Influence of U-shape accelerations of heart rate on very low frequency band and heart rate multifractality

Introduction: Heart rate variability (HRV) fluctuations have a wide frequency band and are a result of multiple contributing factors. In this study, we focused on laminar, U-shaped accelerations of heart rate - approximately 20-40 seconds long - in the nighttime recordings. Methods: The influence of U-shape accelerations on HRV parameters was evaluated based on night-time 34 RR interval time series obtained from healthy adults. We performed calculations on recordings without top 5% intervals (in terms of RR duration), without bottom 5% intervals and on recordings with randomly shuffled RR intervals. For each recording, the VLF frequency band, Shannon and Sample Entropy, standard deviation, mean, RMSSD, Multiscale Multifractal Analysis (MMA), and symbolic analysis parameters were calculated using signals with and without modifications. Results: U-shape accelerations accounted for 5.3±3.3% of the total recordings duration. There were significant differences between the signals without modification and without the smallest 5% intervals (thus without majority of U-shape periods) of the values of HF, LF and VLF frequency bands power, Shannon and Sample Entropy, standard deviation and symbolic analysis parameters (p<;0.05). We also observed a significant reduction of multifractality using Multiscale Multifractal Analysis (MMA) after removing the U-shape accelerations from the recording. These effects did not occur in the surrogate data with shuffled samples (p<;0.05 was obtained only for symbolic analysis parameters), and in case of the signals with rejected the largest 5% of intervals (p<;0.05 only for symbolic analysis parameters and Shannon entropy). Conclusions: Characteristic U-shape accelerations in nighttime RR time series, despite a short total duration, have a significant influence on the VLF frequency band, multifractality and the complexity of the signal. The nature of these events is not clear and requires further study.