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From the Department of Anesthesiology, Mayo Clinic College of Medicine, Rochester, Minnesota.
Address correspondence and reprint requests to Terese T. Horlocker, MD, Department of Anesthesiology, Mayo Clinic, Rochester, MN, 55905. Address e-mail to horlocker.terese{at}mayo.edu.
In Christian legend, the Holy Grail was the cup used by Jesus at the Last Supper. It was said to possess miraculous powers, and in Medieval times its acquisition became the quest of every worthy knight, requiring dedication, single-mindedness, and vision. Not surprisingly, with such lofty prerequisites, the prize remained unclaimed. Today, the concept of the Holy Grail has been applied to all-but-unobtainable ultimate quests or goals of a person, organization, or field of study. For instance, a cure for cancer or a vaccine for the common cold may be characterized as the "holy grail" of medicine. In regional anesthesia, the "holy grail" might be providing neural blockade with rapid onset, long duration, certain success, and without complications. Lofty goals indeed! In recent years, advocates heralded ultrasound technology as the method to achieve these objectives. This issue of Anesthesia & Analgesia includes investigations evaluating the efficacy of ultrasound-guided regional anesthesia during peripheral techniques of the upper (1) and lower (2) extremities, the potential safety in "special" (anesthetized and anticoagulated) patient populations (3), and the sequelae of intentional intraneuronal injection (4). Finally, the current state-of-the art as well as future directions are explored in a Special Article by Marhofer and Chan (5). These well-conducted clinical and laboratory studies add to the rapidly expanding body of literature involving ultrasound-guided regional anesthesia.
Ultrasound-guided blockade is a popular topic at conferences, workshops, and exhibits. Even an entrenched Luddite would be seduced by the appeal of direct visualization of the nerves and adjacent structures during block placement. The technology is rapidly improving, resulting in improved portability and image resolution with an increasingly favorable size-to-cost ratio. Many anesthesiologists already have experience with intraoperative transesophageal echocardiography and ultrasound-guided line placement; the application of this increasingly available technology to neural blockade is a small lateral step. Experts in the technique extol its safety, educational benefits, clinical efficacy, and broad applicability. Increasingly, these experts, while largely self-taught, are recommending certification for practitioners who wish to add this skill to their nerve block armamentarium. Although data regarding the benefits of ultrasound-guided nerve blocks are sparse and inconclusive, many clinicians, both academic and private, have already embraced this technology in their practice.
Comparison to similar intraoperative technological advances may be helpful in understanding the debate surrounding this topic. Echocardiography, particularly transesophageal echocardiography, has become an important part of the cardiac anesthesia training program. This is a sophisticated diagnostic tool, which results in critical decisions affecting clinical management of complex patients. The cardiac anesthesiologists represented by their subspecialty society, recognizing the importance of maintaining high clinical standards for this procedure, have collaborated with their cardiology colleagues to provide training and credentialing via a standardized examination process for individuals who wish to perform this procedure. Most hospitals now require some evidence of training in this technique by anesthesiologists and cardiologists who will perform and bill for echocardiography. Note three important elements in the example of echocardiography that have driven the credentialing process: 1) A diagnostic procedure affecting patient outcome; 2) Billing for a specific procedure; 3) Competition for a procedure from another group of physicians (cardiologists in this case).
Another example affecting the operating room and intensive care unit is the use of ultrasound for placement of vascular lines. Heated debate has surrounded the recommendations from the National Institute for Clinical Excellence (6) that ultrasound guidance be used for all central venous catheterizations. A review of the literature does little to settle the question. Most studies show little or no difference in complication rates between ultrasound-guided techniques and landmark-guided "blind" techniques, particularly with experienced anesthesiologists (7–9). The advantages are primarily associated with real-time ultrasound guidance and novice learners (10). Arguments for the use of ultrasound center around potential reduction of the rare but serious major complications associated with this procedure, as well as increased patient comfort due to decreased needle punctures (11). Opponents to mandating its use cite the added cost, possible increased risk of infection, lengthened procedure time, and the lack of convincing data confirming risk reduction. Further technological advances, such as biplanar ultrasound, which would provide a long axis view of the needle and possibly help control needle depth placement, may enhance the usefulness of this technique. A final concern, relevant to ultrasound guidance in regional anesthesia as well, is the possibility that learners who use ultrasound-guided techniques may not be competent in performing the technique in a setting where ultrasound is not available. Hence, at least for the present, both ultrasound-guided and more traditional techniques should continue to be taught. Perhaps one of the unanticipated benefits of ultrasound-guided regional anesthesia is the training in needle advancement and redirection allowed by using a simulated patient or "phantom," (12) which may potentially facilitate mastery of regional techniques.
An examination of the literature involving ultrasound-guided regional anesthesia provides numerous examples of clinical applications for this technique, but little data supporting its superiority over standard approaches. Similar enthusiasm (and skepticism) were demonstrated over three decades ago when electrical stimulation with unsheathed needles was introduced as an alternative method of identifying neural structures (13). Over time, the technology improved, including the development of insulated needles, stimulating catheters, and reliable nerve stimulators. Neural stimulation became the routine method for performing peripheral techniques, particularly lower extremity blockade. Despite these advancements and widespread application, definitive data supporting improved safety or efficacy compared to paresthesia and transarterial techniques are still lacking (14–16).
In recent years, the number of prospective randomized studies comparing ultrasound-guided and electrical stimulation techniques has steadily increased. In general, onset times and number of needle passes required to obtain a motor response are reduced (1,2,5,17). Ultrasound guidance increases the success rate, provided 1) the needle is redirected (as necessary) to achieve uniform spread around the neural structure and 2) blocks in the control group are performed using a single injection (1). The ultrasound-guided redirection, with injection at multiple sites, makes this a somewhat unfair comparison; multiple injection techniques have consistently been associated with a higher success rate than single injection techniques (14,15). However, the ultrasound images of the U- or doughnut-shaped local anesthetic distribution have provided clinicians with a visualization of successful block (5,17,18) and documented why single injection techniques are unreliable. These observations are fundamental to maximizing the regional anesthetic success rate, regardless of technique of neural localization utilized.
It has been theorized that visualization of needle advancement would decrease the frequency (or severity) of needle misadventures. However, preliminary results do not support the hypothesis that ultrasound guidance increases regional anesthesia safety. For example, the small patient numbers included precludes determination of either the absolute or relative frequency of neurologic complications. Both intraneural needle placement and injection have been reported during ultrasound (19,20). Amazingly, histologic dysplasia (4) and long-term sequelae are not certain (20)! Again, while ultrasound may be a useful tool to detect (and not necessarily prevent) intraneural injection, its greater benefit may be to characterize the pathophysiology and evolution of nerve injury associated with regional anesthesia.
Although vessels are the easiest anatomic structures to identify with ultrasound, vascular punctures are not uncommon (17). The frequency of vascular puncture may be reduced using ultrasound guidance, but complete avoidance cannot be assumed. Similarly, unless color Doppler is used, ultrasound visualization does not prohibit a partial intravascular injection (although needle tip placement entirely within the vessel would presumably be detected by lack of neural sheath expansion). Thus, ultrasound guidance is not a substitute for adequate hemostasis or the patient's ability to describe paresthesias or intravascular injection.
Undoubtedly, ultrasound has illuminated our knowledge of regional anesthesia and increased our understanding of block success and neurologic complications. Additional studies are required to identify its role in teaching and certification. However, even with ultrasound guidance, there will be vascular puncture and other needle-related trauma, intravascular and intraneuronal injections, and failed blocks. Ultrasound-guided regional anesthesia is not a metaphysical experience, it is physics expertly applied to the art of neural blockade. As the physics and our expertise improve, so may our outcomes. In his classic text, Regional Anesthesia: Its Technic and Clinical Application, Labat (21) concluded, "Regional anesthesia is an art. Remembering that even experts may fail, we should try often and again, observing scrupulously its principles, until we succeed." The "holy grail" of regional anesthesia remains elusive—but then that's the challenge.
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