Supraventricular tachycardia (SVT)

Supraventricular tachycardia refers to a heart condition which has episodes of abnormally fast heart rhythms. These are caused by abnormal electrical impulses, which originate in the areas of the heart above the ventricles. Supraventricular tachycardia can occur in healthy young children as well as people with underlying heart disease.

The heart

Your heart has two ventricles - the left and right - that work to pump blood around the body. Above the ventricles are two more chambers called the left and right atriums (known collectively as the atria). Inside the right atrium is the sinoatrial (SA) node, which is the heart's natural pacemaker. The SA node starts every heartbeat by creating an electrical impulse. An area between the atria and ventricles - the atrioventricular (AV) node - is responsible for controlling the rate of electrical impulses that reach the ventricles. The synchronised electrical impulses that travel through the heart are responsible for the coordinated contraction of the heart muscles.

The electrical system of the heart. 

Causes

Supraventricular tachycardia is caused by abnormal pathways within the electrical system of the heart. These pathways may be present from birth, or result from heart damage. They can predispose to rapid electrical impulses being generated within the atria, which then cause the rapid contraction of the atrial muscles. The atrioventricular node usually prevents these rapid electrical impulses from causing rapid contraction of ventricles, which is a potentially fatal condition.

Cross-section of a heart with atrial tachycardia. 

Supraventricular tachycardia refers to a broad range of rhythms that cause the rapid contraction of the atria. Consensus varies on the exact individual rhythms that are broadly classified as supraventricular tachycardia. Mostly commonly, supraventricular tachycardia refers to a specific set of rhythms that occur in episodes of varying duration and frequency, rather than rhythms that persist. Your doctor will be able to explain in more detail which rhythm you specifically have and its implication in terms of treatment.

Signs and symptoms

The common signs and symptoms of supraventricular tachycardia include:

• Palpitations;
• Chest discomfort;
• Anxiety;
• Light-headedness;
• Pounding in the neck, and;
• Shortness of breath.

Methods for diagnosis

To detect and diagnose supraventricular tachycardia, the following tests may be used:

Electrocardiography

During electrocardiography (ECG), electrodes are attached to your chest while you lie on your back. These record your heart's electrical activity and can be used to determine if there are any abnormalities in your heart's rhythm. To see how your heart responds to stress, an electrocardiogram can be performed while you jog on a treadmill or in response to certain medications.

An electrocardiography procedure. 

Blood tests

Your doctor will probably arrange for you to have a blood test, particularly looking at the salts in your blood, your kidney function and thyroid function. Abnormalities in these tests can identify underlying conditions that may trigger supraventricular tachycardia.

Holter monitor

A Holter monitor is a device that is able to record the electrical activity and heart rate over time (e.g, 24 hours). It is worn under your clothing and stays attached while you sleep. This is used if supraventricular tachycardia is suspected but episodes do not last long enough to get to your doctor and be captured by an ECG.

Event recorder

This device is similar to a Holter monitor, but only transmits signals when you are experiencing symptoms. An event recorder may be worn for up to a month and is useful in diagnosing rhythm disturbances that occur at unpredictable times.

Electrophysiological test

An electrophysiological test is used to confirm the diagnosis of tachycardia, or to pinpoint the location of problems in your heart's circuitry.

This test is performed as part of an angiogram. This involves inserting thin flexible tubes (called catheters) which have electrodes on their tips through the groin, arm or neck, through blood vessels to various places in your heart. During each heartbeat, these electrodes map the spread of electrical impulses and identify any abnormalities in the heart's electrical circuitry. If abnormalities are found, these may be treated at the same time using catheter ablation (see below). This procedure can be performed under local anaesthesia with light sedation, or general anaesthesia.

Types of treatment

Immediate treatment

The immediate aims of treating supraventricular tachycardia are to slow the heart rate and return it back to its normal rhythm. This can be achieved by:

Slowing a fast heart rate

Vagal manoeuvre

During an episode of supraventricular tachycardia, your doctor may ask you to perform a vagal manoeuvre. These actions affect the vagus nerve in your neck, the nerve that helps to regulate your heartbeat. These can include:

• Massaging the carotid sinus in your neck;
• Applying cold packs to your face and neck, and;
• Performing a valsalva manoeuvre, which is where you try to forcibly breathe out against a closed mouth and nose.

Medications

If vagal manoeuvres does not work, medications may be used to restore your heart's normal rate. These medications, called antiarrhythmics, are generally given by injection and administered at a hospital. Your doctor may also prescribe ongoing medications in tablet form.

Electrical cardioversion

Electrical cardioversion is a technique used to restore the heart's rhythm using electric shock therapy. While under sedation, the heart's electrical system can be safely reset by a controlled electric shock to your chest. To help the heart to work normally after the treatment, medication may be needed over the long-term.

Preventing further episodes

Ongoing treatment may not be necessary if episodes are infrequent, unproblematic and easily controlled with vagal manoeuvres. However, treatments are available to prevent further episodes of supraventricular tachycardia for individuals who:

• Experience recurrent episodes that affect their quality of life;
• Engage in activities where even infrequent episodes can put them at risk of harm (e.g. pilot or diver), or;
• Are at risk of having very unstable heart rhythms that may lead to complications.

These treatments include:

Medications

When taken regularly, anti-arrhythmic medications may prevent a fast heart rate.

Catheter ablation

This procedure aims to destroy the area of abnormal circuitry in heart tissue that is causing supraventricular tachycardia. It is often performed after an electrophysiological study has clearly identified an abnormal area. Radio waves are then used to carefully burn this area. This procedure is most commonly recommended for individuals, and offers the possibility of a cure. However, a potential risk is that treatment may lead to heart block, where sufficient electrical impulses do not reach the ventricles, leading to reduced cardiac activity. This can cause dizziness and/or blackouts. A pacemaker, which is a small device implanted into the chest, may need to be implanted to help the heart regain its normal rhythm.

Surgery

Very rarely, open heart surgery may be used to destroy the extra electrical pathway causing the tachycardia. One method is known as a maze operation and is used to 'trap' the source of your tachycardia using a similar procedure to catheter ablation. When performed, it is often combined with other procedures for related heart conditions.

Potential complications

Supraventricular tachycardia can vary in severity. This depends on the rate and duration of a rapid heart rate and whether you have other heart conditions. Some possible complications may include:

• The formation of blood clots that can cause a stroke or heart attack;
• The inability of your heart to pump enough blood (heart failure), and;
• Sudden death - some of the less common forms of supraventricular tachycardia can transform into very unstable heart rhythms. These forms may be detected by your doctor when they are looking at your electrocardiogram (ECG). If you are at risk of such rhythms, your doctor may recommend an implantable defibrillator, a small device implanted into the chest, which detects electrical impulses of the heart, and can stop potentially fatal rhythms.

Prognosis

In some cases, supraventricular tachycardia may be cured with treatment. In other cases, it can be managed with vagal manoeuvres and medication.

Prevention

The following strategies can help reduce the frequency of supraventricular tachycardia episodes:

• A healthy diet;
• Exercising and maintaining a healthy body weight;
• Quitting smoking;
• Limiting alcohol and caffeine;
• Controlling stress, and;
• Keeping your blood pressure low.