Introduction

Supraventricular tachycardia (SVT) is a heart condition featuring episodes of an abnormally fast heart rate.

The heart will suddenly start racing, then stop racing or slow down abruptly.

Episodes can last for seconds, minutes, hours or (in rare cases) days. They may occur regularly, several times a day, or very infrequently, once or twice a year.

The heart rate may be as high as 250 beats per minute, but is usually between 140 and 180 (a normal heartbeat should be 60-100 beats per minute at rest).

What it means

  • 'Supraventricular' means that the problem occurs in the upper chambers (atria) of the heart.
  • 'Tachycardia' means an abnormally rapid heart rhythm. 

What happens

When the heart beats normally, its muscular walls contract (tighten and squeeze) to force blood out and around the body. They then relax, so the heart can fill with blood again. This process is repeated for every heartbeat.

In SVT, the heart muscle is contracting so fast that it cannot relax between contractions. This reduces the amount of blood being pumped around the body, which can make you feel dizzy and short of breath.

You usually feel heart palpitations (noticeable heartbeats) and a fast pulse. 

Why it happens

SVT is caused by abnormal electrical impulses that start suddenly in the upper chambers of your heart (the atria). These impulses override your heart's natural rhythm.

It is often a short circuit in the electrical system of your heart that causes these spontaneous impulses. The short circuit causes an electrical signal to travel continuously around in a circle. Each time the signal completes the circuit, the impulse spreads out to the rest of your heart, forcing it to beat rapidly.

SVT attacks often happen for no obvious reason. However, they may be triggered by a change in posture, exertion, emotional upset, coffee or alcohol.

Who is affected

SVT can occur in anyone at any age, but it often occurs for the first time in children or young adults.

Outlook

In the vast majority of cases, attacks of SVT are harmless, do not last long and settle on their own without treatment.

If necessary, SVT can be treated with drugs that correct the abnormal heart rate. It can also be permanently treated with a very effective surgical procedure called catheter ablation, which interrupts the abnormal electrical circuits.

Symptoms

If you have supraventricular tachycardia (SVT) you'll usually feel your heart racing in your chest or throat and a very fast pulse (140-180 beats per minute).

You may also feel:

  • chest pain 
  • dizziness 
  • light-headedness
  • fatigue (tiredness)
  • breathlessness 

On rare occasions, you may faint because of a drop in blood pressure.

How long do symptoms last?

Symptoms of SVT can last for seconds, minutes, hours, or (in rare cases) days.

They may occur regularly, several times a day, or very infrequently, once or twice a year.

How is it different from atrial fibrillation?

SVT features a fast, regular heart rate whereas atrial fibrillation features a fast but irregular heart rate ('flutters')

Read more on atrial fibrillation

Causes

An episode of supraventricular tachycardia (SVT) occurs when abnormal electrical impulses suddenly start in the upper chambers of the heart, and override the heart's natural rhythm.

SVT is sometimes called paroxysmal supraventricular tachycardia (PSVT). Paroxysm means a sudden temporary disturbance of heart rhythm.

PSVT is usually caused by a short circuit in the electrical system of the heart, which causes an electrical signal to travel rapidly and continuously around in a circle, forcing the heart to beat each time it completes the circuit.

Another type of SVT is called Wolff-Parkinson-White syndrome, where an abnormal electrical connection occurs between the atria (upper heart chambers) and ventricles (lower heart chambers). People with Wolff-Parkinson-White syndrome are born with a strand of extra muscle tissue between these chambers. This produces a short circuit, which causes the fast heartbeat.

SVT 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
  • stress or emotional upset
  • a change in posture
  • smoking lots of cigarettes

How the heart works

The heart has two upper chambers (the left and right atria) and two lower chambers (the left and right ventricles).

The atria and ventricles have walls of muscle. A heartbeat happens when this muscle suddenly contracts (tightens) so that the chambers become smaller and the blood inside is squeezed out.

The control of the heartbeat starts with a small clump of cells in the right atrium, called the sinoatrial node (the heart's natural pacemaker). This sends out electrical impulses through the atrial muscle to another clump of cells called the atrioventricular (AV) node, found between the atria and ventricles. The impulse then continues through the AV node down fibres that conduct the impulse into the muscle of the ventricles.

The AV node determines the rate of contraction of the ventricles. The pulse felt at the wrist is due to the contraction of the ventricles.

Diagnosis

If your GP suspects that you have supraventricular tachycardia (SVT), they may ask you to have an electrocardiogram and refer you to a heart specialist.

An electrocardiogram (ECG) is a test that records the rhythm and electrical activity of your heart.

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 SVT may be as high as 250 beats per minute, but is usually between 140 and 180.

ECGs are usually done in hospital or in your GP's surgery. It takes about five minutes and is painless. 

If you manage to have the test done during an attack of SVT, the ECG will record your abnormal heart rate. The diagnosis of SVT can then be confirmed (and other conditions ruled out).

However, it may be difficult to capture an attack. So your doctor may ask you to wear a small, portable electrocardiogram monitor that will record your heart rate either continuously over 24 hours, or when you switch it on (at the start of an attack).

Further tests

Further tests may be done once episodes of SVT are confirmed. These aim to determine the exact location within the heart that is triggering the episodes of SVT.

For example, you may be asked to take part in a electrophysiology study done under sedation, in which doctors pass electrodes (soft flexible wires) up through a vein in your leg  to your heart. These electrodes measure the electrical signals in your heart and enable doctors to determine where the abnormal signals are coming from.

Treatment

In many cases, the symptoms of supraventricular tachycardia (SVT) stop quickly and no treatment is needed.

If treatment is needed, you will have to go to hospital. Treatment for SVT cannot be given until your diagnosis has been confirmed from an electrocardiogram (ECG).

Medication

If symptoms last for a long time or are severe, a medicine – usually adenosine – can be injected into your vein, which blocks the abnormal electrical impulses in your heart.

When adenosine is not recommended – for example, if you have asthma – an injection of verapamil can be given instead.

Medication may be also prescribed to prevent further episodes of SVT. This includes:

  • digoxin 
  • verapamil
  • beta blockers, which slow the electrical impulses in the heart

Electric shock treatment

Electric shock treatment (called cardioversion) is occasionally used to stop an episode of SVT. A machine called a defibrillator applies an electrical current to your chest. This is usually done under general anaesthetic (you are put to sleep).

Catheter ablation

Catheter ablation is an extremely effective procedure that produces small scars in your heart that block the electrical signal travelling around the abnormal electrical circuit. It prevents further episodes of SVT.

A catheter (a thin, soft wire) is guided through one of your veins into your heart, where it records electrical activity. When the precise position of the circuit is found, either heat generated by an electric current or cold (cryoablation) is transmitted to the catheter tip to destroy a small mass of muscle, producing a small scar.

This procedure is usually done under a local anaesthetic (the area is numbed), with some sedation.

Non-drug treatments

The following non-drug methods may be tried to stop SVT:

  • Hold your nose, close your mouth and try to blow out hard (called the Valsalva manoeuvre). This should feel like straining on the toilet.
  • A health professional can massage an area in your neck called the carotid sinus.
  • Dip your face into cold water during an attack.