Propagation of Sinus Waves in the Atrial Architecture
When Laminar Electrical Fluxes Turn Turbulent
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See Article by Mouws et al
The propagation of the electric wave fronts in the atrial myocardium is complex and depends on discontinuities at multiple size scales, from the ionic dimension to the macroscopic structure. In some regions of the human atria, the myocardial fibers organize in well-demarcated structures, where fiber orientation favors a highly anisotropic condition forcing the electric impulse to propagate mostly along the long axis of the muscle bundles. The latter organize in fascicles of variable thickness and number, which contributes to fast electric propagation and enables efficient electromechanical coupling of both atria during each normal sinus beat.1 Bachmann’s bundle and the septopulmonary bundle are 2 major examples of bundles that sustain fast and laminar fluxes of electric waves moving away from the sinus node. However, whereas Bachmann’s bundle extends mostly subepicardially across the interatrial groove, the septopulmonary bundle occupies a deeper level,1 which implies that even during sinus rhythm the propagation of waves is not uniform across the atrial wall. In fact, a certain degree of dissociation exists between the epicardium and the endocardium even for well-structured myocardial layers. Moreover, in some areas of the atrial wall, the uniform distribution of myocardial fibers is disrupted by multiple intricate crossings of fibers (Figure),2 which underlie the nonuniform anisotropic propagation that characterizes, for example, the crista terminalis and the antrum of the pulmonary veins (PVs). Altogether, such an intricate architecture is responsible for the efficient but highly nonlinear propagation of sinus waves, but has also been demonstrated to underlie the initiation and maintenance of cardiac arrhythmias.3,4