Fig. 6.

Spiral wave formation after spatially discordant Ca_i²⁺ alternans. A: Ca²⁺ map of propagation after the last beat (beat 3) before spiral wave formation. The Ca²⁺ wave was blocked near site c (double solid lines), propagated around the block (arrow, 170 ms) and reentered the mapping field near site c (290 ms), initiating a spiral wave. Time is in milliseconds after the beat 3 stimulus. B: Ca_i²⁺ alternans for the preceding two beats showing spatially discordant alternans. The site of propagation block falls on the nodal line. C: Ca_i²⁺ transients near the site of block and across the nodal line. Sites a–c show the direction of propagation (arrow), the site of block (double solid lines), and spiral wave formation. Site d is out of phase with sites a and b, showing spatial discordance. Large- and small-amplitude Ca_i²⁺ transients are shown. D: space-time plot, along the axis joining sites a–c, showing conduction block between sites b and c. The pulse symbol shows the stimulus site in A and the time of stimulation in C and D (along with the red lines in C).