Abstract 17455: Qt Interval Adaptation to Sudden Change of Pacing Rate — Effect of β-Adrenergic Stimulation
Introduction: QT interval correction formulas are based on instantaneous heart rate (HR), but QT interval adaptation to sudden HR change occurs gradually. This results in QT interval ”hysteresis”. In humans, QT interval has been reported to reach a new steady-state (SS) value in about 2 minutes. The effect of adrenergic tone on this process is not well understood.
Hypothesis: QT interval adaptation is affected by β-adrenergic stimulation.
Methods: 10 subjects (42.1±15.3 yr, 3 males) undergoing radiofrequency ablation of SVT were studied. Atrial pacing for 5 min at HR ranging from 60 to140 bpm was performed before and during dobutamine (D) infusion (10 µg/kg/min). QT response to sudden HR change was evaluated.
Results: QT response to sudden HR change is best described by a combination of instantaneous response (IR) followed by a gradual monoexponential course to the new SS value (Fig). Linear function results in inferior fit of QT adaptation course (p<0.05 compared to exponential). The time-constant of the exponential is approximately 1 min (49.3±11.4 s). The IR magnitude is ∼ 3% of RR interval change (2.6±2.0%), i.e. 13% of the total QT change to a new SS value. D shortens SS QT at given HR (by 12.9±5.8 ms at 120 bpm; p<0.005) and increases IR magnitude (to 11.3±9.0% of RR change; p<0.01).
Conclusion: QT adaptation has 2 distinct phases, which might have different mechanism. The effect of β-adrenergic stimulation on IR may attenuate hysteresis (by decreasing the relative contribution of slow adaptation) and increase QT variability by increasing the IR to heart rate variability. These results may be important in assessment of drug effects on repolarization and for understanding of QT variability.
- © 2010 by American Heart Association, Inc.