Quantitative Aspects of Ventricular Repolarization

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Abstract
Introduction:QT dispersion assesses repolarization inhomogeneity on 12‐lead standard ECG. However, the implications of the electrical cardiac vector during the repolarization phase (the T wave loop) with the genesis of this phenomenon are unknown.Methods and Results:The aim of this study was to explore conventional 12‐lead resting ECG QT dispersion and the corresponding morphology of the spatial three‐dimensional (3‐D) T wave loop in 25 normals subjects, 30 postmyocardial infarction (Ml) patients, and in 17 individuals with congenital long QT syndrome (LQTS). Standard and XYZ ECG leads were simultaneously digitized (250 Hz) and automatically analyzed. Ventricular repolarization dispersion was estimated by the range (RAN12o) and standard deviation (SD12o) of the 12 rate corrected QTo intervals (between the Q wave onset and the T wave offset). Spatial T wave loops were extracted from XYZ data and analyzed with a 3‐D algorithm which provides quantitative parameters related to the loop morphology. All scalar measurements of dispersion were significantly larger in the two pathological populations; however none of them could discriminate post‐MI from LQTS groups (RAN12o = 33.3, 61.4, and 62.7 ms respectively, for the three populations). Conversely, a loss of planarity and an increased roundness of the T wave loop were observed in the two pathological groups, with the former effect more pronounced in the LQTS (P = 0.04 compared to post‐MI) and the latter in the post‐MI group (P = 0.02 compared to LQTS). Furthermore, multiple regression and principal component analyses showed that planarity and roundness are independently involved with QT dispersion.Conclusion:Changes in the morphology of the spatial T wave loop associated with QT dispersion were identified. These changes discriminate different substrates of repolarization inhomogeneity. The use of a 3‐D technique to assess repolarization inhomogeneity may bring additional information on the intrinsic nature of this disorder.