Supraventricular Tachycardia (SVT)

Key points to remember about supraventricular tachycardia (SVT)

• an abnormally fast heart rhythm is called tachycardia
•  tachycardias can arise from the upper or lower chambers of the heart
• tachycardias that involve the upper chambers are called supraventricular tachycardia
• supraventricular tachycardias are usually not dangerous
• they are not due to a 'heart attack' and in children with an otherwise healthy heart, do not cause sudden death

What is supraventricular tachycardia (SVT)?

A normal heartbeat originates from the sinus node, the heart's pacemaker. The electrical signal passes to the bottom of the heart through a special junction, called the AV node.

An abnormally fast heart rhythm (tachycardia) can arise from the upper or lower chambers of the heart, or be a 'circuit' made up of the upper and lower chambers.  Doctors can see the heart's electrical activity by using an electrocardiograph (ECG).

Tachycardias that originate from the lower chambers (the ventricles) are called ventricular tachycardia. Those that involve the upper chambers (the atria) are termed supraventricular tachycardia (SVT).

How dangerous is SVT?

Supraventricular tachycardias are usually not dangerous. They are not due to a 'heart attack' and in children with an otherwise healthy heart, do not cause sudden death. However, if they occur very often or for long periods of time (hours to days), then they can cause difficulty with the pumping action of the heart. This can be dangerous if untreated.

What are the types of supraventricular tachycardia?

Atrial tachycardia

An area of the upper chambers takes over the pacemaker activity of the heart. This is relatively uncommon in children.

Atrial Flutter

A large area of the upper chamber forms an electrical circuit. Children who have had previous heart surgery involving the upper chambers usually display this rhythm. Some children with atrial flutter are at risk of developing clots because the blood flow in these chambers is slow and disorganised.

Atrio-ventricular re-entrant tachycardia (AVRT) 

An extra electrical connection (called an 'accessory pathway') exists between the upper and lower chambers. The wave of electricity that normally passes from the top to the bottom of the heart can now pass back up through the abnormal pathway, forming a re-entry circuit. This is the most common form of SVT in children under 8 years of age. The majority of infants with atrio-ventricular re-entrant tachycardia 'outgrow' the tachycardia during their first year but it may come back in later childhood.

Specific diagnoses falling into this category include Wolff-Parkinson-White Syndrome (WPW) and permanent junctional reciprocating tachycardia (PJRT). In older children, WPW is can in rare cases be life-threatening. Your child's doctor may recommend special tests and curative treatment known as an ablation. This is a type of 'keyhole' procedure where a surgeon will access the heart via the veins at the top of the leg.

Atrio-ventricular nodal re-entrant tachycardia (AVNRT) 

The atrio-ventricular node is located between the upper and lower chambers of the heart. It is normally the only area that allows the electrical activity of the heart to pass from the upper chambers to the lower chambers. Sometimes this area can become the source for a tachycardia. This is the most common form of SVT in children over 8 years of age.

How do I recognise if my child has SVT?

Older children and adolescents often experience a fast heart rate as palpitations. They may feel their heart racing at unexpected times such as resting, doing homework, after exercise or eating dinner. Younger children may have difficulty describing this sensation and may complain of chest pain. SVT may rarely cause children or adolescents to pass out (syncope).

Some newborns can be quite unwell if they have had SVT in the womb for a long time. In most infants, however, SVT is well tolerated as long as it is detected within a few hours of it starting.

You may notice the fast heart rate might be while cuddling your baby or during feedings. Some infants develop poor feeding, irritability, unnatural paleness and fast breathing if the SVT continues. If your baby has a fast heart rate very often or shows any of these signs, you should seek medical advice.

When should I seek help urgently for SVT?

During a bad attack, your child may become dizzy/less alert, pass out, feel cold, look pale and/or sweaty. Learning to take your child's pulse is an important skill that will help identify when the rhythm is too fast. If you feel a fast heart rate is causing your child to become seriously unwell, call an ambulance.

Always call an ambulance if your child passes out with an attack. If it is SVT, they should wake up quickly as soon as they lie down. Try 'vagal manoeuvres' (see 'How is an SVT attack stopped?' below). Keep your child lying down, or sitting until fully recovered.

In an emergency, dial 111 within New Zealand (use the appropriate emergency number in other countries).

How can SVT attacks be prevented?

There are different medicines which your child can take by mouth (oral). These medicines prevent attacks when taken regularly. The choice of medication depends on the type of tachycardia. Common medicines include:

• Sotalol
• Flecainide
• Atenolol and Propranolol
• Amiodarone

How is an SVT attack stopped?

'Vagal manoeuvres' can stop an attack of SVT. These work by stimulating the vagus nerve in the chest causing the heart to slow. In an infant, you can try a cold (iced) flannel over their eyes for about 10 seconds. In hospital (only), doctors may immerse a baby's face under cold water for a few seconds.

Children can put their thumb in their mouth and blow hard on it - until they go red in the face. A very cold drink, or standing on their head can work too.

Doctors and nurses use intravenous medications (medicines put directly into a vein) in hospital to stop an attack. The usual medications are Adenosine and Amiodarone.

Electrical cardioversion is rarely needed. This involves using a controlled electrical shock to 'jolt' the heart back to a normal rhythm and happens under a brief general anaesthetic.

Can SVT be cured?

If the tachycardia lasts to school age, the SVT can be cured by physically disabling the part of the heart causing the problem. This is called radio-frequency ablation and involves a cardiac catheter study where the electrical pathways of the heart are clearly mapped and the problem area identified and disabled. It is usually done under a general anaesthetic, and the catheters (special wires) are passed to the heart from veins in the top of the leg.