Effect of Absorbent Patch Saturation Level on Local Sweating Rate and Sweat [Na+] during Exercise

Absorbent patches are often used to determine local sweat sodium concentration ([Na+]); however, the effect of methodological variations, such as the level of patch saturation (absorbed sweat mass), on sweat [Na+] has not been studied. PURPOSE: To determine the effect of low (L), moderate (M), and high (H) patch saturation on local sweat [Na+] during exercise. METHODS: 20 male athletes (24 ± 10 y, 79 ± 16 kg) completed 5 X 30-min sets of intermittent, moderate intensity exercise (76% HRmax) in a temperate room (23°C, 62% rh). After 30 min exercise, 3 sterile absorbent patches (14.5 cm2 absorbent pad, 42 cm2 total patch including tegaderm; 3M) were placed on the right dorsal forearm (L, M, and H patches placed in randomized order from proximal to distal). Patches were removed after reaching the appropriate level of saturation as determined by visual inspection. Absorbed sweat mass and local sweating rate were determined gravimetrically. The L, M, and H patches corresponded with 0.23 ± 0.07 g, 0.47 ± 0.08 g, and 0.92 ± 0.08 g of absorbed sweat (p < 0.001) and 15.7 ± 6.4 min, 32.2 ± 11.8 min, and 88.8 ± 16.7 min skin adherence time, respectively (p < 0.001). Local sweat [Na+] was measured using ion chromatography. Repeated measures ANOVA with Bonferroni adjustment for multiple comparisons was used to determine the impact of patch saturation level on local sweating rate and local sweat [Na+]. Data are expressed as mean ± SD. RESULTS: Sweat [Na+] was significantly different among all patch saturation levels (L: 71.6 ± 31.1 mmol/L, M: 53.9 ± 26.9 mmol/L, and H: 44.0 ± 25.6 mmol/L; p < 0.001). There was also a significant effect of patch saturation level on local sweating rate, with H (0.74 ± 0.17 mg/cm2/min) significantly (p < 0.01) less than both L (1.09 ± 0.47 mg/cm2/min) and M (1.17 ± 0.53 mg/cm2/min). CONCLUSION: An increase in sweat patch saturation level was associated with a decrease in local sweat [Na+], which also corresponded with a decrease in local sweating rate; suggesting a possible hidromeiosis effect with higher absorbed sweat mass and/or adherence time on skin. To develop best practices in sweat testing, future research is needed to determine the interaction effects of absorbed sweat mass and patch adherence time on local sweat [Na+] and to cross-validate results against sweat [Na+] obtained using the whole body wash-down technique.