Dispersion of QT intervals: A measure of dispersion of repolarization or simply a projection effect?

QT interval dispersion may provide little information about repolarization dispersion. Some clinical measurements demonstrate an association between h igh QT interval dispersion and high morbidity and mortality, but what is be ing measured is not clear This study was designed to help resolve this dile mma. We compared the association between different clinical measures of QT interval dispersion and the ECG lead amplitudes derived from a heart vector model of repolarization with no repolarization dispersion whatsoever, We c ompared our clinical QT interval dispersion data obtained from 25 subjects without cardiac disease with similar data from published studies, and corre lated these QT dispersion results with the distribution of lead amplitudes derived from the projection of the heart vector onto the body surface durin g repolarization. Published results were available for mean relative QT int ervals and mean differences from the maximum QT interval. The leads were de rived from Uijen and Dower lead vector data. Using the Uijen lead vector da ta, the correlation between measurements of dispersion and derived lead amp litudes ranged from 0.78 to 0.99 for limb leads, and using the Dower values ranged from 0.81 to 0.94 for the precordial leads. These results show a cl ear association between the measured QT interval dispersion and the variati on in ECG lead amplitudes derived front a simple heart vector model of repo larization with no regional information. Therefore, measured QT dispersion is related mostly to a projection effect and is not a true measure of repol arization dispersion. Our existing interpretation of QT dispersion must be reexamined, and other measurements that provide true repolarization dispers ion data investigated.