Fehmi Keçe

General Introduction 15 1 1.2.4 Rotors Rotors or spiral waves describe a specific type of functional re-entry. The re-entry is in this case not circular, but curved or spiral. The wave front and wave tail meet at a focal point called a ‘phase singularity’. The wave front is in contrast to the ‘leading circle model’ not constant but depends on the wave front curvature. The phase singularity region has the highest curvature and therefore the slowest wave front conduction velocity. The tissue core forms an area of functional block, similar to the centre of the leading circle model. The tissue core is not excitable, because the propagating wave front is unable to invade a core of tissue in the centre of the rotor, due the wave front curvature that is very high and the conduction velocity therefore very slow (14). 1.2.5 Endo-epicardial asynchrony A relatively newly describedmechanism is that of endo-epicardial asynchrony, characterized by dissociation of electrical activity not only within the epicardial layer but also between the epicardial layer and the endocardial bundle network. This concept may play a role in the maintenance of AF. During the first 6 months of AF, endo- and epicardial layers of the atrial wall become progressively dissociated. After that time, fibrillation waves in the endo- and epicardial layers often propagate at different speed and in different directions and endo-epicardial breakthroughs become more abundant (15). Dissociated layers of fibrillation waves will stabilize the fibrillatory process, because as soon as fibrillation waves die out, they can be replaced by breakthroughs from the opposite site. In this case, ectopic AF becomes 3-dimensional as a result of structural remodelling with a probably lower response to medication and ablation therapy, explaining why an early rhythm control strategy often has better results (16, 17). 1.2.6 Anatomical re-entry and remodelling of the atria Re-entry occurs in the presence of unidirectional block and slow conduction making the wave length shorter than the length of the circuit. Anatomical re-entry occurs commonly in patients with atrial remodelling (18). Remodelling of the atria can be structural, electrical and autonomic. 1.2.7 Fibrosis (Structural Remodelling) In patients with AF, structural remodelling of the atria occurs with activation of fibroblasts, enhanced connective tissue deposition and fibrosis (19, 20). In addition, inflammation, fatty infiltration, hypertrophy, necrosis and amyloidosis can be detected in patients with (a predisposition for) AF (21). Factors inducing structural remodelling are structural heart