Quantitative assessment of sudomotor activity by capacitance hygrometry

A commercial capacitive hygrometer device manufactured for use in technical or chemical laboratory environments has been used for quantitative and dynamic assessment of sweat gland activity in selected skin areas. For this purpose the hygrometer device was supplemented by a chamber attached to the skin for collecting evaporated water and a supply of dry nitrogen gas providing a gas flow through the chamber and through the hygrometer capsule. The accuracy of the technique was proven in pilot experiments in which fixed amounts of water were evaporated. A positive correlation was found between the weight of the water and the water evaporation computed from the hygrometer readings (r = 0.997). The time constant of the device was in the range of 10 s. This time constant appears to be fast enough for recording physiological changes in the sweating rate of human subjects. In experiments on healthy subjects sudomotor reflexes were assessed and compared to vasoconstrictor responses and to thermographically measured temperature changes of the skin during the Valsalva manoeuvre and a painful mechanical stimulus. Direct measurement of water evaporation such as by this technique may provide information on sympathetic reactions which could be utilized in both physiological and pathophysiological states.