EFFECT OF EXERCISE-INDUCED ISCHEMIA ON QT INTERVAL DISPERSION

An increased spatial dispersion of ventricular repolarization duration (QT dispersion) is associated with an increased vulnerability to arrh ythmias. This study was designed to examine the effect of exercise on QT dispersion in ischemic heart disease (IHD). QT dispersion, correcte d QT dispersion, and percentage change in uncorrected and corrected QT dispersion between rest and peak exercise were examined in 14 members of a control group, 17 patients with IHD, and 14 patients with IHD wh o were receiving P-blockers (IHD-B). All subjects had undergone a stan dard Bruce protocol exercise test, and QT intervals were measured at r est and peak exercise with a digitizing tablet interfaced to a persona l computer. QT dispersion at rest was markedly increased in the IHD gr oup compared with that in the control and IHD-B groups, respectively ( corrected QT dispersion in milliseconds), 74 +/- 7, 40 +/- 4, 49 +/- 5 , p < 0.03). The corrected QT dispersion at peak exercise was greater in the IHD group compared with that in the control group (57 +/- 5 vs 26 +/- 3 msec, p < 0.03). The percentage change in QT dispersion with exercise was significantly higher in the IHD group (52% +/- 5%) compar ed with that in both the control group (28% +/- 4%, p < 0.002) and the IHD-B group (30% +/- 3%, p < 0.01). A larger mean QT dispersion at pe ak exercise and an increased percentage change in QT dispersion with e xercise may help explain the increased susceptibility of the IHD group for arrhythmias. The cardioprotective action of beta-blockers may be explained by their blunting effect on exercise-related changes in QT d ispersion.