Researchers have known for a while that heart disease and depression influence each other. However, a new study investigates the impact of depression on heart disease over a long period of time, and finds the psychological disorder to increase mortality risk.

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New research looks at the long-term effects of depression on people who have also experienced a heart attack.

The Centers for Disease Control and Prevention (CDC) estimate that every 42 seconds, an adult in the United States has a heart attack. Heart disease remains the leading cause of death worldwide, and in the U.S., a heart disease-related death occurs every minute.

Depression is also a major health concern. According to the Anxiety and Depression Association of America, the disorder affects more than 15 million U.S. adults. Additionally, major depressive disorder is the leading cause of disability among those aged between 15 and 44.

New research - to be presented at the American College of Cardiology's 66th Annual Scientific Session - suggests that there may be a link between these two conditions.

The study focused on people who received a diagnosis of heart attack, stable angina, or unstable angina. These conditions are forms of coronary heart disease - the most widespread type of heart disorder - and were responsible for approximately 365,000 deaths in 2014, according to the CDC's latest statistics.

Depression doubles mortality risk for people with heart attack

The researchers - led by Dr. Heidi May, Ph.D., a cardiovascular epidemiologist at the Intermountain Medical Center Heart Institute in Salt Lake City, UT - investigated the health records of nearly 25,000 patients registered with the Intermountain Health System. These patients were also clinically followed for an average of almost 10 years after receiving their initial diagnosis of coronary heart disease.

Overall, 15 percent of these patients were also diagnosed with depression after their heart disease diagnosis. This proportion is considerably larger compared with the general population estimate of 7.5 to 10 percent.

Additionally, the researchers adjusted for risk factors, age, gender, medications, and depression onset, as well as heart attack, chest pain, and follow-up complications.

After all of the adjustments, depression was revealed as the strongest predictor of death in the group. In fact, people with depression were twice as likely to die compared with people without depression.

Of the 3,646 people who were diagnosed with depression, 50 percent died during the study. By comparison, only 38 percent of those who did not receive a depression diagnosis died over the course of the study.

Clinicians should look out for depression in cardiovascular patients

Previous studies have examined the relationship between depression and heart disease, and researchers noticed that the two tend to influence each other. While previous research has looked at the effect of depression within the first few months after a heart attack, the study by May and team is the first to investigate the long-term effects of the depressive disorder.

The authors say that their findings should prompt physicians to better identify depression - either by using assessment tools such as questionnaires, or simply by being more vigilant regarding symptoms of depression in their patients.

"It can be devastating to be diagnosed with coronary artery disease," says the lead author of the study. "Clinicians need to pay attention to the things their patients are expressing, in terms of both physical symptoms as well as emotional and nonverbal factors."

May goes on to underscore the importance of depression screening for patients with coronary heart disease:

"This study shows that it does not matter if depression emerges in the short-term or a few years down the road - it is a risk factor that continually needs to be assessed. I think the take-home message is that patients with coronary disease need to be continuously screened for depression, and if found to be depressed, they need to receive adequate treatment and continued follow-up."

Learn how treating depression may reduce heart disease risk.