Therapy strategies for atrial fibrillation based on electric characterization are becoming viable personalized medicine approaches to treat a notoriously difficult disease. In light of these approaches that rely on high-density surface mapping, this study aims to evaluate the presence of 3-dimensional electric substrate variations within the transmural wall during acute episodes of atrial fibrillation.

Optical signals were simultaneously acquired from the epicardial and endocardial tissue during acute fibrillation in ovine isolated left atria. Dominant frequency, regularity index, propagation angles, and phase dynamics were assessed and correlated across imaging planes to gauge the synchrony of the activation patterns compared with paced rhythms. Static frequency parameters were well correlated spatially between the endocardium and the epicardium (dominant frequency, 0.79±0.06 and regularity index, 0.93±0.009). However, dynamic tracking of propagation vectors and phase singularity trajectories revealed discordant activity across the transmural wall. The absolute value of the difference in the number, spatial stability, and temporal stability of phase singularities between the epicardial and the endocardial planes was significantly >0 with a median difference of 1.0, 9.27%, and 19.75%, respectively. The number of wavefronts with respect to time was significantly less correlated and the difference in propagation angle was significantly larger in fibrillation compared with paced rhythms.

Atrial fibrillation substrates are dynamic 3-dimensional structures with a range of discordance between the epicardial and the endocardial tissue. The results of this study suggest that transmural propagation may play a role in atrial fibrillation maintenance mechanisms.