Rheumatoid arthritis seems to raise the risk of silent heart disease in patients -- even before they know they have the chronic condition, U.S. researchers reported.

They found patients who later were diagnosed with rheumatoid arthritis were three times more likely to have been hospitalized for an acute heart attack and five times more likely to have an unrecognized heart attack.

After diagnosis, the rheumatoid arthritis patients were twice as likely to find out they had a heart attack and did not even notice, or to die suddenly from a heart condition, the researchers wrote in the journal Arthritis & Rheumatism.

The increased risk could not be accounted for by other factors such as elevated cholesterol, blood pressure, weight or alcohol abuse, said Dr. Hilal Maradit Kremers of the Mayo Clinic in Rochester, Minnesota, who led the study of 600 patients.

"What we are finding is that though traditional cardiovascular risk factors are important, they are less important for those with rheumatoid arthritis," Maradit Kremers said in a statement.

"Something else is going on. It could be that rheumatoid arthritis and heart disease have a common origin. What we do know is that the cause cannot be explained by just one factor."

Rheumatoid arthritis, which affects more than 2 million Americans, is an autoimmune disease caused when immune cells mistakenly attack the joints and organs.

It causes chronic pain and disability and there is no cure.

The pain may make them unaware of having heart attacks, Maradit Kremers said. The heart attacks are diagnosed later during heart examinations.

"It's possible that people suffering from rheumatoid arthritis have so much pain in their joints and are receiving so many painkillers that they either don't feel the chest pain in the same way as those without rheumatoid arthritis or don't appreciate its importance," she said.