Complexity and predictability of the heartbeat time series
in normal and transplanted subjects
R. Balocchi 
, M. Barbi 
, C. Carpeggiani , S. Chillemi ,
A. Di Garbo
C. Michelassi
Institute of Clinical Physiology, via Trieste, 41, 56100-Pisa, Italy
Institute of Biophysics, via S. Lorenzo, 26, 56100-Pisa, Italy
The spontaneous changes in heart rate, known as sinus arrhythmia, are due to autonomic activity on the sinus node, highly modulated by baroreceptor activity and respiration. Reduction in heart rate variability (HRV) as a function of age or disease has been demonstrated by many investigators, together with its importance as prognostic indicator in pathophysiological situations. Here, the non linear behaviour and predictability of the 24-hour HRV have been investigated in healthy subjects and heart transplanted subjects, two apparently opposite conditions as for the influence of the autonomic nervous system.
In more detail, the first differences of the RR (interbeat time interval) time series have been processed with both the well established non linear forecasting methods [1] and the more recently proposed S-map approach [2].
The agreement between predicted and observed values was quantified by both
linear correlation coefficients and normalised error (error/variance).
Preliminary results show, in both groups: a) short term predictability,
significantly higher during sleep than awake period; b) no strong evidence
of nonlinearity; furthermore, a stochastic-like behaviour seems to
characterise transplanted subjects. Investigation continues on a higher
number of subjects