What is supraventricular tachycardia?
Supraventricular tachycardia (SVT), also known as paroxysmal narrow complex tachycardia (PNCT), is a disturbance of heart rhythm as a result of rapid electrical activity of the upper parts of the heart. In these attacks the heart is beating very fast, usually at a rate between 140 and 240 per minute. In most cases, the heart is normal and the heart rhythm is benign although uncomfortable. Palpitation is much the commonest symptom, but there also may be dizziness or even, very
occasionally, fainting. Attacks usually start in youth and may recur over many years, tending to reduce as the individual gets older. Some people find that certain things act as triggers, such as emotional upset, a variety of foods and drinks (especially coffee) and alcohol. Individual attacks may only last seconds or minutes, but can last hours.
Supraventricular tachycardia is a condition which causes the heart to beat very quickly (200-300 beats a minute) for an extended period of time. It is the most common cause of a heart rate this fast in children and occurs in one of 250-1,000 kids. But one must first understand how the heart normally beats to understand what happens when it goes awry.
The heart is an amazing organ in that it contains millions of individual cells, yet they all normally contract in a rhythmic manner co-ordinated with each other. This beating is initiated by a natural pacemaker called the sinoatrial node. This SA node, located in the upper half of the heart, generates an impulse at a rate which depends on the age of the child and the level of activity in which the child is involved. Once the impulse is generated, it follows a very specific path toward the lower half of the heart, passing through the atrioventricular node. From there, the impulse travels through the lower half of the heart. This impulse does not travel in the most direct route from top to bottom, resulting in the cells on the top half of the heart contracting just before those on the bottom. This, in turn, allows for efficient pumping of the blood to the rest of the body.
Supraventricular tachycardias start in the upper part of the heart. There are many types of supraventricular tachycardias, including atrial fibrillation and atrial flutter, the most common forms. For information about atrial fibrillation or atrial flutter, see the topic Atrial Fibrillation. This topic covers other supraventricular tachycardias, including Wolff-Parkinson-White syndrome. In supraventricular tachycardia (SVT), the impulse takes a more direct route from the top to the bottom of the heart. In the most common type of SVT in children, this impulse travels quickly in a circle from top to bottom but then continues in a route back to where it started, stimulating that impulse all over again. You can see how this would cause a fast heart rate because the impulse is feeding back on itself.
An episode of SVT usually starts suddenly for no apparent reason. It may last just a few minutes, but can last several hours. It then stops just as suddenly as it started. Rarely, an episode lasts longer than a few hours. The time between episodes of SVT can vary greatly. In some cases, short bursts of SVT occur several times a day. At the other extreme, an episode of SVT may occur just once or twice a year. In most cases it is somewhere in between, and an episode ('paroxysm') of SVT occurs now and again.