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Unstable angina is the pain you feel in your chest when your heart is not getting enough oxygen. It may be a worsening of the chest pain you were already getting with a condition called stable angina. Or it may be a brand new chest pain.

Stable angina usually happens only when you are active. It typically stops when you rest for a few minutes and take your angina medication. But unstable angina can happen even when you are resting, and it may not go away when you take your angina medication. The pain is usually worse than in stable angina, and it is likely to go on for at least 20 minutes.¹

Doctors often think of unstable angina as the middle step between stable angina and a heart attack. The lack of oxygen is worse than in stable angina. But it isn't as bad as in a heart attack. A heart attack can cause permanent damage to your heart.

If you get unstable angina, it is an emergency. You need treatment right away. It's important to dial 911 and get emergency medical help.

In the longer term, taking medication and changing how you live can help you have fewer and milder attacks. It can also lower your risk of having a heart attack.

• Unstable angina usually happens when the arteries in the heart get narrow from a buildup of fat. Doctors call this coronary artery disease.
• People often describe unstable angina as a discomfort, an ache or pain, or a burning feeling or heavy pressure. It usually lasts at least 20 minutes.
• You are most likely to get these symptoms in your chest. But you can also get them in your back, arm, jaw or throat.
• This can start without any warning, even if you are resting.
• Your doctor should check out any new chest discomfort or pain, even if it goes away.
• If it is unstable angina, you need to go to the hospital right away for treatment. This is to lower your risk of having a heart attack.
• You will also need to take other medications, some for life, to lower your risk of having more attacks of unstable angina.
• Making some changes in your daily life also will lower these risks. If you smoke, now is the time to stop.

To understand what happens when you have unstable angina, it helps to know something about your heart and how it works.²

Your heart is in the middle of your chest. It is a muscle shaped like a fist. It works automatically, so you don't have to think about making it beat.

Your heart works like a pump. Every time it beats, it pushes blood around your body.

Blood carries oxygen and food (nutrients) to your cells. The cells in your body will die if they don't get a constant supply of both. For more, see How your heart moves blood around your body.

Your heart works harder, and for longer, than any other organ in your body. If your heart stops working, every other part of your body will die within minutes.

Your heart has thick, muscular walls. It's divided into two sides, right and left. Each side has an upper part and a lower part. Doctors call these chambers.

The two chambers on the right side of your heart pump blood to your lungs, where the blood picks up oxygen. Blood then returns to the two chambers on the left side. They pump the blood to the rest of your body, taking oxygen and food to your cells.

The two upper chambers of your heart are called the right atrium and the left atrium. The two lower chambers are larger. They are called the right ventricle and the left ventricle.

For more, see What happens when your heart beats.

To do its job, your heart needs its own supply of blood. The blood carries oxygen to your heart's cells, so that it can do its work.

Your heart gets its blood from your coronary arteries. They supply the heart's muscular wall with blood that is rich in oxygen.

Your left coronary artery has two main branches.

• One is called the left anterior descending artery (or LAD for short). It supplies most of the blood to your left ventricle, the part of your heart that pumps blood out to your body. This job makes this artery very important to your health.
• The other one is called the left circumflex artery.

Your right coronary artery is smaller. Its branches carry blood to the back of your heart. These arteries split into a network of smaller vessels that take blood deeper into your heart muscle.

For more, see How blood moves through your heart.

Most people get unstable angina because of a condition called coronary artery disease. This is when clumps of fat (called plaques) build up on the smooth lining of a coronary artery. Over time, they make the artery narrower, stiffer and rougher.

This is called atherosclerosis. It's very common. It can happen in arteries anywhere in your body. If you have atherosclerosis in your coronary arteries, you have coronary artery disease. You might hear doctors call this ischemic heart disease or coronary heart disease.

Here is what happens in unstable angina.

• A plaque in one of your coronary arteries breaks open and tears.
• A blood clot forms over the tear to try to patch it up, just like a scab forms on your skin if you cut yourself.¹ Doctors call this clot a thrombus.
• The clot partly blocks the artery. Then there isn't enough blood getting through to your heart.
• Not enough blood means not enough oxygen. This is what causes the bad pain and discomfort of unstable angina.
• The clot can get bigger. If the clot totally blocks off the artery and there isn't any oxygen getting to your heart, it's called a heart attack. This can permanently damage your heart. For more, see Heart attack.

Sometimes, because the symptoms are the same, it can be hard for doctors to tell if someone has unstable angina or a type of heart attack. They can find out by doing certain tests. For more, see How do doctors diagnose unstable angina?

You may hear doctors use the words acute coronary syndrome (ACS for short). This is a general term. It covers both unstable angina and some types of mild heart attack. For more, see Acute coronary syndrome.

We have looked at unstable angina caused by coronary artery disease. But some people have angina caused by another condition that stops their heart from getting enough oxygen. For more, see Other causes of angina. If your angina is caused by something else, talk to your doctor about how it should be treated.

There are different types of angina, too. On these pages, we look at unstable angina. For more on the others, see Other types of angina.

We don't know exactly why fatty clumps (called plaques) build up in some people's arteries and cause angina. And we don't know why some people's angina is stable while other people's angina is unstable. But we do know that some things make you more likely to get coronary artery disease and angina. Doctors call these things risk factors. Having a risk factor doesn't mean you'll get angina for sure. It just means you are more likely to get it than someone who doesn't have that risk factor.

The risk factors for angina are the same as the risk factors for heart disease. We've listed the main ones below.³

• Being male
• Being middle-aged or older
• Having heart attacks or coronary disease run in your family
• Smoking
• Being overweight or obese
• Having high cholesterol
• Having high blood pressure
• Having diabetes.

The more risk factors you have, the more likely you are to get unstable angina. But you and your doctor can work to make your risk factors less dangerous. This can reduce your chance of getting an attack of unstable angina again or of having a heart attack.

You can't do anything about some risk factors, like conditions that run in your family or getting older. But you can do something about others, like smoking. If you smoke, now is the time to stop. And eating better and exercising will both help.

Being under stress may affect some risk factors. For example, if you're stressed, you might eat or smoke more than usual. We don't know if stress by itself makes you more likely to get angina.²

For more on steps you can take to stay as healthy as possible, see Unstable angina: what you can do to help yourself.

You can also get good treatments for some of the things that put you at risk for getting angina. For more, see our articles on these topics:

• Diabetes
• High blood pressure
• High cholesterol
• Obesity
• Smoking.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.