Effect of myocardial ischemia and type I antiarrhythmic drug therapy on exercise QT intervals.

Direct measurement of the QT interval during exercise is the preferred method to assess duration of this interval with changes in heart rate. The influence of exercise induced ischemia or type I antiarrhythmic drugs on the exercise QT interval has not been well studied. Exercise electrocardiograms and thallium scintigraphy were performed in three groups-normals, patients with coronary artery disease, and patients taking type I antiarrhythmic drugs. The change in QT interval with heart rate increases during exercise was compared among the three groups. The QT interval shortened progressively in the normal group. Demonstrable myocardial ischemia (positive thallium scan) had no effect on the QT interval. Type I antiarrhythmic drugs prolong the QT interval at rest compared to normals, but the interval shortened to the same degree as in normals during exercise. Thus, normal values can be used to compare QT intervals in patients with coronary artery disease. These values do not apply to patients receiving type I antiarrhythmic drugs.