Unstable angina is often exacerbated by the formation of temporary blood clots within the coronary arteries. If the blood clots persist, a heart attack can occur. It's not surprising, then, that unstable angina often occurs before a heart attack, and should be treated aggressively.
Stable Angina is the name given to the symptoms (usually chest pain or chest discomfort) produced when the heart muscle is in a state of ischemia - that is, when the heart muscle is not getting enough blood supply. Stable angina occurs when there is a partial blockage in a coronary artery that limits the maximum blood flow to the heart muscle.
Patients with stable angina usually have no symptoms during rest or with mild activity, since blood flow to the heart muscle is adequate under these conditions. However, during periods of exercise or other conditions of stress in which the heart muscle requires more oxygen, the partial blockage prevents an adequate increase in blood flow. This lack of sufficient blood flow under these circumstances produces ischemia, thus leading to angina.
This condition is called "stable" angina because the symptoms occur in a predictable fashion - that is, they occur only under conditions such as exercise or other stresses that require the heart to work harder.
In contrast, in unstable angina symptoms occur unpredictably, most often at rest.