Oscillatory functions affecting outcome of coronary heart disease: the hazard of too much or too little stability

Heart rate and catecholamine contribution to QT interval shortening on exercise

BACKGROUND

QT interval shortens with exercise. Some 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.

QT interval and mortality from coronary artery disease

Abnormalities in the QT interval can be divided into 3 types, prolongation of the QT interval, increases in the dispersion of the QT interval, and abnormalities in the heart rate dependent behavior of the QT interval. Abnormalities may be found in short or long-term recordings. Prolongation of the QT interval may reflect factors associated with an adverse prognosis in coronary disease and may in itself be arrhythmogenic. The data to date suggest that there is an association between adverse prognosis and QT interval prolongation in coronary disease, both before and after acute myocardial infarctions. This relationship is weak, however, and is not clinically useful. The data as to whether increased QT dispersion postmyocardial infarction relates to adverse prognosis is weak because there is no convincing evidence yet. If there is a relationship it is weak. Abnormalities in the rate dependent behavior of the QT interval are widely found, but as no large scale prospective study with mortality as an endpoint has yet been undertaken the significance of rate dependent abnormalities is uncertain. The widespread introduction of beat-to-beat QT analysis of 24 hour Holter tapes may take QT intervalology into the realm of clinical practice.