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ORLANDO -- A study by Duke University Medical Center researchers suggests that the cumulative effect of the daily mental and emotional stresses of life reduces the heart's ability to respond appropriately to the outside world.

This insight, the researchers said, could help explain the mechanism behind the known links between mental stress and heart disease. The researchers also said their findings emphasize the importance of stress reduction techniques and exercise in maintaining a healthy environment for the heart.

By continually monitoring the electrical activity of the hearts of 135 patients with coronary artery disease during a 48-hour period, the researchers found that higher levels of negative emotions were strongly associated with a reduction in the ability of the heart to respond to stress and negative emotions.

Duke researcher Simon Bacon, Ph.D., presented the results of the Duke analysis today (March 4, 2004) at the annual meeting of the American Psychosomatic Society. The research was supported by the National Institutes of Health (NIH).

"While we have known that emotional stresses have been linked to the development and progression of coronary artery disease, it has not been clear why this is so," Bacon said. "Past laboratory studies in humans and animals have demonstrated that mental stress can negatively affect the autonomic control over the heart." Autonomic control is the reflexive control of heart action over which people have no conscious control.

"What we have shown for the first time, using detailed cardiac measurements during everyday life, is that such negative emotions as anger, stress or sadness were associated with a reduction in autonomic control of the heart," Bacon continued. "These findings may help explain how acute stress may contribute to the increased risk of clinical events in patients with coronary artery disease."

For the study, the researchers attached portable electrocardiogram (ECG) machines to 135 heart patients, who wore them continually for two days. While the monitors recorded the heart's electrical activity minute-by-minute during this period, a tone would sound every 20 minutes during waking hours. This signal prompted patients to record in a portable diary their activities and emotions at that moment.

"In this way, we were able to correlate what each patient was experiencing during everyday life with what was happening with their hearts," Bacon explained. "The monitors showed that during periods of mental stress and negative emotions, the hearts showed a reduced capacity to respond."

The monitors measured each patient's heart rate variability -- a phenomenon by which the brain controls the rate at which our heart beats. For example, when humans encounter danger, the brain triggers the sympathetic nervous system, which in turn makes the heart beat faster. Once the danger has passed, the brain initiates the parasympathetic nervous system, which then slows the heart rate down. In normal daily life these two systems, the sympathetic and parasympathetic nervous systems, help maintain the heart rate at an equilibrium guarding against sudden cardiac events.

"Sick hearts show very little heart rate variability, so they are not as responsive, leaving them vulnerable," Bacon said. "Healthy hearts have a better ability to respond to anything that occurs. The bottom line is that stress we experience throughout the course of the day can be bad for our hearts. Our study showed that even small fluctuations can have a cumulative impact."

Such techniques as stress and anger management, or behavioral modification techniques, may be effective in reducing the body's unhealthy reactions to stress. Exercise may also be an effective method of stress reduction, Bacon said.

According to Bacon, future research should be directed at intervention strategies for minimizing the effects of mental stress. At Duke, the NIH-funded SMART HEART trial is currently evaluating the potential benefits of exercise training and stress management programs for patients with coronary artery disease.

Other colleagues on the study included Lana Watkins, Ph.D., Michael Babyak, Ph.D., Andrew Sherwood, Ph.D., Robert Waugh, M.D., and James Blumenthal, Ph.D. Other members of the team were Alan Hinderliter, M.D., University of North Carolina, Chapel Hill and Junichiro Hayano, M.D., Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan.