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Figure 1 Simultaneous measurement of effective refractory period (ERP) and action potential duration at 70% repolarization (APD70) and 90% repolarization (APD90) with the combination catheter. S1 and S2 denote the basic and extrastimulus artifacts. Repolarization levels (n%) at which extrastimuli are superimposed onto the preceding repolarization phase are visualized.