Predicting sudden cardiac death from T wave alternans of the surface electrocardiogram: promise and pitfalls

Reference for Electrocardiographic Imaging-Based T-Wave Alternans Estimation

Sudden cardiac death causes multiple deaths annually, and T-wave alternans are a reliable predictor of this fatal event. Detecting alternans is crucial for reducing disease incidence, and electrocardiographic imaging is a promising tool, providing spatial-temporal insights. The absence of references and segmentation methods specific to these data may complicate progress in the field. Therefore, this work aimed to develop a reference for evaluating estimation methods. Initially, a novel T-wave segmentation procedure specific to these data was introduced and compared with a commonly used method. Subsequently, a reference for assessing alternans estimation methods was created by integrating alternans into epicardial signals through a spatial-temporal Gaussian function. Finally, a bootstrap-based classifier for detecting alternans was developed. Results underscored the superiority of the novel T-wave segmentation procedure, with the lowest 95% confidence interval being [ 16.57 μ V , 18.80 μ V ], indicating significant disparities between the two segmentation methodologies. Furthermore, the generated reference demonstrated the distinguishability of T-wave alternans with an amplitude of approximately 55 μ V from noise. Additionally, the classifier exhibited consistency with previous findings, demonstrating its ability to detect alternans with amplitudes around 50 μ V . In conclusion, this study provides a spatial-temporal reference for proper evaluation of estimation methods, contributing to establishing a gold standard.

Machine learning based detection of T-wave alternans in real ambulatory conditions

BACKGROUND AND OBJECTIVE

T-wave alternans (TWA) is a fluctuation in the repolarization morphology of the ECG. It is associated with cardiac instability and sudden cardiac death risk. Diverse methods have been proposed for TWA analysis. However, TWA detection in ambulatory settings remains a challenge due to the absence of standardized evaluation metrics and detection thresholds.

METHODS

In this work we use traditional TWA analysis signal processing-based methods for feature extraction, and two machine learning (ML) methods, namely, K-nearest-neighbor (KNN) and random forest (RF), for TWA detection, addressing hyper-parameter tuning and feature selection. The final goal is the detection in ambulatory recordings of short, non-sustained and sparse TWA events.

RESULTS

We train ML methods to detect a wide variety of alternant voltage from 20 to 100 μV, i.e., ranging from non-visible micro-alternans to TWA of higher amplitudes, to recognize a wide range in concordance to risk stratification. In classification, RF outperforms significantly the recall in comparison with the signal processing methods, at the expense of a small lost in precision. Despite ambulatory detection stands for an imbalanced category context, the trained ML systems always outperform signal processing methods.

CONCLUSIONS

We propose a comprehensive integration of multiple variables inspired by TWA signal processing methods to fed learning-based methods. ML models consistently outperform the best signal processing methods, yielding superior recall scores.

Biomarker Periodic Repolarization Dynamics Indicates Enhanced Risk for Arrhythmias and Sudden Cardiac Death in Myocardial Infarction in Pigs

BACKGROUND

Periodic repolarization dynamics (PRD) is an electrocardiographic biomarker that captures repolarization instability in the low frequency spectrum and is believed to estimate the sympathetic effect on the ventricular myocardium. High PRD indicates an increased risk for postischemic sudden cardiac death (SCD). However, a direct link between PRD and proarrhythmogenic autonomic remodeling has not yet been shown.

METHODS AND RESULTS

We investigated autonomic remodeling in pigs with myocardial infarction (MI)-related ischemic heart failure induced by balloon occlusion of the left anterior descending artery (n=17) compared with pigs without MI (n=11). Thirty days after MI, pigs demonstrated enhanced sympathetic innervation in the infarct area, border zone, and remote left ventricle paralleled by altered expression of autonomic marker genes/proteins. PRD was enhanced 30 days after MI compared with baseline (pre-MI versus post-MI: 1.75±0.30 deg2 versus 3.29±0.79 deg2, P<0.05) reflecting pronounced autonomic alterations on the level of the ventricular myocardium. Pigs with MI-related ventricular fibrillation and SCD had significantly higher pre-MI PRD than pigs without tachyarrhythmias, suggesting a potential role for PRD as a predictive biomarker for ischemia-related arrhythmias (no ventricular fibrillation versus ventricular fibrillation: 1.50±0.39 deg2 versus 3.18±0.53 deg2 [P<0.05]; no SCD versus SCD: 1.67±0.32 deg2 versus 3.91±0.63 deg2 [P<0.01]).

CONCLUSIONS

We demonstrate that ischemic heart failure leads to significant proarrhythmogenic autonomic remodeling. The concomitant elevation of PRD levels in pigs with ischemic heart failure and pigs with MI-related ventricular fibrillation/SCD suggests PRD as a biomarker for autonomic remodeling and as a potential predictive biomarker for ventricular arrhythmias/survival in the context of MI.

Cardiac Alternans: From Bedside to Bench and Back

Cardiac alternans arises from dynamical instabilities in the electrical and calcium cycling systems of the heart, and often precedes ventricular arrhythmias and sudden cardiac death. In this review, we integrate clinical observations with theory and experiment to paint a holistic portrait of cardiac alternans: the underlying mechanisms, arrhythmic manifestations and electrocardiographic signatures. We first summarize the cellular and tissue mechanisms of alternans that have been demonstrated both theoretically and experimentally, including 3 voltage-driven and 2 calcium-driven alternans mechanisms. Based on experimental and simulation results, we describe their relevance to mechanisms of arrhythmogenesis under different disease conditions, and their link to electrocardiographic characteristics of alternans observed in patients. Our major conclusion is that alternans is not only a predictor, but also a causal mechanism of potentially lethal ventricular and atrial arrhythmias across the full spectrum of arrhythmia mechanisms that culminate in functional reentry, although less important for anatomic reentry and focal arrhythmias.

Machine Learning approach for TWA detection relying on ensemble data design

BACKGROUND AND OBJECTIVE

T-wave alternans (TWA) is a fluctuation of the ST-T complex of the surface electrocardiogram (ECG) on an every-other-beat basis. It has been shown to be clinically helpful for sudden cardiac death stratification, though the lack of a gold standard to benchmark detection methods limits its application and impairs the development of alternative techniques. In this work, a novel approach based on machine learning for TWA detection is proposed. Additionally, a complete experimental setup is presented for TWA detection methods benchmarking.

METHODS

The proposed experimental setup is based on the use of open-source databases to enable experiment replication and the use of real ECG signals with added TWA episodes. Also, intra-patient overfitting and class imbalance have been carefully avoided. The Spectral Method (SM), the Modified Moving Average Method (MMA), and the Time Domain Method (TM) are used to obtain input features to the Machine Learning (ML) algorithms, namely, K Nearest Neighbor, Decision Trees, Random Forest, Support Vector Machine and Multi-Layer Perceptron.

RESULTS

There were not found large differences in the performance of the different ML algorithms. Decision Trees showed the best overall performance (accuracy 0.88 ± 0.04 , precision 0.89 ± 0.05 , Recall 0.90 ± 0.05 , F1 score 0.89 ± 0.03 ). Compared to the SM (accuracy 0.79, precision 0.93, Recall 0.64, F1 score 0.76) there was an improvement in every metric except for the precision.

CONCLUSIONS

In this work, a realistic database to test the presence of TWA using ML algorithms was assembled. The ML algorithms overall outperformed the SM used as a gold standard. Learning from data to identify alternans elicits a substantial detection growth at the expense of a small increment of the false alarm.

Spatial-Temporal Signals and Clinical Indices in Electrocardiographic Imaging (II): Electrogram Clustering and T-wave Alternans

During the last years, attention and controversy have been present for the first commercially available equipment being used in Electrocardiographic Imaging (ECGI), a new cardiac diagnostic tool which opens up a new field of diagnostic possibilities. Previous knowledge and criteria of cardiologists using intracardiac Electrograms (EGM) should be revisited from the newly available spatial-temporal potentials, and digital signal processing should be readapted to this new data structure. Aiming to contribute to the usefulness of ECGI recordings in the current knowledge and methods of cardiac electrophysiology, we previously presented two results: First, spatial consistency can be observed even for very basic cardiac signal processing stages (such as baseline wander and low-pass filtering); second, useful bipolar EGMs can be obtained by a digital processing operator searching for the maximum amplitude and including a time delay. In addition, this work aims to demonstrate the functionality of ECGI for cardiac electrophysiology from a twofold view, namely, through the analysis of the EGM waveforms, and by studying the ventricular repolarization properties. The former is scrutinized in terms of the clustering properties of the unipolar an bipolar EGM waveforms, in control and myocardial infarction subjects, and the latter is analyzed using the properties of T-wave alternans (TWA) in control and in Long-QT syndrome (LQTS) example subjects. Clustered regions of the EGMs were spatially consistent and congruent with the presence of infarcted tissue in unipolar EGMs, and bipolar EGMs with adequate signal processing operators hold this consistency and yielded a larger, yet moderate, number of spatial-temporal regions. TWA was not present in control compared with an LQTS subject in terms of the estimated alternans amplitude from the unipolar EGMs, however, higher spatial-temporal variation was present in LQTS torso and epicardium measurements, which was consistent through three different methods of alternans estimation. We conclude that spatial-temporal analysis of EGMs in ECGI will pave the way towards enhanced usefulness in the clinical practice, so that atomic signal processing approach should be conveniently revisited to be able to deal with the great amount of information that ECGI conveys for the clinician.

T-Wave Alternans in Nonpathological Preterm Infants

BACKGROUND

Sudden infant death syndrome is more frequent in preterm infants (PTI) than term infants and may be due to cardiac repolarization instability, which may manifest as T-wave alternans (TWA) on the electrocardiogram (ECG). Therefore, the aim of the present work was to analyze TWA in nonpathological PTI and to open an issue on its physiological interpretation.

METHODS

Clinical population consisted of ten nonpathological PTI (gestational age ranging from 29 3 7 to 34 2 7  weeks; birth weight ranging from 0.84 to 2.10 kg) from whom ECG recordings were obtained ("Preterm infant cardio-respiratory signals database" by Physionet). TWA was identified through the heart-rate adapting match filter method and characterized in terms of mean amplitude values (TWAA). TWA correlation with several other clinical and ECG features, among which gestational age-birth weight ratio, RR interval, heart-rate variability, and QT interval, was also performed.

RESULTS

TWA was variable among infants (TWAA = 26 ± 11 µV). Significant correlations were found between TWAA versus birth weight (ρ = -0.72, p = .02), TWAA versus gestational age-birth weight ratio (ρ = 0.76, p = .02) and TWAA versus heart-rate variability (ρ = -0.71, p = .02).

CONCLUSIONS

Our preliminary retrospective study suggests that nonpathological PTI show TWA of few tens of µV, the interpretation of which is still an open issue but could indicate a condition of cardiac risk possibly related to the low development status of the infant. Further investigations are needed to solve this issue.

Cardiac electrophysiological adaptations in the equine athlete-Restitution analysis of electrocardiographic features

Exercising horses uniquely accommodate 7–8-fold increases in heart rate (HR). The present experiments for the first time analysed the related adaptations in action potential (AP) restitution properties recorded by in vivo telemetric electrocardiography from Thoroughbred horses. The horses were subjected to a period of acceleration from walk to canter. The QRS durations, and QT and TQ intervals yielded AP conduction velocities, AP durations (APDs) and diastolic intervals respectively. From these, indices of active, λ = QT/(QRS duration), and resting, λ₀ = TQ/(QRS duration), AP wavelengths were calculated. Critical values of QT and TQ intervals, and of λ and λ₀ at which plots of these respective pairs of functions showed unity slope, were obtained. These were reduced by 38.9±2.7% and 86.2±1.8%, and 34.1±3.3% and 85.9±1.2%, relative to their resting values respectively. The changes in λ were attributable to falls in QT interval rather than QRS duration. These findings both suggested large differences between the corresponding critical (129.1±10.8 or 117.4±5.6 bpm respectively) and baseline HRs (32.9±2.1 (n = 7) bpm). These restitution analyses thus separately identified concordant parameters whose adaptations ensure the wide range of HRs over which electrophysiological activation takes place in an absence of heart block or arrhythmias in equine hearts. Since the horse is amenable to this in vivo electrophysiological analysis and displays a unique wide range of heart rates, it could be a novel cardiac electrophysiology animal model for the study of sudden cardiac death in human athletes.

Benchmarking of a T-wave alternans detection method based on empirical mode decomposition

BACKGROUND AND OBJECTIVE

T-wave alternans (TWA) is a fluctuation of the ST-T complex occurring on an every-other-beat basis of the surface electrocardiogram (ECG). It has been shown to be an informative risk stratifier for sudden cardiac death, though the lack of gold standard to benchmark detection methods has promoted the use of synthetic signals. This work proposes a novel signal model to study the performance of a TWA detection. Additionally, the methodological validation of a denoising technique based on empirical mode decomposition (EMD), which is used here along with the spectral method, is also tackled.

METHODS

The proposed test bed system is based on the following guidelines: (1) use of open source databases to enable experimental replication; (2) use of real ECG signals and physiological noise; (3) inclusion of randomized TWA episodes. Both sensitivity (Se) and specificity (Sp) are separately analyzed. Also a nonparametric hypothesis test, based on Bootstrap resampling, is used to determine whether the presence of the EMD block actually improves the performance.

RESULTS

The results show an outstanding specificity when the EMD block is used, even in very noisy conditions (0.96 compared to 0.72 for SNR = 8 dB), being always superior than that of the conventional SM alone. Regarding the sensitivity, using the EMD method also outperforms in noisy conditions (0.57 compared to 0.46 for SNR=8 dB), while it decreases in noiseless conditions.

CONCLUSIONS

The proposed test setting designed to analyze the performance guarantees that the actual physiological variability of the cardiac system is reproduced. The use of the EMD-based block in noisy environment enables the identification of most patients with fatal arrhythmias.

Sudden Cardiac Risk Stratification with Electrocardiographic Indices - A Review on Computational Processing, Technology Transfer, and Scientific Evidence

Great effort has been devoted in recent years to the development of sudden cardiac risk predictors as a function of electric cardiac signals, mainly obtained from the electrocardiogram (ECG) analysis. But these prediction techniques are still seldom used in clinical practice, partly due to its limited diagnostic accuracy and to the lack of consensus about the appropriate computational signal processing implementation. This paper addresses a three-fold approach, based on ECG indices, to structure this review on sudden cardiac risk stratification. First, throughout the computational techniques that had been widely proposed for obtaining these indices in technical literature. Second, over the scientific evidence, that although is supported by observational clinical studies, they are not always representative enough. And third, via the limited technology transfer of academy-accepted algorithms, requiring further meditation for future systems. We focus on three families of ECG derived indices which are tackled from the aforementioned viewpoints, namely, heart rate turbulence (HRT), heart rate variability (HRV), and T-wave alternans. In terms of computational algorithms, we still need clearer scientific evidence, standardizing, and benchmarking, siting on advanced algorithms applied over large and representative datasets. New scenarios like electronic health recordings, big data, long-term monitoring, and cloud databases, will eventually open new frameworks to foresee suitable new paradigms in the near future.

Arrhythmogenic Substrates in Sleep-Disordered Breathing with Arterial Hypertension

BACKGROUND

Sleep-disordered breathing (SDB) is highly associated with arterial hypertension (HT). Sympathetic hypertonia increases the risk of sudden cardiac death in patients with sleep apnea. This study aims to noninvasively investigate the electrophysiological features in SDB patients with and without arterial HT.

METHODS

Fifty-three patients with SDB were classified into two groups: SDB group and SDB + HT group. Twenty subjects with arterial HT were enrolled as controls (HT group). To assess arrhythmogenic vulnerability, high-resolution 24-hour ambulatory electrocardiograms were obtained for analyzing continuous late potential (LP), T-wave amplitude variability (TAV), and heart rate variability (HRV).

RESULTS

A higher incidence of positive LP was observed in the SDB + HT (85%) group than that observed in the SDB (50%) and HT (20%) groups (P < 0.01). TAV was highest in the SDB + HT group (78 μV) compared with the SDB (61 μV) and HT groups (42 μV; P < 0.01). Positive LP and TAV values were observed at night in the SDB + HT and SDB groups. The low-frequency/high-frequency of the HRV analysis was highest in the SDB + HT (4.7) group compared with that in the SDB (2.9) and HT (2.9) groups (P = 0.01).

CONCLUSION

Nocturnal LP, TAV, and HRV examinations were useful to investigate arrhythmogenesis. SDB patients with arterial HT showed a high prevalence of depolarization and repolarization abnormalities and relative sympathetic hyperactivity. This suggests that an electrophysiological instability is more prevalent in SDB patients with arterial HT.

[Microvolt T-wave alternans. Ischemic vs. nonischemic dilated cardiomyopathy]

The use of implantable cardioverter defibrillators (ICD) for primary preventive therapy of sudden arrhythmogenic death has become a mainstay in selected patients with systolic congestive heart failure, particularly in the setting of ischemic and nonischemic cardiomyopathy (Moss et al., N Engl J Med 346:877–883, 2002; Bardy et al., N Engl J Med 352:225–237, 2005). However, more accurate identification of high-risk patients is desirable in order to avoid unnecessary ICD implants. Since currently available risk stratification methods have limited predictive accuracy, development of new techniques is important in order to noninvasively assess arrhythmogenic risk in patients prone to sudden death.Microvolt level T-wave alternans (mTWA) has recently been proposed to assess abnormalities in ventricular repolarization favoring the occurrence of reentrant arrhythmias (Adam et al., J Electrocardiol 17:209–218, 1984; Pastore et al., Circulation 99:1385–1394, 1999). In 1994, a preliminary clinical study by Rosenbaum et al. convincingly demonstrated that mTWA is closely related to arrhythmia induction in the electrophysiology laboratory as well as to the occurrence of spontaneous ventricular tachyarrhythmias during follow-up (Rosenbaum et al., N Engl J Med 330:235–241,1994). More recently, a number of clinical studies have examined its clinical applicability in ischemic and nonischemic cardiomyopathy.

Nonparametric signal processing validation in T-wave alternans detection and estimation

Although a number of methods have been proposed for T-Wave Alternans (TWA) detection and estimation, their performance strongly depends on their signal processing stages and on their free parameters tuning. The dependence of the system quality with respect to the main signal processing stages in TWA algorithms has not yet been studied. This study seeks to optimize the final performance of the system by successive comparisons of pairs of TWA analysis systems, with one single processing difference between them. For this purpose, a set of decision statistics are proposed to evaluate the performance, and a nonparametric hypothesis test (from Bootstrap resampling) is used to make systematic decisions. Both the temporal method (TM) and the spectral method (SM) are analyzed in this study. The experiments were carried out in two datasets: first, in semisynthetic signals with artificial alternant waves and added noise; second, in two public Holter databases with different documented risk of sudden cardiac death. For semisynthetic signals (SNR = 15 dB), after the optimization procedure, a reduction of 34.0% (TM) and 5.2% (SM) of the power of TWA amplitude estimation errors was achieved, and the power of error probability was reduced by 74.7% (SM). For Holter databases, appropriate tuning of several processing blocks, led to a larger intergroup separation between the two populations for TWA amplitude estimation. Our proposal can be used as a systematic procedure for signal processing block optimization in TWA algorithmic implementations.

Detection of T-wave alternans in fetal magnetocardiography using the generalized likelihood ratio test

T-wave alternans (TWA) is an indicator of cardiac instability and is associated with life-threatening ventricular arrhythmias. Detection of TWA in the adult has been widely investigated and is used routinely for cardiac risk assessment. Detection of TWA in the fetus, however, is much more difficult due to the low amplitude and variable configuration of the signal, the presence of strong interferences, and the brevity of fetal TWA episodes. In this paper, we present a statistical detector based on the generalized likelihood ratio test that is designed for detection of TWA in the fetus. The performance of the detector is evaluated by constructing receiver-operator characteristic curves, using simulated data and real data from subjects with macroscopic TWA. The detector is capable of detecting TWA episodes as brief as 20 beats. The detection performance is improved significantly by modeling the fetal T-wave as a low-rank, low bandwidth signal, and using maximum likelihood estimation to estimate the model parameters. This approach enables all of the data to be used to estimate the noise statistics, providing highly effective suppression of interference, including maternal interference. The method is suitable for routine use because it can be applied to raw, unprocessed recordings, allowing automated analysis of extended fetal magnetocardiography recordings.

Microvolt T-wave alternans in short QT syndrome

BACKGROUND

T-wave alternans (TWA) is an accepted marker of risk for malignant ventricular arrhythmias, for which prognosis value has been established in different populations. Short QT syndrome (SQTS) is a very rare primary electrical disease carrying the risk of ventricular fibrillation. TWA in SQTS has not been evaluated yet.

METHODS

Thirteen patients with SQTS (QT = 308 ± 16 ms, QTc = 329 ± 10 ms, heart rate = 69 ± 8 beats/min) underwent microvolt TWA measurement using spectral analysis. TWA testing was performed using Heartwave II (Cambridge Heart™, Inc., Bedford, MA, USA) during bicycle exercice and classified as negative, positive, or indeterminate according to the published standards for clinical interpretation.

RESULTS

Twelve patients were male (mean age 23 ± 5 years). Five were asymptomatic, three presented with aborted sudden cardiac death, and five with unexplained syncope. Six patients belonged to two unrelated families, while familial cases of SQTS were present for two other patients. A familial history of sudden death (SD) was present for seven patients. Ventricular fibrillation was inducible in three patients. Four patients were implanted with an implantable cardioverter-defibrillator and one presented with polymorphic ventricular tachycardia during follow-up. TWA was negative in each but one patient (indeterminate). Maximal negative heart rate was 118 ± 12 beats/min. Patients with previous SD displayed significant shorter QT and higher resting heart rate compared to the remaining cases.

CONCLUSIONS

TWA testing is negative in 12 of 13 SQTS patients, even in the symptomatic or inducible ones. Measurement of TWA using conventional protocol and criteria for risk stratification in SQTS seems therefore useless.

Comparison of standard versus orthogonal ECG leads for T-wave alternans identification

T-wave alternans (TWA), an electrophysiologic phenomenon associated with ventricular arrhythmias, is usually detected from selected ECG leads. TWA amplitude measured in the 12-standard and the 3-orthogonal (vectorcardiographic) leads were compared here to identify which lead system yields a more adequate detection of TWA as a noninvasive marker for cardiac vulnerability to ventricular arrhythmias. Our adaptive match filter (AMF) was applied to exercise ECG tracings from 58 patients with an implanted cardiac defibrillator, 29 of which had ventricular tachycardia or fibrillation during follow-up (cases), while the remaining 29 were used as controls. Two kinds of TWA indexes were considered, the single-lead indexes, defined as the mean TWA amplitude over each lead (MTWAA), and lead-system indexes, defined as the mean and the maximum MTWAA values over the standard leads and over the orthogonal leads. Significantly (P < 0.05) higher TWA in the cases versus controls was identified only occasionally by the single-lead indexes (odds ratio: 1.0-9.9, sensitivity: 24-76%, specificity: 76-86%), and consistently by the lead-system indexes (odds ratio: 4.5-8.3, sensitivity: 57-72%, specificity: 76%). The latter indexes also showed a significant correlation (0.65-0.83) between standard and orthogonal leads. Hence, when using the AMF, TWA should be detected in all leads of a system to compute the lead-system indexes, which provide a more reliable TWA identification than single-lead indexes, and a better discrimination of patients at increased risk of cardiac instability. The standard and the orthogonal leads can be considered equivalent for TWA identification, so that TWA analysis can be limited to one-lead system.

On the estimation of T-wave alternans using the spectral fast fourier transform method

BACKGROUND

T-wave alternans (TWA) has been associated with increased vulnerability to ventricular tachyarrhythmias and sudden cardiac death. However, both random (white) noise and (patho)physiologic processes (ie, premature ventricular contractions and heart and respiration rates) may hamper TWA estimation and therefore lessen its clinical utility for risk stratification.

OBJECTIVE

To investigate the effect of random noise and certain (patho)physiologic processes on the estimation of TWA by using the fast Fourier transform method and to develop methods to overcome these potential sources of error.

METHODS

We used a combination of human electrocardiogram data and computer simulations to assess the effects of a premature ventricular contraction and random and colored noise on the accuracy of TWA estimation.

RESULTS

We quantitatively demonstrate that replacing a "bad" beat with an odd/even median beat is a more accurate approach than replacing it with the overall average or the overall median beat. We also show that phase resetting may have a significant effect on alternans estimation and that estimation of alternans by using frequencies >0.4922 cycles/beat in a 128-point fast Fourier transform provides the most accurate approach for estimating the alternans when phase resetting is likely to occur. In addition, our data demonstrate that the number of indeterminate TWA tests due to high levels of noise can be reduced when the alternans voltage exceeds a new higher threshold. Furthermore, the amplitude of random noise has a significant effect on alternans estimation and the alternans voltage threshold should be adjusted for noise levels >1.8 μV. Finally, we quantitatively demonstrate that colored noise may lead to a false-positive or a false-negative result. We propose methods to estimate the effect of these (patho)physiologic processes on the alternans estimation in order to determine whether a TWA test is likely to be a true positive or a true negative.

CONCLUSION

This study introduces novel methods to overcome potential sources of error in the estimation of TWA. These methods may improve the utility of TWA either for ambulatory monitoring or for clinical risk stratification for ventricular arrhythmias and sudden cardiac death.

Repolarization alternans heterogeneity in healthy subjects and acute myocardial infarction patients

An association between heterogeneity of repolarization alternans (RA) and cardiac electrical instability has been reported. Characterization of RA in health and identification of physiological RA heterogeneity may help discrimination of abnormal RA cases more likely associated to arrhythmic events. Thus, aim of the present study was the identification of a physiological RA region in terms of mean temporal location (MRAD) with respect to the T apex, and mean amplitude (MRAA), by application of our heart-rate adaptive match filter method to clinical ECG recordings from 51 control healthy (CH) subjects and 43 acute myocardial infarction (AMI) patients. Results indicate that RA occurring within the first half of the T wave is dominant in both CH and AMI populations (74.5% and 53.5% of cases, respectively; P<0.05). Definition of physiological RA region in the MRAD vs. MRAA plane (-83 ms ≤ MRAD ≤ 23 ms, 0≤ MRAA ≤ 30 μV) provided 0% and 32.6% abnormal RA cases among the CH subjects and AMI patients, respectively. We conclude that myocardial infarction may associate with an RA occurring early (MRAD<-83 ms) or late (MRAD >23 ms) along the JT segment, in addition or in alternative to an abnormally high RA amplitude (MRAA >30 μV).

Microvolt T-wave alternant: pathomechanism and evaluation of a new marker of arrhythmia risk

A mikrovolt T-hullám-alternáns, angolul microvolt T-wave alternant (µV-TWA), a T-hullám amplitúdójának ütésről ütésre történő mikrovoltszinten mérhető váltakozása. Mivel ez a változás igen kicsi, csak finom, érzékeny digitális jelfeldolgozó technikával mérhető. Jelenlegi álláspont szerint a µV-TWA megjelenése előre jelezheti a letális, malignus kamrai tachyarrythmiák kialakulásának valószínűségét, a hirtelen szívhalál bekövetkeztét. Az elmúlt tíz évben experimentális és klinikai vizsgálatok próbálták magyarázni a µV-TWA kialakulásának patomechanizmusát és a mögöttes sejtszintű folyamatokat. Azonban a mai napig nem sikerült a µV-TWA-t kialakító celluláris folyamatokat megfelelően tisztázni. Összefoglaló tanulmányunkban áttekintjük azokat a témával foglalkozó közleményeket, amelyek a folyamatban szerepet játszó akciós potenciál repolarizációjában fontosak voltak és áttörést jelentettek az elmúlt években. Részletezzük az akciós potenciál és ionáram-fluktuáció, a citoplazmatikus kalciumkoncentráció-szabályozás, a béta-adrenerg receptorok, valamint a connexinek szerepét a µV-TWA és a következményes kamrai tachyarrhythmiák kialakításában. Ismertetjük továbbá a µV-TWA detektálására jelenleg alkalmazott technikákat, azok felhasználási lehetőségeit a hirtelen szívhalál veszélyeztetettségének felmérésében.

Nonlinear trend estimation of the ventricular repolarization segment for T-wave alternans detection

Repolarization alternans or T-wave alternans (TWA) is a subject of great interest as it has been shown as a risk stratifier for sudden cardiac death. As TWA consists of subtle and nonvisible variations of the ST-T complex, its detection may become more difficult in noisy environments, such as stress testing or Holter recordings. In this paper, a technique based on the empirical-mode decomposition (EMD) to separate the useful information of the ST-T complex from noise and artifacts is proposed. The identification of the useful part of the signal is based on the study of complexity in the EMD domain by means of the Hjorth descriptors. As a result, a robust technique to extract the trend of the ST-T complex has been achieved. The evaluation of the method is carried out with the spectral method (SM) over several public domain databases with ECGs sampled at different frequencies. The results show that the SM with the proposed technique outperforms the traditional SM by more than 2 dB. Also, the robustness of this technique is guaranteed as it does not introduce any additional distortion to the detector in noiseless conditions.

Elevated myocardial repolarization lability and arterial baroreflex dysfunction in healthy individuals with nondipping blood pressure pattern

BACKGROUND

The "nondipping" pattern (NDP) of blood pressure, characterized by the absence of the reduction in blood pressure (BP) that typically takes place during the night time, has been associated with elevated cardiovascular morbidity, including stroke, myocardial infarction, and sudden cardiac-related death. In various cardiovascular diseases, reduced vagal control of cardiac function and elevated lability of cardiac repolarization have been associated with increased risk for ventricular arrhythmia and sudden death. The aim of this study was to assess temporal QT variability, arterial baroreflex sensitivity (BRS), and baroreflex effectiveness index (BEI) in two groups of otherwise healthy subjects, one group consisting of those with a normal BP pattern and the other with a nondipping BP pattern.

METHODS

Ninety-five healthy subjects underwent 24-h ambulatory BP (AMBP) monitoring. A minimum of 10% reduction in BP during the night relative to daytime levels is considered normal (as found in "dippers"). These individuals were classified as "dippers" (n = 59) and individuals without 10% reduction in BP during night were classified as "nondippers" (n = 36). Electrocardiogram (ECG) readings and beat-to-beat BP were recorded at 1,000 Hz with the subjects at rest in the supine posture for 20 min. BRS, BEI, and QT variability index (QTVI) were calculated.

RESULTS

There were no differences between the study groups with respect to age, gender, and average BP. Nondippers showed an increase in QTVI (-1.28 +/- 0.48 in nondippers vs. -1.52 +/- 0.29 in dippers, P < 0.05) and a decrease in BEI (0.34 +/- 0.17 in nondippers vs. 0.43 +/- 0.17 in dippers, P < 0.05), whereas BRS did not differ between the groups.

CONCLUSION

A nondipping BP pattern in healthy subjects is associated with elevated myocardial repolarization lability and impaired baroreflex function, suggesting dysfunction of the autonomic nervous system.

Noninvasive diagnosis of cardiac sarcoidosis using microvolt T-wave alternans

Sarcoidosis is a systemic granulomatous disorder whose prognosis worsens when the heart is involved, and early diagnosis is important. Endomyocardial biopsy is the most helpful diagnostic examination, but suffers from low sensitivity and low specificity. Microvolt T wave alternans (MVTWA) is utilized in noninvasive examinations to detect beat-to-beat changes in the shape of the T wave at the microvolt level. Such beat-to-beat T wave changes arise from beat-to-beat changes in the transmural gradient of action potential duration. We speculate that the granulomatous changes of cardiac sarcoidosis produce cell-to-cell uncoupling and augment the transmural gradients of action potential duration. To examine the clinical significance of MVTWA in the prediction of cardiac involvement in sarcoidosis patients, we obtained MVTWA in a total of 35 sarcoidosis patients with and without cardiac involvement. All patients underwent electrocardiography (ECG), ambulatory electrocardiography, chest radiography, transthoracic echocardiography, and MVTWA examination using a CH 2000 system. We diagnosed cardiac sarcoidosis in 7 patients according to the accepted diagnostic criteria. MVTWA was detected in 6 out of 7 cardiac sarcoidosis patients (85.7%) as opposed to in 2 out of 28 patients without cardiac involvement (7.1%). The difference between the two groups was statistically significant (P < 0.001). The sensitivity and specificity of MVTWA in cardiac sarcoidosis detection were 85.7% and 92.8%, respectively. The positive and negative predictive values were 75% and 96.3%, respectively, with an overall accuracy of 91.4%. Noninvasive examination of MVTWA using a CH 2000 is a useful diagnostic tool for detecting cardiac involvement in patients with sarcoidosis.

Assessment of microvolt T-wave alternans in high-risk patients with the congenital long-QT syndrome

BACKGROUND

Microvolt T-wave alternans (MTWA) has been used for arrhythmogenic risk stratification in cardiac disease conditions associated with increased risk of sudden cardiac death. Macroscopic T-wave alternans has been observed in patients with congenital long-QT syndrome (LQTS). The role of MTWA testing in patients with LQTS has not been established.

OBJECTIVE

To determine the diagnostic value of MTWA testing in high-risk patients with LQTS.

METHODS AND RESULTS

We assessed MTWA in 10 consecutive LQTS index patients who survived cardiac arrest or had documented torsade de pointes tachycardia and 6 first-degree family members with congenital LQTS which had been genotyped in 13 of 16 subjects (7 index patients, 6 family members). No LQTS-causing mutation was identified in 3 index patients with overt QT prolongation. MTWA was assessed during standardized bicycle exercise testing using the spectral method and yielded negative (n = 8) or indeterminate (n = 2) results in index patients, respectively. Similarly, all first-degree family members tested MTWA negative except for one indeterminate result. Two genotype positive family members could not be tested (two children-4 and 9 years of age).

CONCLUSION

In patients with congenital LQTS, free from structural heart disease and with a history of life-threatening cardiac arrhythmias, assessment of MTWA does not yield diagnostic value. Hence, determination of MTWA in lower risk LQTS patients without spontaneous arrhythmic events is likely not to be useful for arrhythmia risk stratification.

Adrenergic stimulation promotes T-wave alternans and arrhythmia inducibility in a TNF-alpha genetic mouse model of congestive heart failure

T-wave alternans (TWA) is a proarrhythmic repolarization instability that is common in congestive heart failure (CHF). Although transgenic mice are commonly used to study the mechanisms of arrhythmogenesis in CHF, little is known about the dynamics of TWA in these species. We hypothesized that TWA is present in a TNF-alpha model of CHF and can be further promoted by adrenergic stimulation. We studied 16 TNF-alpha mice and 12 FVB controls using 1) in vivo intracardiac electrophysiological testing and 2) ambulatory telemetry during 30 min before and after an intraperitoneal injection of isoproterenol. TWA was examined using both linear and nonlinear filtering applied in the time domain. In addition, changes in the mean amplitude of the T wave and area under the T wave were computed. During intracardiac electrophysiological testing, none of the animals had TWA or inducible arrhythmias before the injection of isoproterenol. After the injection, sustained TWA and inducible ventricular tachyarrhythmias were observed in TNF-alpha mice but not in FVB mice. In ambulatory telemetry, before the isoproterenol injection, the cardiac cycle length (CL) was longer in TNF-alpha mice than in FVB mice (98 +/- 9 and 88 +/- 3 ms, P = 0.04). After the injection of isoproterenol, the CL became 8% and 6% shorter in TNF-alpha and FVB mice (P < 10(-4)); however, the 2% difference between the groups in the magnitude of CL changes was not significant. In TNF-alpha mice, the magnitude of TWA was 1.5-2 times greater than in FVB mice both before and after the isoproterenol injection. The magnitude of TWA increased significantly after the isoproterenol injection compared with the baseline in TNF-alpha mice (P = 0.003) but not in FVB mice. The mean amplitude of the T wave and area under the T wave increased 60% and 80% in FVB mice (P = 0.006 and 0.009) but not in TNF-alpha mice. In conclusion, TWA is present in a TNF-alpha model of CHF and can be further promoted by adrenergic stimulation, along with the enhanced susceptibility for ventricular arrhythmias.

Comparative analysis of methods for automatic detection and quantification of microvolt T-wave alternans

Microvolt T-wave alternans (TWA), consisting of every-other-beat changes in ECG T-wave morphology, is an index of susceptibility to malignant ventricular arrhythmias, requiring automatic techniques to be identified. Five of these, namely, fast-Fourier-transform spectral method (FFTSM), complex-demodulation method (CDM), modified-moving-average method (MMAM), Laplacian-likelihood-ratio method (LLRM) and adaptive-match-filter method (AMFM), were applied here to simulated and sample clinical data. The aim was to compare individual methods ability to properly identify stationary and time-varying TWA, avoiding false-positive detections. The MMAM provided false-positive TWA when applied to simulated ECGs affected by amplitude variability, but TWA. Stationary TWA was properly quantified by the MMAM and, occasionally, underestimated by all other methods. The AMFM properly identified time-varying TWA. By contrast, the FFTSM detected not-stationary TWA as stationary, the MMAM introduced a time-delay in the estimated TWA-amplitude signal, while the CDM and LLRM were reliable only in the presence of slow-varying TWA. Altogether, the AMFM accomplished the best compromise between the needs to avoid false-positive TWA and to detect and characterize true-positive TWA. Results of our simulation approach were useful to explain different TWA levels measured by each competing methods applied to sample Holter ECGs from healthy subjects and coronary artery disease patients.

The many faces of repolarization instability: which one is prognostic?

Instabilities of the STT segment's magnitude, and particularly the 0.5 beat/cycle oscillations (T-wave alternans, or TWA), have been linked to the heightened risk of ventricular tachyarrhythmias (VTA) and sudden cardiac death (SCD). During the last decade theoretical, experimental and clinical research efforts have focused primarily on TWA, examining its mechanisms and predictive value using time-invariant cutoff values. However, recent evidence suggests that such a single-snapshot test of a single-frequency (TWA) oscillation using a constant cutoff value might be suboptimal for risk stratification because of several reasons. First, it is well known that the risk of VTA/SCD evolves over time with changes in electrophysiologic substrate, environmental and physiologic triggers, and the impact of other physiologic (eg, circadian) rhythmicity. Hence, the outcome of TWA testing might depend on the time of day, as Holter-based TWA studies have demonstrated. Furthermore, currently used single-snapshot testing with a binary cutoff value may not coincide with the periods of heightened risk for VTA/SCD and may not yield prognostic information, as a recent TWA substudy of the sudden cardiac death in heart failure trial has showed. Second, the analysis focused on TWA alone ignores the existence of multiple (alternating and nonalternating) forms of repolarization instability that have been shown to arise or increase before the onset of VTA/SCD. Summarizing, recent studies have identified multiple forms of repolarization instabilities modulated by distinct mechanisms, which might have different prognostic values. Therefore, the assessment of TWA needs to be dynamic and personalized to take into account the time evolution of risk and individual history.

The cardiovascular response to sexual activity: do we know enough?

Interest in comprehensive cardiac rehabilitation over the past 25 years spawned a series of small investigations concerning the heart rate, blood pressure, and ischemic response to sexual intercourse. This information was adequate for advising patients about return to sexual activity after a myocardial infarction or cardiac surgery. However, the introduction of medications for erectile dysfunction enabled impotent cardiac patients to engage in sexual activity and has highlighted the need for more detailed information concerning cardiovascular physiology during coitus. Review of the medical literature indicates a remarkable paucity of such data despite dramatic advances in most other aspects of cardiovascular physiology and pathophysiology. This brief paper gives an overview of the current knowledge of the cardiovascular response to sexual activity and, within the framework of advances in cardiology, highlights areas where it appears important to fill in the knowledge gap.

Clinical significance of microvolt T-wave alternans

Several studies have recently proven that primary preventive therapy of sudden arrhythmogenic death is possible in selected patients with congestive heart failure, particularly in the setting of ischemic cardiomyopathy [1, 2]. However, a number needed to treat between 11 and 17 to save one life over three years in these studies indicates that a more accurate identification of high risk patients is desirable in order to avoid unnecessary implants of cardioverter/defibrillators (ICD). Since currently available risk stratification methods have limited predictive accuracy, development of new techniques is important in order to non-invasively assess arrhythmogenic risk in patients prone to sudden death. Microvolt level T-wave alternans (mTWA) has recently been proposed to assess abnormalities in ventricular repolarization favoring the occurrence of reentrant arrhythmias [3, 4]. In 1994, a first clinical study by Rosenbaum and coworkers [5] convincingly demonstrated that mTWA is closely related to arrhythmia induction in the electrophysiology (EP) laboratory as well as to the occurrence of spontaneous ventricular tachyarrhythmias during follow-up [5]. More recently, a number of clinical studies has examined its clinical applicability [4-7]. The present review summarizes currently available clinical data on TWA with a particular focus on risk stratifying patients with congestive heart failure and myocardial infarction.

Beat-to-beat repolarization variability measured by T wave spectral variance index in chronic infarcted animals

BACKGROUND

Recent experimental and clinical studies have shown that beat-to-beat variability of repolarization morphology is associated with an increased risk for developing malignant ventricular arrhythmias. However, few data exist on the relationship between beat-to-beat repolarization variability and myocardial infarction itself.

METHODS AND RESULTS

In the present work we tested the algorithm of T wave spectral variance (TWSV) using the two dimension fast Fourier transform, in an animal model of myocardial infarction to extend the evidences that support the existence of beat-to-beat alteration in repolarization during the chronic stage of myocardial infarction. Thirty-four New Zealand rabbits were included in the study and divided in two groups. Group I (N = 24) exposed to surgical ligation of the left anterior descendent coronary artery. Group II (N = 10) sham operated animals. The TWSV index was calculated before and after 15 and 45 days of surgery. Both groups showed significant increments in TWSV after 15 days postsurgery. However, while the sham animals return to its control value, the infarcted group exhibited values of the TWSV index that remains significantly high after 45 days of surgery, with a mean increment of 28.7% (P < 0.05 against sham). Moreover, when the infarcted group was qualitatively divided in three subgroups, according to its infarction areas, a trend was found in the correlation between the magnitude of the infarcted area and the TWSV index.

CONCLUSION

This noninvasive measure confirms the presence of temporal repolarization variability associated with chronic myocardial infarction and further contributes to identify the infarcted animals.

Optimal target heart rate for exercise-induced T-wave alternans

OBJECTIVES

This study was conducted to determine the optimal target heart rate (HR) for the use of exercise-induced T-wave alternans (TWA) as an index for risk of malignant ventricular tachyarrhythmias.

BACKGROUND

Rate-dependent TWA is an index of vulnerability to ventricular tachyarrhythmias. However, false positive TWA was reported to occur in normal subjects at high HR.

METHODS

Two groups were evaluated: Group I: 50 patients with malignant ventricular tachyarrhythmias, who received an implantable cardioverter-defibrillator (ICD); and Group II: 55 age-matched normal subjects. In both Groups, TWA was evaluated during symptom-limited bicycle exercise test.

RESULTS

Peak HR during exercise test was 103 +/- 17 beats/min in Group I, versus 124 +/- 18 beats/min in Group II (P < 0.001). In Group I, 4 patients were excluded from analysis, due to high noise level or frequent ectopy during exercise. Out of the remaining 46 patients, TWA was present in 28 patients (61%), and absent in 18 (39%). In group II, TWA was present in four subjects (7%), and absent in 51 (93%). HR at the onset of TWA was 91 +/- 11/min in Group I, and 119 +/- 12/min in Group II (P < 0.001). Receiver operated characteristics curves demonstrated that a HR of 115 beats/min was the cutoff with the best sensitivity and specificity for TWA (100 and 96%, respectively). None of the patients in Group I developed TWA at HR > 115 beats/min, while two out of four in Group II had TWA at HR > 115/minutes. However, 13 patients in Group I who had no TWA were unable to exercise to a peak HR > 115 beats/min, compared to nine subjects in Group II.

CONCLUSIONS

A target HR of 115 beats/min was highly sensitive and specific for determination of exercise-induced TWA as an index of risk of malignant ventricular tachyarrhythmias. However, a significant number of patients may not be able to achieve this target HR, resulting in an indeterminate test. The value of pharmacologic testing in this group should be assessed.

Predicting the recurrence of ventricular tachyarrhythmias from T-wave alternans assessed on antiarrhythmic pharmacotherapy: a prospective study in patients with dilated cardiomyopathy

BACKGROUND

Microvolt T-wave alternans (TWA) has been proposed as a useful index to identify patients at risk of ventricular tachyarrhythmias. Recent studies have demonstrated that antiarrhythmic drugs, such as amiodarone and procainamide, decrease the prevalence of TWA. In this study, we tested whether TWA in patients on antiarrhythmic pharmacotherapy significantly predicts the recurrence of ventricular tachyarrhythmias in patients with dilated cardiomyopathy.

METHODS

To evaluate the ability to predict the recurrence of ventricular tachyarrhythmias, determinate TWA and left ventricular ejection fraction (LVEF) were prospectively assessed in 49 patients with ischemic or nonischemic dilated cardiomyopathy on antiarrhythmic pharmacotherapy for sustained ventricular tachycardia (VT) or ventricular fibrillation (VF). The pharmacotherapy consisted of class I (17 patients), III (29 patients), and IV (3 patients) antiarrhythmic drugs. The study endpoint was the first recurrence of sustained VT or VF on treatment during the follow-up period.

RESULTS

TWA was positive on antiarrhythmic pharmacotherapy in 30 patients (61%). During a follow-up of 13 +/- 11 months, the sustained VT or VF recurred in 21 of the 41 patients (51%) with available follow-up data. The sensitivity of TWA and LVEF for predicting recurrence of ventricular tachyarrhythmias was 76 and 38%, specificity was 60 and 70%, positive predictive value was 67 and 57%, and negative predictive value was 71 and 52%. Kaplan-Meier event-free analysis revealed that TWA was a significant risk stratifier (P = 0.02), whereas LVEF was not.

CONCLUSIONS

This prospective study suggests that TWA significantly predicts the recurrence of ventricular tachyarrhythmias, even on antiarrhythmic pharmacotherapy, in patients with dilated cardiomyopathy. TWA may also be a useful marker for evaluating the efficacy of antiarrhythmic drugs for ventricular tachyarrhythmias.

Value of microvolt T-wave alternans for predicting patients who would benefit from implantable cardioverter-defibrillator therapy

Despite considerable progress in the management of coronary artery disease and dilated cardiomyopathy, a substantial proportion of patients remains at the risk of life-threatening arrhythmic events. The Multicenter Automatic Defibrillator Implantation II and Sudden Cardiac Death Heart Failure studies have conclusively demonstrated that prophylactic implantable cardioverter-defibrillator (ICD) therapy reduces mortality among subjects with ischemic and nonischemic cardiomyopathy but at the expense of potentially unnecessary ICD implantation in a large percentage of patients. Microvolt T-wave alternans (MTWA), with a negative predictive value greater than 90%, holds promise for selecting the patients who would likely and patients not likely to benefit from ICD implantation. Accurate identification of high-risk patients by noninvasive MTWA may allow for improved widespread screening for sudden death prevention in the general population.

Detection of T-wave alternans using an implantable cardioverter-defibrillator

BACKGROUND

Microvolt T-wave alternans (TWA) increases acutely prior to ventricular tachycardia (VT) or ventricular fibrillation (VF) in computer simulations and animal models, suggesting that TWA may provide a warning for VT/VF in patients with an implantable cardioverter-defibrillator (ICD).

OBJECTIVES

The purposes of this study were to develop a method for analyzing TWA recorded from ICD electrograms (EGMs) and to evaluate the degree of concordance between EGM TWA and TWA recorded from the surface ECG.

METHODS

We developed a software program to measure EGM TWA in the frequency domain and then used simulated EGMs to determine the effects of ICD signal processing, electrical noise, and variation in the EGM fiducial point on the recorded amplitude and K score (signal-to-noise ratio) of TWA. We then applied this method to analyze TWA simultaneously using both surface ECGs and ICD EGMs during incremental pacing in 25 ICD patients. Pacing modes and EGM sources were varied in repeated trials. EGMs with dynamic range adjusted to achieve a large T wave were telemetered to a digital Holter recorder and measured offline. ECG TWA was analyzed using a commercial system. A positive (+) ECG test had sustained alternans >or=1.9 microV with K score >or=3. Stored EGMs were reviewed for VT/VF during a 6-month follow-up period.

RESULTS

Simulations demonstrated that the EGM method accurately identified TWA >or=10 microV. Overall, 10 (40%) patients had at least one ECG TWA+ test and 15 patients (60%) had no ECG TWA+ tests. The maximum value of TWA was greater in EGMs than in ECGs (median 64 microV vs 2.2 microV, P <.0001). EGM TWA was greater in ECG TWA+ tests than in ECG TWA- tests (169 +/- 175 microV vs 71 +/- 61 microV, P <.001). Using a sustained EGM TWA threshold of 30 microV, EGM TWA was concordant with ECG TWA in 63 (84%) of 75 analyzed tests (P <.0001) and predicted ECG TWA results with 85% sensitivity and 84% specificity. Both ECG and EGM TWA predicted VT/VF during follow-up (ECG: P = .006; EGM: P = .035).

CONCLUSION

The amplitude of TWA is at least 10 times greater on ICD EGMs than on surface ECGs. EGM and ECG TWA have substantial concordance and comparable predictive value for spontaneous VT/VF. These observations support the hypothesis that ECG and EGM TWA detect the same electrical alternans phenomenon.

Upsurge in T-Wave Alternans and Nonalternating Repolarization Instability Precedes Spontaneous Initiation of Ventricular Tachyarrhythmias in Humans

Background— Analysis of repolarization instability, manifested by T-wave alternans (TWA), has proved useful for arrhythmia risk assessment. However, temporal relations between TWA and the spontaneous initiation of ventricular tachyarrhythmias (VTA) in humans are unknown. We examined continuous dynamics of repolarization in Holter electrocardiograms with spontaneous sustained (>30 seconds) VTA.

Methods and Results— Ambulatory electrocardiograms from 42 patients (79% with ischemic heart disease; left ventricular ejection fraction, 37±15%) were digitized, and the lead with the highest magnitude of the T wave was selected for analysis. TWA was examined by the modified moving average and intrabeat average analyses. To examine non-TWA (longer-period) oscillations in the repolarization segment, spectral energy of oscillations of consecutive T-wave amplitudes was calculated with the use of the short-time Fourier transform. Heart rate variability was assessed with the Fourier transform as well. TWA increased before the onset of VTA and reached a peak value of 23.6±11.7 μV 10 minutes before the event ( P =0.0007). Spectral power of the oscillations of consecutive T-wave amplitudes increased nonuniformly, with the greatest increase in the respiratory range (2.6 μV 2 ; P =0.005). In the TWA range, the change was smaller but highly pronounced relative to the 60- to 120-minute level (65%; P =0.003). The low-frequency and high-frequency heart rate variability power declined before the arrhythmia ( P =0.04 and P =0.06, respectively).

Conclusions— The magnitude of repolarization instability, manifested by TWA and beat-to-beat oscillations of T-wave amplitudes at other frequencies, increased before the onset of VTA. Tracking of these dynamics can facilitate timely detection of high-risk periods and may be useful for initiation of preventive treatments.

Discordant repolarization alternans-induced atrial fibrillation is suppressed by verapamil

BACKGROUND

Ventricular alternans of repolarization produces serious ventricular arrhythmias in experimental models. The present study investigated the role of alternans of atrial repolarization in patients with atrial fibrillation (AF).

METHODS AND RESULTS

Electrophysiological studies were performed in 19 patients without structural heart disease. Monophasic action potentials (MAP) were recorded with 2 Franz catheters during steady state pacing, starting at a cycle length (CL) of 400 ms with subsequent decrements of 10 ms. Duration from the onset of upstroke to 90% repolarization of the MAP were measured. If discordant alternans (DA) was present during pacing, verapamil was administrated, and MAP measurements were repeated. Rapid pacing resulted in concordant alternans to DA in 13 of 19 (68%) patients. AF was initiated after the induction of DA in 8 of 13 patients (p=0.012). Verapamil treatment resulted in a significant decrease in the longest pacing CL at which DA was induced (207+/-19 vs 178+/-17 ms, p<0.0001).

CONCLUSIONS

Rapid atrial pacing induced DA and was associated with initiation of AF. Furthermore, induction of DA was suppressed by verapamil. Reducing the spatiotemporal repolarization heterogeneity may be how the calcium-channel blockade prevents initiation of AF.

DYNAMIC BEHAVIOR OF A PACED CARDIAC FIBER

Consider a typical experimental protocol in which one end of a one-dimensional fiber of cardiac tissue is periodically stimulated, or paced, resulting in a train of propagating action potentials. There is evidence that a sudden change in the pacing period can initiate abnormal cardiac rhythms. In this paper, we analyze how the fiber responds to such a change in a regime without arrhythmias. In particular, given a fiber length L and a tolerance eta, we estimate the number of beats N = N(eta, L) required for the fiber to achieve approximate steady-state in the sense that spatial variation in the diastolic interval (DI) is bounded by eta. We track spatial DI variation using an infinite sequence of linear integral equations which we derive from a standard kinematic model of wave propagation. The integral equations can be solved in terms of generalized Laguerre polynomials. We then estimate N by applying an asymptotic estimate for generalized Laguerre polynomials. We find that, for fiber lengths characteristic of cardiac tissue, it is often the case that N effectively exhibits no dependence on L. More exactly, (i) there is a critical fiber length L* such that, if L < L*, the convergence to steady-state is slowest at the pacing site, and (ii) often, L* is substantially larger than the diameter of the whole heart.