AGE-RELATED ALTERATIONS IN THE FRACTAL SCALING OF CARDIAC INTERBEAT INTERVAL DYNAMICS

We postulated that aging is associated with disruption in the fractall ike long-range correlations that characterize healthy sinus rhythm car diac 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 interbe at interval time series using standard time and frequency domain stati stics 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 expone nts (alpha) from the fluctuation analysis close to a value of 1.0. In the group of healthy elderly subjects, the interbeat interval time ser ies 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 you ng (alpha(2) = 1.12 +/- 0.19 vs. 0.90 +/- 0.14, respectively, P = 0.00 9; 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 character istic time scale may be dominating heartbeat control in these subjects . The age-related loss of fractal organization in heartbeat dynamics m ay reflect-the degradation of integrated physiological regulatory syst ems and may impair an individual's ability to adapt to stress.