Autonomic nervous system influences on qt interval in normal subjects
Anthony R. Magnano, MD*,
Steve Holleran, BS ,
Rajasekhar Ramakrishnan, PhD,
James A. Reiffel, MD, FACC* and
Daniel M. Bloomfield, MD, FACC*,*
* Department of Medicine, Division of Cardiology, New York, New York, USA
Department of Pediatrics, Columbia University College of Physicians and Surgeons, New York, New York, USA
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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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Figure 2 Effects of exercise, atropine and isoproterenol on the electrocardiogram (ECG) complex at 100 beats/min. As heart rate increases in response to each experimental intervention, less decrement in the QT interval was observed during isoproterenol as compared to exercise or atropine. The changes in the ECG complex of lead V2 are shown for one typical subject at 100 beats/min.
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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).
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Figure 4 Effect of isoproterenol on the U-wave. In a representative subject, isoproterenol infusion produced a dose-dependent increase in U-wave amplitude in all subjects studied. The U-wave in this subject was present at baseline, but visibly increased in amplitude during low dose isoproterenol. As the infusion rate increased, the U-wave (tracked by arrows in the figure) merged with the T-wave forming a single T-wave complex. At higher doses of isoproterenol, the T-wave is characterized by a prolonged, flattened terminal downslope during isoproterenol. In contrast, these U-wave changes were not observed during exercise (not shown) or atropine infusion. The end of T-wave is indicated by a tick mark for each electrocardiogram complex.
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