Heart Attack (Myocardial Infarction)

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A heart attack (myocardial infarction) is usually caused by a blood clot, which stops the blood flowing to a part of your heart muscle. You should call for an ambulance immediately if you develop severe chest pain. Treatment with a clot-busting medicine or an emergency procedure to restore the blood flow through the blocked blood vessel are usually done as soon as possible. This is to prevent or minimise any damage to your heart muscle. Other treatments help to ease the pain and to prevent complications. Reducing various risk factors can help to prevent a heart attack.

The heart is mainly made of special muscle (myocardium). 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. See separate leaflet called The Heart and Blood Vessels for more details.

If you have a heart attack, 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 if the vessel is blocked. This part of the heart muscle is at risk of dying unless the blockage is quickly removed. When a part of the heart muscle is damaged it is said to be infarcted. The term myocardial infarction (MI) means damaged heart muscle.

If a main coronary artery is blocked, a large part of the heart muscle is affected. If a smaller branch artery is blocked, a smaller amount of heart muscle is affected. After a heart attack, if part of the heart muscle has died, it is replaced by scar tissue over the following few weeks.

Some newer terms used by doctors

A heart attack and myocardial infarction (MI) are commonly used terms, and mean the same thing. However, the term MI is used less often now by doctors. This is because there are actually a range of conditions that can be caused by a sudden reduction in blood flow in a coronary artery. This range of conditions has an overall term called acute coronary syndrome (ACS).

Two main sub types of ACS can be diagnosed by what is seen on your heart tracing (electrocardiograph, or ECG). The two main types are called ST-elevation MI (STEMI) and non-ST-elevation MI (NSTEMI). The ST elevation refers to a section on the ECG tracing.

NSTEMI can also include unstable angina. Most people with angina chest pains only have pain with a certain amount of exertion. This is called stable angina. If the pattern of your pain changes suddenly and the pains develop after minimal exertion, or while you are resting, this is called unstable angina. This is an emergency and needs immediate medical care. See separate leaflet called Angina for further information.

In STEMI, the artery supplying an area of the heart muscle is completely blocked. However, in NSTEMI, the artery is only partly blocked, so only part of the heart muscle supplied by the affected artery is affected. Your treatment can depend upon which type you have - STEMI or NSTEMI.

This leaflet mainly discusses STEMI - which is just referred to as a heart attack from now on. See separate leaflet called Acute Coronary Syndrome for more details on NSTEMI, unstable angina and ACS.

Blood clot (thrombosis) - the cause in most cases

The most common cause of a heart attack is a blood clot that forms inside a coronary artery, or one of its branches. This blocks the blood flow to a part of the heart.

Blood clots do not usually form in normal arteries. However, a clot may form if there is 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.

What happens is that a crack develops in the outer shell of the atheroma plaque. This is called plaque rupture. This exposes the softer inner core of the plaque to blood. This can trigger the clotting mechanism in the blood to form a blood clot. Therefore, a build-up of atheroma is the root problem that leads to most cases of ACS. (The diagram below shows four patches of atheroma as an example. However, atheroma may develop in any section of the coronary arteries.)

Cross-section diagram of the heart

Treatment with clot-busting medication or a procedure called angioplasty (see below) can break up the clot and restore blood flow through the artery. If treatment is given quickly enough this prevents damage to the heart muscle, or limits the extent of the damage.

Uncommon causes

Various other uncommon conditions can 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.

The rest of this leaflet deals only with the common cause - thrombosis over an atheroma plaque.

Heart attacks are common. About 146,000 people in the UK have a heart attack every year. Most occur in people aged over 50 and it becomes more common with increasing age. Sometimes younger people are affected.

A heart attack is three times more common in young men than in young women. However, after the menopause, the female hormones no longer protect the heart so the risk is then the same for men and women.

A heart attack may occur in people known to have heart disease, such as people with chest pain called angina. It can also happen out of the blue in people with no previous symptoms of heart disease. This is because fatty patches or plaques (called atheroma) often develop  without any symptoms at first.

Certain risk factors increase the risk of more atheroma forming. This can lead to ACS. See separate leaflet called Preventing Cardiovascular Diseases for more details.

Briefly, risk factors that can be modified and may help to prevent a heart attack include:

The most common symptom is severe chest pain, which often feels like a heavy pressure feeling 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 angina, but it is usually more severe and lasts longer. (Angina usually goes off after a few minutes. Heart attack pain usually lasts more than 15 minutes - sometimes several hours.)

However, some people have only a mild discomfort in their chest. The pain can sometimes feel like indigestion or heartburn.

Occasionally, a heart attack happens without causing any pain. This is usually diagnosed when you have a heart tracing (electrocardiograph, or ECG) at a later stage.

Some people collapse and die suddenly, if they have a large portion of heart muscle damaged. This is not very common.

Dial 999/112/911 for an ambulance immediately.

Then, if you have some, take one aspirin tablet (see below for the reason for this). You will normally be admitted straight to hospital.

Many people develop chest pains that are not due to a heart attack. For example, you can have quite bad chest pains with heartburn, gallbladder problems or with pains from conditions of the muscles in the chest wall. Therefore, tests are usually done to confirm a heart attack. These are:

  • A heart tracing (electrocardiograph, or ECG). There are typical changes to the normal pattern of the ECG in a heart attack. Patterns that occur include things called pathological Q waves and ST elevation. However, it is possible to have a normal ECG even if you have had a heart attack.
  • Blood tests. A blood test that measures a chemical called troponin is the usual test that confirms a heart attack. This chemical is present in heart muscle cells. Damage to heart muscle cells releases troponin into the bloodstream. In a heart attack the blood level of troponin increases within 3-12 hours from the onset of chest pain, peaks at 24-48 hours and returns to a normal level over 5-14 days.

A rough idea as to the severity of the heart attack (the amount of heart muscle that is damaged) can be gauged by:

  • The degree of abnormality of the ECG; and
  • The level of troponin in the blood.

Another chemical that may be measured in a blood test is called creatine kinase. This too is released from heart muscle cells during a heart attack.

Your ECG will 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 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 (echocardiogram) or a test called a myocardial perfusion scan may be done.

Also, before discharge from hospital, you may be advised to have tests to assess the severity of the fatty patches or plaques (atheroma) in the coronary arteries - for example:

The following is a typical situation and mentions the common treatments that are usually offered. However, each case is different and treatments may vary depending on your situation.

Aspirin and other antiplatelet medicines

As soon as possible after a heart attack 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.

Other antiplatelet medicines called clopidogrel or ticagrelor may be given. They work in a different way to aspirin and help reduce platelet stickiness.

Injections of heparin or a similar medicine

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

Pain relief

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

Treatment to restore blood flow in the blocked coronary artery

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

There are two treatments that can restore blood flow back through the blocked artery:

  • Emergency angioplasty. Ideally this is 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 more details.
  • An injection of a clot-busting medicine is an alternative to emergency angioplasty. It can be given easily and quickly in most situations. Some ambulance crews are trained to give this. Note: a common clot-busting 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 heart attack in the future. This is because antibodies develop to it and it will not work so well a second time. An alternative clot-busting medicine should be given.

Both the above treatments usually work well to restore blood flow and greatly improve the outlook. The most crucial factor is the speed at which one or other treatment is given after symptoms have started.

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 and they also help to prevent abnormal heart rhythms from developing. Beta-blocker medicines will also help to prevent another heart attack.


Some people have a raised blood sugar level when they have a heart attack, even if they do not have diabetes. If this occurs then your blood sugar (glucose) 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.

Normally you will be advised to take regular medication for the rest of your life. See separate leaflet called Medication After a Heart Attack (Myocardial Infarction) for more details. Many people recover well from a heart attack 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 heart attack as much as possible.

This often depends on the amount of heart muscle that is damaged. In many cases, only a small part of the heart muscle is damaged and then heals as a small patch of scar tissue. The heart can usually function normally with a small patch of scar tissue. A larger heart attack is more likely to be life-threatening or cause complications.

Even before treatments became available to restore blood flow, many people made a full recovery. With the help of modern treatment, particularly if you are given treatment within a few hours to restore blood flow, a higher percentage of people now make a full recovery.

Some possible complications include the following:

  • Heart failure. If a large area of the heart muscle is damaged then the pumping ability of the heart may be reduced. Less blood than usual is then pumped around the body, especially when extra blood is needed when you exercise. Symptoms such as breathlessness, tiredness and swollen ankles may develop. Mild heart failure can often be treated with medication. Severe heart failure can be serious and even life-threatening. See separate leaflet called Heart Failure for more details.
  • Abnormal heart rhythms may occur if the electrical activity of the heart is affected. The main risk of this happening is within the first few hours after a heart attack. Sudden, chaotic, fast heartbeats may occur. This is called ventricular fibrillation and is the common cause of cardiac arrest. This needs immediate treatment with an electrical shock given by a defibrillator. Otherwise, collapse and sudden death are likely. Other less serious abnormal heart rhythms can also occur which can often be treated with medicines. See separate leaflet called Abnormal Heart Rhythms (Arrhythmias) for more details.
  • A further heart attack may occur sometime in the future. This is more likely if the coronary arteries are badly affected with atheroma, or further build-up of atheroma continues. If the risk of this is thought to be high then surgery may be advised to bypass or widen severely narrowed coronary arteries.

The most crucial time is during the first day or so. If no complications arise and you are well after a couple of weeks then you have a good chance of making a full recovery. A main objective then is to get back into normal life and to minimise the risk of a further heart attack.

When recovering from a heart attack, it is natural to wonder if there are any dos and don'ts. In the past, well-meaning but bad advice to 'rest and take it easy from now on' caused some people to become over-anxious about their hearts. Some people gave up their jobs, hobbies, and any activity that caused exertion, for fear of straining the heart.

However, quite the opposite is true for most people who recover from a heart attack. Regular exercise and getting back to normal work and life are usually advised. Much can be done to reduce the risk of a further heart attack. See separate leaflet called After a Heart Attack (Myocardial Infarction) for more details.

Original Author:
Dr Tim Kenny
Current Version:
Peer Reviewer:
Dr John Cox
Document ID:
4301 (v46)
Last Checked:
Next Review:
The Information Standard - certified member
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