February
17, 2001
Complex
Hearts
by Pearl Tesler
Here's
a test. Forget about genetics and your HMO and imagine for a second
that you really are in control of your health. You can choose one
of these two resting heart rates to be yours. Which do you want, the
nice smooth one or the wacky bumpy one?
Surprise:
that nice smooth pattern belongs to someone with severe congestive
heart failure. The bumpy pattern is a healthy heart.
Dr.
Ary Goldberger, Professor of Medicine at Harvard Medical School, explains
that the heart is a complex system, one that displays the characteristics
common to complexity: nonlinearity, emergent phenomena, and fractal
structure.
The
bumps in a healthy heart rate pattern are examples of a fractal pattern.
By definition, a fractal is a pattern that displays selfsimilarity
and scale invariance. What this means is, the closer you look, the
more structure you see, and these smaller structures are little copies
of the larger structures. A frequentlycited example is a tree. Each
branch mimics the structure of the entire tree, and each branch branches
off into smaller branches that continue the pattern.
Showing
graphs of a healthy heartbeat pattern, Goldberger notes that these
graphs show fractal structure. A graph that shows the meanderings
of heart rate over 300 minutes looks just like the one that spans
only 30 minutes. Zooming in again, the graph that covers only 3 minutes
looks roughly identical.
What
is Complexity?
Complexity, also called "chaos" or "nonlinear dynamics," refers
to systems that have lots of interdependent factors. These systems
tend to display the following traits:
• nonlinearity
(output isn't proportional to input)
• bifurcations, or abrupt changes
• fractal structure (scale invariance and selfsimilarity)
• period doubling
• nonlinear waves (solitons, spirals, scrolls)
• chaos
• emergent
phenomena (strange attractors)

When
this healthy randomness breaks down, watch out. When discernible patterns
begin to show up, it often signals disease. In graphs of patients
experiencing altitude sickness and sleep apnea, patterns like sinusoidal
waves appear.
Goldberger
says this trait isn't exclusive to hearts. "The output of many systems
actually becomes more regular with pathologies. Patients with a wide
range of disorders often display strikingly ordered dynamics." As
examples, he cites Parkinson's, nystagmus (periodic motions of the
eyes), and obsessive compulsive disorder.
Goldberger
hopes that research into the complex dynamics of biological systems
can translate into useful diagnostics. A better understanding of "healthy
disorder" and its ominous disappearance could lead to "really smart"
heart monitors or, in neuroscience, a device to forecast seizures.