Biomedical Engineering Reference
In-Depth Information
Although it is often difficult to separate the effects of aging from those of
disease and such lifestyle changes as reduced physical activity, healthy aging in
the absence of such confounding factors appears to have a profound impact on
physiologic processes. Because of the progressive age-related degeneration of
various tissues and organs, and the interruption of communication pathways
between them, complex physiologic networks break down, become discon-
nected, and lose some of their capacity to adapt to stress.
There is considerable redundancy in many of the biologic and physiologic
systems in higher organisms; for example, humans have far more muscle mass,
neuronal circuitry, renal nephrons, and hormonal stores than they need to sur-
vive. This creates a
physiologic reserve
that allows most individuals to compen-
sate effectively for age-related changes. Because the network structure of
physiologic systems also enables alternate pathways to be used to achieve the
same functions, physiologic changes that result from aging alone usually do not
have much impact on everyday life. However, these changes may become mani-
fest at times of increased demand, when the body is subjected to high levels of
physiologic stress. For this reason, elderly individuals are particularly vulnerable
to adverse events such as falls, confusion, and incontinence when exposed to
environmental, pharmacologic, or emotional stresses.
2.
MEASURES OF COMPLEXITY LOSS
With the development of monitoring devices that can measure the output of
regulatory processes on a moment-to-moment basis, it has become apparent that
the dynamics of many systems lose complexity with advancing age (5). The
continuous heart rate time series of a healthy young subject and healthy elderly
subject shown in Figure 2 provides a good example. The average heart rate over
the 8-minute period of recording is 64.7 beats per minute in the young subject
and 64.5 in the older subject—nearly identical. Furthermore, the standard devia-
tion of the heart rate is also nearly identical: 3.9 in the young subject and 3.8 in
the elderly subject. However, it is apparent that the dynamics of the heart rate
time series are strikingly different. Until recently, scientists have lacked the
tools to describe these dynamics. However, new advances in the fields of
nonlinear dynamics, chaos theory, and complex systems (see Part II, chapters 1
(by Shalizi) and 2 (by Socolar) this volume) have provided new ways to quan-
tify the aging process and understand its mechanisms. One particularly useful
concept that can be used to quantify the complexity of various anatomic struc-
tures or physiologic processes is the concept of fractals.
The definition of a fractal, first described by Mandelbrot (6), is a geometric
object with "self-similarly" over multiple measurement scales. For example,
many anatomic structures demonstrate self-similarity in their structures and have
fractal properties. The branches upon branches upon branches of bronchi in the
