Combinations of S1 S2 interval (CLS1S2) and S2 S3 interval (CLS2S3) resulting in wavebreak and re-entry in models with (a) steep APD restitution, and (b) flat APD restitution. Red points show the upper limit of S2 S3 interval that resulted in wavebreak, above this threshold the S3 action potential propagated in region R1 without wavebreak. Blue points show the lower limit of S2 S3 interval that resulted in wavebreak, below this threshold the S3 action potential was blocked in region R2 at the stimulus site. The vulnerable regions where re-entry and wavebreak were initiated are bounded by these upper and lower limits, and are shown shaded grey. The dashed line in (a) indicates the shortest CLS1S2 that produced a propagating action potential.