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Anatomy of a Pacemaker
 Above, a normal, healthy heart beart circulates blood efficiently.
In atrial fibrillation, the heart's two upper chambers quiver instead of beating in coordination. The heart's ability to circulate blood to the rest of the body effectively is reduced because the blood does not pump out of the atria completely.
Impaired circulation through the heart may cause blood to clot, increasing the risk for a stroke. The likelihood of developing atrial fibrillation increases with age.
Ventricular Fibrillation
In ventricular fibrillation, the heart's electrical activity becomes disordered, and the lower chambers of the heart contract rapidly, out of synchronization, rendering the heart unable to effectively circulate blood to the body. Unless medical help is provided immediately, collapse and sudden cardiac death will follow in minutes.
Usually caused by ischaemic heart disease (reduced blood supply to the heart muscle) or other types of heart disease, ventricular fibrillation may also occur -- though very rarely -- in healthy hearts.
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