There's an interesting post on Medscape entitled "Is Calcium Good or Bad":
This has relevance to AFibbers as arterial calcification (atherosclerosis) is a risk with warfarin use, particularly where Vit K intake is reduced (as some old fashioned doctors and nurses still recommend). It now appears as though there is serious medical evidence that Vit K2 is effective in preventing and treating arterial calcification. - i.e. making sure that the calcium goes where it should in the bones and not in the arteries and kidneys.
I find that Vit K2 has the added benefit of stabilising INR.