Figure 9. Time sequence of the development of reentry initiated by an early afterdepolarization (EAD) (approximating torsade de pointes). Blue represent the peak membrane potential, red is the plateau potential, and yellow is the rest potential. The time sequence is left to right, top to bottom. The top left frame shows a wave front (vertical color bars) propagating from left to right. The blue rectangle of cells represents an EAD that occurs late during the action potential plateau. In the next frame, the EAD wave front begins to develop in the retrograde direction (green region) and fails to develop in the antegrade direction, demonstrating unidirectional block. The cells in the tip of the newly formed wave fragment depolarize adjoining cells slowly because of the increased load, and thus the wave front propagates more slowly at the tip than near the edge of the medium. The wave front develops in a counterclockwise manner and evolves into a spiral wave. As the excitation wave front propagates away from the spiral, the spiral appears to rotate as a result of the curvature of the wave front. The tip propagates very slowly, and its path is indicated by the black line. From one reentry cycle to the next, the tip rotates around a different region of unexcited cells (spiral core). If the region of block were stationary, the tip would trace the same path from one rotation to the next and the ECG would appear monomorphic. When the tip rotates about a nonstationary core, a polymorphic ECG is produced.
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