The normal heartbeat originates from the heart's normal pacemaker called the sinus node. An arrhythmia occurs when the electrical activity of the heart originates from a location other than this normal pacemaker. This location can be a site in the upper or lower chambers of the heart, or it can be a "circuit" composed of parts of the upper chambers, the lower chambers, or both.
The normal heart rate varies with age and activity. Babies have a faster heart rate than adults. For each age group, normal ranges have been established. If the heart rate exceeds this limit, a fast rhythm (tachycardia) exists.
If the electrical activity for this fast rhythm originates from the normal pacemaker of the heart, the rhythm is "sinus tachycardia". If the mechanism is not normal, a fast arrhythmia or "tachy-arrhythmia" exists.
There are two basic mechanisms for fast rhythms: automatic and re-entry. Arrhythmias are also divided based on location of their origin. Fast arrhythmias that originate from the lower chambers (i.e., the ventricles) are called ventricular tachycardias. Those that originate from the upper chambers (i.e., the atria) are termed supra-ventricular tachycardias (SVT).
After diagnosis of fast arrhythmias, the first objective needs to be consultation with your cardiologists regarding the risks of the fast rhythm and the risks and benefits of its treatment. If the fast rhythm has a "benign" course with little impact on daily life, close observation may be warranted. On the other hand, an infrequent fast rhythm that is a cause for concern to either the patient, family, or physician may require therapy.
There are many medications available for treatment of fast arrhythmias. The choice of medication depends on the mechanism of the fast rhythm. In selected cases, a catheterization procedure can be performed to treat the arrhythmia.
Supra-ventricular tachycardias are usually not dangerous. If they occur very frequently or for prolonged periods of time (hours to days) then they can cause difficulty with the pumping action of the heart. They are not due to a "heart attack" and do not cause sudden death.
Types of supra-ventricular tachycardias include:
Atrial tachycardia - A location or an area of the upper chambers takes over the pacemaker activity of the heart. This is an automatic tachycardia and is relatively uncommon in children.
Atrial Flutter/Atrial Fibrillation - When a large area of the upper chamber is involved in a circuit pattern, atrial flutter can develop. This rhythm can be seen in children who have had previous heart surgery involving the upper chambers, for example the Fontan procedure.
Children with atrial flutter are at risk for developing clots in the upper chambers because the flow in these chambers is slow and disorganized. There is often swirling of blood in the atrium seen on echocardiogram.
Atrio-ventricular re-entrant tachycardia (AVRT) - An extra electrical connection (called "accessory pathway") between the upper and lower chamber allows the formation of a circuit that conducts electrical activity faster than the normal pacemaker. This is the most common form of fast arrhythmias in infancy.
The majority of infants with atrio-ventricular re-entrant tachycardia "outgrow" the tachycardia during the first year of life. The specific diagnoses falling in this category include Wolff-Parkinson-White Syndrome (WPW) and Permanent Junctional Reciprocating Tachycardia (PJRT).
Atrio-ventricular nodal re-entrant tachycardia (AVNRT) - The atrio-ventricular node (A-V node) is located between the upper and lower chambers of the heart. It is the only area that normally allows the electrical activity of the heart to pass from the upper chambers to the lower chambers. Sometimes the region of the A-V node can become a source for a tachycardia. This is the most common form of fast arrhythmias in adolescence.
Junctional tachycardia - The origin of the tachycardia is the "junction" between the upper and lower chambers. This is an automatic tachycardia. This tachycardia is seen in patients who have had recent surgery involving this area, for example repair of a ventricular septal defect (VSD), atrioventricular septal defect or Tetralogy of Fallott.
Ventricular tachycardia are more serious than supra-ventricular tachycardias and can lead to serious heart dysfunction. When the source of the fast heart rate is the lower chambers (ventricles), ventricular tachycardia (VT) is present. These types of tachycardias are usually associated with symptoms such as passing out, lightheadedness, or dizziness.
Ventricular tachycardia is relatively uncommon in children. It can be seen in patients with congenital heart disease, especially those who, despite surgery, continue to have problems with heart function. Ventricular tachycardia is also seen with other conditions. Among the most common are prolonged QT syndrome, Hypertrophic Cardiomyopathy, and Myocarditis.
In older children and adolescents, the fast heart rate is often felt as palpitations. They recognize that a fast heart rate is occurring at an inappropriate time such as while at rest, doing homework, or eating dinner. Fast arrhythmias may also cause children or adolescents to pass out (syncope). Younger children may have difficulty describing this sensation and may complain of chest pain.
In infants, fast arrhythmias are often better tolerated. The infant's parents become concerned when they note the fast heart rate while cuddling their infant or during feedings. Some infants may develop poor feeding, irritability, or pallor (unnatural paleness).