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

Introduction: The variability observed in the heart rate may reflect fundam ental aspects of cardiac activity. It has been under discussion whether hea rt rate variability (HRV) Is due to noise or chaos, which is irregular beha vior occurring in deterministic nonlinear systems. Methods and Results: Using chaos analysis techniques, me analyzed HRV of fi ve normal and five human cardiac transplant subjects at rest. HRV is studie d using the beat-to-beat RR interval time series extracted from the EGG. Th e cardiac transplant subjects exhibited a much smaller HRV than the normal subjects because of heart denervation. We present the map and correlation d imension estimation for the RR;time series. To test for nonlinear correlati ons In the dynamics, we built surrogate time series that have the same powe r spectra as the experimental time series, but also have randomized phases. The experimental and the surrogate data were compared using the correlatio n integral. No correlation dimension was found for the RR time series of ei ther the normal or the cardiac transplant subjects. Nevertheless, nonlinear correlations were detected in the HRV of the normal subjects but not in HR V of the cardiac transplant subjects. For the latter, no significant change s were observed in the correlation integral as a function of time after tra nsplantation. Conclusion: We found no evidence of low-dimensional chaos in the HRV of nor mal and cardiac transplant subjects. However, some nonlinear correlations w ere detected in the HRV of the normal subjects, which may be associated wit h autonomic nervous system influence.