Hi Eddie, I talked to my wife (a nurse practitioner) about this.
She said the warfarin would be to thin your wife's blood and reduce the risk of blood clots; a very important thing to do with someone with afib. She should also be on drugs to slow the heart rate eg beta blocker. See this web site
http://www.americanheart.org/presenter.jhtml?identifier=9 Best wishes
Graham
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Graham, Korg Pa1000, Korg G1 Air, Countryman E6, Roland BA330, 2 x Roland CM-30, , Mackie SRM150