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Figure 1 (A) Examples of monophasic action potentials recorded at the high lateral wall of the right atrium by S1S2 in patients with chronic atrial fibrillation. A single extrastimulus (S2) was given at progressively shorter S1S2 intervals. As diastolic interval decreased from 138 to 0 ms, the action potential duration (APD) at 90% repolarization decreased from 240 to 158 ms, and then reached the effective refractory period with a coupling interval of 200 ms. (B) The right atrium was paced at progressively shorter S1S1 intervals until APD alternans and then 2:1 atrial capture failure occurred. Pacing with cycle lengths of 220 and 190 ms resulted in APD and diastolic interval alternans.
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