Objectives: This study sought to develop a regression-based method for characterization of QT interval behavior during exercise and to define the normal range of the resulting "dynamic" measures of repolarization during submaximal treadmill testing in men and women.
Background: The Bazett-corrected QT (QTc) interval during exercise has been used as a marker for ischemic disease, arrhythmogenic substrate and the long QT syndrome. However, recent studies indicate that the QTc interval is nonlinear with respect to heart rate during exercise, making the end-exercise QTc interval dependent on peak work load achieved.
A mathematic description of the behavior of the Bazett-corrected QTc interval during exercise was developed from the underlying relationship between the unadjusted QT interval and heart rate in 94 normal men. Measurements were made from digitized precordial lead V5 complexes that were averaged by computer over 20-second periods at upright control (mean rate, 78 beats/min), during moderate exercise (mean rate, 125 beats/min), and at peak effort (mean rate, 162 beats/min), using a gently graded treadmill protocol that produces small heart rate increments between 2-minute stages. Although the group mean QTc interval increased during early exercise and decreased during higher exercise workloads, the mean unadjusted QT interval decreased throughout exercise in a strongly linear relationship with increasing heart rate: QT[ms] = 481 - 1.
View Article and Find Full Text PDFNormal values for simple and rate-corrected measures of the duration and dispersion of electrocardiographic (ECG) repolarization during treadmill exercise were obtained in 94 clinically normal men, and the behavior of these measures during ischemia was examined in 79 men with catheterization-proven disease or with stable angina, all of whom had normal resting ECGs. Temporal measures of repolarization, exemplified by QTc, were found to be moderately sensitive for disease only when highly specific criteria were derived from data at peak exercise heart rates that were significantly higher in normal subjects than in patients; sensitivities of rate-corrected measures declined markedly when criteria were derived from matched heart rate data in normal subjects. In contrast, the standard deviation of the mean measured J-point to T-wave peaks across the V1 to V6 precordial leads (SD-SoTm), a measure of precordial dispersion of repolarization, was unrelated to exercise heart rate in normal subjects; SD-SoTm exceeding the 95% value at matched intraexercise heart rates in normal subjects was found at peak exercise in 38% of men with coronary disease, including some patients with otherwise false-negative standard exercise test responses.
View Article and Find Full Text PDFThe BCG vaccination is not recommended for health-care workers in the United States. The current strategy against tuberculosis in tuberculin-negative house staff is an annual tuberculin screening test followed by chemoprophylaxis with isoniazid for a positive result. We performed a decision analysis that unequivocally concluded that the BCG vaccination leads to fewer cases of tuberculosis in this population over a 10-yr period.
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