Supraventricular tachycardia (SVT) is the most common arrhythmia (abnormal heart rhythm) diagnosed in children. It is said to occur in up to 1 in 2500 children. While the problem is often congenital, meaning it is present at birth, the onset and severity of symptoms varies. Most of the time, the problem occurs in children with otherwise normal hearts but it can occur along with other congenital heart problems.