Abnormal rhythm of the heartbeat. Most commonly seen after someone has had a myocardial infarction, but also present in some normal individuals – especially if they have taken a lot of coffee or other stimulant – and in those with a congenital abnormality of the conducting system of the heart muscle. The cause is interference in the generation or transmission of electrical impulses through the heart's conducting system. Occasional isolated irregular beats (ectopic beats) do not necessarily mean that conduction is faulty. Arrhythmias can be classified as tachycardias (more than 100 beats a minute) or bradycardias (slower than 60 beats a minute). The pulse may be regular or irregular. (See HEART, DISEASES OF.)