Supraventricular tachycardia (SVT) is the name given to an abnormality of heart rhythm, characterised by a fast heartbeat.
You will usually feel your heart suddenly start racing and going very fast, then stop or slow down abruptly. You may also experience:
Episodes can last for seconds, minutes, hours or even days in rare cases. They may occur several times a day, or only once or twice a year.
When to see a doctor
Although SVT is rarely life-threatening, you should contact your GP as soon as possible if you experience symptoms of the condition. A test called an electrocardiogram (ECG) can be used to confirm a diagnosis of the condition if the heart is still racing. Occasionally an ECG can show abnormalities that suggest you have SVT even when your heart rhythm returns to normal.
As SVT can sometimes be difficult to diagnose, your GP may refer you to a cardiologist (heart specialist) who specialises in the treatment of heart rhythm disorders, called an electrophysiologist.
If you experience severe chest pain or breathing difficulties, or you feel very faint, call 999 for an ambulance.
Why it happens
SVT occurs when there is a problem with the electrical system in the heart that controls your heart rhythm. This causes the heart to beat much faster than normal.
In some people, this is due to a problem with the structure of the heart from birth (Wolff-Parkinson-White syndrome), but it can also occur in hearts that are otherwise normal.
This is why SVT can occur in anyone at any age and why it often occurs for the first time in children or young adults. Generally the fast heart rate occurs more frequently and lasts longer the older you get.
SVT is can be triggered by the extra heartbeats called ectopic heartbeats that most people experience, but some episodes are brought on by things like drinking large amount of alcohol or caffeine, stress, or smoking lots of cigarettes.
Most episodes of SVT are harmless, do not last long and settle on their own without treatment. However, treatments are available to stop ongoing episodes and prevent them occurring in the future if necessary.
Ongoing episodes of SVT can sometimes be stopped using manoeuvres that help slow down the electrical impulses in the heart by stimulating a nerve called the vagus nerve, such as the Valsalva manoeuvre. This manoeuvre usually involves holding your nose, closing your mouth and trying to exhale hard while straining as if you were on the toilet, or giving birth.
If manoeuvres like this are ineffective, injections of medication or an electric shock treatment called cardioversion can usually help stop ongoing SVT episodes.
In order to prevent future episodes, medication can be prescribed to be taken every day or a procedure called catheter ablation may be used. This procedure involves destroying the tiny areas of the heart causing problems and it is very effective at preventing future episodes.
Symptoms of supraventricular tachycardia (SVT)
If you have supraventricular tachycardia (SVT), you will usually have periods where you feel your heart beating very quickly.
Even though your heart is beating faster, the gaps between heartbeats should still be regular. If your heartbeat is fast and irregular, it is more likely you have a different heart rhythm problem called atrial fibrillation.
Other symptoms of supraventricular tachycardia include:
On rare occasions, you may faint because of a drop in blood pressure.
Symptoms of SVT can last for seconds, minutes, hours or longer (in rare cases). Some people experience symptoms several times a day, whereas others may only have episodes once or twice a year.
When to see a doctor
Although SVT is rarely life-threatening, you should contact your GP as soon as possible if you experience symptoms of the condition.
If you experience severe chest pain or breathing difficulties, or you feel very faint, call 999 for an ambulance.
Causes of supraventricular tachycardia (SVT)
Episodes of supraventricular tachycardia (SVT) occur because of a problem with electrical impulses in the heart.
Heartbeats are normally initiated by a small group of cells located in the top of the heart called the sinoatrial node, which acts as the heart's natural pacemaker. The sinoatrial node produces electrical signals that are sent through muscles of the upper heart chambers (atria), causing them to contract and pump blood into the lower heart chambers (ventricles).
The signal then passes into another group of cells located in the middle of the heart called the atrioventricular (AV) node. From here, the signals travel into the ventricle muscles, causing them to contract and pump blood out of the heart.
Episodes of supraventricular tachycardia occur when a problem develops in this system. This causes faster signals to be sent around the heart, increasing the speed at which the heart beats.
In most cases the problem is temporary, lasting for a few seconds, minutes or hours.
Types of supraventricular tachycardia
There are several different types of SVT, which are classified by the specific problem in the heart that disrupts the electrical system.
A type of SVT called Wolff-Parkinson-White syndrome occurs due to an abnormal electrical connection between the atria and ventricles, which creates a short circuit in the heart's electrical system. This happens because people with Wolff-Parkinson-White syndrome are born with a strand of extra muscle tissue between these chambers.
In other cases, a short circuit occurs even though the heart is otherwise normal and some cases are caused by an electrical signal from another part of the heart overriding the signal from the sinoatrial node.
Supraventricular tachycardia triggers
SVT is usually triggered by extra heartbeats (ectopic beats), which occur in all of us but may also be triggered by:
some medications, including asthma medications, herbal supplements and cold remedies
drinking large amounts of caffeine or alcohol
tiredness, stress or emotional upset
smoking lots of cigarettes
Diagnosing supraventricular tachycardia (SVT)
Your GP may suspect you have supraventricular tachycardia (SVT) from a description of your symptoms.
If so, you may be asked to have a test called an electrocardiogram or be referred to a cardiologist (heart specialist).
An electrocardiogram (ECG) is a test that records the rhythm and electrical activity of your heart. It is usually carried out in hospital or in your GP's surgery. It takes about five minutes and is painless
During an ECG, small stickers (electrodes) are stuck to your arms, legs and chest, and connected via wires to an ECG machine.
Every time your heart beats, it produces tiny electrical signals. An ECG machine traces these signals onto paper.
Your heart rate during an episode of SVT will usually be between 140 and 250 beats per minute (bpm), compared to a normal heartbeat of 60-100bpm.
If you manage to have the test done during an episode of supraventricular tachycardia, the ECG will record your abnormal heart rate. This confirms SVT and rules out other conditions.
However, it may be difficult to capture an episode, so your doctor may ask you to wear a small, portable electrocardiogram monitor to record your heart rate either continuously over 24 hours, or from when you switch it on at the start of an episode.
Further tests may be done once episodes of SVT are confirmed. These are to determine the exact location of the problem in heart.
For example, you may be asked to take part in an electrophysiology study done under sedation, in which doctors pass soft flexible wires up a vein in your leg and into your heart. These wires measure the electrical signals in your heart and enable doctors to locate the problem.
Treating supraventricular tachycardia (SVT)
In many cases, symptoms of supraventricular tachycardia (SVT) stop quickly and no treatment is needed.
However, treatment is available to stop an episode of SVT and prevent future episodes if necessary.
Stopping an SVT episode
If possible, medication and surgery will not be used as the first treatment for ongoing episodes of supraventricular tachycardia. Instead, a technique called vagal manoeuvres may be attempted first.
Vagal manoeuvres are techniques designed to stimulate the vagus nerve. Stimulating this nerve can help reduce the speed of the electrical impulses in your heart and stop episodes of supraventricular tachycardia. However, these techniques work in less than 1 in 3 cases.
One of the main vagal manoeuvres used is called the Valsalva manoeuvre. There is no standard way to perform this manoeuvre, but it often involves holding your nose, closing your mouth and trying to exhale hard while straining as if you were on the toilet. If you are in hospital, you may be asked to blow hard into a tube instead.
You can perform a simple version of the Valsalva manoeuvre at home to try to stop an episode of SVT. You can also try dipping your face into a bowl of cold water, as this can have a similar effect.
An alternative vagal manoeuvre is a carotid sinus massage. This involves massaging an area of your neck called the carotid sinus in an attempt to stimulate the vagus nerve. However, this should only be carried out by a health professional and should not be attempted at home.
If vagal manoeuvres are unsuccessful, you may require an injection of a medication called adenosine in hospital. This medication blocks the abnormal electrical impulses in your heart.
Side effects of adenosine are relatively common, but usually short-lived. After an injection you may experience nausea (feeling sick),dizziness, chest tightness or shortness of breath.
An injection of a medication called verapamil may sometimes be used instead of adenosine, but only usually if adenosine is unsuitable (for example, if you have asthma). This is because there is a risk of more serious side effects including low blood pressure.
If a prolonged episode of supraventricular tachycardia doesn't respond to vagal manoeuvres or medication, or if these treatments are unsuitable, a treatment called cardioversion may be used.
Cardioversion is a relatively simple procedure that uses a machine called a defibrillator to apply an electrical current to your chest. This shocks the heart back into a normal rhythm.
It's usually carried out under general anaesthetic and you should be able to go home the same day.
Cardioversion is a very effective procedure and serious complications are uncommon. However, your chest muscles may feel sore afterwards and areas of skin where the electrical shocks were applied may be red and irritated for a few days.
Preventing future episodes
There are also some treatments that reduce your chances of having further SVT episodes.
Some supraventricular tachycardia episodes are triggered by things like tiredness, drinking lots of alcohol or caffeine, or smoking lots of cigarettes.
Cutting down on the amount of caffeine or alcohol you drink, stopping or limiting how many cigarettes you smoke, and making sure you get enough rest can help reduce your chances of having further episodes.
If necessary, medication can be prescribed to prevent further episodes of SVT by slowing down the electrical impulses in your heart. These medications are taken as a daily tablet and include digoxin, verapamiland beta blockers.
Common side effects of these medications can include dizziness,diarrhoea and blurred vision. Tiredness can occur with beta blockers and men may experience problems getting erections. Less common side effects include difficulty getting to sleep (insomnia) anddepression.
If the medication you're prescribed doesn't work or has unpleasant side effects, a more suitable alternative can often be found.
If episodes of SVT occur repeatedly, an operation called catheter ablation is the recommended treatment option. This prevents further episodes of SVT by destroying the tiny parts of the heart causing the problems in the heart's electrical system. This is a safe and highly effective treatment and means you no longer need to take medication.
During the procedure, a thin wire called a catheter is inserted into a vein in your upper leg or groin, before being guided to your heart. When the wire reaches the heart, it records the electrical activity to pinpoint the precise location of the problem.
When the problem area is found, high-frequency radiowaves are transmitted to the catheter tip to destroy it, producing a small scar.
You will remain awake during this procedure, but will be given a sedative to relax you. Local anaesthetic is used to numb the area where the catheter is inserted.
The procedure normally lasts about an hour and a half and you can normally go home the same day you have the procedure. However, sometimes you may need to stay in hospital overnight, for example, if you are operated on in the late afternoon.
Catheter ablation is very effective at preventing future episodes of SVT (19 out of every 20 people treated will never have the problem again), but like all operations it carries a risk of complications. These include bruising and bleeding where the catheter was inserted. Typically any bruising will be small, but even if the bruise is large it will require no treatment and will disappear within two weeks.
There is also a small risk (less than 1 in 100) of the heart's normal electrical system being damaged (heart block). If this happens, you may need a permanent pacemaker to control your heart rhythm.
You should discuss potential benefits and risks of catheter ablation with your surgeon, called an electrophysiologist, before the procedure.