http://www.hrspatients.org/patients/heart_disorders/default.asp
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Millions of people experience irregular heartbeats at some point in their lives, called arrhythmias, most of which are harmless and happen in healthy people free of heart disease. Sometimes, however, rhythm disturbances can be serious or even fatal. This document describes the three disease categories of heart diseases and disorders, including electrical, circulatory, and structural.