Nerve stimulators used for peripheral nerve blocks vary in their electrical characteristics

Background: Nerve stimulation with a low-intensity electrical current has become a vital part of the performance of peripheral nerve blockade. The purpose of this study was to compare the accuracy and characteristics of peripheral nerve stimulators used in clinical practice in the United States. Methods: Fifteen peripheral nerve stimulators were fitted with fresh batteries and set to deliver currents ranging from 0.1 to 4.0 mA into a series of high-tolerance resistance loads ranging from 1 to 100 kOmega. The current output, stimulus duration, morphology, frequency, and maximum voltage output were studied using a factory-calibrated oscilloscope. Results: All peripheral nerve stimulators performed uniformly well when set to deliver currents of 1.0 MA or more into a standard resistance load of 1 or 2 kOmega. However, at lower currents, the median error (%) increased from 2.4 (-5-144%) at 0.5 mA to 10.4 (-24-180%) at 0.1 mA into a I kOmega load. The morphology of the stimulus Was characterized by a regular monophasic square pulse at current outputs of up to 1 mA and at a resistance of 1 kOmega. The stimulus waveform became particularly distorted as the impedance load was increased. The duration of the default stimulus set by the manufacturer varied from 34.8 to 460 mus among the peripheral nerve stimulators tested. The maximum voltage output ranged from 7.4 to 336 volts. Conclusions: Nerve stimulators used for regional anesthesia vary greatly in accuracy of current output and in manufacturer-selected electrical characteristics (e.g., current duration, stimulating frequency, maximum voltage output).