af-nest af-nest an af-nest or atrial fibrillation nest (afn) is a locus or cluster in the atrial wall with distinct electrical features and properties originated by fibrillar myocardium. it plays as an "electrical multiplier" re-feeding the atrial fibrillation. one of the currently existing techniques to treat atrial fibrillation (af) is based on thermo-coagulation of afn. they are typically numerous at the pulmonary veins antrum. several evidences have shown that the afns represent the true substrate of the af. a large number of congenital and acquired conditions may cause this type of myocardium. the higher the number of afns the easier the initiation and the longer the af maintenance. despite being fundamental in the af physiopathology the long-lasting af depends on additional factors. the most important is the "background tachycardia" (bkt) which is a focal reentrant tachycardia caused by "fractal micro-reentry". this special tachycardia exists even during af, keeping the afns in a high frequency activation. this tachycardia has a unique mechanism of "protection" that prevents it from being reverted by the large amount of surrounding stimuli generated by the af itself. recent studies have shown that the bkt occurs within a more developed afn or when there is a confluence of two or more of these elements. by using spectral analysis, it is possible to observe that the "fractal micro-reentry" phenomenon tends to occur depending on a critical amount of fibrillar myocardium. this is a small point in the atrial wall with numerous micro-reentries (in a three-dimensional model) inside the afn, caused by "cellular electrical disconnection" with progressive dichotomy (biological fractal phenomenon), even without the presence of fibrosis and without the need of major histological changes. by this way, it may be present even in normal hearts that explains the "lone atrial fibrillation". this intense micro-electrical activity stimulates the surrounding atrial myocardium that accepts the activation according to its refractory period. this produces a slightly irregular focal tachycardia, known as "background tachycardia" responsible for af maintenance with or without the contribution of many others afns or even of other bkt. the greater the number of these elements is the longer the af may last even becoming permanent.