Methodological considerations in calculating heart rate variability based on wearable device heart rate samples

Heart rate variability (HRV) analysis has recently been incorporated into wearable device application. The data source of HRV is the time series of heart beat intervals extracted from electrocardiogram or photoplethysmogram. These intervals are non-uniformly sampled signals and not suitable for spectral HRV analysis, which usually uses uniformly resampled heart beat intervals before calculating the spectral domain parameters. Such a practice is not applicable to heart rate data obtained from wearable devices that usually display and export the beat per minute (BPM) time series data at 1 Hz. The preferred resampling rate to calculate spectral domain parameters is 4 Hz. We compare the spectral HRV results with the 1 Hz and 4 Hz resampling rates in order to validate the use of 1 Hz resampled-RRI data to represent wearable devices BPM time series data for HRV analysis. Our results show that, using a specific combination of signal processing techniques, the lowest mean relative error in spectral domain parameters of normalized low-frequency power (LFnu), normalized high-frequency power (HFnu) and the ratio of normalized low-frequency power to normalized high-frequency power (LFnu/HFnu) between 1 Hz and 4 Hz are 3.7%, 15.3% and 16.4%, respectively. We conclude that using the heart rate data sampled at 1 Hz produces a reasonable estimation of sympathetic activity but a poor estimation of parasympathetic activity.