New electrocardiographic indices of risk stratification

Abstract Sudden cardiac death is caused mainly by ventricular tachyarrhythmias, which occur as a result of interplay between the autonomic nervous system, myocardial substrate, and triggering factors. Digital electrocardiology, including 12-lead ECG, exercise testing, signal-averaged ECG, and Holter monitoring, provides vast opportunities to determine risk of ventricular arrhythmias. This noninvasive ECG testing is of particular use in patients with coronary artery disease and myocardial infarction, who account for over 80% of sudden death cases. The influence of the autonomic nervous system on the heart and circulatory system has been evaluated for years using the time-domain and frequency-domain parameters of heart rate variability. Although prognostic significance of heart rate variability remains strong, new parameters seem to be gaining more attention because of their superior predictive value. Nonlinear dynamic analysis of RR patterns reflects more direct beat-to-beat RR variability than classical HRV parameters. Beat-to-beat behavior of RR around premature ventricular beats, i.e. heart rate turbulence, also shows better predictive value than classical HRV parameters. These new parameters providing insight into autonomic regulation of the heart and baroflex sensitivity can be obtained from standard Holter recordings. The myocardial substrate is usually evaluated in ECG by quantifying QRS duration, magnitude of left ventricular hypertrophy, and presence of late potentials in signal-averaged ECG. These parameters describe myocardial substrate during the depolarization process, however, repolarization plays an even more pivotal role in arrhythmogenesis. Heterogeneity of repolarization is a critical factor contributing to arrhythmic cascade in ventricular myocardium. QT dispersion has been explored as a parameter reflecting heterogeneity of repolarization. However, recent studies indicate that QT dispersion does not reflect heterogenous repolarization well, and there are overall conceptual and technical limitations of QT dispersion. Instead, various methods quantifying T-wave morphology or T-loop morphology are being developed based on observations that T-wave morphology is a reflection of repolarization wavefront in the myocardium. Prognostic value for predicting sudden death of some of these parameters has already been described, whereas others await confirmation in ongoing and future studies. Heterogeneity of repolarization is a very dynamic phenomenon, and beat-to-beat changes in repolarization morphology is one of the critical factors triggering sudden death. Such beat-to-beat changes can be augmented by ventricular premature beats, by ischemia, or even by increased heart rate. T-wave alternans, consisting of 2:1 changes in repolarization morphology is a new and very promising index of risk, especially in postinfarction patients. T-wave variability with non-2:1 pattern of changes seems to indicate risk of sudden death also, although more studies are needed to determine the prognostic value of T-wave variability in various patient populations. The new ECG parameters described above constitute a powerful noninvasive risk stratification tool that could be applied utilizing routine digital ECG techniques.