The electrical impulse travels first through the atria. These contract
and pump blood into the ventricles. The impulse is held up slightly at
the atrioventricular node (AV node) which acts like a 'junction box'.
It then travels through the atrioventricular bundle (AV bundle) which
acts like a 'wire' that takes the impulse to the ventricles. This makes
the ventricles contract to pump blood into the arteries. (See separate
leaflet called '
How the Heart Works' which explains this in more detail.)
What is a supraventricular tachycardia (SVT)?
Tachycardia means a fast heart rate. Supraventricular
means 'coming from above the ventricle'. During an episode of SVT, the
heartbeat is not controlled by the SA node (the normal timer of the
heart). Another part of the heart overrides this timer with faster
impulses. The source or 'trigger' of the impulse in an SVT is somewhere
above ('supra') the ventricles, but the impulse then spreads to the
ventricles. The heart then contacts (beats) faster than normal. For
example:
- A 'short circuit' in the heart's electrical
pathways may develop. The most common cause of SVT is an abnormal extra
pathway from the AV node to the ventricles. This can make electrical
impulses go round and round this section of the heart.
- A small area in one of the atria may become more 'excitable' than usual and start to produce electrical impulses.
An episode of SVT usually starts suddenly for no apparent
reason. It may last just a few minutes, but can last several hours. It
then stops just as suddenly as it started. Rarely, an episode lasts
longer than a few hours.
The time between episodes of SVT can
vary greatly. In some cases, short bursts of SVT occur several times a
day. At the other extreme, an episode of SVT may occur just once or
twice a year. In most cases it is somewhere in between, and an episode
('paroxysm') of SVT occurs now and again.
Note: the term SVT is usually only used when the heart rate is fast and regular. Another condition which causes a fast but irregular
heart rate, and is caused by abnormal impulses in the the atria, is
called atrial fibrillation. This is not dealt with further in this
leaflet. See separate leaflet on 'Atrial Fibrillation'.
Who gets supraventricular tachycardia (SVT)?
In most cases, the first episode of SVT begins in childhood or early
adulthood. However, a first episode of SVT can occur at any age. It is
an uncommon condition, but the exact number of people affected is not
known.
What are the symptoms of a supraventricular tachycardia (SVT)?
Symptoms last as long as the episode of SVT lasts. This may be seconds, minutes, hours or, rarely, longer.
Possible symptoms include the following.
- Your pulse rate becomes 140 - 200 per minute. Sometimes even faster.
- Palpitations (feeling your heart beat).
- Dizziness, or 'feeling light headed'.
- You may become breathless.
- If you have angina, then an angina pain may be triggered by an episode of SVT.
- You may have no symptoms, or are just 'aware' that your heart is beating fast.
- Sometimes your blood pressure may become low with too fast a heart
rate, especially if it persists for several hours. In some cases this
causes a faint or collapse. This is more likely if you are older and
have other heart or lung problems.
Do I need any tests?
Electrocardiograph (ECG)
This traces the
rhythm and electrical activity of your heart. It is a painless test and
takes about five minutes to do. Small metal patches are put on your
arms, legs, and chest and are connected to the ECG machine to take a
reading.
If an ECG is done during an episode of SVT, it can
usually confirm the diagnosis and rule out other causes of a fast heart
rate. (For example, a small area within a ventricle sometimes triggers
a tachycardia. It is important to rule out a 'ventricular tachycardia'
as this tends to be more serious than SVT, and has different
treatments.)
The ECG between episodes of SVT is usually normal.
So, doing an ECG between episodes of symptoms may not be much help.
However, if SVT is suspected, you may be asked to wear a small portable
ECG recorder. Some types record an ECG continuously over 24 hours.
Others are designed so that you can switch it on to record when you
have symptoms.
Specialist tests
Once it is confirmed that
you have episodes of SVT, a number of special tests are sometimes
advised. These aim to find the exact location of the 'excitable' part
in your heart which is triggering the episodes of SVT.
What are the treatment options for supraventricular tachycardia (SVT)?
Stopping an episode of SVT
Many episodes of
SVT soon stop on their own, and no treatment is then needed. If an
episode of SVT lasts a long time or is severe, you may need to be
admitted to hospital to stop it.
- Medicines which are given
by injection into a vein will usually stop an SVT. Adenosine is
commonly used. It works by blocking electrical impulses in the heart.
Verapamil is an alternative if adenosine is not advised. For example,
some people with asthma cannot have adenosine.
- Electric shock treatment is sometimes used to stop an episode of SVT.
Preventing episodes of SVT
Options include the following.
-
You can take medication
every day to prevent episodes of SVT. Various medicines can interfere
with the electrical impulses in your heart. They include: digoxin,
verapamil, and beta-blockers - but there are others. If one does not
work or causes side-effects, another can often be found to suit.
-
Catheter ablation (destruction) treatment may be an option
for some types of SVT. A catheter (small wire) is passed via a large
vein in your leg into the chambers of your heart. It is guided by
special x-ray techniques. The tip of the catheter can destroy a tiny
section of heart tissue that is the source or 'trigger' of the abnormal
electrical impulses. This is only suitable if the exact site of the
trigger can be found by special tests, and be located accurately by the
catheter tip. It can be very successful, and after the procedure you do
not need to take medication to prevent SVT.
-
Open heart surgery is rarely needed to destroy a 'trigger' of abnormal electrical impulses which is not controlled by the above two options.
-
Not treating is an option if episodes of SVT are infrequent,
only last a short time, or cause few symptoms. The treatments above
have to be balanced against the possible side-effects and risks. Some
people prefer to put up with symptoms if they not too bad and only
occur now and then.
Further help and information
Arrhythmia Alliance
PO Box 3697 Stratford Upon Avon Warwickshire CV37 8YL
Tel: 01789 450787 Web: www.arrhythmiaalliance.org.uk
British Heart Foundation
14 Fitzhardinge Street London W1H 4DH
Tel (Heart Help Line): 08450 70 80 70 Web: www.bhf.org.uk
References
© EMIS and PiP 2008 Updated: 18 Mar 2008
