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Definition

Broadly defined, cardiac arrhythmias are any abnormality or perturbation in the normal activation sequence of the myocardium. The sinus node, displaying properties of automaticity, spontaneously depolarizes, sending a depolarization wave over the atrium, depolarizing the atrioventricular (AV) node, propagating over the His-Purkinje system, and depolarizing the ventricle in systematic fashion. There are hundreds of different types of cardiac arrhythmias. The normal rhythm of the heart, so-called normal sinus rhythm, can be disturbed through failure of automaticity, such as sick sinus syndrome, or through overactivity, such as inappropriate sinus tachycardia. Ectopic foci prematurely exciting the myocardium on a single or continuous basis results in premature atrial contractions (PACs) and premature ventricular contractions (PVCs). Sustained tachyarrhythmias in the atria, such as atrial fibrillation, paroxysmal atrial tachycardia (PAT), and supraventricular tachycardia (SVT), originate because of micro- or macro re-entry. In general, the seriousness of cardiac arrhythmias depends on the presence or absence of structural heart disease.

The most common example of a relatively benign arrhythmia is atrial fibrillation (see the chapter “Atrial Fibrillation)”. Similarly common are PACs and PVCs, which, although a nuisance, generally are benign in the absence of structural heart disease. In contrast, the presence of nonsustained ventricular tachycardia (VT) or syncope in patients with coronary artery disease (CAD) or severe left ventricular (LV) dysfunction may be a harbinger of subsequent sudden cardiac death and must not be ignored.

http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/cardiology/cardiac-arrhythmias/Default.htm