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International Conference on Complex Systems (ICCS2006)

Broken asymmetry of the human heartbeat: loss of time irreversibility in aging and disease

Madalena Costa
Beth Israel Deaconess Medical Center / Harvard Medical Schoo

Ary Goldberger
Beth Israel Deaconess Medical Center / Harvard Medical School

C.-K. Peng
Beth Israel Deaconess Medical Center / Harvard Medical School

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     Last modified: April 25, 2006

Broken Asymmetry of the Human Heartbeat: Loss of Time Irreversibility in Aging and Disease

Madalena D. Costa, Ary L. Goldberger, C.-K. Peng
Beth Israel Deaconess Medical Center, Harvard Medical School, Massachusetts 02215, USA

Living systems function in conditions far from equilibrium. They utilize energy to evolve to more hierarchically ordered structural configurations and less entropic states in comparison with the surrounding environment. In extreme cases presaging death, a state approaching maximum equilibrium is reached.
Time irreversibility, defined as lack of invariance of the statistical properties under the operation of temporal inversion, is a fundamental property of non-equilibrium systems. Surprisingly, relatively little work has been published on practical implementation of time reversibility to biologic time series.
To the extent that loss of self-organizing capability is associated with aging and disease, loss of temporal irreversibility may be a marker of pathology.
We introduce a time irreversibility measure and show the results of applying it to the analysis of ~24hr human cardiac interbeat interval time series for groups of healthy young (n=26) and healthy elderly (n=46) subjects, those with congestive heart failure in sinus rhythm (n=43), and those with atrial fibrillation (n=9) from an open-access database (www.physionet.org).
The time asymmetry index is significantly higher for young healthy subjects than for both healthy elderly subjects and subjects with pathology (p<0.005). Furthermore, the time asymmetry index is significantly higher for the elderly healthy group than for the pathologic groups (p<0.005).
These findings provide, for the first time, evidence that supports the hypotheses that: 1) time irreversibility is greatest for healthy physiologic systems, which exhibit the most complex dynamics; 2) time irreversibility decreases with aging or pathology, offering a new marker of loss of nonlinear functionality and adaptability. Our results also suggest that a readily implemented quantitative measure of time irreversibility complement traditional indexes of biologic variability.

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