Diurnal changes in local skin water assessed via tissue dielectric constant at 300 MHz

Abstract

Objective. Skin tissue dielectric constant (TDC) measurements are largely dependent on tissue water content. Because TDC measurements and their inter-side ratios are used for a variety of clinical and research purposes we believe that knowledge of potential TDC diurnal variations could be useful. Because of the strong water-dependency of TDC values, our operative hypothesis was that the absolute TDC measured in upper body areas would progressively decrease during the day whereas TDC values in lower extremities would progressively increase during the day. Our goal was to test this hypothesis and determine the degree of inter-side symmetry. Approach. TDC values to a skin depth of 2.0–2.5 mm were self-measured hourly starting at 0800 and ending at 2000 h at four anatomical sites by 12 women who were trained and experienced in the measurement method. Measurements were done bilaterally in triplicate below the eye, mid-cheek, forearm and calf. An assessment of temporal changes in TDC values and inter-side TDC ratios was done via regression and ANOVA. Main results. TDC values from the eye, cheek and forearm progressively decreased (correlation coefficient, R, 0.708 to 0.941). Morning-to-night decreases were 11.2 ± 8.3%, 6.8 ± 5.7% and 5.6 ± 6.0% respectively. TDC values from the calf progressively increased (R = 0.864) with a morning-to-night TDC increase of 9.3 ± 10.7%. In contrast, TDC inter-side ratios (dominant side to nondominant side) were stable over the 13 h monitoring interval with no evidence of significant temporal change. Morning TDC side-averaged values were for the eye, cheek, forearm and calf values averaged 53.8 ± 8.8, 56.7 ± 9.8, 40.6 ± 6.7 and 41.1 ± 4.9 respectively. Significance. Three major outcomes of the present study emerge: (1) the findings demonstrate a previously unestablished gravity dependent skin-water shift via TDC measurement, (2) an estimate of time-of-day TDC differences is provided, and (3) the insensitivity of inter-side TDC ratios to time-of-day is demonstrated, thereby providing confidence in TDC ratios obtained at different times of day.