Your Heart Rhythm

A Primer On Cardiac Arrhythmias


Arrhythmia Terminology

Arrhythmias are named according to three main features: 1) whether the heart rate is to fast or too slow; 2) which part of the heart the arrhythmia is coming from; and 3) what specific physiological derangement may be causing the arrhythmia. All the most common arrhythmias will be explained in upcoming sections.

1. Rate Terminology ­ The technical name for an abnormally slow heart rhythm is "bradycardia" [pronounced BRAY-DEE-CAR-DEE-UH], while an abnormally fast heart rhythm is called "tachycardia" [pronounced TACK-EE-CAR-DEE-UH]. Most often, a "normal" heart rate is taken as between 60 to 100 beats per minute (bpm), so arrhythmias that cause the heart to beat less than 60 bpm are considered bradyarrhythmias and arrhythmias that cause the heart to accelerate over 100 bpm are sometimes referred to as tachyarrhythmias. Keep in mind that one has to consider the body's needs, and a normal rhythm often exceeds 100 bpm. For example, a rate of 120-150 bpm is expected if someone is running on a treadmill. This is not abnormal under the circumstances, so that heart rate is not an indication of a problem.

2. Anatomical Terminology ­ Arrhythmias are generally classified into two broad groups, depending on which part of the heart gives rise to the rhythm disturbance. Ventricular arrhythmias, obviously, arise from the lower chambers of the heart. On the other hand, "supraventricular" arrhythmias arise from tissues above the ventricles ("supra" means "above"). Supraventricular tachycardia, for example, refers to any rapid beat that does not arise from the ventricles. This could include arrhythmias coming from the sinus node, the atrium, or even the AV node.

3. Electrophysiologic Mechanism ­ Electrophysiologists classify arrhythmias into two mechanisms: Disorders of Impulse Conduction and Disorders of Impulse Generation. Impulse generation refers to the creation of an electrical signal in the heart, and disorders of impulse generation include arrhythmias that occur due to either an inadequate rate of "firing" of the electrical signals (resulting in bradycardia) or an excessive rate of generation of electrical signals. For example, if the sinus node (which is the natural pacemaker of the heart) has trouble generating electrical impulses, the heart will beat too slowly. This condition is called Sinus Bradycardia and is a disorder of impulse generation. Conversely, if another area in the atrium starts generating rapid, abnormal signals which "take over" as the predominant pacemaker, the tachycardia that results is another example of a disorder of impulse generation. On the other hand, disorders of impulse conduction include arrhythmias caused by abnormalities in the path of movement of the electrical signals through the various heart tissue. A "loose connection" in the AV node that results in failure of conduction of the signal from the atrium to the ventricles (known as AV Block) is one example. A common cause of tachycardia results if the electrical signal gets "stuck" traveling around and around in a circular pathway. This phenomenon of the signal chasing it's own tail is referred to as "Reentry" and is another type of disorder of impulse conduction.