Women are almost twice as likely to die within 30 days of a heart attack compared with men, but the difference disappears when age, medical history and disease severity are taken into account.

Researchers found 9.6 per cent of women died within a month of acute coronary syndrome — heart attacks and chest pain or angina caused when the heart doesn't get enough oxygen — compared with 5.3 per cent of men.

Heart attack survivor Claire Coutu regrets waiting a day before going to the hospital after feeling tightness in her chest. (CBC)

Cardiovascular disease is the leading cause of death in both men and women, accounting for one-third of all deaths. The researchers aimed to find out why women tend to fare more poorly than men.

In Tuesday's issue of the Journal of the American Medical Association, Dr. Jeffrey Berger of New York University Medical Center and his colleagues reported much of the difference was accounted for by clinical differences in baseline characteristics such as age, as well as the amount of blockage in arteries.

Berger's team analyzed factors such as the clinical classification of the syndrome and severity of disease, based on 136,247 patients. Of those studied, 28 per cent were women who had an average age of 68 when they had a heart attack or angina, versus age 60 for the men.

"Perhaps the most striking findings in our analyses relate to the examination of mortality according to type of ACS," the study's authors wrote.

"We found a significant interaction between sex and type of ACS, such that 30-day mortality risk among women was modestly higher than men only for those patients presenting with STEMI," a certain pattern on an electrocardiogram after a heart attack that can be more serious.

In patients without STEMI and unstable angina, women had a lower 30-day mortality risk than men.

Watch blood pressure, cholesterol levels

Women tended to be older and have a greater incidence of high blood pressure, high cholesterol and diabetes, the researchers found. The study shows why it's important for women to pay attention to these risk factors, cardiologists said.

Overall, women who had angiographic data (X-rays to study the inside of the heart and blood vessels, also called cardiac catheterization) were more likely to have non-obstructive coronary artery disease and less likely to have two or three diseased vessels involved compared with men, the team found.

"Understanding and considering these differences may lead to better risk stratification and treatment of all patients with ACS," the team concluded.

"It's sometimes hard to sort out if there is a bias of treating women differently or if there's a difference in biology between men and women," said Dr. Beth Abramson, a Toronto cardiologist and a spokesperson for the Heart and Stroke Foundation.

The lastest findings point to biology. Women may experience more subtle symptoms such as shortness of breath and tightness in the chest, compared with crushing chest pain in men, and some treatments are less effective and riskier in women.

Don't ignore tightness in chest

"There may well be some form of bias, which is a legitimate misinterpretation because of a failure to understand that women present differently," said cardiologist and study author Dr. Paul Armstrong of the University of Alberta in Edmonton.

Claire Coutu of Toronto shrugged off the tightness in her chest that she felt while camping, swimming and kayaking as over-exerting herself. The active 59-year-old suffered a minor heart attack.

"I really just got out of breath and I got weak and again the pain in the chest started again and I thought, 'I'd better go to the hospital,'" she recalled. Coutu went to the hospital the next day, a delay she now calls foolish.

The study was supported by the American Heart Association and the Duke Clinical Research Institute.