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Figure 4 Representative examples of action potential duration restitution (APDR) by S₁S₂ (left panels) and rapid pacing (right panels). The slope of APDR by rapid pacing was slightly steeper than that of restitution by S₁S₂ in each patient. APD₉₀ = action potential duration at 90% repolarization; CAF = chronic atrial fibrillation; DI = diastolic interval; PAF = paroxysmal atrial fibrillation; Slope_max = maximal slope.