The sequence of each normal heartbeat is as follows.
- The sinoatrial node (SA node) in the right atrium is a tiny
in-built 'timer'. It fires off an electrical impulse at regular
intervals. (About 60-80 per minute when you rest, and faster when you
exercise. This controls the heart rate.) Each impulse spreads across
both atria. This causes them to contract and pump blood through one way
valves into the ventricles.
- The electrical impulse gets
to the atrioventricular node (AV node) at the lower right atrium. This
acts like a 'junction box' and the impulse is delayed slightly. Most of
the tissue between the atria and ventricles does not conduct the
impulse. However, a thin band of conducting fibres called the
atrioventricular bundle (AV bundle) acts like 'wires' and carries the
impulse from the AV node to the ventricles.
- The AV
bundle splits into two - a right and left branch. These then split into
many tiny fibres (the Purkinje system) which conducts the electrical
impulse throughout the ventricles. This makes the ventricles contract
and pump blood through one way valves into large arteries.
- The artery going from the right ventricle (pulmonary artery) takes blood to the lungs.
- The artery going from the left ventricle (aorta) takes blood to the rest of the body.
- The heart then rests for a short time (diastole). Blood
coming back to the heart from the large veins fill the atria during
diastole.
- The veins coming into the left atria bring blood from the lungs (full of oxygen).
- The veins coming into the right atria bring blood from the body (needing oxygen).
What is atrial fibrillation?
If you have atrial fibrillation (AF) then:
- Your heart rate is usually a lot faster than normal.
- Your heart beat is irregular. This is called an abnormal heart rhythm or an 'arrhythmia'.
- The force of each heart beat can vary in intensity.
What happens is that the normal controlling 'timer' in the
heart is over-ridden by many random electrical impulses that 'fire off'
from the heart muscle in the atria. The atria then 'fibrillate'. This
means that the atria only partially contract - but very rapidly (up to
400 times per minute). Only some of these impulses pass through to the
ventricles in a haphazard way. Therefore, the ventricles contract
anywhere between 50 and 180 times a minute, but usually between 140 and
180 times a minute. However, the ventricles contract in an irregular
way and with varying force.
Therefore, if you have AF and feel
your pulse, you may count up to 180 beats per minute. Also, the force
of each beat can vary, and the pulse feels erratic.
Classification of AF
AF is commonly classified in the following way:
-
Paroxysmal AF.
The word paroxysmal means 'recurring sudden episodes of symptoms'. If
you have paroxysmal AF it means that you have episodes of AF that come
and go. Each episode comes on suddenly, but will stop without any
treatment within seven days (usually within two days). Each episode
stops just as suddenly as it starts and the heart beat goes back to a
normal rate and rhythm. The period of time between each episode (each
paroxysm) can vary greatly from case to case. Although paroxysmal AF
means that it will stop on its own, some people with paroxysmal AF take
treatment as soon as the AF develops to stop it as quickly as possible
after it starts.
-
Persistent AF. This means AF that lasts longer than seven
days and is unlikely to revert back to normal without treatment.
However, the heart beat can be reverted back to a normal rhythm with
'cardioversion' treatment (see later). Persistent AF tends to be
recurrent so it may come back again at some point after successful
cardioversion treatment.
-
Permanent AF. This means that the AF is present long-term
and the heart beat has not been reverted back to a normal rhythm. This
may be because cardioversion treatment was tried and was not
successful, or because cardioversion has not been tried. People with
permanent AF are treated to bring their heart rate back down to normal,
but the rhythm remains irregular (see below). Permanent AF is sometimes
called 'established AF'.
Most people with AF have permanent AF.
How common is atrial fibrillation and what causes it?
It is common, but mainly occurs in older people. Nearly 50,000 cases
are diagnosed each year in the UK. It becomes more common with
increasing age. About 1 in 200 people aged 50-60 have AF. This rises to
just under 1 in 10 people aged 80-90. It is uncommon in younger people
unless you have certain heart conditions.
What causes atrial fibrillation?
Causes of AF include the following:
- High blood pressure is the most common cause. (High blood pressure puts a strain on the heart muscle.)
- AF is a common complication of various heart conditions. For
example, AF is a complication of ischaemic heart disease. This is the
condition that causes angina and heart attacks and is common in older
people. Various other heart problems may also trigger AF to develop.
For example, AF occurs in some people with heart valve problems,
cardiomyopathy, and pericardial disease.
- Other conditions and situations that may trigger AF to develop
include: an overactive thyroid gland (hyperthyroidism); pneumonia;
pulmonary embolus; obesity; lung cancer; drinking a lot of alcohol;
drinking a lot of caffeine (tea, coffee, etc).
- In about 1 in 9 cases of AF there is no apparent cause. The heart
is otherwise fine and there are no other diseases to account for it.
This is called 'lone AF'.
What are the symptoms of atrial fibrillation?
Symptoms often develop quickly, soon after the AF develops. Possible symptoms include:
-
Palpitations. This means that you become aware of your heart. You may feel it beating in a fast and irregular way.
-
Dizziness.
-
Angina (chest pains) may develop. In particular, the pains tend to occur when you exert yourself, but they may occur even when you are resting.
-
Breathlessness is often the first symptom that develops. It
may occur all the time, but you may become breathless just when you
exert yourself such as when you walk up stairs.
The reason why breathlessness, dizziness, and angina may
develop is because when the heart beats too fast, it becomes less
efficient. Small amounts of blood pumped faster by the heart are not as
good as larger amounts that are pumped at the slower normal rate. This
can lead to a pooling of blood in the veins of the lungs, and a reduced
output of blood from the heart which can lead to these symptoms.
However,
many people with AF have no symptoms, particularly if their heart rate
is not very fast. The AF may then be diagnosed by chance when a doctor
or nurse feels your pulse.
Are any tests needed?
- A 'heart tracing' called an electrocardiogram (ECG) can confirm
the diagnosis. This test can also rule out other causes of an erratic
or fast heart rate.
- Other tests such as blood tests and an echocardiogram (ultrasound
scan of the heart) may be advised. These tests look for an underlying
cause of AF such as a heart problem or an overactive thyroid gland.
- Often an underlying cause is already known about. For example, you
may already have angina. You may not need any further tests if AF
develops as a complication.
What are the possible complications of atrial fibrillation?
An increased risk of having a stroke (or other blood clot problem)
The main complication of AF is an increased risk of having a stroke. AF causes turbulent blood flow in the heart chambers. This sometimes
leads to a small blood clot forming in a heart chamber. A clot can
travel in the blood vessels until it gets stuck in a smaller blood
vessel in the brain (or sometimes in another part of the body). Part of
the blood supply to the brain may then be cut off, which causes a
stroke.
The risk of developing a blood clot and having a stroke
varies, depending on various factors. The level of risk is divided into
three categories: high, medium and low risk.
-
High risk
means that, without treatment, you have about a 6-12 in 100 chance
(sometimes higher) of having a stroke in the next year. People in the
high risk group include those:
- who have already had a stroke or known blood clot, or
- are aged 75 years or older who also have one of the following 'risk
factors': high blood pressure, diabetes or a cardiovascular disease
(such as angina, heart attack, peripheral vascular disease), or
- who have a heart valve problem, or
- who have heart failure or poor heart function shown on a heart scan.
-
Moderate risk means that you have about a 3-5 in
100 chance of having a stroke in the next year. People in the moderate
risk group include those:
- aged 65 years or older (with no high risk factors), or
- who are of any age (up to age 75 when the risk is high) but who
also have one of the following 'risk factors': high blood pressure,
diabetes or a cardiovascular disease (such as angina, heart attack,
peripheral vascular disease).
-
Low risk means that you have about a 1-2 in 100
chance or less of having a stroke in the next year. People in the low
risk group are all people with AF aged less than 65 and who do not have
any risk factors that put them in the high or moderate risk category.
Other complications
Less common complications of AF include the following:
- Heart failure develops in some cases. See separate leaflet called 'Heart Failure'.
- Cardiomyopathy. There are various causes of cardiomyopathy and AF
with a fast heart rate is an uncommon cause. Cardiomyopathy means
"weakness of the heart muscle". The reason why cardiomyopathy should
develop in some people with AF is not clear.
- Angina pains may get worse if you have angina.
What are the treatment options for atrial fibrillation?
Treatments that may be considered include:
- Rate
control. This means bringing the heart rate back down to normal. This
is done for all people with AF who have fast heart rate (that is, most
cases).
- Rhythm control. This means converting the irregular rhythm back to a normal regular rhythm. This is only possible in some cases.
- Anticoagulation treatment which aims to prevent a stroke.
- Other treatments in certain circumstances.
Each of these are now discussed further.
Rate control treatment
If the heart rate is
brought down to normal the heart becomes efficient again, and the
symptoms usually improve. The pulse may still feel irregular, but not
fast.
Several drugs can slow the heart rate down. They include
beta-blocker drugs (such as atenolol, metoprolol and propranolol),
diltiazem, verapamil, and digoxin. These drugs work by interfering with
the electrical impulses of the heart. The drug chosen may depend on
factors such as other heart problems that you may have.
In
untreated AF, the heart rate may be as fast as 180 beats per minute,
although it is more commonly between 120 and 160 beats per minute. The
aim of medication is to bring the heart rate back down to normal
(ideally, to less than 90 beats per minute when resting). Treatment is
usually successful, but the dose needed can vary from case to case.
Also, in some cases a combination of drugs may be needed if the heart
rate is not brought down low enough with a single drug.
Rhythm control treatment
Rhythm control
means reverting the erratic heart beat back to a normal regular rhythm.
This is called 'cardioversion'. This treatment is not tried in most
cases as treatment to control the rate of the heart (described above)
usually works well to control symptoms. However, cardioversion may be
considered in certain situations. For example, if drugs to control the
fast heart rate do not work very well, or if the irregular heart beat
is causing unpleasant symptoms.
One method of cardioversion is
to give your heart an 'electric shock'. Another method is to use drugs
that may convert the heart rhythm back to a regular beat. Both of these
methods have only limited success. For example, after cardioversion,
within a year in about half of cases the heart has reverted back to AF.
Cardioversion is more likely to be considered certain situations, for example:
- If your AF developed recently.
- If you are younger than 65. (Age is no bar to cardioversion, but it is less likely to an option the older you become.)
- If an underlying cause of the AF has been successfully treated (and
so AF is unlikely to come back again once the normal heart rhythm has
been restored).
- If you have no other heart abnormality. (That is, if you have 'lone AF' described earlier.)
- If you have acute heart failure or unstable angina which is being made worse by the irregular heart beat of AF.
Cardioversion is usually not an option in certain situations. For example:
- If
you have certain heart diseases that include a structural problem to
the heart (for example, certain valve problems such as mitral stenosis).
- If you have had AF a long time (usually more than 12 months).
- If you have had several previous attempts at cardioversion which
have not worked, or only worked for a short time before the heart
reverted back to AF.
A newer technique to restore the heart rhythm is called
catheter ablation. In this procedure a catheter (a long thin wire) is
passed into the chambers of the heart via a large blood vessel in a
leg. The tip of the catheter can destroy tiny sections of heart tissue
that may be the source or 'trigger' of the abnormal electrical
impulses. This treatment is only suitable in certain cases and is not a
routine treatment. It does not always work and there is a small risk of
serious complications.
Anticoagulation - usually with warfarin
Anticoagulation
means that you take a drug to reduce the chance of forming a blood
clot. Therefore, anticoagulation helps to prevent a stroke from
occurring. Some people call anticoagulation 'thinning the blood'
although the blood is not actually made any thinner. The most commonly
used anticoagulant drug is called warfarin. Warfarin interferes with
certain chemicals in the blood to prevent blood clots forming so easily.
Overall,
warfarin reduces the risk of stroke by nearly two-thirds. In other
words, warfarin treatment can prevent about 6 in 10 strokes that would
have occured in people with AF. The greatest benefit is seen in those
people who are in the 'high risk' category of having a stroke
(described above).
As with all treatments, there is a small risk
if you take warfarin. The main risk is that a bleeding problem may
develop as the blood will not clot so well. Over a period of one year
of treatment, about nine in a thousand people who take warfarin for AF
are likely to have a serious bleeding problem. For example, some people
develop a serious bleeding ulcer in the gut. If you have a serious
bleed you are likely to need to be admitted to hospital, often needing
a blood transfusion, and it can even result in death.
Most
people with AF who have a high or medium risk of having a stroke are
advised to take warfarin. However, some people with a moderate risk may
be treated with aspirin rather than warfarin (see below), particularly
if the risks of taking warfarin are higher than average. People with a
low risk of having a stroke are not usually advised to take warfarin.
This is because the benefit does not usually outweigh the risk of
serious bleeding problems with taking warfarin. In short, the decision
to take warfarin is a joint decision between you and your doctor. It
involves weighing up the risk of having a stroke against the small risk
of a complication from taking warfarin.
If you take warfarin you
will need regular blood tests to check on how quickly your blood clots.
Blood tests may be needed quite often at first, but should become less
often quite quickly. The aim is to get the dose of warfarin just right
so your blood does not clot as easily as normal, but not so much as to
cause bleeding problems.
Aspirin is another drug that helps to
prevent blood clots forming. It is not as effective as warfarin, but is
less likely to cause problems. It is usually advised if you only have a
low risk of stroke, or if you cannot take warfarin.
Other treatments
Other treatments may be
advised, depending on the need to treat any underlying problems such as
angina, heart valve problems, high blood pressure, overactive thyroid,
etc.
Further help and information
British Heart Foundation
14 Fitzhardinge Street, London, W1H 6DH
Tel (Heart Help Line): 08450 70 80 70 Web: www.bhf.org.uk
Anticoagulation Europe
PO Box 405, Bromley, Kent, BR2 9WP
Tel: 020 8289 6875 Web: www.anticoagulationeurope.org
A charity providing information and advice to people on oral anticoagulation treatment.
References
© EMIS and PiP 2008 Updated: 17 Mar 2008
