Discordant alternans has the potential to produce reentrantarrhythmia than concordant alternans, but its mechanism is unknown. Right atrial free wall was dissected from 22 dogs (6-10years old). Using a voltage sensitive dye and high-resolution video imaging (5400 pixels, 300 fr/sec), action potential propagation was mapped in a small area (2.8 x 4.2 mm) of the atrial epicardium where fiber orientation was parallel, during pacing with a thin unipolar electrode (100 Nm in diameter). Concordant repolarization alternans was observed when cycle length (CL) was reduced to 200ms. At shorter CLs, depolarization alternans also apgeared. Transverse block was observed during concordant alternans with rapid pacing al CLs less than l50 ms. When transverse block occurred, it took a long time f6r the sites distal to the block line to be excited due to the detour propagation. At the next beat, diastolic interval and action potential duration at the sites proximal to the block line were long, while they were short at the sites distal to the line. Thus, discorv dant alternans was initiated. In anisotropic atrial tissues, conduction block occurred during concordant alternans, and the occurrence of conduction block initiated discordant alternans.

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