EVIDENCE OF PHASE-TRANSITIONS IN HEART PERIOD DYNAMICS

Complexity measures of non-linear dynamics are a useful tool for quant ifying observed stretching, folding, scaling and mixing processes in t he Takens-reconstructed state space of heart period dynamics. Although such measures are not suited to provide evidence of deterministic cha os or to estimate true fractal dimensions and Lyapunov spectra in hear t period time series, they allow the classification of RR dynamics and the identification of changes in RR complexity (RRC). The aim of this study was to develop appropriate measures and examine their utility i n identifying the physiological effect of changes between the sleeping and waking state. Twenty-four hour electrocardiography (EGG) recordin gs and diaries noting their waking/sleeping period were obtained from 78 healthy subjects, aged 20 to 55 years. The approximate information dimension (ApD1) and the approximate Kolmogorov entropy (ApEn), introd uced by Pincus, Kaplan and others, were modified in order to allow the calculation of strictly local values. That is, the local or pointwise dimensions and entropies were calculated for each reference vector wi th respect to its symmetric neighbourhood in time. For each subject th e values for the local measures were averaged for 10-min periods, resu lting in 144 global values over 24 h. Similarly, low-and high-frequenc y spectral parameters were calculated. All measures were examined and compared for the waking and the sleeping periods. All complexity measu res as well as to a lesser degree high-frequency power showed a linear dependency on mean RR interval with a large individual variation. For the RRC measures this linear correlation was separated into two diffe rent clusters corresponding to the sleeping and waking periods. In alm ost all cases the correlation was greater in the waking period. In par ticular, in many cases no correlation was observed in the sleeping per iod. However, the tau values for LF were appreciably lower and indicat ed solely a weak relationship to the RR interval in the waking period. Analysis of variance combining mean RR interval with RRC or spectral parameters singly and in couples revealed that the best separation wit h respect to physiological state could be achieved with the complexity measures, in particular with ApEn. The results show evidence of at le ast two dynamical regimes (phases) of heart period dynamics and a clos e but different functional relationship within the phases between RR i nterval and RR complexity. The separation between these regimes and th e relatively sudden shift from one regime to the other suggest the exi stence of a phase transition with respect to waking and sleeping perio ds in terms of synergetics.