Myocardial infarction (MI) is usually caused by a blood clot that stops blood flow in a heart (coronary) artery. Call for an ambulance immediately if you develop severe chest pain. Treatment with a 'clot busting' drug or an emergency procedure to restore blood flow through the blocked artery are usually done as soon as possible to prevent damage to heart muscle. Other treatments help to ease the pain and prevent complications. Reducing risk factors can help to prevent an MI.
What is a myocardial infarction?
Myocardial infarction (MI) means that part of the heart muscle suddenly loses its blood supply. Without prompt treatment, this can lead to damage to the affected part of the heart. An MI is sometimes called a heart attack or a coronary thrombosis. An MI is part of a range or disorders called 'acute coronary syndromes'.
The heart is mainly made of special muscle. The heart pumps blood into arteries (blood vessels) 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.
What happens when you have a myocardial infarction?
If you have an MI, 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.)
Thrombosis - the cause in most cases
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 MI.
Treatment with 'clot busting' drugs or a procedure called angioplasty 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 and cause an MI. 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; cocaine abuse which can cause a coronary artery to go into spasm; complications from heart surgery; and some other rare heart problems. There are not dealt with further.
Prevalance
MI is common. About 180,000 people in the UK are admitted to hospital each year with an MI. Most MIs occur in people over 50, and become more common with increasing age. Sometimes younger people are affected. An MI is three times more common in men than women. An MI may occur in people known to have heart disease such as angina. It can also happen 'out of the blue' in people with no previous symptoms of heart disease. (Atheroma often develops without any symptoms at first.)
What are the symptoms of a myocardial infarction?
A small MI occasionally happens without causing pain (a 'silent MI'). It may be truly pain-free, or sometimes the pain is mild and you may think it is just heartburn or 'wind'.
Some people collapse and die suddenly if they have a large or severe MI.
What should I do if I suspect I am having a myocardial infarction?
How is myocardial infarction diagnosed and assessed?
A heart tracing called an ECG (electrocardiograph). There are typical changes to the normal pattern of the heart tracing if you have an MI. Patterns that occur with an MI include things called 'pathological Q waves' and 'ST elevation'. However, it is possible to have a normal ECG even if you have had an MI.
Blood tests. A blood test that measures a chemical called troponin is the usual test that confirms an MI. This chemical is present in heart muscle cells and damage to heart muscle cells releases troponin into the bloodstream. 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 MI (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 creatinine kinase. This too is released from heart muscle cells during an MI.
Your heart tracing will be monitored for a few days to check on the heart rhythm. Various blood tests will be done to check on your general wellbeing.
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 echocardiogram (an ultrasound scan of the heart) or a test called myocardial perfusion scintigraphy may be done.
Also, before discharge from hospital, you may be advised to have tests to assess the severity of atheroma in the coronary arteries. For example, an ECG taken whilst you exercise on a treadmill or bike ('exercise-ECG'). Or, angiography of the coronary arteries. In this test a dye is injected into the coronary arteries. The dye can be seen by special X-ray equipment. This shows up the structure of the arteries (like a road map) and can show the location and severity of any atheroma.
What is the treatment for myocardial infarction?
Aspirin and other antiplatelet drugs
As soon as possible after an MI 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 onto a patch of atheroma inside an artery that go on to form the clot (thrombosis) of an MI. Another antiplatelet drug called clopidogrel is also usually given as soon as possible. This works in a different way to aspirin and adds to the action of reducing platelet stickiness.
Pain relief
A strong pain killer given by injection into a vein will ease the pain.
Treatment to restore blood flow in the blocked coronary 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.
An injection of a 'clot busting' drug 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: the common 'clot buster' drug used in the UK is called streptokinase. If you are given this drug you should not be given it again if you have another MI in the future. This is because antibodies develop to it and it will not work so well a second time. An alternative 'clot buster' drug should be given if you have another MI in the future.
Both the above treatments usually work well to restore blood flow and greatly improve the outlook. The most crucial factor is the quickness in which one or other treatment is given after symptoms have developed.
A betablocker drug
Beta-blockers 'block' the action of certain hormones such as adrenaline. These hormones increase the rate and force of the heartbeat. Beta-blockers have some protective effect on the heart muscle and they also help to prevent abnormal heart rhythms from developing.
These are usually given for a few days to help prevent further blood clots.
Treatment after you have had a myocardial infarction
Once you have had an MI, you will normally be advised to take regular medication for the rest of your life. Medication after an MI is discussed more fully in another leaflet called 'Medication After a Myocardial Infarction'. Briefly, the following four drugs are commonly prescribed to prevent a further MI, 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 be able to take aspirin then an alternative anti-platelet drug 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 ACE inhibitor (angiotensin converting enzyme inhibitor). ACE inhibitors have a number of actions including having a protective effect on the heart.
A statin drug to lower the cholesterol level in your blood. This helps to prevent the build-up of atheroma.
Also, you will normally be advised to take the antiplatelet drug clopidogrel in addition to aspirin. However, this is usually only advised for a certain number of weeks or months, depending on the type and severity of the MI.
Many people recover well from an MI 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 below). This advice aims to reduce your risk of a future MI as much as possible.
Can myocardial infarction be prevented?
Everybody has a risk of developing atheroma which can lead to an MI. However, certain 'risk factors' increase the risk and include:
Preventable or treatable risk factors:
smoking
hypertension (high blood pressure)
high cholesterol level
lack of exercise
a poor diet
obesity
excess alcohol
Having diabetes. But if you have diabetes, the increased risk of heart disease is minimised by good control of the blood sugar level, and reducing blood pressure if it is high.
Risk factors that are fixed and you cannot change:
being male.
ethnic group (for example, British Asians have an increased risk).
