Radial Pulse and Electrocardiography Modulation by Mild Thermal Stresses Applied to Feet: An Exploratory Study with Randomized, Crossover Design

Objective To investigate the changes in radial pulse induced by thermal stresses (TSs). Methods Sixty subjects were enrolled. Using an open-label, 2×2 crossover randomization design, both feet of each subject were immersed in 15 °C water for cold stress (CS) and in 40 °C water for heat stress (HS) for 5 min each. Radial pulse, respiration and electrocardiogram (ECG) signals were recorded before, during and immediately after the TSs. Results The analysis of heart rate variability revealed that CS increased the low-frequency (LF) and high-frequency (HF) components ( P <0.05) and that HS reduced the LF and HF components ( P <0.01). Both TSs reduced the normalized LF, increased the normalized HF, and reduced the LF/HF ratio. The differences in the ECG signals were more dominant during the TS sessions, but those in the radial pulse signals became more dominant immediately after the TS sessions. CS decreased the pulse depth ( P <0.01) and increased the radial augmentation index ( P <0.1), and HS increased the pulse pressure ( P <0.1) and subendocardial viability ratio ( P <0.01). There were no significant differences in pulse rate during the three time sequences of each TS. The respiration rate was increased ( P <0.1), and the pulse rate per respiration (P/R ratio) was significantly decreased ( P <0.05) with CS. The HF region (10–30 Hz) of the pulse spectral density was suppressed during both TSs. Conclusions CS induced vasoconstriction and sympathetic reactions, and HS induced vasodilation and parasympathetic reactions. Based on definitions used in pulse diagnosis, we made the novel discoveries that the pulse became slower (decreased P/R ratio), more floating and tenser under CS and that the HF region of the spectral power decreased significantly under both TSs.