Temporally localized contributions to measures of large-scale heart rate variability

Abstract

The purpose of this work was to determine the temporal origins of the standard deviation of successive 5-min mean heart period sequences (SDANN) and the power of the ultralow-frequency (ULF) spectral band (<0.0033 Hz). We hypothesized that SDANN and ULF might have their origins in changes in human activity rather than slow oscillatory rhythms. Heart period sequences were obtained from 24-h Holter electrocardiograms of 10 healthy ambulatory subjects. There was no evidence of any persistent oscillation within the ULF band. Using moving 4-h windows in short-time Fourier transforms, we showed that the amplitude of ULF fluctuated markedly, particularly during times bordering sleep. The local ULF amplitude correlated ( r = 0.59 ± 0.09) with large-scale changes in heart period quantified with 2- and 4-h wavelet transforms. Local SDANN also fluctuated, mainly around times of sleep. Although the 24-h SDANN and ULF values correlated highly, there was little correlation between their temporal distributions ( r = 0.10 ± 0.25). The temporal distributions of measures of long-range heart period variability suggest that they reflect changes in human activity levels.

Dissection of long-range heart rate variability

“…Hypothesized causes include oscillatory and aperiodic, deterministic changes (8,9), as well as heart period fluctuations due to peripheral vasomotor, thermoregulatory, or reninangiotensin systems (10). We have shown in healthy ambulatory subjects that SDANN arises from epochs of time when patients have local mean heart period values that differ from the 24-h mean heart period (i.e., mainly at night) (11). Similarly, ULF power occurs predominantly when patients are changing their behavior from passive to active, and vice versa.…”

“…The standard deviation of all heart periods (SDNN) was calculated as the standard deviation of the sequence of all heart periods of normal beats. For power spectral density functions, we first computed a 24-h Fourier transformation (11). Using linear interpolation, the 24-h heart period sequences were uniformly sampled 2 18 times using a sample interval of 0.329 s. The interpolated sequences had their mean values removed, were Hanning windowed, and then transformed to the frequency domain using a fast Fourier transform.…”

“…We calculated the contributions of the specific activities in each 5-min epoch to the total SDANN of the scripted activity day (11). First, we replaced the mean heart period value of each successive 300-s epoch with the 4-h mean heart period and then recalculated the SDANN of this altered 4-h sequence.…”

Coupling of sympathetic nerve traffic and BP at very low frequencies is mediated by large-amplitude events

“…However, we recently described nonstationarity in rat BP recordings in the form of erratic, large-amplitude events (5). In addition, Roach et al (21) described temporally localized contributions to human heart rate variability at ultralow frequencies (Ͻ0.0033 Hz). It appears that, rather than being stationary, very-low-frequency variability in cardiovascular variables is dominated by temporally localized, large-amplitude events (5,15,21,22).…”

“…In addition, Roach et al (21) described temporally localized contributions to human heart rate variability at ultralow frequencies (Ͻ0.0033 Hz). It appears that, rather than being stationary, very-low-frequency variability in cardiovascular variables is dominated by temporally localized, large-amplitude events (5,15,21,22). Consequently, conclusions based on standard spectral analysis of cardiovascular variables within the low-frequency range, including that of a weak functional relationship between sympathetic nerve activity and BP, can be misleading (5).…”