Why Is Only Type 1 Electrocardiogram Diagnostic of Brugada Syndrome? Mechanistic Insights From Computer Modeling

Long-term follow-up of patients with Brugada syndrome: Foremost risk factors associated with overall arrhythmic events

Brugada syndrome (BS) is characterized by ST segment elevation in right precordial leads (V1-V3), ventricular tachycardia (VT), ventricular fibrillation (VF), and sudden cardiac death (SCD) in individuals without structural heart disease. The aim of this study is to contribute to the controversial issue of finding the most valuable marker that can predict poor prognosis during follow-up in patients with a diagnosis of BS. A total of 68 patients diagnosed with BS or had Brugada-type ECG change between January 1997 and July 2012 at the Department of Cardiology of Başkent University Faculty of Medicine, Ankara, Turkey, were included in this cohort study. Patients were screened every 6 months for arrhythmia-related syncope, SCD, appropriate and inappropriate defibrillation (shock), AF development and death; collectively defined as "arrhythmic events" and were the primary endpoints. Patients with and without arrhythmic events were compared. The mean age was 34.9 ± 12.2 years (9-71 years), and 52 (76.5%) patients were male. Mean follow-up was 49.6 ± 37.6 months (4-188 months). Univariate analysis showed that male sex (P = .004), type 1 electrocardiographic pattern (P = .008), SCD (P = .036), VT/VF history (P = .046), requirement for electrophysiological studies (P = .034), implantable cardioverter-defibrillator placement (P = .014) were found to demonstrate significant differences in patients with and without arrhythmic events. In multivariable analyzes, spontaneous type 1 ECG presence (HR = 8.54, 95% CI: 0.38-26.37; P = .003) and VT/VF history (HR = 9.21, 95% CI: 0.004-1.88; P = .002) were found to be independently associated with arrhythmic events. We found the presence of spontaneous type 1 ECG and a history of VT/VF to be associated with increased likelihood of overall arrhythmic events in BS. Given the higher risk of poor prognosis, we recommend additional measures in patients with BS who have these features.

Intracellular ion accumulation in the genesis of complex action potential dynamics under cardiac diseases

Intracellular ions, including sodium (Na^{+}), calcium (Ca^{2+}), and potassium (K^{+}), etc., accumulate slowly after a change of the state of the heart, such as a change of the heart rate. The goal of this study is to understand the roles of slow ion accumulation in the genesis of cardiac memory and complex action-potential duration (APD) dynamics that can lead to lethal cardiac arrhythmias. We carry out numerical simulations of a detailed action potential model of ventricular myocytes under normal and diseased conditions, which exhibit memory effects and complex APD dynamics. We develop a low-dimensional iterated map (IM) model to describe the dynamics of Na^{+}, Ca^{2+}, and APD and use it to uncover the underlying dynamical mechanisms. The development of the IM model is informed by simulation results under the normal condition. We then use the IM model to perform linear stability analyses and computer simulations to investigate the bifurcations and complex APD dynamics, which depend on the feedback loops between APD and intracellular Ca^{2+} and Na^{+} concentrations and the steepness of the APD response to the ion concentrations. When the feedback between APD and Ca^{2+} concentration is positive, a Hopf bifurcation leading to periodic oscillatory behavior occurs as the steepness of the APD response to the ion concentrations increases. The negative feedback loop between APD and Na^{+} concentration is required for the Hopf bifurcation. When the feedback between APD and Ca^{2+} concentration is negative, period-doubling bifurcations leading to high periodicity and chaos occurs. In this case, Na^{+} accumulation plays little role in the dynamics. Finally, we carry out simulations of the detailed action potential model under two diseased conditions, which exhibit steep APD responses to ion concentrations. Under both conditions, Hopf bifurcations leading to slow oscillations or period-doubling bifurcations leading to high periodicity and chaotic APD dynamics occur, depending on the strength of the ion pump-Na^{+}-Ca^{2+} exchanger. Using functions reconstructed from the simulation data, the IM model accurately captures the bifurcations and dynamics under the two diseased conditions. In conclusion, besides using computer simulations of a detailed high-dimensional action-potential model to investigate the effects of slow ion accumulation and short-term memory on bifurcations and genesis of complex APD dynamics in cardiac myocytes under diseased conditions, this study also provides a low-dimensional mathematical tool, i.e., the IM model, to allow stability analyses for uncovering the underlying mechanisms.

Phase 2 Re-Entry Without Ito: Role of Sodium Channel Kinetics in Brugada Syndrome Arrhythmias

BACKGROUND

In Brugada syndrome (BrS), phase 2 re-excitation/re-entry (P2R) induced by the transient outward potassium current (Ito) is a proposed arrhythmia mechanism; yet, the most common genetic defects are loss-of-function sodium channel mutations.

OBJECTIVES

The authors used computer simulations to investigate how sodium channel dysfunction affects P2R-mediated arrhythmogenesis in the presence and absence of Ito.

METHODS

Computer simulations were carried out in 1-dimensional cables and 2-dimensional tissue using guinea pig and human ventricular action potential models.

RESULTS

In the presence of Ito sufficient to generate robust P2R, reducing sodium current (INa) peak amplitude alone only slightly potentiated P2R. When INa inactivation kinetics were also altered to simulate reported effects of BrS mutations and sodium channel blockers, however, P2R occurred even in the absence of Ito. These effects could be potentiated by delaying L-type calcium channel activation or increasing ATP-sensitive potassium current, consistent with experimental and clinical findings. INa-mediated P2R also accounted for sex-related, day and night-related, and fever-related differences in arrhythmia risk in BrS patients.

CONCLUSIONS

Altered INa kinetics synergize powerfully with reduced INa amplitude to promote P2R-induced arrhythmias in BrS in the absence of Ito, establishing a robust mechanistic link between altered INa kinetics and the P2R-mediated arrhythmia mechanism.

Brugada syndrome unmasked by dengue fever

Key Clinical Message

Understanding the circumstances, leading to unmasking of hidden Brugada syndrome is essential for the practicing clinician and the patients so that they are informed adequately to seek prompt medical attention.

Abstract

Brugada syndrome is a genetic arrhythmia syndrome characterized by a coved type of ST-segment elevation in the ECG. The patients are usually asymptomatic, with unmasking of the disease under certain conditions. We are reporting the case of a patient diagnosed with Brugada syndrome, which was unmasked during an attack of dengue fever.

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.

R-on-T and the initiation of reentry revisited: Integrating old and new concepts

Initiation of reentry requires 2 factors: (1) a triggering event, most commonly focal excitations such as premature ventricular complexes (PVCs); and (2) a vulnerable substrate with regional dispersion of refractoriness and/or excitability, such as occurs during the T wave of the electrocardiogram when some areas of the ventricle have repolarized and recovered excitability but others have not. When the R wave of a PVC coincides in time with the T wave of the previous beat, this timing can lead to unidirectional block and initiation of reentry, known as the R-on-T phenomenon. Classically, the PVC triggering reentry has been viewed as arising focally from 1 region and propagating into another region whose recovery is delayed, resulting in unidirectional conduction block and reentry initiation. However, more recent evidence indicates that PVCs also can arise from the T wave itself. In the latter case, the PVC initiating reentry is not a separate event from the T wave but rather is causally generated from the repolarization gradient that manifests as the T wave. We call the former an "R-to-T" mechanism and the latter an "R-from-T" mechanism, which are initiation mechanisms distinct from each other. Both are important components of the R-on-T phenomenon and need to be taken into account when designing antiarrhythmic strategies. Strategies targeting suppression of triggers alone or vulnerable substrate alone may be appropriate in some instances but not in others. Preventing R-from-T arrhythmias requires suppressing the underlying dynamic tissue instabilities responsible for producing both triggers and substrate vulnerability simultaneously. The same principles are likely to apply to supraventricular arrhythmias.