Is Regional Anesthesia Simply an Exercise in Applied Sonoanatomy?

The key steps in any successful regional anesthetic involve identifying the exact position of the nerve, reaching it with a precisely placed needle (without damage to any adjacent structures), and, finally, carefully injecting local anesthetic. Although easy in principle, clinicians are confronted daily with the difficulty of converting this theory into practice. Knowledge of anatomy based on surface landmarks is an essential starting point but is hardly ever satisfactory alone. The introduction of the peripheral nerve stimulator into clinical practice was a major advance. Unfortunately, even with this tool, our performance is still far from perfect, resulting in unpredictable block failures, inadvertent puncture of adjacent structures leading to complications, or frustrating and time-consuming trial-and-error attempts.Alon P. Winnie once predicted:“Sooner or later someone will make a sufficiently close examination of the anatomy involved, so that exact techniques will be developed.”1Ultrasonography may represent just such a method for providing a “sufficiently close examination of the anatomy.” Studies comparing ultrasound guidance with nerve stimulator guidance have found significantly higher success rates, shorter onset times, 2and a decrease in local anesthetic needs 3and complications with the former method. In this issue of Anesthesiology, Perlas et al. 4contribute further to our knowledge with a volunteer study, using the newest-technology high-frequency probes (12-MHz) to identify the brachial plexus in five typical locations and to guide a needle and verify its position with nerve stimulation. With figures of excellent quality and high educational value, this article once more demonstrates the technical feasibility of ultrasound-guided brachial plexus block.Ultrasound-facilitated nerve blocks were first reported in 1978, 5and interest has increased in the past 10 yr owing to progress in transducer technology and image processing. Although early studies were limited to vascular identification by Doppler ultrasound, recent studies have tried to directly visualize the nerves. 6Normal peripheral nerves in transverse scans appear as multiple round or oval hypoechoic areas encircled by a relative hyperechoic background. As with tendons, the connective tissue within the nerves (perineurium and epineurium) displays an anisotropic behavior, depending on the angle of the emitted ultrasound wave relative to the long axis of the nerve. 7This Editorial View accompanies the following article: Perlas A, Chan VWS, Simons M: Brachial plexus examination and localization using ultrasound and electrical stimulation: A volunteer study. Anesthesiology 2003; 99:429–35.Earlier, low-frequency methods were not ideal. However, the move toward higher frequencies, resulting in the 7- to 15-MHz linear transducers of today, has resulted in much higher image resolution. Moreover, ultrasound has become portable, providing increased flexibility and applicability. However, a fundamental rule of ultrasound physics shows that although higher frequency results in higher spatial resolution, the depth of tissue penetration is also reduced. Accordingly, the highest frequency is not automatically the best choice for all applications. Thus, it is not surprising that in Perlas’ study 4the 12-MHz probe offered higher resolution compared to lower-frequency transducers in superficial plexus locations but failed to identify the nerves in 73% of the cases in the infraclavicular region, where they can easily be identified with 5–10 MHz in 3 cm distance. 8Different frequency probes will probably be needed for different purposes. For example, we use 15-MHz probes in experimental laboratory settings 9but 4- to 5-MHz probes for an ultrasound-guided posterior lumbar plexus block with a target deeper than 5 cm. 10To visualize the brachial plexus in 1 cm depth at the interscalene, supraclavicular, or axillary level, the 12-MHz transducer is obviously an excellent choice. 4,11,12However, we reckon that inserting the needle on the outer end of the transducer and advancing it in a lateral to medial direction, as proposed here, contrary to the common practice of inserting it close to the middle of the transducer and guiding it perpendicularly to the ultrasound beam, makes more sense in deep blocks and is questionable for 1-cm deep targets. This may displace the insertion point unnecessarily far from the nerve and thus increase the length of tissue penetration.In performing ultrasonography-guided blocks, observing a homogeneous and complete local anesthetic spread around the nerve is the most reliable predictor of block success. This is not necessarily achieved with the needle tip in the closest proximity to the nerve. This signifies a fundamental difference from the blind, one-dimensional nerve-stimulator technique in which injections are typically performed after the needle is placed as close to the nerve as possible (as defined by a low stimulating current). Interestingly, in the present study, there was no clear correlation between ultrasonographically verified needle-to-nerve contact and nerve stimulated muscle contraction with currents of up to 1.5 mA. We may speculate about the reasons, 13but, above all, this underlines the demand for precisely defined and calibrated nerve stimulators, at least in experimental settings. This is important, because we know, for example, that only with an impulse duration of 0.1 ms will the current correlate with the distance to the nerve. Nevertheless, commercially available devices have recently been identified as being highly variable in their accuracy of current output and preset electrical characteristics. 14Ultrasound has proved helpful for regional anesthesia in two ways: First, it allows the systematic, noninvasive, in vivo assessment of topographic sonoanatomy and its variations. 11Performing careful ultrasound measurements enhances our anatomic understanding, tests the accuracy of common block techniques, and can even result in suggestions to modify them, 8as likewise demonstrated with magnetic resonance imaging. 15We still have much to learn from in vivo ultrasound, inasmuch as anatomy textbooks rely mainly on cadaver dissections. Second, and probably most important, ultrasound helps to individually guide the needle in real time. Advantages of ultrasound-guidance include the direct visualization of the nerves, the entire needle, or the needle tip; identification of adjacent structures to avoid; and, finally, monitoring of local anesthetic spread.In conclusion, the use of ultrasound-guidance in regional anesthesia and interventional pain management is growing. The technique provides information about upper and lower extremity block anatomy, 12facilitates neuraxial methods, 16and is being used to guide the performance of a wide range of other blocks, including blocks of the stellate ganglion, 17coeliac plexus, 18or lumbar facet nerves. 9Currently, many centers have performed thousands of ultrasound-guided procedures (e.g. , n > 2,000, Vienna Study Group), and detailed descriptions of the methods are entering textbooks. 19Ultrasound imaging brings light into regional anesthesia, which has hitherto been the domain of extraordinarily experienced “needle-magicians.” The statement that nobody has an eye on the needle now can be revised: We have a dispassionate eye above the needle to control what we are doing and to give nerve blocks an objective basis. Ultrasound guidance offers anesthetists a unique chance to improve block success and decrease the rate of complications, even if they are less experienced in regional techniques. Devices have become user-friendly, more affordable, and can be shared with other specialties. Studies like the one in this issue of Anesthesiology 4contribute largely to making regional anesthesia more of a science rather than an art. However, despite all the proven advantages of ultrasound-guidance, this method is incredibly underused in daily practice. Consequently, after achieving today's high transmitter frequencies, our next step should be to markedly increase application frequency.