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Figure 1 (A) The same action potential is shown during pacing at 1 Hz for 30 s (APD₉₀ [an index for the magnitude of early afterdepolarization formation] = 173 ms) followed by a 2-s pause (APD₉₀ = 253 ms) and during bigeminal pacing (800:200 ms intervals) for 30 s followed by a 2-s pause (APD₉₀ = 410 ms). (B) An action potential and isometric force are shown for pacing at 1 Hz interrupted by a premature beat and for a 5-Hz pacing train (20 beats) followed by a 1,000-ms pause. Vertical lines mark peak force. (C) An action potential and isometric force are shown for pause intervals (350 and 2,000 ms) after a pacing train (5 Hz for five beats). Vertical calibration bars (B and C) designate 400 dynes.