Any variation from the normal regular rhythm of the heartbeat. The condition is produced by some affection interfering with the mechanism which controls the beating of the heart, and includes the following disorders: sinus arrhythmia; atrial fibrillation; atrial flutter; heart block; extrasystoles; pulsus alternans and paroxysmal atrial tachycardia; ventricular tachycardia; and ventricular fibrillation. (See HEART, DISEASES OF; ELECTROCARDIOGRAM (ECG), ANTIARRHYTHMIC DRUGS.)