Multidimensional rhythm disturbances as a precursor of sustained ventricular tachyarrhythmias

Personalized ECG monitoring and adaptive machine learning

This non-technical review introduces key concepts in personalized ECG monitoring (pECG), which aims to optimize the detection of clinical events and their warning signs as well as the selection of alarm thresholds. We review several pECG methods, including anomaly detection and adaptive machine learning (ML), in which learning is performed sequentially as new data are collected. We describe a distributed-network multiscale pECG system to show how the computational load and time associated with adaptive ML could be optimized. In this architecture, the limited analysis of ECG waveforms is performed locally (e.g., on a smart phone) to determine a small number of clinically important ECG elements, and an adaptive ML engine is located on a remote server (Internet cloud) to determine an individual's "fingerprint" basis patterns and to detect anomalies in those patterns.

Magnetic Resonance Imaging of Contracting Ultrathin Cardiac Tissue

OBJECTIVE

Integrating cardiac-tissue patches into the beating heart and evaluating the long-term effects of such integration on cardiac contractility are two challenges in an emerging field of regenerative medicine. This pilot study presents tools for the imaging of contracting multicellular cardiac tissue constructs (MTCs) in vitro and demonstrates the feasibility of tracking the early development of strand geometry and contractions in ultrathin strands and layers of cardiac tissue using CINE MRI.

APPROACH

Cultured, ultrathin (~50-100-micron) MTCs of rat neonatal cardiomyocytes were plated in rectangular cell chambers (4.5 × 2.0 cm) with and without ultrathin, carbon EP electrodes embedded in the floor of the cell chamber. Two-dimensional, steady-state free precession (SSFP) CINE MRI, cell microscopy, and tissue photography were performed on Days 5-9 of cell development. Potential confounders and MRI artifacts were evaluated using non-contracting cardiac tissues and cell-free chambers filled with the cell-culture medium.

MAIN RESULTS

Synchronized contractions formed by Day 7; individual contracting tissue strands became identifiable by Day 9. The global patterns and details of the strand geometry and movement patterns in the SSFP images were in excellent agreement with microscopic and photographic images. No synchronized movement was identifiable by either microscopy or CINE MRI in the non-contracting MTCs or the cell-free medium. The EP recordings revealed well-defined depolarization and repolarization waveforms; the imaging artifacts generated by the carbon electrodes were small.

SIGNIFICANCE

This pilot study demonstrates the feasibility of imaging cardiac-strand patterns and contractile activity in ultrathin, two-dimensional cardiac tissue in commonly used clinical scanners.

Cardiac autonomic modulation and long-term use of amiodarone in patients with chronic Chagasic cardiopathy

BACKGROUND

Patients with chronic Chagas cardiopathy (CCC), which may be associated with cardiac arrhythmias, frequently use amiodarone, an antiarrhythmic drug that, experimentally, appears to modulate the cardiac autonomic function.

OBJECTIVE

The present cross-sectional observational study aimed to evaluate autonomic cardiac modulation in patients with CCC undergoing chronic amiodarone therapy.

METHODS

Three groups were investigated: Group 1 included patients with CCC not treated with amiodarone (n = 27); Group 2 included patients with CCC with prolonged use (at least 6 months) of amiodarone (n = 16); and Group 3 included non-Chagasic control patients (n = 23). All patients underwent a complete clinical and laboratory assessment, followed by autonomic function tests, consisting of a basal continuous electrocardiogram in the resting supine position for 10 minutes, followed by a change the orthostatic posture for a further 5 minutes. Heart rate variability (HRV) parameters (median and interquartile interval) were quantified using linear methods in the time- and frequency-domains (autoregressive spectral analysis) and nonlinear methods, including symbolic analysis.

RESULTS

Patients with CCC using amiodarone had changes in HRV suggestive of an offset in the sympatho-vagal balance with a vagal modulation predominance (normalized HF, 49.7[27.4] vs 31.1[22.8] [P < 0.05]; and percentage 2V, 40.1 [14.6] vs 21.5 [13.4] [P < 0.05] vs untreated CCC group). These changes were further accompanied by increases in parameters indicative of greater complexity of HRV.

CONCLUSIONS

The deviation in the sympatho-vagal balance and the increase in the complexity of HRV strongly suggest that amiodarone may have a cardioprotective effect, in addition to its antiarrhythmic effects, which could increase the survival of these patients.

Sudden Cardiac Death from the Perspective of Nonlinear Dynamics

There are some reports that sympathetic nerve activity (SNA) shows a characteristic pattern a few hours before the onset of lethal ventricular arrhythmias. If so, it could be possible to predict sudden cardiac death a few hours in advance of its occurrence. Recently, we reported that a previously unidentified V-trough of SNA is a potential precursor of lethal cardiac events by examining 24-hour ambulatory electrocardiograms in which such an event was recorded by chance. In contrast, the chaotic nature of heart rate variability has been noted recently from the viewpoint of nonlinear dynamics. This study models the hemodynamics, consisting of heart rate, SNA, and blood pressure (BP), by modifying a known chaotic electrical circuit, the Chua circuit. A V-trough of the SNA appears when the resistive element between the SNA and BP in the circuit is increased, which corresponds to the impaired regulation of BP by the SNA. This finding is consistent with an acknowledged finding that a depressed baroreflex (a reflex of the BP by SNA) may trigger a lethal arrhythmia. This study indicates that a V-trough of the SNA is a possible precursor of sudden cardiac death on the basis of experimental and clinical findings as well as mathematical modeling.

Increasing sensitivity in the measurement of heart rate variability: the method of non-stationary RR time-frequency analysis

A novel method of the time-frequency analysis of non-stationary heart rate variability (HRV) is developed which introduces the fragmentary spectrum as a measure that brings together the frequency content, timing and duration of HRV segments. The fragmentary spectrum is calculated by the similar basis function algorithm. This numerical tool of the time to frequency and frequency to time Fourier transformations accepts both uniform and non-uniform sampling intervals, and is applicable to signal segments of arbitrary length. Once the fragmentary spectrum is calculated, the inverse transform recovers the original signal and reveals accuracy of spectral estimates. Numerical experiments show that discontinuities at the boundaries of the succession of inter-beat intervals can cause unacceptable distortions of the spectral estimates. We have developed a measure that we call the "RR deltagram" as a form of the HRV data that minimises spectral errors. The analysis of the experimental HRV data from real-life and controlled breathing conditions suggests transient oscillatory components as functionally meaningful elements of highly complex and irregular patterns of HRV.

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.

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.

Orthonormal-Basis Partitioning and Time-Frequency Representation of Cardiac Rhythm Dynamics

Although a number of time-frequency representations have been proposed for the estimation of time-dependent spectra, the time-frequency analysis of multicomponent physiological signals, such as beat-to-beat variations of cardiac rhythm or heart rate variability (HRV), is difficult. We thus propose a simple method for 1) detecting both abrupt and slow changes in the structure of the HRV signal, 2) segmenting the nonstationary signal into the less nonstationary portions, and 3) exposing characteristic patterns of the changes in the time-frequency plane. The method, referred to as orthonormal-basis partitioning and time-frequency representation (OPTR), is validated using simulated signals and actual HRV data. Here we show that OPTR can be applied to long multicomponent ambulatory signals to obtain the signal representation along with its time-varying spectrum.

Forecasting epilepsy from the heart rate signal

Information contained in the R-R interval series, specific to the pre-ictal period, was sought by applying an unsupervised fuzzy clustering algorithm to the N-dimensional phase space of N consecutive interval durations or the absolute value of duration differences. Data sources were individual, complex partial seizures of temporal-lobe epileptics and generalised seizures of rats rendered epileptic with hyperbaric oxygen. Forecasting success was 86% and 82% (zero false positives in resistant rats), respectively, at times ranging from 10 min to 30 s prior to seizure onset Although certain forecasting clusters predominated in the patient group and different ones predominated in the animal group, forecasting on the whole was seizure-specific. The high prediction sensitivity of this method, which matches that of EEG-based methods, seems promising. It is believed that an on-line version of the algorithm, trained on each patient's peri-ictal ECG, could serve as a basis for a simple seizure alarm system.

Incidence of nonsustained and sustained ventricular tachyarrhythmias in patients with an implantable cardioverter defibrillator

INTRODUCTION

Nonsustained ventricular tachycardia (NSVT) is a frequent phenomenon in some patients with heart disease, but its association with sustained ventricular tachycardias (ventricular tachycardia [VT]/ventricular fibrillation [VF]) is still not clear. The aim of this study was to determine whether NSVT incidence was associated with sustained VT/VF in patients with an implantable cardioverter defibrillator (ICD).

METHODS AND RESULTS

Retrospective data analysis was conducted in 923 ICD patients with a mean follow-up of 4 months. NSVT and sustained VT/VF were defined as device-detected tachycardias. The incidence rates of NSVT and sustained VT/VF as well as ICD therapies were determined as episodes per patient. The NSVT index was defined as the product of NSVT episodes/day times the mean number of beats per episode, i.e., total beats/day. The NSVT index peak was defined as the highest value on or prior to the day with sustained VT/VF episodes. Patients (n = 393) with NSVT experienced a higher incidence of sustained VT/VF (17.2 +/- 63.0 episodes/patient) and ICD therapies (15.2 +/- 61.4 episodes/patient) than patients (n = 530) without NSVT (sustained VT/VF: 0.5 +/- 6.6 and therapies: 0.5 +/- 5.6; P < 0.0001). Approximately 74% of NSVT index peaks occurred on the same day or <3 days prior to sustained VT/VF episodes. The index was higher for peaks < or =3 days prior to the day with sustained VT/VF (94.3 +/- 140.1 total beats/day) than for peaks >3 days prior to the day with sustained VT/VF (32.7 +/- 55.9 total beats/day; P < 0.0001).

CONCLUSION

ICD patients with NSVT represent a population more likely to experience sustained VT/VF episodes with a temporal association between an NSVT surge and sustained VT/VF occurrence.

Detecting instabilities of cardiac rhythm

Diminished beat-to-beat variations in cardiac cycle lengths (CLs) are associated with poor prognosis after acute myocardial infarction and in patients with heart failure. Short-long-short sequences of cardiac cycles, or ultra-short rhythm instabilities, precede initiation of ventricular tachyarrhythmias in some patients. However, little is known about clinical or prognostic significance of abrupt short-term instabilities in CL (AICL) that occur minutes to hours before the event, in part because appropriate analytical methods are lacking. Although various techniques have been used to analyze CL changes, methods for analysis of AICL are limited. We compared performance of time domain, spectral, nonlinear, and pattern recognition techniques with respect to the detection and quantification of AICL. Because of high intra- and inter-subject variability of CL, pattern recognition techniques compared favorably to other studied methods. In continuous ambulatory ECG recordings, AICL occurred hours before spontaneous initiation of sustained atrial and ventricular arrhythmias in different patient populations. AICL were also found prior to the onset of spontaneous ventricular arrhythmias in a mouse model of congestive heart failure. To quantify AICL, we used the number of unstable orthogonal projection coefficients; this number gradually increased hours before the event. Removal of ectopic beats reduced but did not eliminate AICL. To illustrate potential physiological effects and temporal evolution of AICL, we used a simple, continuous, two-dimensional model of cardiac tissue governed by the Morris-Lecar equations. Computer simulations in this model showed that AICL may lead to gradual accumulation of spatial irregularities of the propagation wavefront giving rise to the initiation of reentry. Time-frequency analysis of the most significant eigenvectors of cardiac rhythm in subjects undergoing head-up tilt showed that AICL could indicate instabilities and unsuccessful adaptation of autonomic nervous system activity to physiological stimuli.

Study of precursors of ventricular tachycardia from data stored in the memory of a dual chamber implantable cardioverter defibrillator

This study was performed to examine precursors of ventricular tachyarrhythmias in patients who experienced a sustained ventricular tachyarrhythmia and received appropriate therapy by ICD. From an overall consecutive population of 77 patients, 18 patients (1 woman, mean age 61.7 +/- 10.8 years) were selected for having experienced a sustained ventricular tachyarrhythmia and received at least one appropriate ICD therapy preceded by 20 minutes of internal information. The number of premature ventricular complexes (PVCs)/min for each of the 20 minutes preceding the onset of ventricular tachyarrhythmia, the shortest coupling intervals between PVC and normal sinus beat, and the presence of short-long-short (SLS) interval sequences were examined. Data were stratified according to underlying disease, left ventricular ejection fraction, rate of ventricular tachyarrhythmia, and antiarrhythmic therapy. One hundred twenty-eight episodes of spontaneous ventricular tachyarrhythmia were retrieved. Rapid ventricular tachyarrhythmia (>160 beats/min) were preceded by a significantly greater mean number (3.71 +/- 6.36)of PVCs than slower ventricular tachyarrhythmia (<or=160 beats/min) (0.63 +/- 0.88, P = 0.0004). The mean shortest PVC coupling interval was significantly shorter in patients with (588 +/- 99 ms) versus without (643 +/- 111 ms, P = 0.03)ischemic heart disease, before episodes of rapid(527 +/- 55 ms)versus slower (636 +/- 105 ms, P = 0.0001)ventricular tachyarrhythmia, and in the absence (538 +/- 80 ms)versus the presence(620 +/- 105 ms, P = 0.006)of amiodarone. SLS sequences preceded 29% of rapid ventricular tachyarrhythmic episodes, versus 8% of the slower ventricular tachyarrhythmia (P < 0.01). Significant differences were found in the characteristics of PVCs preceding ventricular tachyarrhythmic episodes in accordance to their rate and the underlying cardiomyopathy. Though insufficient in isolation, these findings may be helpful when combined with other observations to develop preventive algorithms, or to refine the programming of implantable devices.

Sympathetic nervous system activity and ventricular tachyarrhythmias: recent advances

Sympathetic nervous system activity (SNSA) is believed to participate in the genesis of ventricular tachyarrhythmias (VTA) but understanding has been impeded by the number and complexity of effects and the paucity of data from humans. New information from studies of genetic disorders, animal models, and spontaneous human arrhythmias indicates the importance of the temporal pattern of SNSA in arrhythmia development. The proarrhythmic effects of short-term elevations of SNSA are exemplified by genetic disorders and include enhancement of early and delayed afterdepolarizations and increased dispersion of repolarization. The role of long-term elevations of SNSA is suggested by animal models of enhanced SNSA signaling that results in apoptosis, hypertrophy, and fibrosis, and sympathetic nerve sprouting caused by infusion of nerve growth factor. Processes that overlap short- and long-term effects are suggested by changes in R-R interval variability (RRV) that precede VTA in patients by several hours. SNSA-mediated alterations in gene expression of ion channels may account for some intermediate-term effects. The propensity for VTA is highest when short-, intermediate, and long-term changes are superimposed. Because the proarrhythmic effects are related to the duration and intensity of SNSA, normal regulatory processes such as parasympathetic activity that inhibits SNSA, and oscillations that continuously vary the intensity of SNSA may provide vital antiarrhythmic protection that is lost in severe heart failure and other disorders. These observations may have therapeutic implications. The recommended use of beta-adrenergic receptor blockers to achieve a constant level of inhibition does not take into account the temporal patterns and regional heterogeneity of SNSA, the proarrhythmic effects of alpha-adrenergic receptor stimulation, or the potential proarrhythmic effects of beta-adrenergic receptor blockade. Further research is needed to determine if other approaches to SNSA modulation can enhance the antiarrhythmic effects.

Strain-specific patterns of autonomic nervous system activity and heart failure susceptibility in mice

Transgenic mice are widely used to study cardiac function, but strain-dependent differences in autonomic nervous system activity (ANSA) have not been explored. We compared 1) short-term pharmacological responses of cardiac rhythm in FVB vs. C57Black6/SV129 wild-type mice and 2) long-term physiological dynamics of cardiac rhythm and survival in tumor necrosis factor (TNF)-alpha transgenic mice with heart failure (TNF-alpha mice) on defined backgrounds. Ambulatory telemetry electrocardiographic recordings and response to saline, adrenergic, and cholinergic agents were examined in FVB and C57Black6/SV129 mice. In FVB mice, baseline heart rate (HR) was higher and did not change after injection of isoproterenol or atropine but decreased with propranolol. In C57Black6/SV129 mice, HR did not change with propranolol but increased with isoproterenol or atropine. Mean HR, but not indexes of HR variability, was an excellent predictor of response to autonomic agents. The proportion of surviving animals was higher in TNF-alpha mice on an FVB background than on a mixed FVB/C57Black6 background. The homeostatic states of ANSA are strain specific, which can explain the interstrain differences in mean HR, pharmacological responses, and survival of animals with congestive heart failure. Strain-specific differences should be considered in selecting the strains of mice used for transgenic and gene targeting experiments.