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Figure 3 The QT interval (QT) at 100 beats/min during exercise (EX), atropine (AT) and isoproterenol (ISO). At a heart rate of 100 beats/min, the QT during isoproterenol was significantly (p < 0.001) longer than the QT during either exercise or atropine. The difference in QT between exercise and atropine was also statistically significant (p < 0.005).