Dual-layer model of the atrial wall. (A) Model structure with two layers (epi- and endocardium) and transmural connections (grey cylinders). (B) Dissociation of electrical activity between the two layers and transmural conduction resulting in a breakthrough wave (black arrow). (C) Simulation protocol (see Methods section for details). (D) Number of waves per second for a control and Na⁺-channel blockade simulation with 12 connections. The first 1 s represents Step 1 of the simulation protocol. Time points between 2 s and 7 s represent Step 2 of the simulation protocol. Time points between 7s and 13 s represent Steps 3 and 4 of the simulation protocol.