Measurement, interpretation and clinical potential of QT dispersion
Marek Malik, PhD, MD, FACCa and
Velislav N. Batchvarov, MD, PhDa
a Department of Cardiological Sciences, St. George’s Hospital Medical School, London, United Kingdom
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Figure 1 Effect of the shape of the T loop on the QT interval measurement in a hypothetical lead. Projections of T loops with different shapes and at different angles to the axis of the lead result in T waves with different amplitude and morphology. Only an insignificant proportion of the final part of a T wave with high amplitude may be unmeasurable because of falling into the noise band (A); T waves with smaller amplitude as a result of wider T loop (C) or elongated loop at different angle (B), have a greater proportion of their final parts falling into the noise band. Thus, the measurable QT interval can almost coincide with the real end of repolarization (A), or be significantly smaller (B,C). Points 1, 2 and 3 indicate three time instants of the T loop and of the T wave. (Reproduced with permission from Kors et al. QT dispersion as an attribute of T-Loop morphology. Circulation 1999;99:458–63.)
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Figure 2 QT dispersion as a result of both different real duration and different measurable duration of QT intervals. Two hypothetical T waves of the same amplitude have different offset (dashed lines) when the heart vector becomes perpendicular to the axis of one of the leads. This results in "real" dispersion of the QT intervals (vertical dashed lines). In addition, different proportion of the final part of the two T waves is below the threshold level (e.g., with an automatic threshold method). This leads to the measured dispersion of the QT intervals (vertical solid line), which is different from the real dispersion.
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Figure 3 Main automatic QT measurement techniques. From top to bottom: threshold method applied to the original T wave (TH), or to its differential (DTH), tangent method with a tangent to the steepest point of the descending limb of the T wave (SI), tangent method with a line through the T wave peak and the maximum slope point (PSI). (Reproduced with permission from McLaughlin NB, et al. Comparison of automatic QT measurement techniques in the normal 12 lead electrocardiogram. Br Heart J 1995;74:84–89).
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Figure 4 Effect of the shape of the descending part of the T wave on the QT interval measured with a tangent method. The two hypothetical T waves have a common offset (vertical dashed line), but significantly different shape of the descending part. As a result, a tangent to the steepest point may significantly underestimate (top panel) or overestimate (bottom panel) the T wave offset.
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Figure 5 Agreement between automatic and manual measurement of QT dispersion in patients with hypertrophic cardiomyopathy. The differences between the measurements are plotted against the mean value from the two measurements. Most of the differences are within 2 SD from the mean differences (dashed line), which is approximately ±60 ms, obviously an unacceptably high measurement error. There is also no correlation between the two sets of measurements (r2 = 0.00). (Reproduced with permission from Savelieva I, et al. Agreement and reproducibility of automatic versus manual measurement of QT interval and QT dispersion. Am J Cardiol 1998;81:471–7).
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Figure 6 Weighted mean ± SD values of QT dispersion (in milliseconds) from reviewed studies in normal subjects, patients with chronic myocardial infarction (chr.MI), left ventricular hypertrophy (LVH) of various etiology except hypertrophic cardiomyopathy, in heart failure and dilated cardiomyopathy (HF,DCM), in hypertrophic cardiomyopathy (HCM), in acute myocardial infarction (acute MI), and in long-QT syndrome (LQTS). See text for details.
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Figure 7 Mean±SD of QT dispersion (in milliseconds) from the reviewed studies of normal subjects, and patients with chronic myocardial infarction, left ventricular hypertrophy, heart failure and dilated cardiomyopathy, hypertrophic cardiomyopathy, acute myocardial infarction, and long QT syndrome. Abbreviations as in Figure 6. See text for details.
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Figure 8 Mean and standard deviations of QT dispersion in patients with (closed circles) and without (open circles) serious ventricular arrhythmias. In one study patients with ventricular fibrillation (closed diamond) and sustained monomorphic ventricular tachycardias (closed circle) were compared separately with patients without sustained ventricular arrhythmias. *p < 0.05 between groups with and without serious ventricular arrhythmias; NS: statistically nonsignificant difference between groups with and without serious ventricular arrhythmias.
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