Heart patients who become depressed have a higher risk of developing heart failure, regardless of whether they take antidepressants, U.S. researchers said on Monday.

They said the study is the first to look at whether depression raises the risk for heart failure, a chronic condition affecting 5 million Americans in which the heart gradually loses its ability to pump blood efficiently.

"Our data suggest that depression is an important and emerging risk factor for heart failure among patients with coronary heart disease," Heidi May of Intermountain Medical Center in Utah, whose study appears in the Journal of the American College of Cardiology, said in a statement.

Prior studies have shown that depression is about three times more common after a heart attack and depressed patients are at higher risk of a second heart attack.

May wanted to see if heart disease and depression had an effect on heart failure, which strikes more than 550,000 people in the United States each year at an annual cost of about $35 billion a year, according to the American Heart Association.

The researchers studied nearly 14,000 people with clogged heart arteries. None of them had heart failure or depression at the time of their diagnosis. Patients were tracked until they developed heart failure or died.

When they checked heart failure rates among the 1,377 people who eventually developed depression, the researchers found much higher rates than among those who were not depressed.

The heart failure rate was 3.6 percent per 100 people among those who did not develop depression, but it was 16.4 percent in the group that did. While many of the patients took antidepressants, this did nothing to reduce heart failure risks.

"This finding may indicate that antidepressants may not be able to alter the physical or behavioral risks associated with depression and heart failure, despite a potential improvement in depressive symptoms," May said.

Studies have shown that depressed heart patients are more likely than others to stop taking their heart medications and are less likely to stay on heart-healthy diets or get regular exercise.

Depression can also bring about changes in the body, including reduced heart rate and increases in blood factors that encourage the formation of blood clots.

May said the findings suggest that although symptoms of depression may improve, the heart risks related to depression might not.