Some people have ischemia, which is, reduced oxygen flow to the heart without pain. As many as 4 million Americans may have ischemic episodes without knowing it, a condition called "silent ischemia". They may also have a heart attack with no symptoms. People with angina may also have undiagnosed episodes of silent ischemia. Tests such as an exercise test or a 24-hour portable electrocardiogram (Holter monitor) are used to diagnose silent ischemia.

Silent ischemia, like all ischemia, is caused by a blockage in a coronary artery. In this case, however, ischemia may be caused by a brief or less severe blockage than those that lead to a heart attack, or ischemia may be silent because of decreased pain perception in some people.

Ischemia can be entirely silent, or without any symptoms, and still be severe enough to cause a heart attack. One large study of the population of Framingham, Mass., found that 35% of heart attacks in women and 28% in men are not recognized as heart attacks; about half were silent, and about half caused symptoms the victims incorrectly attributed to other causes. Many people with a typical heart attack have no prior warning symptoms despite having had coronary artery disease for some time. Even people with symptoms of angina are often found, upon examination, to have had episodes of ischemia without angina.

The bottom line: People with silent ischemia have the same risk of a future heart attack as people with typical symptoms.