REVIEW ARTICLE Peripheral Nerve Stimulation in Regional Anesthesia Stephen M. Klein, MD,* M. Steve Melton, MD,* Warren M. Grill, PhD,Þ and Karen C. Nielsen, MD* the patient; (2) improved patient comfort, because a paresthesia Abstract: Peripheral nerve stimulation has a long history in regional technique can be painful or unpleasant compared with nerve anesthesia. Despite the advent of ultrasound-guided peripheral nerve stimulation; and (3) a perceived improvement in accuracy.4Y6 blockade, nerve stimulation remains a popular technique used alone or, This transition in clinical practice preceded a great deal of the now, in combination with ultrasound-guided techniques. In light of this literature that describes the principles of electrical nerve locali- evolving utility of nerve stimulation, this is an appropriate time to review zation. Today, a similar, more rapid evolution in technology and the basic concepts and knowledge base of this historically important tool. practice is occurring between nerve stimulation and ultrasound- Electrical nerve stimulation facilitates nerve localization, using threshold guided regional anesthesia. However, despite the popularity current as a surrogate for needle-to-nerve distance. Preferential activation of ultrasound-guided peripheral nerve blocks, nerve stimulation of motor nerves is possible because motor nerve fibers are more readily remains a common technique used alone or in combination activated with a shorter duration of current compared with sensory with ultrasound guidance. The developing utility of combining nerves. The association between current and needle-to-nerve distance these 2 techniques signals an opportunity to review the history predicts that less current is needed to evoke a motor response as the and present the status of nerve stimulation. This review gives needle moves closer to the nerve. Thus, an elicited motor response at an account of events that led to the development and imple- or below 0.5 mA is considered a common end point for successful neural mentation of peripheral nerve stimulation and summarizes the blockade. However, current magnitude is neither 100% sensitive nor fundamental principles of nerve stimulation as well as the neu- specific. Independent of technical ability, both the biological environ- rostimulation literature that applies to regional anesthesia, in- ment and the equipment used impact the current-distance relationship. cluding the controversies therein. Thus, successful electrical nerve stimulation is dependent on an anes- thesiologist with a solid foundation in anatomy and a thorough under- standing of electrophysiology. METHODS (Reg Anesth Pain Med 2012;37: 383Y392) To conduct the literature search for this review, we used the PubMed database queried before December 2010. Be- cause words such as nerve and stimulation produce vast lists of references, we used single terms of interest limited to the title lectrical nerve stimulation is used extensively to facilitate and field as keywords to achieve search results containing fewer Eenhance peripheral nerve block performance. Studies using than 400 articles per search, which was deemed manageable for nerve stimulatorYbased techniques report a high degree of effi- selective individual title, abstract, and/or article review for rel- cacy and reproducibility, making it one of the most prevalent evance. Successful terms using this strategy included nerve 1Y3 regional anesthesia techniques. By providing an objective stimulator (327 results) and nerve localization (25 results). estimate of needle-to-nerve distance, nerve stimulation enables Unsuccessful terms limited to the title field were adjusted and practitioners to deposit local anesthetic with a high degree of combined using the ‘‘AND’’ function. One successful combi- precision. The technique capitalizes on physiology that allows nation was stimulation (title) AND block (title) (352 results). electrical current passing from the needle tip to depolarize a Lastly, we used the ‘‘AND’’ function to search a combination mixed nerve without causing a painful sensory response. As the of terms open to all fields. These terms included stimulation needle is guided closer to the nerve, less current is required to AND block AND nerve stimulator (299 results). In addition, elicit a motor response of equal magnitude. This relationship we reviewed reference lists from the PubMed articles and serves to approximate needle-to-nerve distance. Whereas nerve found additional articles of interest that were not captured in stimulation is straightforward in concept and widely applied, the our original search. Such articles of interest provided addi- fundamental principles surrounding this technique are more tional information about historical milestones and physiologic complex. A host of confounding physical and electrical inter- concepts included in this review. Lastly, we included rele- actions influences nerve stimulation. vant references that were familiar to us but were not otherwise When introduced into clinical practice, nerve stimulation captured. An English language restriction was applied to all was quickly adopted to replace paresthesia techniques because of searches. Commentaries, letters to the editor, reviews, and nonY numerous benefits including (1) the ability to perform regional peer-reviewed articles were excluded. anesthesia with visual feedback rather than verbal feedback from HISTORY From the Departments of *Anesthesiology and †Biomedical Engineering, The first concept of nerve conduction can be traced back to Duke University Medical Center, Durham, NC. 260 BC and the hollow nerve doctrine, which described the nerve Accepted for publication March 20, 2012. as a cavity through which messages travel.7 Over time, tre- Address correspondence to: M. Steve Melton, MD, Department of mendous philosophical speculation was devoted to the mecha- Anesthesiology, Box 3094, Duke University Medical Center, Durham, NC 27710 (e-mail: [email protected]). nism responsible for the transmission of messages through the The authors declare no conflict of interest. nerve. In the 18th century, when electricity was accepted as a Departmental financial support was received for this study. biological property, electricity became the most tangible means * Copyright 2012 by American Society of Regional Anesthesia and Pain to explain neuromuscular conduction. In 1791, Galvani8 pub- Medicine ISSN: 1098-7339 lished his theory of ‘‘animal electricity,’’ which contended that DOI: 10.1097/AAP.0b013e3182576647 muscle contraction resulted from latent electricity within the Regional Anesthesia and Pain Medicine & Volume 37, Number 4, July-August 2012 383 Copyright © 2012 American Society of Regional Anesthesia and Pain Medicine. Unauthorized reproduction of this article is prohibited. Klein et al Regional Anesthesia and Pain Medicine & Volume 37, Number 4, July-August 2012 nerve. In 1850, however, Helmholtz demonstrated the temporal In 1973, Montgomery et al4 described a nerve stimula- nature of nerve conduction, distinguishing it from electric cur- tion technique using standard unsheathed needles, commonly rent.9 This was the beginning of our present notion of nerve used for paresthesia-guided techniques, and a readily available function, paving the way for the elucidation of peripheral nerve battery-powered transistor-based peripheral nerve stimulator. stimulation physiology.10 Although the success rate was not reported, this technique was In 1912, von Perthes11 was the first to describe the use of described as safe and successful over an 18-month period and nerve stimulation. Likely because of the complexity of the involved approximately 1000 peripheral nerve blocks, including equipment at the time, this ‘‘impractical’’ technique was not interscalene, supraclavicular, infraclavicular, axillary, sciatic, pop- adopted. In 1955, more than 40 years later, Pearson12 published a liteal, femoral, obturator, median, ulnar, and radial nerve blocks. modern description of a neurostimulator-guided peripheral nerve In a separate study, this group reported a 95% success rate in block, and in 1962, Greenblatt and Denson13 described a system more than 200 cases of electrical stimulation-guided infracla- similar to the system used today. They constructed a small vicular blocks.5 The reasons for failure included inexperience, portable transistor-based device similar in design and appearance a change in needle position after successful nerve localization to modern nerve stimulators. This device delivered a square- before injection of local anesthetic, and technical difficulties such wave impulse of 0.1 millisecond in duration at 1-Hz frequency, as inadequate battery power or bad wiring of the stimulator.5 with an adjustable output of 0.3 to 30 V. The block technique These early reports4Y6 facilitated the gradual transition from described used a needle, insulated with plastic paint except at the mechanical paresthesia to electrical stimulation for nerve lo- tip, as a stimulation probe. Local anesthetic was injected when an calization. Stated benefits of electrical stimulation included appropriate motor response was achieved with 2- to 5-V stim- perceived improvement in accuracy and visual feedback instead ulation amplitude. Using this technique, 87 blocks were per- of relying on verbal input from the patient. Despite near universal formed, including sciatic, femoral, axillary, obturator, median, adoption, conclusive evidence for the efficacy and outcome radial, ulnar, intercostal, and paravertebral