Acute Coronary Syndrome

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Acute coronary syndrome (ACS) is a term given to various heart conditions by doctors, including a heart attack (myocardial infarction) and unstable angina. These conditions are due to there being a reduced amount of blood flowing to a part of the heart. Various treatments are given and these usually depend on the type of ACS. Treatments help to ease the pain, improve the blood flow and to prevent any future complications.

The term 'acute coronary syndrome' covers a range of disorders, including heart attack (myocardial infarction) and unstable angina that are caused by the same underlying problem.

The underlying problem is a sudden reduction of blood flow to part of the heart muscle. This is usually caused by a blood clot that forms on a patch of atheroma within a coronary artery (which is described below).

The types of problems range from unstable angina to an actual myocardial infarction. In unstable angina a blood clot causes reduced blood flow, but not a total blockage, so the heart muscle supplied by the affected artery does not die (infarct). The location of the blockage, the length of time that blood flow is blocked, and the amount of damage that occurs determine the type of ACS.


The heart is made mainly of special muscle. The heart pumps blood into blood vessels (arteries) which take the blood to every part of the body.

Like any other muscle, the heart muscle needs a good blood supply. The coronary arteries take blood to the heart muscle. The main coronary arteries branch off from the aorta. The aorta is the large artery which takes oxygen-rich blood from the heart chambers to the body. The main coronary arteries divide into smaller branches which take blood to all parts of the heart muscle.

ACS ranges from a heart attack (myocardial infarction) to unstable angina.

Myocardial infarction

If you have a myocardial infarction, a coronary artery or one of its smaller branches is suddenly blocked. The part of the heart muscle supplied by this artery loses its blood (and oxygen) supply. This part of the heart muscle is at risk of dying unless the blockage is quickly undone. (The word infarction means death of some tissue due to a blocked artery which stops blood from getting past.) In addition to being known as a heart attack, a myocardial infarction is sometimes called a coronary thrombosis.

Cross-section diagram of the heart

There are different types of myocardial infarction which are based on what is seen on your heart tracing (also called an electrocardiograph (ECG)). The two main types are called ST elevation myocardial infarction (STEMI) and non-ST elevation myocardial infarction (NSTEMI). In a STEMI, the artery supplying an area of the heart muscle is completely blocked. However, in an NSTEMI, the artery is only partly blocked, so only part of the heart muscle being supplied by the affected artery is affected.

Unstable angina

Unstable angina occurs when the blood clot causes a reduced blood flow, but not a total blockage. This means that the heart muscle supplied by the affected artery does not die (infarct).

Note: this article discusses only NSTEMI and unstable angina. For information on STEMI, see separate leaflet called Myocardial Infarction (Heart Attack).

The majority of cases are due to there being some narrowing in the blood vessels supplying the heart. This is usually due to the presence of some atheroma within the lining of the artery. Atheroma is like fatty patches or plaques that develop within the inside lining of arteries. (This is similar to water pipes that get furred up.)

Plaques of atheroma may gradually form over a number of years in one or more places in the coronary arteries. Each plaque has an outer firm shell with a soft inner fatty core. Atheroma leads to the blood vessels narrowing.

Various other uncommon conditions can also block a coronary artery. For example:

  • Inflammation of the coronary arteries (rare).
  • A stab wound to the heart.
  • A blood clot forming elsewhere in the body (for example, in a heart chamber) and travelling to a coronary artery where it gets stuck.
  • Taking cocaine, which can cause a coronary artery to go into spasm.
  • Complications from heart surgery.
  • Some other rare heart problems.

These are not dealt with further in this leaflet.

ACS is common. About 114,000 people in the UK are admitted to hospital with an ACS each year. Most occur in people aged over 50 and become more common with increasing age. Sometimes younger people are affected.

The risk factors for having an ACS are actually the same as the risk factors for having a heart attack (myocardial infarction) or cardiovascular disease. See separate leaflet called Preventing Cardiovascular Diseases which discusses these in more detail.

Briefly, risk factors that can be modified and may help to prevent an ACS include:

The most common symptom of a ACS is having severe chest pain. The pain often feels like a heavy pressure on your chest. The pain may also travel up into your jaw and down your left arm, or down both arms. You may also sweat, feel sick and feel faint. You may also feel short of breath.

The pain may be similar to a bout of normal (stable) angina, but it is usually more severe and lasts longer. (In people who have stable angina, an angina pain usually goes off after a few minutes. ACS pain usually lasts more than 15 minutes - sometimes several hours.)

Some people with an ACS may not have any chest pain, particularly those who are elderly or those who have diabetes.

It can sometimes be difficult for doctors to distinguish between acute coronary syndrome (ACS) and other causes of pains in the chest. If you are suspected of having ACS then you should be referred urgently to hospital. On admission to hospital, various tests are usually done. These include:

  • A heart tracing called an electrocardiograph (ECG). There may be typical changes to the normal pattern of the heart tracing if you have an ACS. However, an ECG can also be normal in some cases.
  • Blood tests. A blood test that measures a chemical called troponin is the usual test that is done. This chemical is present in heart muscle cells. Damage to heart muscle cells releases troponin into the bloodstream. The level of troponin is raised if you have a heart attack (myocardial infarction) but it is not raised in unstable angina.

Your heart tracing may be monitored for a few days to check on the heart rhythm. Various blood tests will be done to check on your general well-being. Other tests may also be done in some cases. This may be to clarify the diagnosis (if the diagnosis is not certain) or to diagnose complications such as heart failure if this is suspected. For example, an ultrasound scan of the heart (an echocardiogram).

Also, before discharge from hospital, you may be advised to have tests to assess the severity of atheroma in the coronary arteries. These are tests similar to those used to assess the severity of angina and are discussed more fully in the separate leaflet called Angina.

The treatment of ACS varies between cases. A heart attack (myocardial infarction) is treated differently to unstable angina. Treatments may vary depending on your situation.

If you have had a STEMI then you will be treated the same as those who have had a myocardial infarction (again, see separate leaflet called Myocardial Infarction (Heart Attack) for more details). Treatment of people with unstable angina or NSTEMI consists of two phases:

  • Relief of any pain.
  • Preventing progression to, or limiting the extent of, a myocardial infarction.

Your treatment usually varies depending on your risk score. This is a risk score for a myocardial infarction in the future. Various factors are taken into account for this score, including:

  • Your age.
  • Your other risk factors for cardiovascular disease (for example, if you smoke, have raised cholesterol or have high blood pressure or diabetes).
  • Your blood test results.
  • What your ECG looks like when you first attend the hospital.

Aspirin and other antiplatelet medicines

As soon as possible after an ACS is suspected you will be given a dose of aspirin. Aspirin reduces the stickiness of platelets. Platelets are tiny particles in the blood that trigger the blood to clot. It is the platelets that become stuck on to a patch of atheroma inside an artery that go on to form the clot (thrombosis) of a myocardial infarction.

Another antiplatelet medicine called clopidogrel is also sometimes given. This works in a different way to aspirin and adds to the action of reducing platelet stickiness. It is usually given with aspirin if there is a change in your heart rhythm or if your troponin level is raised in your blood test. In some cases it is given as an alternative to aspirin (for example, if you are allergic to aspirin).

Injections of heparin or a similar medicine

These are usually given for a few days to help prevent further blood clots forming.

Pain relief

A strong painkiller given by injection into a vein is given to ease the pain.

Glycoprotein llb/lla receptor antagonist

If the doctors think that you have a high risk of having a myocardial infarction, you may be given a medicine called a glycoprotein llb/lla receptor antagonist. This can help relieve your pain and also works to reduce the chances of blood clots completely blocking your arteries. This medicine is given to you as a drip, directly into your veins. This medicine is also used if you are going to have a treatment to help widen your arteries (for example, an angioplasty - see below).

Treatment to restore blood flow in the blocked coronary artery

The part of the heart muscle starved of blood does not die (infarct) immediately during a myocardial infarction. If blood flow is restored within a few hours, much of the heart muscle that would have been damaged will survive. This is why a myocardial infarction is a medical emergency, and treatment is given urgently. The quicker the blood flow is restored, the better the outlook.

Not all cases need this treatment. However, for some people this is the most appropriate treatment. There are two treatments that can be done to restore blood flow back through the blocked artery:

  • Emergency angioplasty is, ideally, the best treatment if it is available and can be done within a few hours of symptoms starting. In this procedure a tiny wire with a balloon at the end is put into a large artery in the groin or arm. It is then passed up to the heart and into the blocked section of a coronary artery, using special X-ray guidance. The balloon is blown up inside the blocked part of the artery to open it wide again. A stent may be left in the widened section of the artery. A stent is like a wire mesh tube which gives support to the artery and helps to keep the artery widened. See separate leaflet called Coronary Angioplasty for details.
  • An injection of a clot-busting medicine is an alternative to emergency angioplasty. In reality, this is the more common treatment, as it can be given easily and quickly in most situations. Some ambulance crews are trained to give this treatment. Note: a common clot-buster medicine used in the UK is called streptokinase. If you are given this medicine you should not be given it again if you have another myocardial infarction in the future. This is because antibodies develop to it and it will not work so well a second time. An alternative clot-buster medicine should be given if you have another myocardial infarction in the future.

A beta-blocker medicine

Beta-blocker medicines block the action of certain hormones such as adrenaline (epinephrine). These hormones increase the rate and force of the heartbeat. Beta-blocker medicines have some protective effect on the heart muscle. They also help to prevent abnormal heart rhythms from developing. A beta-blocker medicine will also help to reduce the risk of you having a myocardial infarction in the future.


Some people have a raised blood sugar level when they have an ACS, even if they do not have diabetes. If this occurs then your blood sugar levels may need to be controlled with insulin. If you have diabetes then it is also likely that you will need to be treated with insulin to control your blood glucose levels when you are in hospital.


You may be given oxygen which works to reduce the risk of damage to your heart muscle.

Once you have had an ACS, you will normally be advised to take regular medication for the rest of your life. Long-term medication is designed to reduce the risk of a future heart attack (myocardial infarction) and so is advised if you have had unstable angina as well as if you have already had a myocardial infarction. See separate leaflet called Medication after a Myocardial Infarction which discusses this more fully.

Briefly, the following four medicines are commonly prescribed to prevent a further myocardial infarction, and to help prevent complications:

  • Aspirin - to reduce the stickiness of platelets in the blood, which helps to prevent blood clots forming. If you are not able to take aspirin then an alternative antiplatelet medicine such as clopidogrel may be advised.
  • A beta-blocker - to slow the heart rate and to reduce the chance of abnormal heart rhythms developing.
  • An angiotensin-converting enzyme (ACE) inhibitor. ACE inhibitors have a number of actions, including having a protective effect on the heart.
  • A statin medicine to lower the cholesterol level in your blood. This helps to prevent the build-up of atheroma.

Many people recover well from an ACS and have no complications. Before discharge from hospital it is common for a doctor or nurse to advise you how to reduce any risk factors (see above). This advice aims to reduce your risk of a future ACS or myocardial infarction as much as possible.

After recovering from an ACS, it is natural to wonder if there are any dos and don'ts. However, regular exercise and getting back to normal work and life are usually advised. Much can be done to reduce the risk of a further ACS or a heart attack (myocardial infarction). See separate leaflet called After a Myocardial Infarction which discusses this more fully.

Original Author:
Dr Louise Newson
Current Version:
Peer Reviewer:
Dr Hannah Gronow
Document ID:
13357 (v4)
Last Checked:
Next Review:
The Information Standard - certified member
See also: Acute Coronary Syndrome written for professionals
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