Challenges of C arrhythmias raised by the data: 1-Most frequent cause of embolic stroke (AF), with evolution to chronic disease. Major cause of natural death (VF), about 5% only are saved. 2-Hysteresis: strike twice and patient resuscitates. 3-Most models refer to chaotic dynamics, but this does not describe spontaneous reversions to lesser C arrhythmias. Some features suggest intrinsic disorder: 1-The data show intermittency of beat-to-beat intervals and peaks amplitude. 2-Slow evolution of vulnerability (the susceptibility), so called remodeling. Conduction velocity is not known to be very much affected. Intermittency may instead reveal nucleation and contamination processes à la Pomeau. However, the medium is excitable and excitability is a robust cycle to any homo- or heteroclinic nucleation. Alteration of conduction, e.g. on a nearly percolating fibrotic substrate, yields fancy anatomical reentries, but no such intermittent behavior. We believe that the data is suggestive of fragmented pulses, following from the desynchronization of the network of electrically coupled excitable cells.