Decreases in Sympathetic Activity Due to Low-Intensity Extremely Low-Frequency Electric Field Treatment Revealed by Measurement of Spontaneous Fluctuations in Skin Conductance in Healthy Subjects

(1) Background: Our previous studies indicated that low-intensity extremely low-frequency electric field (ELF-EF) treatment alters autonomic activities, as revealed through heart rate variability (HRV) analysis. However, the high-frequency (HF) component of HRV that reflects parasympathetic activity showed no changes either during or after the end of the treatment, suggesting the involvement of sympathetic nerves. (2) Methods: To examine this issue in the present study, the effect of ELF-EF on skin conductance (SC), which is controlled solely by sympathetic nerves, was analyzed. Twelve healthy subjects underwent a 20 min ELF-EF treatment (applied voltage: 9 kV, induced current density: below 6 mA/m2) and a sham treatment in a random order with an interval of more than 2 weeks. SC and HRV were recorded under the eyes-open condition during a 2 min period both before and after the treatment. (3) Results: The number of spontaneous fluctuations in skin conductance (SC-SFs) significantly decreased after the ELF-EF treatment, suggesting psychological changes, including relaxation. The skin conductance level, heart rate, and HRV indices did not change after the ELF-EF treatment. (4) Conclusion: The results support the idea that low-intensity ELF-EF affects autonomic nerves by reducing sympathetic activity, as reflected by SC-SFs.