Heart rate and catecholamine contribution to QT interval shortening on exercise

Background: QT interval shortens with exercise. St,me of this shortening is due to an increase in heart rate, and some is due to other effects of exercise, probably mostly neuroendocrine effects. Data from subjects with cardiac transplants have suggested that non-heart rate-related changes in QT interval on exercise are due to the effects of circulating catecholamines. Hypothesis: We sought to determine whether changes in plasma catecholamine levels with exercise are an important contributor to non-heart rate-related QT interval shortening. Methods: Subjects with DDD pacemakers were recruited. Subjects had QT intervals measured at rest, during a low fixed level exercise test designed to increase heart rate to about 110 beats/min, and, after resting, during pacing at a heart rate of 110 beats/min. Catecholamine levels were measured at each stage of the study. Results: QT interval at rest was 420 +/- 12 ms, during pacing 366 +/- 16 ms, and on exercise 325 +/- 14 ms. This then gave the proportion of QT interval shortening due to heart rate as 68.6 +/- 9.3% of total QT shortening, with the range between 35 and 95.6%. There was no proportionality between the degree of QT interval shortening on exercise that was not due to increases in heart rate and changes in plasma catecholamine levels. Conclusion: Two-thirds of exercise-induced QT interval shortening are due to an increase in heart rate, and one-third to other effects. Changes in plasma catecholamine levels on exercise were not closely related to changes in the QT interval on exercise.