When placing needles for peripheral nerve blocks, some anesthesiologists rely on patient feedback (the ‘paresthesia technique'). Here at Columbia University Medical Center, we prefer the nerve stimulator technique. While both approaches appear safe and effective, we find that nerve stimulators offer several benefits.
Unlike the paresthesia technique, in which the anesthesiologist must contact the target nerve with the needle tip in order to locate it, nerve stimulators allow us to find the nerve without necessarily making contact. A low electrical current emanating from the tip of the insulated needle stimulates the nerve and causes the target muscle to twitch. In this way, we can isolate the target nerve without the discomfort of paresthesia. Nerve stimulators also allow us to sedate anxious patients without risking block accuracy, because they do not rely on subjective patient feedback. We can also perform blocks on patients who speak foreign languages or have other communication barriers.
Nerve stimulators are not without risk, however. A current that is too high can cause a sudden, massive muscle contraction. This can cause patients to jump and, possibly, lead to nerve impalement. In addition, nerve stimulators don't guarantee against needle-nerve contact or subsequent nerve injury. During nerve stimulation, the anesthesiologist must slowly advance the needle until an appropriate twitch occurs, then reduce the current in a stepwise fashion until theappropriate motor response is elicited at or below 0.5 mA. I usually start seeking the nerve with a current of 1.2 mA, then dial down and inject when I can elicit an appropriate response at or below 0.5 mA.
Stimulator design can also influence block success. Although most stimulators are accurate, there are performance differences between devices.1 The stimulator must provide an accurate reading of electrical output throughout its full current range, because both over- and under-readings can cause problems. If the actual current is lower than the reading, the risk of hitting and damaging the nerve increases. If the reading is artificially high, the injection will not be close enough to the nerve to be effective. There must also be safeguards against operator error. The stimulator we use alerts us whenever we've made an improper connection and/or the circuit is incomplete. It also prevents us from switching the positive (anode) and negative (cathode) electrode connections. Negative-to-needle-to-nerve is the correct route of connection, and this is important because it keeps the current requirements low.
During regional anesthesia, correct needle placement is essential. A misplaced needle can not only fail to anesthetize the target area; it can enter an artery or vein and cause anything from pre-seizure CNS excitation to cardiac arrest. And while practitioner training and skill—not the technique of nerve stimulation or paresthesia per se—is the primary determinant of block accuracy, our routine use of nerve stimulators has helped us achieve a high level of block efficacy and a low complication rate, while ensuring the comfort of every patient we treat.
1. Hadzic A, Vloka J, Hadzic N, et al. Nerve stimulators used for peripheral nerve blocks vary in their electrical characteristics. Anesthesiology. 2003 Apr;98(4):969-74.
Dr. Brown is Clinical Professor of Anesthesiology, Director of Orthopedic & Regional Anesthesia, and Director of Ambulatory Anesthesia Services with New York-Presbyterian Hospital—Columbia University Medical Center, New York, NY.