It has been appreciated for a long time that atrial flutter and atrial fibrillation have a clinical relationship. Now, with the technological advances that permit more sophisticated electrophysiological studies, especially mapping studies, we have significantly advanced our understanding of this interrelationship. Regarding the relationship at atrial fibrillation to atrial flutter: Atrial fibrillation of variable duration (very brief to prolonged episodes) precedes the onset of atrial flutter in most instances. It seems that during the period of atrial fibrillation, the functional components of the atrial flutter reentrant circuit are formed. This is principally a line of block between the venae cavae. If this line of block does not form, classical atrial flutter does not form. And if this line of block shortens or disappears, classical atrial flutter disappear as well. In fact, it might be said that the major difference in whether classical atrial flutter or atrial fibrillation develops is whether a line of block forms between the venae cavae. Regarding the relationship of atrial flutter to atrial fibrillation: Studies have demonstrated that a driver (a single focus or reentrant circuit of very short cycle length) can be responsible for causing atrial fibrillation by producing fibrillatory conduction to the rest of the atria. In experimental models and now beginning to be demonstrated in patients, this driver may be a stable reentrant circuit of very short cycle length, i.e., a fast form of atrial flutter, if you will. In fact, there is probably a spectrum of these short cycle lengths that depend, in part, on where the reentrant circuit (i.e., "atrial flutter") exists. When the stable reentrant circuit is of sufficiently short cycle length, it will only activate small portions of the atria in a 1 : 1 manner. The rest of the atria will be activated irregularly, resulting in atrial fibrillation. Unstable reentrant circuits can also do the same thing. In short, it appears that there are several mechanisms of atrial fibrillation, one of which is due to a form of very rapid atrial flutter.