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

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.