[The function of peripheral nerve stimulators in the implementation of nerve and plexus blocks]

A selection of nine instruments supplied by eight different manufacturers for carrying out peripheral nerve stimulation were checked for their suitability, safety and ease of operation, and were compared and contrasted with reference to a spectrum of characteristics that appear desirable in theoretical and practical terms. Measurements at Ohm's resistance showed that in the clinically relevant range of impulse amplitudes (0.1-1.0 mA) the quality of adjustment of the instruments varied widely. The actual electrical impulse delivered by some of the instruments deviated so widely from the adjusted theoretical value that they must be regarded as unsuitable. The duration of the impulse corresponded to the manufacturer's specifications for only two instruments. Four instruments did not generate a monophasic square-wave signal despite assertions to the contrary in the instructions for use, and one instrument did generate such a signal although a "special biphasic asymmetrical" impulse is described by the manufacturer. Impulse-like overshooting at the beginning of the signal, oscillations into the positive range at the end of the signal and fall in current during the course of the signal were the form variants indicating technically inadequate design in the other instruments. Resistance to the square-wave current impulse engendered by the complex body resistance (impedance), a good approximation to the mathematical e-function to be expected, could be demonstrated for the rising signal flank, whereas the signal curve in the lower part of the descending flank was flatter than expected owing to polarization effects in the body tissue. The characteristic voltage and time values calculated for the signal curves are shown in tables.(ABSTRACT TRUNCATED AT 250 WORDS)