An abnormal heart rhythm where the heart beats too fast, too slow, or irregularly. Can range from harmless to life-threatening depending on the type and underlying heart condition.
From Greek 'a-' (without) + 'rhythmos' (rhythm). The term emerged in the early 20th century as electrocardiography allowed precise measurement of heart rhythms. Previously, irregular heartbeats were simply called 'palpitations' or 'heart flutter' without understanding the electrical basis.
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