Global electrical heterogeneity: A review of the spatial ventricular gradient

Abstract

The ventricular gradient, an electrocardiographic concept calculated by integrating the area under the QRS complex and T-wave, represents the degree and direction of myocardial electrical heterogeneity. Although the concept of the ventricular gradient was first introduced in the 1930s, it has not yet found a place in routine electrocardiography. In the modern era, it is relatively simple to calculate the ventricular gradient in three dimensions (the spatial ventricular gradient (SVG)), and there is now renewed interest in using the SVG as a tool for risk stratification of ventricular arrhythmias and sudden cardiac death. This manuscript will review the history of the ventricular gradient, describe its electrophysiological meaning and significance, and discuss its clinical utility.

Keywords: Spatial ventricular gradient; Vectorcardiogram.

Figure 1

A. Manual calculation of the ventricular gradient in the frontal plane by Wilson et al. [1] The area of the QRS complex and T-wave in leads I, II, and III, were used to create vectors representing the mean electrical QRS axis, the mean electrical T-wave axis, and the ventricular gradient (the mean QRST axis/the vector sum of the mean QRS axis and mean T-wave axis). See text for details. Reproduced with permission from Wilson et al. [1] B. An example of the spatial ventricular gradient (SVG). The SVG is a vector (red) which is the sum of the mean QRS area vector (dark blue) and mean T-wave area vector (dark green).

Figure 1

A. Manual calculation of the ventricular gradient in the frontal plane by Wilson et al. [1] The area of the QRS complex and T-wave in leads I, II, and III, were used to create vectors representing the mean electrical QRS axis, the mean electrical T-wave axis, and the ventricular gradient (the mean QRST axis/the vector sum of the mean QRS axis and mean T-wave axis). See text for details. Reproduced with permission from Wilson et al. [1] B. An example of the spatial ventricular gradient (SVG). The SVG is a vector (red) which is the sum of the mean QRS area vector (dark blue) and mean T-wave area vector (dark green).

Figure 2

Idealized example in which there is no ventricular gradient in a sphere of myocardium. If depolarization and repolarization occur in the same spatiotemporal order, there is no ventricular gradient. The orientation of leads I, II, and III are shown, as are the “electrocardiogram” tracings that would be observed in each lead. Panels A–E represent depolarization of an idealized sphere of myocardium. The myocardium is stimulated at point 1 and depolarization occurs from left to right inscribing “QRS complexes” in each lead. After a set amount of time, repolarization also begins at point 1 and also proceeds from left to right (panels F–H). The “QRS complex” and “T-wave” in each lead are therefore oriented in the opposite direction, with areas that are equal and opposite. Panel I summarizes the vector projections in each lead with vector magnitude proportional to the area of the QRS complex or T-wave in each lead. In this example, the “QRS” area and “T-wave” area vectors are equal in magnitude and opposite in direction. The vector sum of the “QRS” area and “T-wave” area vectors (the ventricular gradient) is therefore zero. See text for details. Figure adapted from Hurst [20].

Figure 3

Idealized example in which there is a ventricular gradient in a sphere of myocardium. If depolarization and repolarization begin in different parts of the sphere, a ventricular gradient is observed. Panels A–B represent depolarization of the myocardial sphere as shown in Figure 1 panels A–E. In this example, repolarization begins at point 2 and proceeds right to left (panels C–E). The “QRS complex” and “T-wave” in each lead are therefore oriented in the same direction. The net “QRS” area vector and net “T-wave” area vector therefore have the same area/magnitude (M) and point in the same direction (towards the right--0°). The ventricular gradient is oriented towards lead I (0°) with a magnitude of 2M. The ventricular gradient points towards the right of the sphere which is the area that has the shortest duration of the excited state (shortest duration between depolarization and repolarization). See text for details. Figure adapted from Hurst [20].