The scientists tracked 63,000 women from the long-running Nurses’ Health Study between 1992 and 2004. None had signs of heart disease when the study began, but nearly 8 percent had evidence of serious depression.

The depressed women were more than twice as likely to experience sudden cardiac death — death typically caused by an irregular heartbeat, concluded the 12-year study, published Monday in the Journal of the American College of Cardiology. They also had a smaller increased risk of death from other forms of heart disease.

The big surprise: Sudden cardiac death seemed more closely linked with antidepressant use than with the depression symptoms the women reported.

That might simply mean that women who used antidepressants were, appropriately, the most seriously depressed, cautioned lead researcher Dr. William Whang. But he said the finding merited more research.

Studies of the newer antidepressants most often used today so far haven’t signaled a risk of irregular heartbeat, and some even have suggested protection, noted Dr. Redford Williams of Duke University, a specialist in how psychosocial factors affect health.

The drug question aside, Williams said the work adds to growing evidence that depression is an independent risk factor for heart disease — on top of the classic risks of high blood pressure, diabetes, high cholesterol and smoking.

The predominantly white Nurses’ Health Study may underestimate it, Williams said. “If anything, the impact in African-American women is probably greater,” he said, adding that it’s time for the next step: A study testing whether properly treating depression lowers the risk.

Why might depression have that effect? The study found that the more severe the women’s reported depression symptoms, the more likely she was to have traditional heart risk factors. Also, stresses like depression have been linked to such physical effects as a higher resting heart rate.

Perhaps a more straightforward reason: Depression can make people do a worse job taking care of themselves. Indeed, the American Heart Association last year recommended that everyone who already has heart disease be regularly screened for depression — because depressed patients may skip their medications, sit indoors instead of exercising, and eat particularly poorly.