I have been diagnosed with Supra Ventricular Tachycardia (SVT) and I was wondering if you could give me some information on this heart disease.
Tachycardia means rapid heartbeat, and the strict definition of the term requires three consecutive rapid beats at a rate greater than 100 beats per minute. Supraventricular means “above the ventricles,” and in this case means that the stimulus for the rapid heartbeat is arising above the ventricles.
The normal heart rhythm is called sinus rhythm. This means that the initial electrical impulse comes from a little clump of rate-setting cells called the sinus node which is located in the left atrium, the upper chamber on the left side of the heart. The stimulus propagates through the muscle of the left atrium, causing it and the right atrium to contract, pumping blood into the lower chambers of the heart, the ventricles.
The stimulus is picked up from the contracting atrial muscle by another node, called the atrioventricular node (AV node), where it is delayed for approximately 0.16 seconds. This slight delay enables the atria to complete their contraction before the ventricles begin to contract. From the AV node, the stimulus spreads through a bundle of specialized cells called the “bundle of His” into the ventricular muscle, causing both the right and left ventricles to contract. These events are all accompanied by small electrical currents, which is what we are able to visualize on an electrocardiogram (EKG). On an EKG, one can distinguish the contraction of the atria, the delay in the AV node, and the contraction of the ventricles.
There are many types of tachycardias. The simplest is sinus tachycardia, in which the impulse for each beat originates normally in the sinus node. The sinus node causes the heart to beat more rapidly in response to need, say during exercise, but may also cause more rapid heartbeats in response to stress, anger, or other stimulating events. Sinus tachycardia is usually not apparent to the person having it, and is not considered an arrhythmia, or abnormal heart rhythm.
Supraventricular tachycardia (SVT) is any tachycardia other than sinus in which the rapid stimulus to the heart originates above the ventricles. Such a stimulus may be in a part of the atrial muscle apart from the sinus node, or may arise in a diseased sinus node, or in the AV node. The beginning of such a tachycardia is usually abrupt, and is caused by a premature, or early atrial contraction (APC). Through a variety of different mechanisms, such a premature contraction may initiate a type of circular electrical rhythm which then maintains the tachycardia.
The Wolff-Parkinson-White (WPW) syndrome is a fairly common cause of supraventricular tachycardias. People with WPW syndrome have an extra pathway conducting the stimulus from the atria to the ventricles, a pathway that bypasses the AV node. The syndrome can therefore often be diagnosed by EKG even when the person is not having a tachycardia, unlike other types of SVT in which the EKG is normal between attacks. Figuring out the origin and mechanism of most SVTs is therefore difficult, and people with uncontrolled recurrent attacks may require a cardiac catheterization to record the electrical currents within the atria themselves. If an abnormal focus is found it can be treated through the same catheter. This type of testing and treatment is becoming common as more centers develop the expertise.
Finally, there are ventricular tachycardias, caused by an abnormal focus in the ventricles which stimulate the tachycardia. As in the case of SVTs, a ventricular tachycardia often starts with a ventricular premature contraction (VPC), similar to the APCs that occur in the atria. Ventricular tachycardia is often associated with structural heart disease.
SVTs may be very frightening, with a rapid pounding heartbeat. People commonly believe that they are dying. However, an SVT arising in someone with a healthy heart is rarely life threatening, and the heart can go on beating rapidly literally for hours. Therefore, it is seldom a cause of sudden death, and a person having such an attack usually has plenty of time to get to an emergency room for treatment.
Ventricular tachycardias on the other hand are much more dangerous. They seldom occur in a healthy heart, and the tachycardia therefore will compound any other heart problem already present. They also can degenerate into ventricular fibrillation, which is a common cause of sudden death in people who have had a heart attack. A particular form of ventricular tachycardia called torsade de pointes can be caused by various drug interactions, even in people with normal hearts, and was the reason that cisapride (Propulsid) was taken off the market.
Before ending this posting, I want to reassure people about the APCs and VPCs that I mentioned as the initiators of many tachycardias. Most of us have APCs and/or VPCs fairly frequently but we don’t get a tachycardia, and in someone with an otherwise normal heart they are benign and require no treatment. Many people are unaware of them, and are surprised when their doctor mentions after an EKG that they had some on the tracing. Some people are more sensitive to the sensation that they cause — an extra beat which may feel stronger than usual. (It’s actually the following normal beat that is stronger than usual, and is therefore felt.) Caffeine and other stimulants may increase the numbers of these extra beats, and if they are troublesome, stopping caffeine may reduce their frequency. But — whether or not they are from caffeine — they are harmless, do not indicate that any heart disease is present, do not themselves damage the heart, and seldom lead to an SVT or ventricular tachycardia. The only exception to this is someone who has recently had a heart attack, in which case the extra beats should be suppressed with medication.