Analysis of speed, curvature, planarity and frequency characteristics of heart vector movement to evaluate the electrophysiological substrate associated with ventricular tachycardia

Abstract

Background: We developed a novel method of assessing ventricular conduction using the surface ECG.

Methods: Orthogonal ECGs of 81 healthy controls (age 39.0±14.2 y; 51.8% males; 94% white), were compared with iDower-transformed 12-lead ECGs (both 1000Hz), recorded in 8 patients with infarct-cardiomyopathy and sustained monomorphic ventricular tachycardia (VT) (age 68.0±7.8y, 37.5% male, mean LVEF 29±12%). Normalized speed at 10 QRS segments was calculated as the distance traveled by the heart vector along the QRS loop in three-dimensional space, divided by 1/10th of the QRS duration. Curvature was calculated as the magnitude of the derivative of the QRS loop tangent vector divided by speed. Planarity was calculated as the mean of the dihedral angles between 2 consecutive planes for all planes generated for the median beat. Orbital frequency (a scalar measure of rotation rate of the QRS vector) was calculated as a product of speed and curvature.

Results: Mixed regression analysis showed that speed was slower [6.6 (95%CI 4.4-8.9) vs. 24.6 (95%CI 11.5-37.7)µV/ms; P<0.0001]; orbital frequency was smaller [1.4 (95%CI 1.2-1.6) vs. 6.8 (95%CI 5.4-8.1)ms(-1); P<0.0001], and planarity was larger by 3.6° (95%CI 1.4°-5.8; P=0.002) in VT cases than in healthy controls. ROC AUC for orbital frequency was 0.940 (95%CI 0.935-0.944) across all frequencies and QRS segments. ROC AUC for planarity at 70-249Hz was 0.995 (95%CI 0.985-1.00). ROC AUC for speed at 70-79Hz was 0.979 (95%CI 0.969-0.989).

Conclusion: This novel method reveals characteristic features of an abnormal electrophysiological substrate associated with VT.

Keywords: Conduction velocity; Curvature; Frequency characteristics; Heart vector; Planarity; Speed; Vectorcardiography; Ventricular tachycardia.

Figure 1

A. Detection of the origin-point of the VCG loop representing a single sinus heartbeat. An unfiltered VCG from a patient with prior MI and MMVT is shown. Points P1 and P2 (green and red circles) are detected as the 2 points closest in three-dimensional space and distant in time during a single heartbeat. The origin-point (star) is defined as the point halfway along a line connecting P1 and P2. B. Removal of respiration effects. Original “raw” presentation of 11 consecutive sinus beats: multiple VCG loops of different size are presented in different locations and different planes in three-dimensional space. The origin points of each beat (stars) are located separately from each other. C. After respiration removal: loops are translocated, rotated, and re-scaled. D. Detection of the onset and offset of VCG loops. Onset and offset of the P, QRS, T, and U loops are marked by colored circles.

Figure 2

The velocity and curvature of the heart vector movement through the QRS loop. Unfiltered sinus rhythm VCG loop from a MMVT patient is shown. Frontal (A), horizontal (B), and sagittal (C) planes are shown, with the color of each segment representing speed (purple = slowest; red=fastest). D. 5 consecutive sinus beats are plotted. The x-axis shows 10 normalized time segments from the onset of the QRS loop. The y-axis shows log₁₀-transformed curvature. The z- axis shows the order of consecutive beats (1–5). The speed color scale is computed automatically using the minimum and maximum data values as shown by the legend.

Figure 3

Boxplots of normalized speed of the QRS vector in healthy controls and patients with infarct cardiomyopathy and ventricular tachycardia (VT cases). Median (horizontal line crossing the box) and interquartile range [IQR] (box) of spatial QRS speed is shown. Whiskers specify the adjacent values, defined as the most extreme values within 1.5 IQR of the nearer quartile. *=P<0.05; **=P<0.001; ***=P<0.0001

Figure 4

Boxplots of normalized log-transformed curvature of the QRS loop in healthy controls and patients with infarct cardiomyopathy and ventricular tachycardia (VT cases). Median (horizontal line crossing the box) and interquartile range [IQR] (box) of QRS loop curvature is shown. Whiskers specify the adjacent values, defined as the most extreme values within 1.5 IQR of the nearer quartile. *=P<0.05; **=P<0.001; ***=P<0.0001

Figure 5

Comparison of the speed and curvature of filtered heart vector movement throughout the QRS in sinus rhythm in healthy subject (left column) and a patient with MMVT (right column). QRS loops filtered at 10–19Hz, 20–29Hz, 30–39Hz, and 40–49Hz are shown in rows 1–4, respectively. The x-axis shows 10 normalized time segments from the onset of the QRS loop. The y-axis shows log₁₀-transformed curvature. The z- axis shows the order of consecutive beats (1–5). The color scale represents speed and is computed using the minimum and maximum velocity values as shown by the legend on each small panel (purple = minimum speed; red = maximum speed).

Figure 6

Boxplots of normalized orbital frequency Ω of the spatial QRS vector movement in Healthy controls and patients with infarct cardiomyopathy and ventricular tachycardia (VT cases). Median (horizontal line crossing the box) and interquartile range [IQR] (box) of orbital frequency is shown. Whiskers specify the adjacent values, defined as the most extreme values within 1.5 IQR of the nearer quartile. *=P<0.05; **=P<0.001; ***=P<0.0001

Figure 7

Boxplots of the planarity angle θ of the QRS loop in Healthy controls and patients with infarct cardiomyopathy and ventricular tachycardia (VT cases). Median (horizontal line crossing the box) and interquartile range [IQR] (box) of QRS loop planarity angle is shown. Whiskers specify the adjacent values, defined as the most extreme values within 1.5 IQR of the nearer quartile. *=P<0.05; **=P<0.001; ***=P<0.0001