Arrhythmic Risk Stratification in the Emergency Department.

Abstract

To the Editor: We have read the article of Berling and Isbister, entitled “The Half RR Rule: A Poor Rule of Thumb and Not a Risk Assessment Tool for QT Interval Prolongation,” with great interest.1 The authors investigated the accuracy of the 1/2 RR rule as a risk assessment tool for drug-induced Torsade de Pointes (TdP), comparing it to the QT nomogram, Bazett's corrected QT, and Fridericia's corrected QT. As a result, they reported that the 1/2 RR rule was not as sensitive as the QT nomogram or QTcB > 500 msec for drug induced TdP. It had poor positive agreement in almost all overdose patients, resulting in over half of patients receiving unnecessary cardiac monitoring and repeat electrocardiograms (ECGs). We congratulate the authors for this study. Torsade de Pointes is a rare but potentially lethal cardiac arrhythmia that is associated with a prolonged QT interval. QT prolongation and the risk of TdP are well recognized in rare congenital long-QT syndromes. More commonly, QT prolongation is associated with medications, predominately in drug overdose.1 In relation to that, the variety of medications are globally increasing and widening as each day passes. Therefore, drugs leading to lethal cardiac arrhythmia are being taken into account more delicately in clinical practice. Nowadays, methods like QT nomogram or QT correction to Bazett's or Fridericia's formula have been used in emergency service and outpatient clinics to calculate QT interval and make risk stratification. However, we suggest that novel methods or electrocardiographic parameters related with arrhythmogenic myocardial potential and electrical heterogeneity could be developed and applicable in clinical practice instead of ancient methods such as QT nomograms or QT correction methods in determining the risk of TdP. T-wave is a waveform which represents the ventricular repolarization and determines the QT interval on surface ECG. Numerous basic and clinical studies documented that T-wave peak to end (Tp-e) is valuable index of transmural dispersion of repolarization.2 Relationship of Tp-e interval and lethal ventricular arrhythmia has been evaluated in numerous studies. However, it should be reminded that Tp-e interval can influence the variation of QT interval induced by various physiological changes. So, ratio of Tp-e/QT may be a more sensitive index for ventricular repolarization because it remains constant around the dynamic physiologic changes of heart rate. More importantly, Tp-e/QT significantly increases prior to TdP. Furthermore, increased myocardial electrical heterogeneity in channelopathies (e.g., J-wave syndrome, long-QT syndrome, and short-QT syndrome) and cathecholaminergic ventricular tachycardia is closely associated with increased Tp-e/QT.3 Thus, it allows better estimation of repolarization dispersion and is validated as a sensitive index of arrhythmogenesis. From those perspectives, Tp-e/QT may be an alternative parameter for risk stratification in arrhythmogenesis. In conclusion, ratio of Tp-e is an exceptional electrocardiographic parameter that can be used effectively alternative to conventional method, e.g., QT nomogram. Moreover, relationship between ratio of Tp-e/QT and TdP should be evaluated in subjects suffering TdP and also in large-scaled studies.