Why Time Slows the Heart
By Kathleen Doheny, HealthScoutNews Reporter
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.
Next, Stratton wants to see if these effects of aging can be positively affected by exercise. He is assigning personal trainers to both young and old people and having them start a supervised cardiovascular exercise program; then he will perform the atropine experiment again.
“We hope that the training will increase the resting parasympathetic tone. We hope that exercise might prove to lessen the decline in the heart’s activity,” he says.
Phyllis K. Stein, an expert in the field who reviewed the study for the journal, calls the research interesting and says it has applications down the road.
Once you determine the effects aging has on the autonomic nervous system, she says, “then you can separate out normal and abnormal aging, and maybe identify people who are at high risk of something because their physiology is aging faster than it should be.”
Stratton’s study is one of the few to focus on the role of the parasympathetic nervous system and how its regulation and the body’s response to it change with age, she says.
“The sexy question is, ‘Can you do anything about this?’” she asks.
While it’s not known for sure how to maintain parasympathetic tone, Stein says, among the list of lifestyle behaviors that might help are: regular exercise, weight control, a healthy diet, good stress management and avoiding or stopping smoking.
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On the Net:
National Dysautonomia Research Foundation
American College of Sports Medicine
Veterans Administration Puget Sound Health Care System
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