Cutaneous sensory threshold stimulation with high frequency square-wave current. I. The relationship of electrode dimensions to the sensory threshold.

Numerous methods of studying the sensibility of the skin have been reported. Martin (1) described the threshold of faradic break shocks; Ollendorf (2) used hair strokes, pin pricks, heat and pressure with a probe in normal and abnormal skin to determine differences in sensation; Mayr (3) utilized the pressure of a water manometer on normal and syphilitic skin; Hardy and Wolff (4) introduced the thermal radiation method in measurements of pain thresholds; Bishop (5) studied the effects of single and repetitive electrical stimuli on sensory units; Hauk and Neuert (6) employed a pulsating direct current with frequencies up to 300 cycles per second produced by the make-and-break of an inductorium to study pain thresholds. In a recently reported preliminary study (7) the use of high frequency square-wave current for threshold stimulation of the skin was described. The same vacuum tube apparatus was used in these electrode experiments. A description of the circuit and apparatus can be found in the previous report. The apparatus delivers unidirectional pulsating square-wave current with frequency up to 25,000 cycles per second (c.p.s.) and peak voltage up to 250v. The square-wave current is characterized by a flow of direct current between the make and the break followed by a period of inactivity between the break and the next make, after which the cycle is repeated. Frequency is then defined as the number of cycles per second and current interval is the duration of the current measured in fractions of a cycle. Frequency, current interval and voltage are the physical variables to be controlled or measured when using square-wave current. All three variables can be read directly from an oscilloscope, but such apparatus being bulky and expensive, its use was circumvented by installing into the circuit a vacuum tube voltmeter (voltage indicator) and a counting ratemeter (frequency indicator). The following clinical investigation was undertaken in order to continue the study of the cutaneous sensory threshold with high frequency square-wave current. It was specifically desired to investigate, subjectively and objectively, the sensory responses to threshold stimulation using five electrodes of different dimensions, and to determine the dimensions of choice for future experiments. More than 100 patients attending the Grace-New Haven Community Hospital skin clinic were tested for subjective reactions with the use of a point electrode and four other electrodes measuring inch square, 1 inch square, 2 inch square and 4 inch square. The frequencies obtained with the four flat electrodes on 13 unselected cases were recorded (Table 1) for objective analysis.