Supraventricular tachycardia (SVT) is an abnormally rapid heart rate originating from the upper chambers of the heart. Children with SVT are treated by the specialists in our Heart Center. There are many different types of SVT. To understand it, it is useful first to understand how the normal heartbeats.
The normal heartbeat usually starts in the top right chamber of the heart, the right atrium, in an area called the sinus node (also called the sinoatrial or SA node). The electrical impulse propagates to the atrioventricular (AV) node in the center of the heart and then travels to the bottom chambers of the heart, called the ventricles. The electrical impulse causes the top chambers (atria) and bottom chambers (ventricles) of the heart to beat in a coordinated, sequential fashion. In normal hearts, the AV node is the only connection between the atria and the ventricles.
What Are the Causes of SVT?
In some cases, such as with an accessory electrical connection, the connection is present from birth. In most other cases, SVT develops over time with changes in the electrical characteristics of the tissue. SVT may occur at rest, with high resting tone from the vagus nerve or, most commonly, with exertion and adrenaline release. In some cases, electrolyte abnormalities or metabolic disturbances may be the triggering factor. Your electrophysiologist will help you and your child discover what triggers these episodes for them.
What Are the Symptoms of SVT?
No matter what kind it is, when SVT happens, your child may complain of chest pain, “heart racing,” palpitations, stomach ache or difficulty breathing. And the good news is there are many effective treatments available for SVT.
Occasionally, no treatment is necessary. About a third of children who present as newborns outgrow the condition by their first birthday. For most children, medications are prescribed. For others, certain vagal techniques can help manage your child's symptoms.
Depending on your child's age, and the severity and frequency of symptoms, the electrophysiologist may recommend a procedure called an intracardiac electrophysiology (EP) study and catheter ablation. This is a procedure done via catheters that are inserted into the heart from the veins in the legs. It does not involve surgery or a chest incision.
During catheter ablation, thin tubes or catheters are inserted into veins and artery and guided to the correct positions using x-rays. Using the electrical signals from these catheters, the electrophysiologists can accurately locate the extra electrical connection. The tip of the ablation catheter is then either heated (using radiofrequency) or cooled (cryoablation where liquid nitrous oxide cools the tip of the catheter) to destroy and eliminate the extra electrical connection. After ablation, further testing will be done to ensure the effectiveness of the ablation lesions. If the procedure is successful, then no further treatment will be necessary. Your child will have to lie flat in bed for six hours after the procedure. Depending on the length of the procedure and where the pathway is, your child may stay overnight in the hospital or may go home the same day. About half of our patients stay in the hospital overnight and go home the next morning.
How Is SVT Diagnosed?
There are special doctors who study heart rhythms called electrophysiologists. The most important piece of information that those electrophysiologists can get is a recording of the heartbeat while your child is having SVT or palpitations. There are many ways to record the heart rhythm when your child is symptomatic. By this time, you or your child may have had a 12-lead electrocardiogram already. It uses a lot of stickers all over the chest and prints out a pink sheet of paper: the 12-lead electrocardiogram (ECG or EKG).
The most common form of SVT occurs when there is an extra electrical connection between the top and bottom chambers of the heart, called accessory electrical connection.
To record the rhythm when your child is symptomatic, we use those 12-lead electrocardiograms. Since you can't have one of those on all the time, we also use 24-hour Holter monitor, event monitors (your child keeps an event monitor for a month) or electrical pacing studies in the hospital. Appropriate documentation of SVT allows for correct diagnosis and appropriate treatment for your child.
Types of SVT
The most common type of rapid heartbeat originating in the upper chambers is sinus tachycardia, which is a normal rhythm. Sinus tachycardia occurs when one is exercising, or during stress. Although this heartbeat may feel fast, it is a normal rhythm depending on the circumstances.
During normal sinus rhythm, if both connections are ON, this results in the appearance of a “delta wave,” which electrophysiologists can see on a 12-lead electrocardiogram.
There are also types of SVT that are due to electrical abnormalities in the heart. The most common form of abnormal SVT in young children occurs when there is an extra electrical connection between the top and bottom chambers of the heart, called accessory electrical connection.
Normally, the AV node is the only connection between the atria and the ventricles. When an accessory electrical connection is present, there are two electrical connections instead of one (the AV node and the accessory electrical connection). This allows electricity to travel from the atria to the ventricles. During normal sinus rhythm, if both connections are ON, this results in the appearance of a “delta wave”, which electrophysiologists can see on a 12-lead electrocardiogram.
At other times, the accessory electrical connection may be OFF in the downward direction, which means that it cannot conduct electricity from atria to ventricles. However, it can be ON in the opposite direction: it can conduct from the ventricles to the atria. When this occurs, an abnormally fast heartbeat may develop, with electricity traveling in a circuit. This circuit most commonly goes down the normal AV node and returns back up through the accessory connection to the atria. It is an electrical “short circuit,” resulting in “narrow QRS SVT.”
Another form of SVT occurs when there is an additional area of electricity near the AV node so that electricity travels from atria to the ventricles over the AV node and simultaneously returns to the atria, forming “AV nodal reentry SVT.”
The two most common types of SVT are described here. There are other kinds as well, like automatic atrial tachycardia, atrial re-entrant tachycardia and junctional tachycardia. If your child has one of these, your electrophysiologist will explain how it works in detail.
All treatments carry certain risks. Some medications may cause side effects like drowsiness, fatigue or transient mood swings. Your physician will explain these to you in detail. Catheter ablations carry similar risks for children as adults: nerve and vascular injury, bleeding, infection, stroke, damage to heart valves, cardiac perforation, heart block and the need for a pacemaker, coronary spasm and heart attack.
Tachycardia can often be stopped using several vagal techniques that stimulate the vagus nerve. Stimulating this nerve momentarily stops conduction in the AV node. When conduction pauses in the AV node, it temporarily halts one part of the circle and stops the electrical short-circuit. When the short circuit stops, the heart will return to its normal beating pattern again and the symptoms of the tachycardia will stop. Practice these techniques with your child before the next episode starts so your child. If SVT occurs within half an hour of your child's medication dose, give the medication early and see if SVT will stop on its own within half an hour. Otherwise, perform one of the vagal maneuvers:
- Ice to the face. This is only recommended for infants. Fill a zip-lock bag with some ice, and put some water in it. Hold the bag firmly in place over your child's forehead, eyes and nose for 20 seconds; remove for one minute; repeat two more times if necessary.
- Bear down. Have your child hold his or her breath and “bear down” as if having a bowel movement.
- Take a deep breath, hold it, then push out. Your child can do this by blowing an actual balloon or pretend to do so by blowing on his or her thumb (making sure there is no air leaking out of the lips or around the balloon or thumb) for as long as their breath lasts.
- Do a headstand. Have your child do a headstand, or you can turn your child upside down by holding onto their feet.
- Gulp down ice cold water. Fill a glass with ice, pour water on the ice, then have your child gulp down the water.
If the vagal maneuvers have not worked after three attempts, bring your child to the Emergency Department.