QT Dispersion: Comparison of Orthogonal, Quasi-orthogonal, and 12-Lead Configurations

Measurement, interpretation and clinical potential of QT dispersion

QT dispersion was originally proposed to measure spatial dispersion of ventricular recovery times. Later, it was shown that QT dispersion does not directly reflect the dispersion of recovery times and that it results mainly from variations in the T loop morphology and the error of QT measurement. The reliability of both automatic and manual measurement of QT dispersion is low and significantly lower than that of the QT interval. The measurement error is of the order of the differences between different patient groups. The agreement between automatic and manual measurement is poor. There is little to choose between various QT dispersion indices, as well as between different lead systems for their measurement. Reported values of QT dispersion vary widely, e.g., normal values from 10 to 71 ms. Although QT dispersion is increased in cardiac patients compared with healthy subjects and prognostic value of QT dispersion has been reported, values are largely overlapping, both between healthy subjects and cardiac patients and between patients with and without adverse outcome. In reality, QT dispersion is a crude and approximate measure of abnormality of the complete course of repolarization. Probably only grossly abnormal values (e.g. > or =100 ms), outside the range of measurement error may potentially have practical value by pointing to a grossly abnormal repolarization. Efforts should be directed toward established as well as new methods for assessment and quantification of repolarization abnormalities, such as principal component analysis of the T wave, T loop descriptors, and T wave morphology and wavefront direction descriptors.

Measurement and interpretation of QT dispersion

QT dispersion was proposed as an index of the spatial inhomogeneity of ventricular recovery times. The results of studies that found significant correlation between dispersion of ventricular recovery times measured with monophasic action potentials and QT dispersion were interpreted as proof of the direct link between QT dispersion and the dispersion of ventricular recovery times. Later it was shown that QT dispersion is not a direct reflection of the spatial variation of the recovery times and cannot be used for quantification of this variation. The interlead variability of the QT intervals is a result of different projections of the spatial T-wave loop into the various electrocardiographic leads. The reliability of both manual and automatic measurement of QT dispersion is low and is often of the order of the differences of Qt dispersion between different patient groups. The measurement reliability is influenced by intrinsic factors (e.g., amplitude of the T wave) and extrinsic factors (e.g., noise, paper speed of recording, instruments for manual measurements, and type of algorithm and interalgorithmic settings for automatic measurement). There is very little to choose between the different indices of expression of QT dispersion, as well as between the different lead configurations used for its measurement. QT dispersion is not simply a result of measurement error, but a crude measure of abnormalities during the whole course of repolarization. Only grossly prolonged QT dispersion (e.g., > or =100 ms), must be interpreted simply as a sign of the abnormal course of the repolarization, and inferences about the actual dispersion of the ventricular recovery times should not be made. Newer concepts of assessment of the morphology of the T wave are already emerging and will probably be of higher clinical value.