include this effect in the model. In the original CMP model, the $1000 \times 1000 $ grid is coarse grained into a $200 \times 200$ grid for computational ease. A $1000 \times 1000 $ grid would approximately account for the total number of cardiomyocytes on the epicardial surface of the atrium. The model then treats each cell in the coarse grained grid as a single individual muscle cell. For single cardiomyocytes in isolation there is little clinical evidence for the cell stochastically failing to depolarise. However, instead of considering the CMP model as a microscopic depiction of atrial conduction we can think about a mesoscopic picture where each cell in the $200 \times 200$ grid represents the average dynamics of the underlying $5 \times 5$ block of cells. These coarse grained blocks can still follow the same branching/connectivity rules between cells as originally formulated for the CMP model.

Within this $5 \times 5$ cell block, we can consider the possibility of unidirectional conduction block due to the imbalance between current sources and sinks - the possibility of such a mechanism has been shown by @bub2002 [@bub2002] in a theoretical model and has also been supported by clinical evidence [@fast1995; @ciaccio2016; 