Bradycardia is an abnormally slow heart rate. In adults, it is defined as a heart rate of less than 60 beats per minute. Different types of bradycardia are collectively referred to as bradyarrhythmias. They include:
Sinus bradycardia—an unusually slow heartbeat due to heart disease, a reaction to medication, or harmless causes, such as excellent fitness or deep relaxation
Sick sinus syndrome—an unusually slow heartbeat due to a malfunction of the sinoatrial node, which is the heart's natural pacemaker
Heart block (atrioventricular block or AV block)—an unusually slow heartbeat due to a slowing or blocking of electrical impulses in the heart’s conduction system
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