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Figure 3 Mechanisms underlying T-wave alternans (TWA). (Left) spatial dispersion of repolarization. Compared to region 2, region 1 has longer action potential duration (APD) and depolarizes every other cycle (beats 1 and 3). (Right) Temporal dispersion of repolarization. Action potential duration alternates between cycles, either from alternans of cytosolic calcium (not shown) or steep APD restitution. Action potential duration restitution (inset) is the relationship of APD to diastolic interval (DI), the interval separating the current action potential from the prior one. If restitution is steep (slope >1), DI shortening abruptly shortens APD, which abruptly lengthens the next DI and APD, leading to APD alternans.