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.)
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.
What causes myocardial infarction?
Thrombosis - the cause in most cases
The
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.)
Treatment with
'clot busting' drugs 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 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 deals only with the common cause - thrombosis over an atheroma plaque.
Who has a myocardial infarction?
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?
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'.
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?
Call 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.
How is myocardial infarction diagnosed and assessed?
Many
people develop chest pains that are not due to an MI. 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. However,
tests can usually confirm MI. These are:
- 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?
The
following is a 'typical' situation and mentions the common treatments
offered. Each case is different and treatments may vary depending on
your situation.
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
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. This is why an
MI 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 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 leaflet
called 'Angioplasty' for details.
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.
Injections of heparin or a similar drug
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.
Other drugs or treatments may be needed if you develop
complications. For example, treatments for heart failure may be needed
if you develop heart failure as a complication after an MI.
How serious is a myocardial infarction?
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 (infarcts or
dies) which heals as a small patch of scar tissue. The heart can
usually function normally with a small patch of scar tissue. A larger
MI is more likely to be life-threatening or cause complications.
Even
before treatments became available to restore blood flow such as 'clot
busting' drugs and angioplasty, many people made a full recovery as
many MIs are small. 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 that may occur after an MI 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 life-threatening.
-
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 an MI. Sudden, chaotic, fast heart beats 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 is
likely.
-
A further MI 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 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 MI.
After having a myocardial infarction
After
recovering from an MI, 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 an MI.
Regular exercise and getting back to normal work and life is usually
advised. Much can be done to reduce the risk of a further MI. This is
discussed more fully in another leaflet called 'After a Myocardial Infarction'.
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:
- a family
history of heart disease or a stroke that occurred in a father or
brother aged below 55, or in a mother or sister aged below 65
- being male.
- ethnic group (for example, British Asians have an increased risk).
Risk factors are discussed more fully in another leaflet called 'Preventing Heart Disease'.
Briefly, if you can reduce any risk factors, it reduces your risk of
having an MI (or of having a further MI if you have already had one).
Some risk factors are fixed and you cannot change them. However, if you
have a fixed risk factor, you may want to make extra effort to reduce
preventable risk factors such as smoking or lack of exercise.
Appendix: What is 'acute coronary syndrome'?
The
term 'acute coronary syndrome' is a term that is used more and more by
doctors. It covers a range of disorders (including MI) 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
caused by a blood clot that forms on a patch of atheroma within a
coronary artery (which is described earlier). If the blood clot causes
a reduced blood flow, but not a total blockage then the heart muscle
supplied by the affected artery does not infarct (die). This situation
causes 'acute coronary syndrome with unstable angina' - and typically
leads to a sudden worsening of angina pains. If there is death of heart
tissue then this is called an 'acute coronary syndrome with MI' (the
subject of this leaflet). There is a third 'in between' category where
just a very small amount of heart tissue infarcts. This is called
'acute coronary syndrome with myocyte necrosis'. In effect, this is
like having a mild MI.
One test that is used to distinguish
between these three acute coronary syndromes is the blood test for
troponin. This test is described earlier. If the level of troponin is
normal, then there is no death of heart tissue. If the level is high,
then it is classed as an MI. If there is just a slight rise in the
level of troponin then this diagnoses 'acute coronary syndrome with
myocyte necrosis'.
Further sources of information and help
British Heart Foundation
14 Fitzhardinge Street, London, W1H 4DH
Tel (Heart Help Line): 08450 70 8070 Web: www.bhf.org.uk
References
© EMIS and PiP 2008 Updated: 18 Mar 2008
