regional nerve block: An in vitro study. Reg Anesth Pain Med. 2004; 29:480-488.

Vincent W.S. Chan, M.D., is Professor of Anesthesiology, University of Toronto, Toronto, Ontario, Canada.