Complexity and predictability of the heartbeat time series in transplanted subjects

Physiological fluctuations in heart rate, known as ''sinus arrhythmia'', are due to the influence of the autonomic nervous system on the sinus node, modulated by baroreceptor activity and respiration. Reduction in heart rate variability (HRV) correlated to age or disease has been reported by several investigators, together with its importance as prognostic indicator in pathophysiological situations. In this paper the heart rhythm is investigated in heart transplanted subjects, an apparently opposite condition with respect to normal hearts as for the influence of the autonomic system. Selected segments of the (first difference) interbeat interval time series are analyzed with an efficient approach [Sugihara, 1994] able to evaluate both the short term predictability and the nonlinearity of these data, Moreover, other more qualitative methods are used to better characterize the experimental sequences. The results indicate that these time series only occasionally exhibit some degree of predictability, as assessed by this metric. A careful examination of the predictability behavior, by means of surrogate data, reveals that it cannot be interpreted as evidence of nonlinearity. Rather, a stochastic-like dynamics seems to characterize the transplanted hearts.