Nonlinear dynamics in heart rate variability

The aim of the present study was to examine the influence of data length on the values of fractal dimension (FD), Lyapunov exponent (LE) and approximate entropy (ApEn) and to apply these techniques in a healthy population. There was no significant difference between long- and short-term recordings in the methods used, except when using FD without a windowing function. There was a significant difference between day and night values of FD (p<0.005) and ApEn showed a difference between standing and supine position (p<0.01). These results indicate a possible relation with the autonomic regulation of heart rate for FD and ApEn, while the LE seems to be an inherent to the system. The results presented indicate that these methods can be used on both long and short recordings. This could be useful in the clinical situation.