There are two main types of angina pectoris. Stable (classical) angina is much more common. About 500,000 new cases of stable angina are reported annually in the United States, according to the American Heart Association.

Stable angina occurs while (or just after) the heart has a need for extra oxygen. The heart needs extra oxygen during a variety of situations that put extra stress on the heart, which include:

Stable angina attacks typically last anywhere from one to 15 minutes, with relief brought on by rest and/or medication. The pain or discomfort associated with episodes of stable angina typically reflects a temporary reduction in blood flow to the heart muscle, rather than permanent damage to the heart muscle.

Chest pain or discomfort that occurs at unpredictable times, including at rest, and does not resolve within 15 minutes (with or without medication) may be a sign of unstable angina. Another indication of unstable angina is an increase in frequency and/or severity of stable angina. Unstable angina is a dangerous medical condition that may signal the progression of underlying coronary artery disease. In some cases, unstable angina will be provoked after a plaque ruptures within the coronary arteries. This causes the artery to narrow, further restricting blood flow to the heart. Unstable angina may also be caused by blood clots that form on damaged plaque.

Episodes of unstable angina can occur at low levels of exertion, even when a person is at rest, and in individuals having no prior history of angina. Indeed, sudden and unrelieved chest pain may cause people to suspect they are having heart attack. Physicians therefore use caution and approach the situation as an emergency. This is because unstable angina can quickly develop to a heart attack, and cardiac enzyme levels may not indicate any heart muscle damage -- or healing -- until days after a heart attack. There is also an increased risk for life-threatening arrhythmias (e.g., ventricular tachycardia and ventricular fibrillation).

It should be noted that some people have episodes of cardiac ischemia that produce no type of angina at all. These episodes are called silent ischemia. This type of cardiac ischemia is usually diagnosed from an exercise stress test. Studies have demonstrated that silent ischemia can be more common among certain ethnic or racial groups. For example, recent reports find that Asian Americans, in comparison to white Americans, experience significantly fewer episodes of ischemic chest pain. Other symptoms, however, occur more frequently, such as shortness of breath, fatigue and palpitations.