The normal heartbeat originates from the heart's normal pacemaker called the sinus node. A fast heart rhythm can originate from this normal pacemaker or from other areas of the heart. Abnormal locations of rhythm origin can be in the upper or lower chambers of the heart or can be a "circuit" composed of parts of the upper chambers, the lower chambers, or both.
Description of a Fast Heart Rhythm
The normal heart rate varies with age and activity, decreasing with increase in age during childhood and adolescence. For each age group, normal ranges have been established. If the heart rate exceeds this limit, a fast heart rhythm (tachycardia) exists.
What are the basic mechanisms of fast rhythms?
There are two basic mechanisms for fast heart rhythms: automatic and re-entry. An automatic mechanism occurs when an area of heart tissue generates electrical activity at a rate faster than normal.
Re-entry occurs from a “circuit” where electrical impulses travel in a circular way, faster than the normal pacemaker. This circuit may involve tissue of the upper chambers, the lower chambers, or both.
Besides the mechanism of the rhythm, arrhythmias are divided based on location of their origin. Fast arrhythmias that originate from the lower chambers (i.e., the ventricles) are called ventricular tachycardias. All others are termed supraventricular tachycardia (SVT).
Signs and Symptoms of Fast Heart Rhythms
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) or, in very rare circumstances, a cardiac arrest.
Younger children may have difficulty describing this sensation and may complain of chest pain or have a general feeling of illness.
In infants, fast arrhythmias are more difficult to detect as they will not complain of symptoms. Some infants may develop poor feeding, irritability, or pallor (unnatural paleness) associated with prolonged fast heart rhythm.
Treatment for Fast Heart Rhythms
There are many medications available for treatment of fast arrhythmias. The choice of medication depends on the mechanism of the fast rhythm and the patient’s response. Medication does not cure the problem but can prevent episodes while it is being taken. In selected cases, a “catheter ablation”
procedure can be performed to cure the arrhythmia.
Types of Supra-Ventricular Tachycardias
- Atrial tachycardia. A location or an area of the upper chambers takes over the pacemaker activity of the heart.
- 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. Children with atrial flutter and fibrillation are at risk for developing clots in the upper chambers because the flow in these chambers is slow.
- 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 between the upper and lower chambers of the heart. This is the most common form of fast heart rhythm in infancy. Many infants with atrio-ventricular re-entrant tachycardia "outgrow" the tachycardia during the first year of life as the accessory pathway becomes unable to function. Some 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. There can be two pathways within the AV node which allows a circuit within it. This results in 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 may be 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 Fallot. It is also rarely seen in children with otherwise normal hearts, often occurring in families.
- Ventricular Tachycardia. When the source of the fast heart rhythm is the lower chambers (ventricles), ventricular tachycardia (VT) is present.
Ventricular tachycardia is relatively uncommon in children. Patients with congenital heart disease may have this, 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.
Diagnosis of Fast Heart Rhythms
Recording the heart rhythm with an electrocardiogram while the patient is having an episode confirms the diagnosis. This can be done by a life squad, in an emergency room or a physician's office capable of performing electrocardiograms.
Other types of monitors can be used to record the heart's electrical activity at home, school or any remote location. In rare circumstances, implanted monitors can be used to record rhythms that are brief and infrequent.
After diagnosis, you should be talk with your cardiologist about 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, 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. If the fast rhythm recurs despite therapy, the symptoms usually would be the same as before therapy.
It is helpful to learn how to take your child's pulse and to know the "normal" heart rate for their age. Your doctor can help you learn this skill. The ability to count your child's heart rate can be reassuring to you and helpful to your doctor when making a diagnosis.