Why is Coumadin used to treat chronic atrial fibrillation?

In atrial fibrillation, the blood from the atria is not fully ejected into the other heart chambers. Blood remaining in the atria could form clots, which could increase the risk of heart attack or stroke (when the clot wedges itself into an artery of the heart or brain). As a blood thinner, Coumadin helps prevents these clots.

Douglas E. Severance, MD
Family Medicine
Since atrial fibrillation increases the risk for blood clots and strokes, Coumadin is prescribed to thin the blood and help prevent blood clots from forming.
Joan Haizlip, MSN
Cardiology (Cardiovascular Disease)

Coumadin is used to treat atrial fibrillation because it is a blood thinner and prevents blood clots. 

When you have A Fib, the upper chambers of your heart (atria) are beating in a very disorganized manner.  They actually are not even beating....they are fibrillating or quivering, so blood is not being pumped out very well.  This increases the chances of blood clots forming.  Then if the clots get pumped out, they can go to your brain and then you are in trouble.  The result can be a stroke or a "brain attack".  This can leave you with stroke symptoms, like difficulty speaking, walking or talking. 

So your healthcare provider will prescribe a blood thinner, like coumadin.  Coumadin prevents blood clots from forming and can help prevent you from getting a stroke.