Novel arrhythmic risk markers incorporating QRS dispersion: QRS_d × (T _peak − T _end)/QRS and QRS_d × (T _peak − T _end)/(QT × QRS)

Dear Editor,

We read the excellent article by Robyns et al. (2015) with great interest, validating the Index of Cardio‐Electrophysiological Balance (iCEB) as a reliable marker for predicting arrhythmogenicity in humans. Risk stratification of patients who might develop life‐threatening ventricular arrhythmias remains difficult. Several risk markers based on repolarization have been proposed. QT interval (corrected, QT_c) prolongation is a widely used marker but its use is limited by a low sensitivity and specificity, arrhythmias can develop despite a normal or even shortened QT interval. Other markers include QT dispersion (QT_d), interval from the peak to the end of the T wave (T _peak − T _end) and (T _peak − T _end)/QT ratio.

However, the major problem with the above repolarization markers is that abnormal depolarization, which contributes to arrhythmogenesis, is largely ignored. For example, in heart failure and Brugada syndrome, conduction velocity (CV) is reduced. This increases the likelihood of reentry by shortening the excitation wavelength, λ (CV × effective refractory period). λ must be determined invasively by electrophysiological studies. Therefore there is a need for noninvasive markers that are good approximates of λ: the index developed by Lu, Yan, and Gallacher (2013) iCEB, is one of such markers.

Based on the concept of λ and iCEB, and the observations that T _peak − T _end and (T _peak − T _end)/QT are superior to the QT interval in predicting arrhythmogenicity, Tse recently proposed two novel indices that may have a higher accuracy in risk stratification: (T _peak − T _end)/QRS and T _peak − T _end/(QT × QRS) (Tse, 2016a,b). Both can easily be determined from the electrocardiogram and are firmly based on electrophysiological principles that λ is critical in determining arrhythmogenicity (Tse, Lai, Tse, & Yeo, 2016; Tse, Lai, Yeo, Tse, & Wong, 2016; Tse, Lai, Yeo, & Yan, 2016; Tse, Sun, Wong, Tse, & Yeo, 2016; Tse, Wong, Tse, & Yeo, 2016a,b). Although these have not been validated clinically, they have the potential of having superior predictive values than ventricular repolarization markers such as QT_c, QT_d, T _peak − T _end, or (T _peak − T _end)/QT ratio.

Nevertheless, a downfall of Tse's indices is that they do not account for increased CV dispersion in arrhythmogenesis. This can refer to phase difference in conduction latencies of neighboring regions, difference in CV across the myocardial wall, and coefficient of dispersion using standard deviation of the mean CV. A method of measuring CV dispersion clinically is increased QRS dispersion (QRS_d). QRS_d has been defined as the maximum difference between QRS durations measured in the right and left precordial leads. Here, we further propose two indices incorporating QRS_d: (1) QRS_d × (T _peak − T _end)/QRS, and (2) QRS_d × (T _peak − T _end)/(QT × QRS). The term QRS_d/QRS is proposed to serve as a surrogate marker of CV dispersion coefficient based on the standard deviation of the mean CV. These indices may have good predictive value for arrhythmic outcome and cardiovascular mortality in clinical conditions with increased CV dispersion, such as heart failure and Brugada syndrome.

In conclusion, clinical markers such as iCEB, Tse's conduction‐repolarization indices of (T _peak − T _end)/QRS and T _peak − T _end/(QT × QRS), as well as the two novel indices presented here will further aid identification of patients at risk of developing ventricular arrhythmias.