FIGURE 9.

Vulnerable windows for different S1 wavefront velocities, but the same spatial APD gradient, with normal and fivefold-slowed Na⁺ channel recovery. (Solid bar) Obtained by solving Eq. 3 with constant S1 wavefront velocity, i.e., . (Shaded bar) Obtained by solving Eq. 3 with S1 wavefront velocity varies as in the discordant alternans, i.e., . (Open bar) From LR1 model in homogeneous 1D cable. Normal recovery: the spatial APD distribution of the S1 beat is , which was fit from the 30th beat (for x = 0–24 mm) of the discordant alternans shown in Fig. 8 A. in which was fit from the 29th beat (for x = 0–24 mm) in Fig. 8 A. PCL = 175 ms. Fivefold slowed recovery: the spatial APD distribution of the S1 beat is , which was fit from the 30th beat (for x = 0–22 mm) of the discordant alternans shown in Fig. 8 B. in which was fit from the 29th beat (for x = 0–22 mm) in Fig. 8 B. PCL = 200 ms. Note that during discordant alternans, the cycle length CL(x) is not constant in space but varies, however, in a magnitude much smaller than APD; therefore, it is reasonable for us to calculate d₁(x) using PCL instead of CL(x).