QT dispersion in 120 electrocardiographic leads in patients with structural heart disease

The clinical significance of QT dispersion (QTd) measured in 12-lead ECGs is controversial. The aim of this study was to clarify factors that determine the QTd and its measurement errors in different lead arrays in patients with structural heart disease. Two blinded observers measured QT intervals on a computer screen from 120-channel ECG recordings in a retrospective set of 257 patients, comprising a group of 121 myocardial infarction (MI) survivors without ventricular tachyarrhythmia during a 6-month follow-up and a group of 136 survivors of ventricular tachyarrhythmia/fibrillation. QTd did not differ in patients with and without ventricular tachyarrhythmia/fibrillation. Eleven ventricular tachyarrhythmia/fibrillation survivors without structural heart disease had the lowest QTd (P < or = 0.02). The strongest factor determining QTd and the magnitude of its measurement error was the lead array (P = 0.0001). Measurement errors had two components. The smallest relative errors were in the total body surface mapping array with one component related to interobserver reproducibility (9.1 +/- 7.6%), and the other component related to accuracy of measurement of the QT interval (36 +/- 16%). The authors estimated that a difference of QTd of at least 50 ms between study groups is required in a 12-lead ECG to draw any conclusions from the studies. In patients with structural heart disease, QTd from limited arrays of ECG leads was not a reliable measure. It correlated with the presence of structural heart disease, but not with arrhythmogenicity. An array consisting of ECG leads covering the entire chest allowed better reproducibility and measurement accuracy of QTd.