Those symptoms, or lack of symptoms, can result in delayed medical care and differences in treatment that might in turn help explain why women in the study were also more likely to die of their heart attacks, according to researchers.

"They might not even know they're having a heart attack," said Dr. John Canto, from the Watson Clinic in Lakeland, Florida, who worked on the report.

According to the Centers for Disease Control and Prevention, close to 800,000 Americans have their first heart attack every year, and heart disease is the leading cause of death for both men and women.

Although the results are based on a study of more than a million heart attack patients, Canto cautioned, they are still preliminary. But, he added, they do challenge the notion that chest pain and discomfort should be considered "the hallmark symptom" for all heart attack patients.

"If our results are in fact true, I would argue that rather than the one-size-fits-all symptom message, we also have to tailor that message to say that women less than 55 are also at higher risk for atypical presentation," which includes jaw or arm pain, Canto told Reuters Health.

He and his colleagues analyzed medical records in a national database of heart attack patients from 1994 to 2006, including about 1.1 million people treated at close to 2,000 hospitals.

They found that 31 percent of male patients didn't have any chest pain or discomfort, compared to 42 percent of women.

The likelihood of having such an "atypical presentation" differed most between younger women and younger men, the researchers reported Tuesday in the Journal of the American Medical Association.

Women under 45 were 30 percent more likely than men in their age group to present without chest pain; between ages 45 and 65 the difference dropped to around 25 percent, and after 75, it all but disappeared.

A similar pattern, with smaller differences between sexes, was seen in the likelihood of death.

Almost 15 percent of women died in the hospital after their heart attack, compared to about 10 percent of men. Younger women with no chest pain were almost 20 percent more likely to die than male counterparts. But after age 65, the women's risk fell below that of men.

Dr. Patrick O'Malley, an internist at the Uniformed Services University of the Health Sciences in Bethesda, Maryland, said at least part of that difference could be due to lack of action by patients and doctors when symptoms are unusual.

"We tend to not think of heart disease in younger women if they're not having chest pain... and therefore we're not going to be as aggressive," he told Reuters Health. "It does delay treatment."

For patients, "because it's not chest pain, they'll be coming later," added O'Malley, who didn't participate in the new research.

Women tend to be older than men when they have a first heart attack, and in this study the average age difference was seven years.

"Young women shouldn't be having heart attacks, so when a young woman has a heart attack, there's something biologically different in that patient," Canto said.

He said those biological differences may include variations in hormones or the way clots form in younger women.

The registry used in the new study was funded by Genentech, and some of the authors report financial partnerships with pharmaceutical companies.

Instead of chest pain, some people having a heart attack may instead have unexplained shortness of breath, or pain in other areas, including the jaw, neck, arms, back and stomach, Canto said.

Women, especially those who are predisposed to heart attacks because they have diabetes, have a family history of heart disease or are smokers, should know that a lack of chest pain doesn't rule out the possibility of a heart attack, he added.

Dr. Gregg Fonarow, a cardiologist at the David Geffen School of Medicine at UCLA, agreed.

The findings, he told Reuters Health in an email, "emphasize the need for women and men of all ages to recognize that heart attacks can present without chest pain and that symptoms including shortness of breath, weakness, a feeling of indigestion, or unexplained fatigue require immediate medical attention."

Fonarow, who also wasn't involved in the new study, added that "an even better approach than early recognition and treatment is to prevent having a heart attack in the first place," by understanding heart risks and adopting a healthier lifestyle.

SOURCE: bit.ly/hwxtTL Journal of the American Medical Association, online February 21, 2012.

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