Myocardial Infarction (Heart Attack)
Myocardial infarction (MI) is usually caused by a blood clot in a heart (coronary) artery. Phone for medical help immediately if you develop severe chest pain. A 'clot busting' drug should be given as soon as possible to prevent damage to heart muscle. Also, 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 it's 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.
Understanding the heart and coronary arteries
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? f you have an MI, a coronary artery or one of it's smaller branches is suddenly blocked. The part of the heart muscle supplied by this artery loses it's 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.)
If one of the main coronary arteries 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. In people who survive an MI, the part of the heart muscle that dies ('infarcts') is replaced by scar tissue over the next few weeks.
causes:Thrombosis - the cause in most casesThe common cause of an MI is a blood clot (thrombosis) 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 MI. (The diagram above shows four patches of atheroma as an example. However, atheroma may develop in any section of the coronary arteries.)
'Clot busting' drugs (see below) can break up the clot and undo the blockage. If given quickly enough this prevents damage to the heart muscle, or limits the extent of the damage.
Uncommon causesVarious 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.
The rest of this leaflet only deals with the common cause - thrombosis over an atheroma plaque
symptoms
Severe chest pain is the usual main symptom. 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. The pain may be similar to angina, but it is usually more severe and lasts longer. (Angina usually goes off after a few minutes. MI pain usually lasts more than 15 minutes - sometimes several hours.)
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'.
Collapse and sudden death may occur with a large or severe MI. What should I do if I suspect I am having a myocardial infarction?
Call an ambulance or doctor immediately. The earlier the treatment, the better the chance of a good outcome. The following describes a typical course of events that then occurs.
You will be given a dose of aspirin immediately to reduce the 'stickiness' of the blood. It helps to prevent further blood clotting.
You will normally be admitted to hospital.
A strong pain killer given by injection will ease the pain.
Heart monitoring (ECG) and blood tests are done to confirm an MI, and to rule out other causes of chest pains.
You will usually be given an injection of a 'clot busting' drug. This dissolves the blood clot. The sooner this is given, the better. The part of the heart muscle starved of blood does not die ('infarct') immediately. If blood flow is restored within a few hours, much of the heart muscle that would have been damaged will survive.
Injections of heparin are usually given for a few days to help prevent further blood clots.
Your heart is monitored for a few days to check on the heart rhythm.
Various tests may be done to look for complications.
You will be advised to take regular medication from now on. Medication after an MI is discussed more fully in another leaflet. 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) - especially if you have any heart failure (see below).
A statin drug to lower the cholesterol level. This helps to prevent the build-up of atheroma.
