LACK OF EVIDENCE FOR LOW-DIMENSIONAL CHAOS IN HEART-RATE-VARIABILITY

Introduction: The term chaos is used to describe erratic or apparently random time-dependent behavior in deterministic systems. It has been suggested that the variability observed in the normal heart rate may b e due to chaos, but this question has not been settled. Methods and Re sults: Heart rate variability was assessed by recordings of consecutiv e RR intervals in ten healthy subjects using ambulatory ECG. All recor dings were performed with the subjects at rest in the supine position. To test for the presence of nonlinearities and/or chaotic dynamics, t en surrogate time series were constructed from each experimental datas et. The surrogate data were tailored to have the same linear dynamics and the same amplitude distribution as the original data. Experimental and surrogate data were then compared using various nonlinear measure s. Power spectral analysis of the RR intervals showed a 1/f pattern. T he correlation dimension differed only slightly between the experiment al and the surrogate data, indicating that linear correlations, and no t a ''strange'' attractor, are the major determinants of the calculate d correlation dimension. A test for nonlinear predictability showed co herent nonlinear dynamic structure in the experimental data, but the p rediction error as a function of the prediction length increased at a slower rate than characteristic of a low-dimensional chaotic system. C onclusion: There is no evidence for low-dimensional chaos in the time series of RR intervals from healthy human subjects. However, nonlinear determinism is present in the data, and various mechanisms that could generate such determinism are discussed.