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[Nonlinear dynamics in regulation of heart rate in healthy infants].

The aim of this study was to investigate qualitatively and quantitatively non-linear and linear characteristics of heart rate identifying their connection to sleep states during the first 6 months of life. 16 newborns were examined polygraphically in a longitudinal study (first day, first and sixth month). After R-peak detection of a limb lead ECG and R-R interval calculation, time series of the instantaneous heart rate (IHR) were constructed. They were analyzed qualitatively with regard to the structure of the Poincaré maps and the occurrence of bifurcations as well as quantitatively by calculating the correlation dimension (D2) and the Lyapunov exponent (LLE). In some cases, bifurcations in the IHR time series occurred. The Poincaré maps showed mainly clear structures in the shape of limited point clusters. D2 was in the range of 1.5 to 2.5 and was larger in quiet than in active sleep. LLE was always positive, although higher values in active sleep than in quiet sleep were exhibited in the first month of life only. D2 and LLE values during active sleep were age-dependent. The results of the study indicate a non-linear component in the heart rate control of healthy newborns. The change in D2 and LLE in connection with postnatal age and sleep states expresses a modulation of these system components during the postnatal maturation and during vegetative sleep organisation.