SHORT-TERM AND LONG-TERM VARIATIONS IN NONLINEAR DYNAMICS OF HEART-RATE-VARIABILITY

Objectives: The purpose of the study was to investigate the short- and long-term variations in the non-linear dynamics of heart rate variabi lity, and to determine the relationships between conventional time and frequency domain methods and the newer nan-linear methods of characte rizing heart rate variability. Methods: Twelve healthy subjects were i nvestigated by 3-h ambulatory ECG recordings repeated on 3 separate da ys. Correlation dimension, non-linear predictability, mean heart rate, and heart rate variability in the time and frequency domains were mea sured and compared with the results from corresponding surrogate time series, Results: A small significant amount of non-linear dynamics exi sts in heart rate variability, Correlation dimensions and non-linear p redictability are relatively specific parameters for each individual e xamined. The correlation dimension is inversely correlated to the hear t rate and describes mainly linear correlations. Non-linear predictabi lity is correlated with heart rate variability measured as the standar d deviation of the R-R intervals and the respiratory activity expresse d as power of the high-frequency band, The dynamics of heart rate vari ability changes suddenly even during resting, supine conditions. The a brupt changes are highly reproducible within the individual subjects. Conclusions: The study confirms that the correlation dimension of the R-R intervals is mostly due to linear correlations in the R-R interval s. A small but significant part is due to non-linear correlations betw een the R-R intervals. The different measures of heart rate variabilit y (correlation dimension, average prediction error, and the standard d eviation of the R-R intervals) characterize different properties of th e signal, and are therefore not redundant measures. Heart rate variabi lity cannot be described as a single chaotic system, Instead heart rat e variability consists of intertwined periods with different non-linea r dynamics. It is hypothesized that the heart rate is governed by a sy stem with multiple ''strange'' attractors.