Supraventricular tachycardia

What is SVT?

Supraventricular tachycardia (SVT) causes your heart to beat very fast in a regular rhythm. It is a type of heart palpitation. During an episode of SVT, the heartbeat is not controlled by the sinoatrial (SA) node (the normal timer of the heart). Another part of the heart overrides this timer with faster impulses. The source of this impulse in SVT is somewhere above (supra) the ventricles and causes a very fast, regular heart rate.

Types of SVT

There are three main types of SVT:

• Atrioventricular junctional tachycardias. The most common type of SVT is atrioventricular nodal re-entry tachycardia (AVNRT), which is in this category. It is most commonly seen in people in their twenties and thirties and is more common in women. It occurs when there is an electrical short circuit in the centre of the heart. An extra impulse starts to race around this short circuit causing your heart to beat very fast.

• Atrial tachycardias. This article does not contain information about atrial fibrillation which is managed differently to SVT. For more information about this condition see atrial fibrillation.

• Atrioventricular re-entrant tachycardia (AVRT).

SVT symptoms

Symptoms last as long as the episode of SVT lasts. This may be seconds, minutes, hours or, rarely, longer. Symptoms start quickly when the SVT begins, and stop rapidly when it ends. Possible symptoms include the following:

• Very fast heart rate. Your heart rate rises to 140-200 beats per minute (bpm) or sometimes faster. (A normal heart rate is 60-100 bpm.)

• 'Thumping heart' sensations (palpitations).

• Dizziness, or feeling light-headed.

• Shortness of breath.

• You may look pale.

• Chest discomfort. You may feel mild chest discomfort.

• Angina. If you have angina then it may be triggered by an episode of SVT.

You may have no signs or symptoms, or just be aware of your fast heartbeat. Sometimes your blood pressure may become low, especially if it continues 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.

The time between episodes of SVT can vary greatly. How often they happen varies between people. Some people have several very short episodes of SVT daily, whilst others have one episode every few years.

Most people who have a first episode of SVT will seek a medical professional, as the symptoms can be distressing. They will then usually be referred to a heart specialist to decide if they need further investigations and treatment.

Triggers for SVT

Episodes of SVT may be triggered by:

• Certain medications - for example, some asthma inhalers, antidepressants, herbal supplements and cold remedies.

• Stimulants such as caffeine

• Alcohol.

• Stress or emotional upset.

• Smoking.

Avoiding these triggers will often reduce the frequency of SVTs.

Treatment for SVT

There are different ways to stop an episode of SVT.

• No treatment. Many episodes of SVT soon stop on their own, and no treatment is needed.

• Vagal manoeuvres. Some people can stop an episode of SVT by stimulating their vagus nerve which can slow your heart rate. If you are diagnosed with SVT you are likely to be taught these manoeuvres.

• Medical help. You may need to be admitted to hospital to stop it. In hospital they may use:

  • Medicine. A medicine called adenosine is given by injection into a vein. It usually stops SVT. It works by blocking electrical impulses in the heart. Alternatives can be given if you cannot have adenosine.

  • Cardioversion. This is when an electrical impulse is applied to the heart at a certain part of the heartbeat.

Driving

In the UK, if you have a Group 1 entitlement (car and motorcycle) you must not drive if the SVT has caused or is likely to cause incapacity (you are unable to control or stop the vehicle). You may be able to resume driving if an underlying cause is identified and the SVT has been controlled for at least 4 weeks. You must tell the DVLA if the SVT caused or is likely to cause incapacity, or it is not controlled for 4 weeks, or an underlying cause is not identified.

If you have a Group 2 entitlement you must notify the DVLA. You must not drive if the SVT caused or is likely to cause incapacity. Driving may be permitted only after an underlying cause has been identified and the SVT has been controlled for at least 3 months and a measure of your heart function meets the requirement.

Preventing SVT

People with SVT are referred to heart specialists when they are diagnosed. They will discuss the options with you. This may involve:

• Avoiding triggers. See above for details.

• Not treating. This is an option if the episodes of SVT are infrequent, short or cause few symptoms.

• Medication. Examples include verapamil and beta-blockers. If one does not work or causes side-effects, another can often be tried.

• Tissue destruction using a catheter (catheter ablation). A small wire (catheter) is passed via a large vein in the top of the leg into the chambers of the heart. The tip of the catheter can destroy a tiny section of heart tissue that is the source of the abnormal electrical signals.