Example snapshots showing wavebreak and re-entry in simulated tissue with circular heterogeneity, and steep APD restitution. The colour scheme is the same as in Figure 3, with brighter colours showing depolarised tissue. The boundary of R1 is shown by the grey dotted line. Each panel shows consecutive snapshots of membrane voltage following stimuli with S1 S2 interval of 200 ms, and S2 S3 interval of 120 ms. (a) Wavebreak and sustained re-entry for R1 with radius of 50 mm. (b) Wavebreak and one cycle of re-entry for R1 with radius of 12.5 mm. Movies of these simulations are included in additional files 3 and 4.