US National Library of Medicine
Medline Plus: Arrhythmias
A fact sheet on arrhythmias tell what they are, what causes them, different types of arrhythmias, risk factors, symptoms, treatment, and prognosis after diagnosis. Numerous diagrams of the heart accompany the article.
Khan Academy
Khan Academy: Cardiac Dysrhythmia and Defibrillators
This passage will test your knowledge on defibrillators.
Texas Heart Institute
Texas Heart Institute: Heart Information Center: Arrhythmia
One-page article provides useful information on arrhythmia, including categories, symptoms, diagnosis, and treatment.