Multilead template-derived residua of surface ECGS for quantitative assessment of arrhythmia risk

Spectrum of clinical applications of interlead ECG heterogeneity assessment: From myocardial ischemia detection to sudden cardiac death risk stratification

Heterogeneity in depolarization and repolarization among regions of cardiac cells has long been recognized as a major factor in cardiac arrhythmogenesis. This fundamental principle has motivated development of noninvasive techniques for quantification of heterogeneity using the surface electrocardiogram (ECG). The initial approaches focused on interval analysis such as interlead QT dispersion and T_peak -T_end difference. However, because of inherent difficulties in measuring the termination point of the T wave and commonly encountered irregularities in the apex of the T wave, additional techniques have been pursued. The newer methods incorporate assessment of the entire morphology of the T wave and in some cases of the R wave as well. This goal has been accomplished using a number of promising vectorial approaches with the resting 12-lead ECG. An important limitation of vectorcardiographic analyses is that they require exquisite stability of the recordings and are not inherently suitable for use in exercise tolerance testing (ETT) and/or ambulatory ECG monitoring for provocative stress testing or evaluation of the influence of daily activities on cardiac electrical instability. The objectives of the present review are to describe a technique that has been under clinical evaluation for nearly a decade, termed "interlead ECG heterogeneity." Preclinical testing data will be briefly reviewed. We will discuss the main clinical findings with regard to sudden cardiac death risk stratification, heart failure evaluation, and myocardial ischemia detection using standard recording platforms including resting 12-lead ECG, ambulatory ECG monitoring, ETT, and pharmacologic stress testing in conjunction with single-photon emission computed tomography myocardial perfusion imaging.

Electrocardiographic conduction and repolarization markers associated with sudden cardiac death: moving along the electrocardiography waveform

The QT interval along with its heart rate corrected form (QTc) are well-established ECG markers that have been found to be associated with malignant ventricular arrhythmogenesis. However, extensive preclinical and clinical investigations over the years have allowed for novel clinical ECG markers to be generated as predictors of arrhythmogenesis and sudden cardiac death. Repolarization markers include the older QTc, QT dispersion and newer Tpeak - Tend intervals, (Tpeak - Tend) / QT ratios, T-wave alternans (TWA), microvolt TWA and T-wave area dispersion. Meanwhile, conduction markers dissecting the QRS complex, such as QRS dispersion (QRSD) and fragmented QRS, were also found to correlate conduction velocity and unidirectional block with re-entrant substrates in various cardiac conditions. Both repolarization and conduction parameters can be combined into the excitation wavelength (λ). A surrogate marker for λ is the index of Cardiac Electrophysiological Balance (iCEB: QT / QRSd). Other markers based on conduction-repolarization are [QRSD x (Tpeak-Tend) / QRSd] and [QRSD x (Tpeak-Tend) / (QRSd x QT)]. Advancement in technology permitted sophisticated electrophysiological analyses such as principal component analysis and periodic repolarization dynamics to further improve risk stratification. This was closely followed by other novel indices including ventricular ectopic QRS interval, the f99 index and EntropyXQT, which integrates mathematical and physical calculations for determining the risk markers. Though proven to be effective in limited patient cohorts, more clinical studies across different cardiac pathologies are required to confirm their validity. As such, this review seeks to encapsulate the development of old and new ECG markers along with their associated utility and shortcomings in clinical practice.

Traditional and novel electrocardiographic conduction and repolarization markers of sudden cardiac death

Sudden cardiac death, frequently due to ventricular arrhythmias, is a significant problem globally. Most affected individuals do not arrive at hospital in time for medical treatment. Therefore, there is an urgent need to identify the most-at-risk patients for insertion of prophylactic implantable cardioverter defibrillators. Clinical risk markers derived from electrocardiography are important for this purpose. They can be based on repolarization, including corrected QT (QTc) interval, QT dispersion (QTD), interval from the peak to the end of the T-wave (Tpeak - Tend), (Tpeak - Tend)/QT, T-wave alternans (TWA), and microvolt TWA. Abnormal repolarization properties can increase the risk of triggered activity and re-entrant arrhythmias. Other risk markers are based solely on conduction, such as QRS duration (QRSd), which is a surrogate marker of conduction velocity (CV) and QRS dispersion (QRSD) reflecting CV dispersion. Conduction abnormalities in the form of reduced CV, unidirectional block, together with a functional or a structural obstacle, are conditions required for circus-type or spiral wave re-entry. Conduction and repolarization can be represented by a single parameter, excitation wavelength (λ = CV × effective refractory period). λ is an important determinant of arrhythmogenesis in different settings. Novel conduction-repolarization markers incorporating λ include Lu et al.' index of cardiac electrophysiological balance (iCEB: QT/QRSd), [QRSD× (Tpeak - Tend)/QRSd] and [QRSD × (Tpeak - Tend)/(QRSd × QT)] recently proposed by Tse and Yan. The aim of this review is to provide up to date information on traditional and novel markers and discuss their utility and downfalls for risk stratification.

Marked exercise-induced T-wave heterogeneity in symptomatic diabetic patients with nonflow-limiting coronary artery stenosis

BACKGROUND

T-wave heterogeneity (TWH) independently predicted cardiovascular mortality in Health Survey 2000 based on 12-lead ECGs recorded at rest. We investigated whether TWH is elevated during exercise tolerance testing (ETT) in symptomatic diabetic patients with nonflow-limiting coronary artery stenosis compared to control subjects without diabetes.

METHODS

Cases were all patients (n = 20) with analyzable ECG recordings during both rest and ETT who were enrolled in the Effects of Ranolazine on Coronary Flow Reserve (CFR) in Symptomatic Patients with Diabetes and Suspected or Known Coronary Artery Disease (RAND-CFR) study (NCT01754259); median CFR was 1.44; 80% of cases had CFR <2. Control subjects (n = 9) were nondiabetic patients who had functional flow reserve (FFR) >0.8, a range not associated with inducible ischemia. TWH was analyzed from precordial leads V4 , V5 , and V6 by second central moment analysis, which assesses the interlead splay of T-waves about a mean waveform.

RESULTS

During exercise to similar rate-pressure products (p = .31), RAND-CFR patients exhibited a 49% increase in TWH during exercise (rest: 49 ± 5 μV; exercise: 73 ± 8 μV, p = .003). By comparison, in control subjects, TWH was not significantly altered (rest: 52 ± 11 μV; ETT: 38 ± 5 μV, p = .19). ETT-induced ST-segment depression >1 mm (p = .11) and Tpeak -Tend (p = .18) and QTc intervals (p = .80) failed to differentiate cases from controls.

CONCLUSIONS

TWH is capable of detecting latent repolarization abnormalities, which are present during ETT in diabetic patients with nonflow-limiting stenosis but not in control subjects. The technique developed in this study permits TWH analysis from archived ECGs and thereby enables mining of extensive databases for retrospective studies and hypothesis testing.

Ranolazine reduces repolarization heterogeneity in symptomatic patients with diabetes and non-flow-limiting coronary artery stenosis

BACKGROUND

Experimental evidence suggests that ranolazine decreases susceptibility to ischemia-induced arrhythmias independent of effects on coronary artery blood flow.

OBJECTIVE

In symptomatic diabetic patients with non-flow-limiting coronary artery stenosis with diffuse atherosclerosis and/or microvascular dysfunction, we explored whether ranolazine reduces T-wave heterogeneity (TWH), an electrocardiographic (ECG) marker of arrhythmogenic repolarization abnormalities shown to predict sudden cardiac death.

METHODS

We studied all 16 patients with analyzable ECG recordings during rest and exercise tolerance testing before and after 4 weeks of ranolazine in the double-blind, crossover, placebo-controlled RAND-CFR trial (NCT01754259). TWH was quantified without knowledge of treatment assignment by second central moment analysis, which assesses the interlead splay of T waves in precordial leads about a mean waveform. Myocardial blood flow (MBF) was measured by positron emission tomography.

RESULTS

At baseline, prior to randomization, TWH during rest was 54 ± 7 μV and was not altered following placebo (47 ± 6 μV, p = .47) but was reduced by 28% (to 39 ± 5 μV, p = .002) after ranolazine. Ranolazine did not increase MBF at rest. Exercise increased TWH after placebo by 49% (to 70 ± 8 μV, p = .03). Ranolazine did not reduce TWH during exercise (to 75 ± 16 μV), and there were no differences among the groups (p = .95, ANOVA). TWH was not correlated with MBF at rest before (r²  = .07, p = .36) or after ranolazine (r²  = .23, p = .06).

CONCLUSIONS

In symptomatic diabetic patients with non-flow-limiting coronary artery stenosis with diffuse atherosclerosis and/or microvascular dysfunction, ranolazine reduced TWH at rest but not during exercise. Reduction in repolarization abnormalities appears to be independent of alterations in MBF.

Tracking interlead heterogeneity of R- and T-wave morphology to disclose latent risk for sudden cardiac death

Sudden cardiac death (SCD) due primarily to ventricular fibrillation claims 1.5 million lives worldwide each year. In 45%-50% of cases, it is the first manifestation of underlying heart disease. Traditional risk factors including smoking, hypertension, age, sex, as well as depressed left ventricular ejection fraction lack sufficient sensitivity and specificity to forewarn of impending life-threatening arrhythmias. There has been a decades-long search for electrocardiographic (ECG) markers of SCD risk. Several interval-based indices such as QT dispersion and T_peak-T_end interval held initial promise but ultimately yielded mixed results. Recently, the focus has been on interlead heterogeneity of R- and T-wave morphology. The new approaches have involved advanced analytical tools including vectorcardiographic techniques and second central moment analysis of QRS-aligned templates to quantify heterogeneity of depolarization and repolarization waveforms. The results of current studies appear to be robust and worthy of further exploration. This review examines the electrophysiological underpinnings of heterogeneity-based risk assessment and provides an update of clinical techniques. We also discuss future directions whereby tracking heterogeneity may help to disclose latent risk for SCD not only in ECG recordings made at rest but also during ambulatory ECG monitoring and exercise tolerance testing.