( CNN ) -- Although many people think of a heart attack as a painful , sometimes fatal event , there are some heart attacks that go entirely unnoticed . Some people may have had heart attacks without knowing it , studies show . Undiagnosed , or `` silent , '' heart attacks affect nearly 200,000 people in the United States annually . As many as 40 to 60 percent of all heart attacks are unrecognized , studies show . By definition , a heart attack usually happens when a clot gets in the way of blood flow from a coronary artery to the heart . This may cause symptoms such as severe chest pain , shortness of breath , fainting and nausea . Anyone who believes that he or she is having a heart attack should seek emergency medical attention . But sometimes a heart attack is not painful , or the person experiencing it does not recognize the symptoms as heart-related , so he or she does not go to a hospital for treatment . Cardiologists have only recently become attuned to the prevalence of these silent heart attacks , and research on treatment is limited . The risk factors for silent heart attacks are the same as for regular heart attacks , experts say , and include smoking , diabetes , stress and family history . Watch CNN Health Files : Heart attacks » . A new study from Duke University Medical Center shows that these silent heart attacks may occur more frequently than physicians thought . Even if a heart attack occurred in the distant past , it may still leave a signature called a Q-wave on an electrocardiogram . But there are silent heart attacks that do not have associated Q-waves . Researchers used a relatively new technique called delayed-enhancement cardiovascular magnetic resonance and then followed up with patients after about two years . The study was done on 185 patients who had never had a diagnosed heart attack but were suspected of having coronary artery disease . The researchers found that 35 percent of patients had evidence of a heart attack and that silent heart attacks without Q-waves were three times more common than those that had Q-waves . Patients with non-Q-wave silent heart attacks also had 11 times higher risk of death from any cause and a 17-fold risk of death from heart problems compared with patients without any heart damage . But experts do not recommend that people generally be screened for silent heart attacks unless they have other heart-related problems . `` Currently , there has not been a study that has demonstrated that early identification and therapy changes how patients with unrecognized heart attacks do in the future , '' said Dr. Han Kim , a cardiologist at Duke University and lead author of the study . `` If you do n't know when an actual event occurred , it becomes difficult to prescribe therapy . '' Although the study was done on a relatively small sample of people at risk of coronary artery disease , meaning the results may not apply to the general population , other cardiologists say the study has merit in adding to the knowledge of silent heart attacks . `` Ultimately , we 're going to need trials to really establish what treatment works and what does n't , '' said Dr. Eric Schelbert , a cardiologist at the University of Pittsburgh School of Medicine who was not involved in the study . Treatment for someone who has had a silent heart attack is usually the same for someone who came to the hospital immediately after a heart attack , Kim said . This may include beta blockers , statin drugs , aspirin or other medications , Schelbert said . Schelbert said he has seen plenty of patients who have had silent heart attacks ; in fact , he has treated some of his own colleagues who have experienced them . `` It 's an incredibly important thing that the physician scientist community needs to explore further , '' he said . Researchers noted that patients with non-Q-wave silent heart attacks were also generally older and were more likely to have diabetes . There needs to be more of a focus on prevention among these risk groups , said Dr. David Wiener , a cardiologist at the Thomas Jefferson University Hospital in Philadelphia , Pennsylvania , who was not involved in the study .
Silent heart attacks affect nearly 200,000 people in the United States annually . Researchers studied 185 people at risk of coronary artery disease . Treatment for `` silent '' heart attacks is similar to that for regular attacks . More research is necessary to determine whether screening is useful .