When a child has SVT, there is usually an extra pathway (see the diagram below) in the heart's electrical system that connects the top chambers and lower chambers of the heart. The information in this section applies to children with WPW but there are a few differences discussed in that section. Wolff-Parkinson-White syndrome (WPW) is one subset of SVT and is the most common type of SVT in young children.

Below is and example of an electrocardiogram (ECG) that was taken in an infant who had a heart rate of 280-300 beats per minute.Sometimes other names are used for SVT such as paroxysmal (starts and stops without warning) atrial tachycardia (PAT) and paroxysmal supraventricular tachycardia (PSVT). Supraventricular tachycardia means fast heart rate coming from the above the ventricles, in the heart's upper chambers (supra = above, ventricular = the lower heart chambers, tachy = fast, cardia = heart). When a child has SVT, the heart suddenly starts to beat very fast, at rates of 180 to 280 beats a minute and up to 300 beats a minute in infants. An arrhythmia is an abnormal heart rhythm caused by a problem in the heart's electrical system, also called the cardiac conduction system. Most of the time, the problem occurs in children with otherwise normal hearts but it can occur along with other congenital heart problems. While the problem is often congenital, meaning it is present at birth, the onset and severity of symptoms varies. It is said to occur in up to 1 in 2500 children. Supraventricular tachycardia (SVT) is the most common arrhythmia (abnormal heart rhythm) diagnosed in children.

Normal heartbeat for child