Each time the heart beats, or contracts, blood is pumped through the ventricles. Oxygenated blood travels from the lungs to the left side of the heart, first through the atrium (top chamber) and then to the ventricle (lower chamber). The left ventricle is the main pumping chamber for the heart, pushing blood through the aorta and into the body. At the same time, de-oxygenated blood travels from the body into the right atrium, into the right ventricle and then to the lungs, where carbon dioxide is exchanged for oxygen.

The heart beat is controlled by a coordinated electrical system. The electrical signal starts out at the top of the right atrium in a group of cells, called the sinus node. The signal moves throughout the top half of the heart, causing the top chambers to contract and push blood into the ventricles. The electrical signal then travels through the atrioventricular (AV) node, a group of cells forming the electrical connection point between the top and bottom chambers of the heart. Next, the signal travels down and splits into two pathways leading to the left and right ventricles. This causes the ventricles to contract and push blood out of the heart to the lungs or the body. The ventricles then relax and the next electrical signal is fired.

Normally, a heart beats 60 to 80 times a minute while a person is at rest. A heart beat that is abnormally fast is called a tachycardia (more than 100 beats a minute). An abnormally slow heart beat is a bradycardia (less than 60 beats a minute).

Kids and Supraventricular Tachycardia

Supraventricular tachycardia (SVT) is a condition in which an abnormal or damaged area of tissue in the heart creates an extra electrical pathway between the atrium and ventricle. When the heart beats normally, the existence of the pathway doesn't usually cause a problem. But if a heart beat is fired too early, the signal can travel through the AV node and then back up to the top chambers through the abnormal pathway. The electrical signal may continue in the abnormal circuit, causing the heart to beat too fast (in children, the heart rate often exceeds 180 beats a minute). The chambers of the heart can't adequately fill with blood. In addition, the heart muscle can't relax or contract strongly enough to supply enough blood for the body. The brain doesn't get an adequate supply of oxygen, causing the patient to feel dizzy, light-headed or faint. Some children will complain of pressure or pain in their chest or a sensation that their heart is racing.

According to the American Heart Association, SVT occurs in about one to four of every 1,000 people. It's the most frequently diagnosed class of arrhythmias in children. One specific type of SVT is Wolff-Parkinson-White syndrome. In this condition, the extra conduction pathway causes the electrical signal to travel to the ventricles too early.

Treating SVT

In many cases, SVT can be controlled with medications. But when arrhythmias are more serious, doctors may recommend a procedure called ablation. The abnormal electrical pathway is located and a small scar is created in the heart tissue (like cutting an electrical wire) to block, or "short-circuit" the electrical flow.

Traditionally, doctors perform ablation using radiofrequency energy. However, many physicians are using cryoablation (freezing to kill a small area of tissue). First, a small cut is made into a blood vessel in the groin. Then, using X-rays for guidance, a catheter is fed through the circulatory system to the heart. Electrodes at the tip of the catheter gather information to "map" the heart's electrical conduction and find the location of the extra pathway.

Once the location of the abnormal tissue is confirmed, the tip of the catheter is aimed at the target and cooled. The initial cooling stops electrical conduction through the pathway, but doesn't damage the heart cells. This allows doctors to determine for sure if they are treating the right area of the heart. It also enables the physicians to avoid injuring any part of the normal conduction pathway. At this point, if surgeons determine they have the wrong location, or are damaging healthy tissue, the cryoablation catheter can be turned off. The tissue warms back up and resumes normal function. But if the abnormality is confirmed, the catheter is cooled further, freezing the tissue and creating a scar.

Lee Beerman, M.D., Director of the Pediatric Arrhythmia Program at Children's Hospital of Pittsburgh, says children have smaller hearts than adults. So there is a higher risk of accidentally damaging healthy heart tissue. If the normal conduction pathways are disrupted, a child may require implantation of a pacemaker to maintain a normal heart beat. Cryoablation is an ideal procedure for children with SVT because surgeons can cool the target area first to "test" the effectiveness, and if necessary, reverse the process.

For general information on arrhythmias and cardiac ablation:

American Heart Association

Heart Rhythm Society

National Heart, Lung and Blood Institute