Dynamic Changes of QT Dispersion as a Predictor of Myocardial Ischemia on Exercise Testing in Patients with Angina Pectoris.

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Abstract
The difference between the maximum and minimum QT intervals on the standard 12-lead ECG (QT dispersion) may be a significant predictor of serious arrhythmias. Dynamic changes in QTd were determined during exercise-induced ischemia in 15 patients with effort angina (≥75% coronary stenosis) and 10 normal individuals. Treadmill exercise testing was performed according to Bruce's protocol and the rate-corrected QT dispersion (QTcd) was calculated using Bazett's formula. The resting QTcd before exercise was similar in the angina patients and the controls. After the first stage of exercise, QTcd was significantly increased in the angina patients (p=0.035), while it remained near baseline in the controls. Five minutes after completing exercise, QTcd was significantly greater in the angina patients than in the controls (p=0.011). Furthermore, QTcd values after the first stage of exercise were significantly correlated with the maximum ST depression observed on completing exercise in the angina patients (r=0.714, p=0.0028). Because QTd may represent the heterogeneity of ventricular repolarization, its significant exercise-induced increase in the angina patients suggests that myocardial ischemia caused repolarization disorders. The significant correlation between QTcd values after the first stage of exercise (before significant ST depression) and the maximum ST depression on completing exercise suggests that an increase in QTcd proceding ischemic ST depression may predict myocardial ischemia. In addition, even daily activities not causing significant ST changes may increase QTcd and the risk of serious arrhythmia in angina patients.