Improved Estimation of Sweating Based on Electrical Properties of Skin
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mproved Estimation of Sweating Based on Electrical Properties of Skin CHRISTIAN TRONSTAD,1 HA˚VARD KALVØY,1 SVERRE GRIMNES,1,2 and ØRJAN G. MARTINSEN2,1 1
Department of Clinical and Biomedical Engineering, Rikshospitalet, Oslo University Hospital, Oslo, Norway; and 2Department of Physics, University of Oslo, Oslo, Norway (Received 14 June 2012; accepted 5 January 2013; published online 17 January 2013) Associate Editor Catherine Disselhorst-Klug oversaw the review of this article.
This provides a very sensitive measurement of sweat gland activity by means of the skin conductance (SC), and is widely used as a physiological parameter within many biomedical applications.3 Sweating also causes changes in the bioelectric potential (SP) at the skin surface, but the characteristics of this parameter are different from SC. While the SC always increases with the sweat duct filling and recovers while sweat is reabsorbed and/or pores are closed, the SP can change in both positive and negative voltage directions and produce biphasic or triphasic responses.3 A third electrical parameter to consider is the skin capacitance, which is positively correlated with the moisture content of the stratum corneum,19 and is most accurately measured by low-frequency skin susceptance (SS).18 Although sweating, here defined as the amount of water loss from the skin by evaporation, and changes in the skin electrical properties both originate from the sweat gland, they are governed by completely different biophysical mechanisms. Sweating includes transport of sweat to the skin surface, evaporation and further transport of water molecules to the air. Some of the sweat is also absorbed in the skin and later evaporates at a decreasing rate, making the evaporation also dependent on the skin sorption characteristics. SC also increases as sweat fills the pores, but this is not necessarily dependent on sweat reaching the skin surface.10 After sweat pore filling, the SC decreases as electrolytes are reabsorbed through the sweat duct wall. The same mechanisms govern the SP response although the waveform is more complex.10 Despite these biophysical differences, mainly the movement of water vs. the movement of electrolytes, high withinsubject correlations have been reported between SC and skin water loss (up to r = 0.88,21 r > 0.859) and between the positive SP response and water loss (r = 0.9312). Between-subjects correlations on the
Abstract—Skin conductance (SC) has previously been reported to correlate strongly with sweat rate (Swr) within subjects, but weakly between subjects. Using a new solution for simultaneous recording of SC, skin susceptance (SS) and skin potential (SP) at the same skin site, the aim of this study was to assess how accurately sweat production can be estimated based on combining these electrical properties of skin. In 40 subjects, SC, SS, SP and Swr by skin water loss was measured during relaxation and mental stress. SC and Swr had high intraindividual correlations (median r = 0.77). Stepwise multilinear regression with bootstrap va
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