For people with heart disease, high levels of stress coupled with depression increase the risk of heart attack and death, according to a new study.

“We found that the combination of high stress and high depression symptoms was particularly harmful for adults with heart disease during an early vulnerability period,” said lead author Carmela Alcantara of Columbia University Medical Center in New York City.

Alcantara and colleagues followed more than 4,400 people age 45 and older with coronary heart disease, a buildup of plaque in the arteries which is the most common form of heart disease.

Between 2003 and 2007, participants had in-home examinations and completed stress and depression questionnaires. For example, they reported how often during the previous week they felt depressed, lonely or cried, and how often during the past months they felt overwhelmed or like life was out of their control.

Almost 12 percent of the participants had high stress, almost 14 percent had high levels of depression and 6 percent reported having both.

After roughly six years of follow-up, 1,337 participants had a heart attack or died.

The 6 percent of people with both high stress and depression were 48 percent more likely to die or have a heart attack within two and a half years of the home visit than people without both of these risk factors.

There was no increased risk over longer periods or for people with either depression or stress, but not both, according to results in the journal Circulation: Cardiovascular Quality and Outcomes.

“We were surprised that high stress, and high depression, alone did not increase the risk of another heart attack or death, in analyses that accounted for important medical, behavioral, and demographic factors,” Alcantara told Reuters Health by email.

Coronary heart disease is the leading cause of death for men and women in the U.S., according to the National Institutes of Health, but it can be managed with medications and lifestyle changes.

Previous studies tied stress and depression to the development of heart disease as well, though this study was limited to people who already had the condition, Alcantara said.

“More research is needed to understand why psychosocial factors like these are so often tied to heart health in particular,” she said.

During periods of stress, the part of the nervous system that regulate the heart and other organs “makes the heart beat harder and faster causing blood pressure to increase, a potential cause of heart attacks and strokes,” Dr. Phil Chowienczyk, who was not part of the Circulation study, wrote in an email to Reuters Health.

In a recent paper in the journal Hypertension, Chowienczyk and colleagues from the King's College London report that normally, the body regulates blood flow by releasing a molecule that lets blood vessels open wider to prevent blood pressure from rising too much.

People with high blood pressure seem to release less of that molecule, particularly during stress, according to their findings - and this may contribute to stress-induced cardiovascular crises.

Behavioral stress and depression management therapies may help improve medical outcomes for people with heart disease, Alcantara said, but more research is needed in that area.