QT dispersion in sinus beats and ventricular extrasystoles in normal hearts

Recent studies have suggested that QT interlead variability (dispersion) on the surface electrocardiogram may have potential as a measure of recovery time dispersion. To test this hypothesis further QT dispersion occurring in sinus beats was compared with that in ventricular extrasystoles.

Simultaneous electrocardiograms were recorded at 50 mm/s during sinus rhythm in a drug free state while ventricular extrastimuli were introduced by programmed right ventricular stimulation at different coupling intervals. QT dispersion, defined as the difference between the maximum and minimum QT, was calculated separately for the extrasystoles and preceding and following sinus complexes. To correct for the influence of the number of measurable leads on QT dispersion, an "adjusted" QT dispersion calculated as QT dispersion/square root of the number of measurable leads, was used to compare sinus complexes and extrasystoles.

Nine patients were studied who were undergoing electrophysiological study for investigation of palpitation and were found to have electrically normal ventricles.

At all coupling intervals tested "adjusted" QT dispersion was significantly greater in the ventricular extrasystoles than in either the preceding or following sinus complexes. For the coupling interval 350 ms, the 95% confidence intervals for the difference between means was 52 to 78 ms (preceding sinus complex) and 56 to 82 ms (following sinus complex) (p less than 0.00001). There was no correlation between the coupling interval and the magnitude of the "adjusted" QT dispersion.

These results accord fully with expected differences in ventricular recovery time dispersion and offer further support for the hypothesis that QT dispersion reflects regional variation in ventricular recovery. If substantiated by invasive studies, these findings have wide implications for both the usefulness and the method of QT measurement.