American Heritage Dictionary
(3)
Century Dictionary
(5)
GNU Webster's 1913
(1)
WordNet
(2)
Elsewhere on the web
The drug is intended to stop atrial fibrillation -- a condition in which the upper chambers of the heart begin quivering rather than beating effectively -- by changing the chambers 'electrical properties to bring normal heart rhythm back.— Salon
There are ongoing phase 3 studies investigating the role of factor Xa inhibitors in patients with atrial fibrillation, although one study has already missed its primary end point.— theHeart.org
For patients taking warfarin for atrial fibrillation, the heart association recommends following a checklist that includes telling your doctor immediately about unusual bleeding or bruising, alerting other doctors and your dentist about your warfarin use, and keeping in close contact with your doctor.
Mr. Emrich was diagnosed with arterial fibrillation, which is a fast and irregular heartbeat that affects two million Americans.— TravelPod.com Recent Updates
Explain to interested patients that atrial fibrillation is associated with a significant increase in the risk of ischemic stroke, but this study suggests that risk is higher for patients with proteinuria and / or chronic kidney disease.— MedPageToday.com - medical news plus CME for physicians

Century Dictionary (1)
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You can expect to see this word several times a year.