(CBS NEWS) Heart disease is the number one killer of women in the U.S.Now, a new study shows heart disease may put older women at risk for dementia.

Ruth Falcon Meyer has heart disease. The 71-year-old uses medication to manage her high blood pressure and irregular heart rate.

"I have it under control and I'm leading a normal life," Meyer says.

New research shows heart disease may also be putting her at risk for dementia.

A study in the Journal of the American Heart Association looked at nearly 6,500 women over 65 and found postmenopausal women with heart disease or vascular disease were about 30% more likely to experience cognitive decline.

"Women who have history of having had a heart attack, their risk was doubled of having either mild cognitive impairment or dementia over time," says Dr. Sylvia Wassertheil-Smoller, a professor at Albert Einstein College of Medicine.

Women who had bypass surgery were at greatest risk. Researchers don't know why heart disease may be linked to dementia in older women, but say it's possible plaque in the arteries may also suggest plaque in vessels feeding the brain.

"The study also found having risk factors like high blood pressure and diabetes increased risk for cognitive decline over time, even in women without heart disease," says Dr. Erica Jones of Weill-Cornell Medical Center. "If you know that you are at risk, then you can do more to help protect yourself."

Ruth says she feels herself slowing down, but she's trying to stay active.

"I'm still working full time and I think that's a big help."

She hopes it will continue to keep her mind healthy.

The study also found that a being overweight or obese may actually protect against cognitive decline.