Nonlinear analysis of continuous ECG during sleep I. Reconstruction

In recent years evidence has accumulated that ECG signals are of a nonlinea r nature. It has been recognized that strictly periodic cardiac rhythms are not accompanied by healthy conditions but, on the contrary, by pathologica l states. Therefore, the application of methods from nonlinear system theor y for the analysis of ECG signals has gained increasing interest. Crucial f or the application of nonlinear methods is the reconstruction (embedding) o f the time series in a phase space with appropriate dimension. In this stud y continuous ECG signals of 12 healthy subjects recorded during different s leep stages were analysed. Proper embedding dimension was determined by app lication of two techniques - the false nearest neighbours method and the sa turation of the correlation dimension. Results for the ECG signals were com pared with findings for simulated data (quasiperiodic dynamics, Lorenz data , white noise) and for phase randomized surrogates. Findings obtained with the two approaches suggest that embedding dimensions from 6 to 8 may be reg arded as suitable for the topologically proper reconstruction of ECG signal s.