Mechanisms of induction of atrial fibrillation: what do we really know?

Background: Insights into the mechanisms of induction or paroxysmal atrial fibrillation hold the potential to develop preventive or even curative therapies for the arrhythmia. Aim of study: To review basic principles of induction of reentrant activation with respect to their applicability to clinical observations. Methods: Literature search regarding relevant clinical and experimental studies. Analysis of the epicardial activation pattern during induction of atrial flutter in dogs with sterile pericarditis using a custom-designed electrode array (128 bipolar electrodes, interelectrode distance 3-8 mm) and a computerized multiplexer-mapping system. Results: Limited clinical data reveal a marked intra- and interindividual variability in the onset mechanisms of atrial fibrillation. Episodes of atrial fibrillation initiated by rapid focal discharges from the pulmonary veins or by increased sympathetic or parasympathetic tone appear to represent rare entities. Induction of reentrant activation generally depends on the occurrence of functional conduction block and regional conduction delay. Conduction block in the atria seems to be primarily based on local dispersion of refractoriness. Refractory periods are modified by various factors, potentially resulting in temporary, nonhomogenous changes. Conclusions: Regional conduction and refractory properties are of major relevance for the initiation or reentry. With a plethora of modifying factors thereof, marked intra- and interindividual variability in the onset mechanisms of atrial fibrillation is more than conceivable.