Unstable angina is a change in your usual pattern of stable angina. Your symptoms do not happen at a predictable time. For example, you may feel angina at rest or with light activity. Your symptoms may be more frequent, severe, or longer-lasting than your usual pattern of stable angina. Your symptoms may not go away when you try your typical ways of relieving them, such as rest or nitroglycerin.

Unstable angina is an emergency. It may mean that you are having a heart attack.

Unstable angina happens when blood flow to the heart is suddenly slowed by narrowed vessels or small blood clots that form in the coronary arteries. A clot may slow blood flow in a coronary artery. There may be no damage to the heart muscle.