Why Time Slows the Heart
By Kathleen Doheny HealthScoutNews Reporter Friday, June 6, 2003
Friday, June 6 (HealthScoutNews) -- Even if you are healthy, your heart slows down with age. Its
pumping power declines, and the maximal heart rate -- the highest number of times your heart can
contract in a minute -- decreases.
Researchers have long known that some of these changes are due to a decline in the sympathetic
nervous system's control of the heart. But in a new study, they have zeroed in on how the
parasympathetic nervous system's control of the heart also decreases and what the effects of that
decline mean.
The two branches of the autonomic nervous system, which control muscles in the heart and elsewhere,
complement each other, with the sympathetic involved in the "fight- or-flight" response and the
parasympathetic involved in relaxation.
What that means, in real life: If you see a thug in a dark alley, the sympathetic nervous system
convinces you to run. Once you're out of harm's way, the parasympathetic calms you down.
"We took away the parasympathetic nervous system activity and observed what the heart did without
it," says Dr. John Stratton, a staff cardiologist at the Veterans Administration Puget Sound Health
Care System and lead author of the study, published in this week's Journal of the American College
of Cardiology.
To "turn off" the parasympathetic system and speed up the heart rate, Stratton and his colleagues
gave the drug atropine to 22 younger people, average age 26, and to 28 older ones, average age 70.
All were healthy, but sedentary.
"The impact of [speeding up the heart rate] was much more deleterious in older than younger
subjects," Stratton says.
The withdrawal of the parasympathetic system caused less of an increase in heart rate in the older
people and less of an increase in their systolic blood pressure.
"The most important finding is that the older heart is under less parasympathetic nervous system
control than the younger heart," Stratton says. Put another way, the responsiveness of the older
heart to the parasympathetic control is diminished.
As heart rate increases, the amount of time the heart has to fill gets shorter, Stratton explains.
And in the study, the negative impact of that was greater for older people. [...] This is only an
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