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 (SD ANN) and the power of the ultralow-frequency (ULF) spectral band (<0.0 033 Hz). We hypothesized that SDANN and ULF might have their origins i n changes in human activity rather than slow oscillatory rhythms. Hear t period sequences were obtained from 24-h Holter electrocardiograms o f 10 healthy ambulatory subjects. There was no evidence of any persist ent oscillation within the ULF band. Using moving 4-h windows in short -time Fourier transforms, we showed that the amplitude of ULF fluctuat ed markedly particularly during times bordering sleep. The local ULF a mplitude correlated (r = 0.59 +/- 0.09) with large-scale changes in he art 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 betwee n their temporal distributions (r = 0.10 +/- 0.25). The temporal distr ibutions of measures of long-range heart period variability suggest th at they reflect changes in human activity levels.