Age-related alterations in the fractal scaling of cardiac interbeat
interval dynamics
N. Iyengar, C. K. Peng, R. Morin, A. L. Goldberger and L. A. Lipsitz
We postulated that aging is associated with disruption in the
fractallike long-range correlations that characterize healthy sinus
rhythm cardiac interval dynamics. Ten young (21-34 yr) and 10 elderly
(68-81 yr) rigorously screened healthy subjects underwent 120 min of
continuous supine resting electrocardiographic recording. We analyzed
the interbeat interval time series using standard time and frequency
domain statistics and using a fractal measure, detrended fluctuation
analysis, to quantify long-range correlation properties. In healthy
young subjects, interbeat intervals demonstrated fractal scaling, with
scaling exponents (alpha) from the fluctuation analysis close to a
value of 1.0. In the group of healthy elderly subjects, the interbeat
interval time series had two scaling regions. Over the short range,
interbeat interval fluctuations resembled a random walk process
(Brownian noise, alpha = 1.5), whereas over the longer range they
resembled white noise (alpha = 0.5). Short (alpha s)- and long-range
(alpha 1) scaling exponents were significantly different in the
elderly subjects compared with young (alpha s = 1.12 +/- 0.19 vs. 0.90
+/- 0.14, respectively, P = 0.009; alpha 1 = 0.75 +/- 0.17 vs. 0.99
+/- 0.10, respectively, P = 0.002). The crossover behavior from one
scaling region to another could be modeled as a first-order
autoregressive process, which closely fit the data from four elderly
subjects. This implies that a single characteristic time scale may be
dominating heartbeat control in these subjects. The age-related loss
of fractal organization in heartbeat dynamics may reflect the
degradation of integrated physiological regulatory systems and may
impair an individual's ability to adapt to stress.