Congenital Heart Defects - Fast Arrhythmias


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.

What defines a fast rhythm?
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.

What are the basic mechanisms of fast rhythms?
There are two basic mechanisms for fast rhythms: automatic and re-entry. The automatic mechanism occurs when a focus of heart tissue begins to generate electrical activity at a rate faster than the normal pacemaker. Due to the faster rate, this site overtakes the normal pacemaker of the heart and a fast rhythm is produced.

Re-entry can occur if there are differences in the electrical properties of the heart tissue. This difference between adjacent heart cells can cause the formation of a "circuit" that generates electrical impulses faster than the normal pacemaker; therefore, a fast rhythm can form. This circuit may involve tissue of the upper chambers, the lower chambers, or both.

For example, if the normal pacemaker is generating an impulse at a rate of 100 beats per minute and another location starts generating impulses at a rate of 150 beats per minute, the heart rate will be 150 because the normal pacemaker is being "outpaced."

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. Those that originate from the upper chambers (i.e., the atria) are termed supra-ventricular tachycardias (SVT).