Predictability and nonlinearity of the heart rhythm

Sequences of interbeat intervals from 24 subjects in relaxed conditions are analysed by the classical nonlinear prediction method as well as by a direct one, able to estimate the time series nonlinearity. The two approaches yield very similar results, showing that a clear nonlinear behaviour is present in most of the examined sequences. Furthermore, a completely different method, detrended fluctuation analysis, is applied to the data. Unexpectedly, the corresponding statistics strongly correlates with the estimators of nonlinearity.