Fractal methods and cardiac interbeat time series

We analyzed cardiac beat-to-beat time series arising from three different groups: healthy young and healthy elderly subjects and patients with congestive heart failure. We briefly describe two methods used to analyze cardiac interbeat series: the power spectral method and the detrended fluctuation analysis (DFA). We also use the Higuchi method to calculate the fractal dimension of these time series. We find that the fractal dimension is different for each group, healthy young subjects can be characterized by a single value of fractal dimension, whereas in the cases of healthy elderly subjects and patients with congestive heart failure a crossover behavior in fractal dimension is oberved. Our results are then qualitatively compared to those found by means of other fractal methods, the power spectrum method and detrended fluctuations analysis, respectively.