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Figure 1 The heart rate (HR)-QT interval (QT) relationship in a single subject during exercise, atropine and isoproterenol. At rest QT was similar during each condition. However, as HR increased, isoproterenol (circle) was associated with less shortening of the QT than exercise (square) or atropine (triangle). As a result, the slope describing the relationship between HR and QT is much less steep for isoproterenol than for exercise or atropine. This individual demonstrated a transient increase in QT as HR increased from 85 to 95 beats/min. Twelve of our 25 subjects demonstrated transient increases ( 50 ms) in QT as HR increased in response to isoproterenol. However, such events were rare, representing only 22 of 3,415 consecutive electrocardiograms during isoproterenol. None of our subjects demonstrated this type of increase in QT in response to atropine or exercise.
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