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The concept of complexity, derived from the field of nonlinear dynamics, can be adapted to measure the output of physiologic processes that generate highly variable fluctuations resembling chaos. We review data suggesting that physiologic aging is associated with a generalized loss of such complex-ity in the dynamics of healthy organ system function and hypothesize that such loss of complexity leads to an impaired ability to adapt to physiologic stress. This hypothesis is supported by observations showing an age-related loss of complex variability in multiple physiologic processes including cardiovascular control, pulsatile hormone release, and electroencephalographic potentials. If further research supports this hypothesis, measures of complexity based on chaos theory and the related geometric concept of fractals may provide new ways to monitor senescence and test the efficacy of specific interventions to modify the age-related decline in adaptive capacity. (JAMA. 1992;267:1806-1809) HEALTHY physiologic function is char¬ acterized by a complex interaction of multiple control mechanisms that en¬ able an individual to adapt to the exi¬ gencies and unpredictable changes of everyday life. The aging process appears to be marked by a progressive impair¬ ment in these mechanisms, resulting in a loss of dynamic range in physiologic function and, consequently, a reduced capacity to adapt to stress. A key question is how to quantitate physiologic aging. Previous investiga¬ tions have focused primarily on age-re¬ lated declines in the mean value of dis¬ crete physiologic variables such as cre-atinine clearance,1 forced expiratory vol¬ ume,2 nerve conduction velocity,3 and insulin sensitivity.4 However, the wide interindividual variance of such mea-
Lewis A. Lipsitz (Wed,) studied this question.