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
Bradycardia. American Heart Association website. Available at: http://www.heart.org/HEARTORG/Conditions/Arrhythmia/AboutArrhythmia/Bradycardia_UCM_302016_Article.jsp. Updated October 25, 2012. Accessed January 18, 2013.
Fleg J. Arrhythmias and conduction disturbances. In: Beers MH, Berkow R, eds. The Merck Manual of Geriatrics (online). Merck & Co.;2000:486.
Explore arrhythmia. National Heart, Lung and Blood Institute—National Institutes of Health website. Available at: http://www.nhlbi.nih.gov/health/health-topics/topics/arr. Updated July 1, 2011. Accessed January 18, 2013.
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