Do you know Vectorcardiogram?

It can help you with aortic stenosis

Patients with moderate and important aortic stenosis usually present changes in T wave on the 12-lead electrocardiogram (ECG).

At one extreme, we have cases with a Strain-type ventricular overload pattern, which delimits particular changes in relation to the physiology and anatomy of the heart, as well as cardiovascular prognosis (we will comment on this topic in a future Post).

However, for the most cases, we have what we usually call “nonspecific changes in ventricular repolarization”, a broad and empty term (actually a label) to designate those T wave changes, which we are unable to determine their nature and/or their implications diagnostic, and prognostic.

When considering that ventricular activation and repolarization are sequential and closely related phenomena, it is logical that both are evaluated together. Then comes a concept elaborated in 1933 by Wilson, the ventricular electrical gradient:

“… if the entire ventricular muscle went through the excitation period at the same time and in the same way, the QRS area and the T area would be equal in absolute magnitude, but with the opposite sign and the QRST area would be zero”.

This theory provided an expansion of the heart electrical phenomena analysis, especially with the emergence of the vectorcardiogram (VCG).

VCG is a diagnostic tool that exposes the vectors resulting from the electrical activation process of the heart. Theoretically, when inserting a third dimension of analysis, we can observe changes that may not be appreciated in the standard electrocardiogram.

After falling into disuse for many years, VCG has been gaining attention in the last decade through computational analysis using different software, as well as conversion matrices that allow us to infer VCG from the ECG, with good accuracy (Figure 1).

When analyzing a VCG, we can observe the P loop, the QRS loop and the T loop, just like the ECG waves. If each loop has magnitude and orientation, we treat them as vector quantities: vector P, vector QRS and vector T (Figure 1).

We have between the QRS vector and the T vector:

  1. a resulting vector: the ventricular gradient vector (VG) and
  2. An angle: the QRS-T spatial angle (SAQRS-T).

Under normal conditions, QRS and T vectors have an almost equal orientation, which produces low normal values for the SAQRS-T.

SAQRS-T is a powerful cardiovascular risk stratifier and is part of a series of parameters that estimate the heterogeneity the global electrical activity of the heart.

Talwar et al. conducted a small and interesting study to evaluate the correlation of SAQRS-T measured by vectocardiography by Frank’s method with invasive hemodynamic parameters of patients with aortic stenosis with no heart failure. The SAQRS-T had a moderate correlation with the systolic gradient between the left ventricle and the aorta (Spearman’s Rho = 0.47 Figure 2) and high correlation with the left ventricular systolic pressure – PSVE (Spearman’s Rho = 0.72; Figure 3).

Even when we utilize SAQRS-T only in the frontal plane (SAQRS-T Frontal), we still obtain relevant information: Kahraman et al. evaluated 289 patients undergoing percutaneous valve replacement (TAVR), where frontal SAQRS-T was an independent predictor of in-hospital mortality (OR: 1,012; CI 95%: 1,004-1,019, p <0.002) and long-term mortality as shown in the Kaplan Meier curve below:

Although the scarce number of studies addressing the topic, the changes in repolarization measured by SAQRS-T in patients with aortic stenosis have an excellent rationale. The left ventricle is subjected to a chronic state of high afterload, which generates a cascade of compensatory changes: increased microstructure activity, increased myocardial fiber thickness, increased number of fibers in parallel, increased oxygen consumption, reduced coronary reserve, reduced myocardial oxygenation index, and changes in ECG repolarization.

Thus, as the physiology of barrier-like heart disease imposed by aortic stenosis worsens, there is an increase in compensatory hypertrophy that increases the magnitude of the QRS vector and displaces it up, back and to the left. Concomitantly, there is a progressive increase in LVSP that results in inversion of the T wave and proportional and progressive opening of the SAQRS-T.

Given this information, we may conclude that:

  • SAQRS-T is a target parameter for further investigations of patients with aortic stenosis and
  • One must be aware of the diagnostic and prognostic implications of changes in ventricular repolarization that occur in aortic stenosis.

            Suggested Literature:

  1. Wilson FN, Macleod AG, Barker PS, Johnston FD. The determination and the significance of the areas of the ventricular deflections of the electrocardiogram. Am Heart J. 1934;
  2. Talwar KK, Mohan JC, Narula J, Kaul U, Bhatia ML. Spatial quantitative vectorcardiography in aortic stenosis: correlation with hemodynamic findings. Int J Cardiol. 1988;18(2):151–8.
  3. Kahraman S, Yilmaz E, Demir AR, Avci Y, Güler A, Kalkan AK, et al. The prognostic value of frontal QRS-T angle in patients undergoing transcatheter aortic valve implantation. J Electrocardiol [Internet]. 2019; Available from: https://doi.org/10.1016/j.jelectrocard.2019.05.003

 

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