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

We 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 $\approx{}8$h of sleep from the SHHS database, and the same subjects recorded 5 years later. We quantify: (1) The average heart rate <RR>; (2) the SD $\sigma_{RR}$ and $\sigma_{\Delta{}RR}$ of the heartbeat intervals RR and their increments $\Delta{}RR$; (3) the long-range correlations in RR as measured by the scaling exponent $\alpha_{RR}$ using the Detrended Fluctuation Analysis; (4) fractal linear and nonlinear properties as represented by the scaling exponents $\alpha^{sign}$ and $\alpha^{mag}$ for the time series of the sign and magnitude of $\Delta{}RR$; (5) the nonlinear fractal dimension $D(k)$ of $RR$ using the Fractal Dimension Analysis. We find: (1) No significant difference in $\left<RR\right>$ (P>0.05); (2) a significant difference in $\sigma_{RR}$ and $\sigma_{\Delta{}RR}$ for the Fantasia groups (P<10^{-4}) but no significant change with age between the elderly SHHS groups (P>0.5); (3) no significant change in the fractal measures $\alpha_{RR}$ (P>0.15), $\alpha^{sign}$ (P>0.2), $\alpha^{mag}$ (P>0.3), and D(k) with age. Our 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.