Supraventricular (originating above the ventricles) tachycardia (SVT) is a series of fast atrial heartbeats that can cause the heart to contract at rates of 250 times per minute or faster. SVT can be uncomfortable and frightening. The type of treatment depends on whether the electrical impulses reenter the atria via a bypass tract (Wolff-Parkinson-White syndrome), through the atrioventricular (AV) node, or are caused by a single abnormal group of cells.
Wolff-Parkinson-White (WPW) Syndrome
Wolff-Parkinson-White Syndrome is a common cause of SVT. In WPW there is an abnormal electrical connection between the atria and ventricles. This extra tissue is a short circuit between these chambers. It provides an extra pathway for electrical impulses to be conducted through the tissue that normally blocks electrical impulses between the atria and ventricles. This short circuit is called an accessory pathway, and it allows electrical impulses to travel between the atria and the ventricles without going through the AV node. In WPW, an SVT is usually started when an impulse travels down the AV node to the ventricles and then up through the short circuit tissue to the atria. This impulse can then travel through the atria and down the AV node before the SA node can start the next heartbeat. If the impulse continues to travel in this repeating, circular pattern, it can cause the heart to beat very rapidly.
AV Nodal Reentrant Tachycardia
AV Nodal Reentrant Tachycardia (AVNRT) is another common form of SVT. In AVNRT, there is an extra electrical pathway in or near the AV node. If an electrical impulse is conducted in this pathway, it may direct the impulse through both the AV node and the extra pathway in a repeating, circular pattern. The AV node and the extra pathway are located essentially in the center of the heart. This causes the upper and lower chambers to beat rapidly at the same time instead of in the normal sequence (upper chambers beating first, followed by the lower chambers).
Rapid AV Nodal Conduction
In some SVTs the atria may spontaneously generate multiple rapid impulses. Many of these impulses can travel through the AV node to the ventricles in an erratic manner. As a result, the heart rhythm can become irregular and rapid. If this happens, the heart will not pump blood efficiently.
Atrial flutter is one of the more common SVTs, where the upper chambers of the heart (atria) beat anywhere from 240 to 320 times per minute. This arrhythmia is similar to atrial fibrillation, as it originates entirely within the upper chambers, but it produces a more organized, regular rhythm. It is usually not harmful, but can result in symptoms such as palpitations, shortness of breath, chest tightness, fatigue, and lightheadedness. If left untreated, atrial flutter may eventually lead to conditions associated with other arrhythmias that result in abnormally high heart rates.
Even though the upper chambers are beating rapidly, the AV node allows only one-half to one-third of the electrical impulses to reach the hearts lower chambers (the ventricles). This prevents the arrhythmia from becoming life-threatening, and keeps the wrist pulse rate at only 100 to 150 beats per minute. This arrhythmia can last for hours or days; therefore, most people with atrial flutter require treatment.
Every normal heart has a normal rhythm. That rhythm varies from person to person. In most healthy people, the heart at rest beats about 60 to 100 times per minute. A small bunch of heart cells called the sinoatrial node keeps time.