Approximately 600,000 women are treated for acute coronary syndrome (ACS) annually in the US. ACS includes heart attack and a milder form called unstable angina. Many of these women have angiograms of which 14-39% show no "significant" coronary artery disease (CAD, cholesterol plaque accumulation in arteries of the heart). The remaining majority of women with ACS have cholesterol plaque buildup which appears severe enough on angiography to limit blood flow to the heart.

It is difficult to advise women with heart attacks and no major heart artery blockages on what to do if chest pain happens again. Additional studies are needed to find out why this sort of heart attack happens and to help doctors understand how to treat patients who have this problem in the best possible way.

Some women with heart attacks who have no major blockage in heart arteries have cholesterol plaque in the arteries of the heart cannot be seen on angiography but can be seen using a newer technique called intravascular ultrasound (IVUS). IVUS involves creating pictures of the artery walls using ultrasound (sound waves) from within the artery itself. In some women without major heart artery blockage, heart attack is caused by low blood flow due to disease of smaller blood vessels which cannot be seen on angiography or IVUS. This problem can be found using magnetic resonance imaging (MRI), which can show blood flow to the heart. MRI may also be used to show where the heart has been damaged. The pattern of damage could suggest that a heart attack in a woman, who has no badly blocked heart arteries, happened for one (or more) of these reasons or another reason.

The Study of Women with ACS and Non-obstructive CAD (SWAN) will use IVUS and MRI to help determine the reasons for heart attacks in women with no major blockages in heart arteries.