Figure 2. Vulnerable period in an array of FitzHugh-Nagumo (FHN) cells with identical refractory properties. A wave of refractoriness was created by an S1 (s1 in the Figure) conditioning stimulus. The boundaries of the vulnerable period were determined by testing excitability with S2 (s2 in the Figure) stimuli timed to occur at different times after the conditioning wave front passed the S2 site. Shown here are the computed ECGs for four different S1S2 (s1-s2) delays. When the S1S2 delay was too short (1.5 arbitrary time units [ATU]), there was no response to response to the S2 stimulus. As the S1S2 delay was prolonged, spiral wave reentry was initiated and the ECG showed a typical monomorphic pattern (delay, 1.7 and 1.9 ATU). When the S1S2 delay was beyond the vulnerable period (delay, 2.5 ATU), a single response was observed.
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