Electrocardiographic measures of ventricular repolarization

Electrocardiographic measures of ventricular repolarization

Abstract: Electrocardiographic measures of ventricular repolarization include QT dispersion, T wave width, spatial QRS-T angle and T peak to T end interval.

Following parameters have been used as electrocardiographic measures of ventricular repolarization:

1. QT variation between different ECG leads (QT dispersion)
2. T peak to T end interval (Tp-Te)
3. T wave width
4. Spatial angle between QRS and T vectors

QT dispersion

Difference in QT interval between the surface electrocardiographic leads is known as QT dispersion. QT dispersion can be defined as the difference between the maximum and minimum QT intervals on body surface electrocardiogram.¹ QT dispersion is thought to reflect the inhomogeneity of repolarization and hence an indicator of arrhythmia potential. Inhomogeneous repolarization of myocardium can predispose to reentrant circuits leading to life threatening ventricular arrhythmias. Reduction of QT dispersion is has also been considered as the mechanism of beneficial effect of a class III antiarrhythmic drug.² 

The validity of QT dispersion as a true indicator of dispersion of ventricular repolarization has been also questioned. It has been mentioned that QT dispersion is just a measure of T loop variation and error in QT measurement.³

T peak to T end interval (Tp-Te)

Interval from the peak of T wave to the end of the T wave (Tp-Te) has been taken as an index of global dispersion of repolarization. Using monophasic action potentials recorded from epicardial and endocardial sites, it has been shown that maximal T peak to T end interval may be taken as a non invasive index of dispersion of ventricular repolarization. Mean Tp-Te interval or Tp-Te from a single lead was not found to be useful. The maximal Tp-Te interval has to be measured.⁴ 

T-peak to T-end interval has also been thought to be a better predictor of arrhythmic risk in certain types of hypertrophic cardiomyopathy than QT dispersion.⁵ 

T wave width

T wave width has also been considered as index for quantification of ventricular repolarization.⁶

Spatial angle between QRS and T vectors

Spatial QRS-T angle has been shown to be an independent predictor of mortality. For calculation of spatial QRS-T angle, QRS and T wave vectors can be determined from XYZ leads derived from standard twelve lead ECG using inverse Dower weighting matrix. In a study involving 46,573 patients with a six year follow up, spatial QRS-T angle was shown to be the most significant predictor of cardiovascular mortality. It was found to be better than all other electrocardiographic measurements.⁷

References

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2. Inoue H, Toda I, Nozaki A, Matsuo H, Sugimoto T. Effects of bretylium tosylate on inhomogeneity of refractoriness and ventricular fibrillation threshold in canine hearts with quinidine-induced long QT interval. Cardiovasc Res 1985;19:655-60.
3. Malik M, Batchvarov VN. Measurement, interpretation and clinical potential of QT dispersion. J Am Coll Cardiol. 2000 Nov 15;36(6):1749-66.
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