Chessboard of atrial fibrillation: reentry or focus? Single or multiple source(s)? Neurogenic or myogenic?

The refractory period may be shortened to one-fifth or one-sixth the normal, although vagus action is a necessary accompaniment of such high-grade shortening. This change makes rapid reexcitation possible and the formation of circuits having diameters of but a few millimetres. Since the refractory phase does not occur simultaneously in all fibers, the circuits must be formed in different and shifting locations.