Fig. 4.

Control-induced transition from concordance (control-off) to discordance (control-on). (a)–(b) Representative APD spatial profiles for AP_n and AP_n+1 (first two action potentials of six-AP sequence) during control-off (a) and control-on (b). (c)–(d) Representative relative repolarization times for AP_n and AP_n+1 during control-off (c; AP₁-σ²: 22.4 ms², AP₂-σ²: 122.2 ms²) and control-on (d; AP₁-σ²: 13.5 ms², AP₂-σ²: 59.9 ms²) conditions. (a–d: Preparation #1 – BCL = 350 ms.) (e) Control-off:control-on (open-arrows and closed-arrows, respectively) repolarization dispersion values ( $\overline{{\sigma }^2}$) derived from cases of concordant:discordant behavior (preparation #1 – BCL = 350, 375 ms; preparation #5 – BCL = 400 ms). Shown is the greater of the two σ²-mean ( $\overline{{\sigma }^2}$) values calculated for each six-AP sequence: ${\text{AP}}_{\text{n}}-\overline{{\sigma }^2}(\text{n}=3)$ or AP_n+1-σ² (n=3).