The muscular network of the sheep right atrium and frequency‐dependent breakdown of wave propagation

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Abstract
The complex branching structure of the right atrium (RA) muscular network may provide the substrate for complex patterns of propagation during atrial fibrillation (AF). As AF results in some cases from stable sources in the left atrium (LA) with fibrillatory conduction toward the RA, we hypothesize that periodic input to the RA at an exceedingly high frequency results in disorganized wave propagation associated with the complex structure of the RA. Optical mapping was performed in isolated coronary‐perfused sheep RA. Rhythmic pacing of Bachmann's bundle allowed well‐controlled and realistic conditions for LA‐driven RA. Pacing at increasingly higher frequencies led to increasing delays in activation distal to major branching sites of the Crista terminalis and pectinate bundles, culminating in spatially distributed intermittent blockade at and above ∼ 6.5 Hz. At this breakdown frequency, the dominant frequencies of the RA response activity became spatially nonuniform. Such frequency‐dependent changes were independent of action potential duration. Rather, the spatial boundaries between proximal and distal frequencies correlated well with branch sites of the pectinate musculature. Thus, there exists a breakdown frequency in the sheep RA below which activity is periodic throughout the atrium and above which it is fibrillation‐like, consistent with the ideas that during AF, high‐frequency activation initiated in the LA undergoes fibrillatory conduction toward the RA, and that sink‐to‐source mismatch effect at branch points of the Crista terminalis and pectinate muscles is important in determining the complexity of the arrhythmia.