@ciaccio2017]. This explains why we might expect any given block of cells to display unidirectional conduction block with some small probability, $\epsilon$, given a suitable geometric arrangement of cells with a small probability, $\delta$. It is also sensible to consider such an effect to be stochastic since leaking current over a number of activation cycles can push a previously blocked group of cells over the depolarisation threshold.

The parameters used in this work are $\nu = 0.2$, $\tau = 50$ and $T = 220$. We fix the coupling fraction to be $\nu = 0.2$ since this is the largest coupling fraction at which we typically observe paroxysmal AF [@christensen2015; @manani2016]. The shortest timescale in the CMP model is associated with the excitation timescale of a single cardiomyocyte of $\Delta t = 0.6 \text{ms}$. In the course grained tissue, this corresponds to an excitation timescale for each cell of $\Delta t^{*} = 5 \cdot 0.6 \text{ms} = 3.0 \text{ms}$. The time step of $3ms$ approximately corresponds to the time taken for an electrical signal to cross a $5 \times 5$ block of cardiomyocytes in the real atrium along a muscle fibre. The refractory period is chosen to be $\tau = 50$ 