The first description of electrical stimulation to locate the brachial plexus was recorded by Perthes in 1912. However, the acceptance of this method to aid in performance of peripheral nerve blocks was not realized until the 1960s when electronic advances and the consequent introduction of more convenient solid-state units were made. Greenblatt and Denson demonstrated that motor nerves can be stimulated without eliciting pain and that the current required to stimulate the nerve depends on the distance between the needle and the target nerve. In the last two decades, peripheral nerve stimulation techniques have largely replaced paresthesia techniques for most major conduction blocks, particularly in lower extremity blockade. This approach is well accepted and is associated with favorable success rates. It is important to realize that nerve stimulators are not used as a replacement for the sound knowledge of anatomy, but to help to position the needle in closer proximity to the nerve without a required contact with the nerve (paresthesia) and with less discomfort to the patient.

In order to propagate a nerve impulse, a certain threshold stimulus must be applied to the nerve. The ability to stimulate a nerve depends on the intensity of the current applied and the duration of the current. In mixed nerves it is possible to stimulate the motor component without eliciting pain by limiting the current intensity and duration. To stimulate motor fibers, a current of shorter duration (0.05 to 0.2 ms) is typically used. The use of a shorter pulse duration increases the likelihood of an increased proximity between the nerve fibers and the unshielded tip of the needle, but makes the localization of the nerve more challenging. Consequently, the nerve stimulator is usually set up with a current of 1 to 1.5 mA and a pulse duration of 0.1 to 0.3 ms. The intensity of the current is decreased along with the pulse duration to adjust the position of the needle. In contrast, the stimulation of sensory fibers requires longer pulse duration time (0.3 to 1.0 ms) than do motor fibers (0.05 to 0.1 ms). With such a setup it is possible to locate sensory nerves such as the radial nerve at the wrist, the lateral femoral cutaneous nerve, and the saphenous nerve by eliciting electrical paresthesia. The use of longer pulse duration is appropriate in patients with peripheral neuropathy, including diabetic patients. In these patients it is often necessary to
stimulate with a pulse duration time of 0.3 to 1.0 ms to elicit a motor response. The access to multiple pulse duration times represents a major improvement of the nerve stimulator presently available. An important principle of peripheral nerve stimulation is the preferential “cathodal stimulation.” In other words, when the nerve is stimulated by an electrode, significantly less current is required to obtain a response to a nerve stimulation when the cathode (negative) rather than the anode (positive) is adjacent to the nerve. This principle has significant clinical applications, and it requires that clinicians pay particular attention to the polarity of the electrodes. Another fundamental principle is that the current intensity required to stimulate the nerve is in relationship to the distance of the needle from the nerve. As the stimulating tip moves away from the nerve, the relationship between the current and the distance from the nerve is governed by Coulombs law:

E = K(Q/r2),

where E is the current required to stimulate and r is the needle–nerve distance. This principle is used to estimate needle–nerve distance by employing a stimulus of known intensity and pulse duration. It should be noted that this relationship is not linear, which means that as the needle–nerve distance increases, a current of substantially greater intensity is required to stimulate the nerve.

It is important to realize that the nerve stimulator-assisted nerve block techniques assume that nerve stimulators are accurate and user-friendly to maneuver during block performance. Skin resistance, electrode surface resistance, and gel conductivity can vary widely. For these reasons, nerve stimulators for use in regional anesthesia should be specifically engineered for that application, rather than be an “all-purpose” unit with neuromuscular block monitoring capabilities. A plethora of features present on some units does little to facilitate their use and adds to the complexity of their operation. Some of the desirable characteristics of nerve stimulators for regional blocks are outlined here (Fig. 2-1).