No evidence of chaos in the heart rate variability of normal and cardiac transplant human subjects.

Abstract

The variability observed in the heart rate may reflect fundamental aspects of cardiac activity. It has been under discussion whether heart rate variability (HRV) is due to noise or chaos, which is irregular behavior occurring in deterministic nonlinear systems. Using chaos analysis techniques, we analyzed HRV of five normal and five human cardiac transplant subjects at rest. HRV is studied using the beat-to-beat RR interval time series extracted from the ECG. The cardiac transplant subjects exhibited a much smaller HRV than the normal subjects because of heart denervation. We present the map and correlation dimension estimation for the RR time series. To test for nonlinear correlations in the dynamics, we built surrogate time series that have the same power spectra as the experimental time series, but also have randomized phases. The experimental and the surrogate data were compared using the correlation integral. No correlation dimension was found for the RR time series of either the normal or the cardiac transplant subjects. Nevertheless, nonlinear correlations were detected in the HRV of the normal subjects but not in HRV of the cardiac transplant subjects. For the latter, no significant changes were observed in the correlation integral as a function of time after transplantation. We found no evidence of low-dimensional chaos in the HRV of normal and cardiac transplant subjects. However, some nonlinear correlations were detected in the HRV of the normal subjects, which may be associated with autonomic nervous system influence.