TITLE PAGE Fractal scale-invariant and nonlinear properties of cardiac dynamics remain stable with advanced age: A new mechanistic picture of cardiac control in healthy elderly Fractal and nonlinear stability of cardiac dynamics with aging

BackgroundHeartbeat fluctuations exhibit temporal structure with robust long-range correlations, fractal and nonlinear features, which have been found to break down with pathologic conditions, reflecting changes in the mechanism of neuroautonomic control. It has been hypothesized that these features change and even break down also with advanced age, suggesting fundamental alterations in cardiac control with aging. Here we test this hypothesis. Methods and ResultsWe analyze heartbeat interval recordings from two independent databases: (a) 19 healthy young (avg. age 25.7 years) and 16 healthy elderly subjects (avg. age 73.8 years) during 2h under resting conditions from the Fantasia database; and (b) 29 healthy elderly subjects (avg. age 75.9 years) during ≈ 8h of sleep from the SHHS database, and the same subjects recorded 5 years later. We quantify: (1) The average heart rate ; (2) the SD σRR and σ∆RR of the heartbeat intervals RR and their Page 1 of 54 Articles in PresS. Am J Physiol Regul Integr Comp Physiol (August 1, 2007). doi:10.1152/ajpregu.00372.2007 Copyright © 2007 by the American Physiological Society. increments ∆RR; (3) the long-range correlations in RR as measured by the scaling exponent αRR using the Detrended Fluctuation Analysis; (4) fractal linear and nonlinear properties as represented by the scaling exponents α and α for the time series of the sign and magnitude of ∆RR; (5) the nonlinear fractal dimension D(k) of RR using the Fractal Dimension Analysis. We find: (1) No significant difference in (P > 0.05); (2) a significant difference in σRR and σ∆RR for the Fantasia groups (P < 10) but no significant change with age between the elderly SHHS groups (P > 0.5); (3) no significant change in the fractal measures αRR (P > 0.15), α (P > 0.2), α (P > 0.3), and D(k) with age. ConclusionsOur findings do not support the hypothesis that fractal linear and nonlinear characteristics of heartbeat dynamics break down with advanced age in healthy subjects. While our results indeed show a reduced SD of heartbeat fluctuations with advanced age, the inherent temporal fractal and nonlinear organization of these fluctuations remains stable. This indicates that the coupled cascade of nonlinear feedback loops, which are believed to underlie cardiac neuroautonomic regulation, remains intact with advanced age.