Cold water stimulation and its effect on heart rate variability: A short time fourier transform analysis

Abstract

This study utilizes cold water consumption as a method of vagus nerve stimulation, investigating how body mass index (BMI) and repeated stimulation affect heart rate variability (HRV) in both time and frequency domains. HRV was measured in 18 subjects (mean age 21.3 ± 1.6 years) over 90 minutes, divided into three equal phases: resting (baseline), first stimulation (FSP), and second stimulation (SSP). Fourier and Short-Time Fourier Transform (STFT) analyses were used to extract the static and time-varying frequency components of HRV. For the normal body mass index (BMI) group, stimulation resulted in statistically significant changes in frequency band power densities compared to their baseline measurements (p<0.05), suggesting that drinking cold water may activate vagal pathways and alter HRV components. For underweight subjects, the very low frequency (VLF) band power density of HRV decreased with stimulation (3.0%) and further depressed with repeated stimulation (6.5%). Low frequency (LF) power density stayed relatively constant with the first stimulation but still increased by 4.1% by shifting of VLF power into this band after the second stimulation, and finally, high frequency (HF) band power density increased after each stimulation up to 2.9%, suggesting that cold water intake shifted the HRV frequency spectrum to higher frequencies in all bands. For normal-weight subjects, after the first stimulation, the LF band power was shifted into both VLF and HF bands. With repetition, the shift was only towards the high frequencies, suggesting that repetition of cold-water intake affects the HRV frequency distribution, and only repetition shifts power to higher frequencies. For obese subjects, the power density shift is towards the low frequencies (5.0% in VLF), and repeated stimulation could only partially recover this shift (4.3%). In all BMI groups, the LF/HF ratio was decreased and was more prominent after the first stimulation. The maximum LF/HF reduction was observed in obese individuals (7.1%), followed by the subsequent highest decline in the normal BMI group (5.2%). This study also highlighted STFT as an effective tool for analyzing temporal changes in HRV, displaying distinct spectral power distribution patterns for each BMI group. Underweight and normal-weight subjects demonstrated an initial increase in HF power density, whereas overweight subjects exhibited a muted response.

Keywords

• BMI
• heart rate variability
• short time fourier transformation
• spectral analysis of heart rate variability
• vagus nerve stimulation
• wavelet spectrogram

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