However ,many researchers agree that the common factor in most types of clubbing is distal digital
vasodilation, which results in increased blood flow to the distal portion of the digits. Whether the vasodilation results from a circulating or local vasodilator, neural mechanism, response to hypoxemia, genetic predisposition, or a combination of these or other mediators is not agreed on currently.
http://emedicine.medscape.com/article/1105946-overview
I hope it helps.