Repolarization Versus Depolarization Defects in Brugada Syndrome

Genetic and Molecular Mechanisms in Brugada Syndrome

Brugada syndrome is a rare hereditary arrhythmia disorder characterized by a distinctive electrocardiogram pattern and an elevated risk of ventricular arrhythmias and sudden cardiac death in young adults. Despite recent advances, it remains a complex condition, encompassing mechanisms, genetics, diagnosis, arrhythmia risk stratification, and management. The underlying electrophysiological mechanism of Brugada syndrome requires further investigation, with current theories focusing on abnormalities in repolarization, depolarization, and current-load match. The genetic basis of the syndrome is strong, with mutations found in genes encoding subunits of cardiac sodium, potassium, and calcium channels, as well as genes involved in channel trafficking and regulation. While the initial discovery of mutations in the SCN5A gene provided valuable insights, Brugada syndrome is now recognized as a multifactorial disease influenced by several loci and environmental factors, challenging the traditional autosomal dominant inheritance model. This comprehensive review aims to provide a current understanding of Brugada syndrome, focusing on its pathophysiology, genetic mechanisms, and novel models of risk stratification. Advancements in these areas hold the potential to facilitate earlier diagnosis, improve risk assessments, and enable more targeted therapeutic interventions.

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.

Clinical significance of J waves with respect to substrate characteristics and ablation outcomes in patients with arrhythmogenic right ventricular cardiomyopathy

AIMS

J-wave syndrome in patients with arrhythmogenic right ventricular cardiomyopathy (ARVC) has been linked to an increased risk of ventricular arrhythmia. We investigated the significance of J waves with respect to substrate manifestations and ablation outcomes in patients with ARVC.

METHODS AND RESULTS

Forty-five patients with ARVC undergoing endocardial/epicardial mapping/ablation were studied. Patients were classified into two groups: 13 (28.9%) and 32 (71.1%) patients with and without J waves, respectively. The baseline characteristics, electrophysiological features, ventricular substrate, and recurrent ventricular tachycardia/fibrillation (VT/VF) were compared. Among the 13 patients with J waves, only the inferior J wave was observed. More ARVC patients with J waves fulfilled the major criteria of ventricular arrhythmias (76.9% vs. 21.9%, P = 0.003). Similar endocardial and epicardial substrate characteristics were observed between the two groups. However, patients with J waves had longer epicardial total activation time than those without (224.7 ± 29.9 vs. 200.8 ± 21.9 ms, P = 0.005). Concordance of latest endo/epicardial activation sites was observed in 29 (90.6%) patients without J waves and in none among those with J waves (P < 0.001). Complete elimination of endocardial/epicardial abnormal potentials resulted in the disappearance of the J wave in 8 of 13 (61.5%) patients. The VT/VF recurrences were not different between ARVC patients with and without J waves.

CONCLUSION

The presence of J waves was associated with the discordance of endocardial/epicardial activation pattern in terms of transmural depolarization discrepancy in patients with ARVC.

Mechanisms of Arrhythmias in the Brugada Syndrome

Arrhythmias in Brugada syndrome patients originate in the right ventricular outflow tract (RVOT). Over the past few decades, the characterization of the unique anatomy and electrophysiology of the RVOT has revealed the arrhythmogenic nature of this region. However, the mechanisms that drive arrhythmias in Brugada syndrome patients remain debated as well as the exact site of their occurrence in the RVOT. Identifying the site of origin and mechanism of Brugada syndrome would greatly benefit the development of mechanism-driven treatment strategies.

Mapping and Ablation of Ventricular Fibrillation Associated With Early Repolarization Syndrome

BACKGROUND

We conducted a multicenter study to evaluate mapping and ablation of ventricular fibrillation (VF) substrates or VF triggers in early repolarization syndromes (ERS) or J-wave syndrome (JWS).

METHODS

We studied 52 patients with ERS (4 women; median age, 35 years) with recurrent VF episodes. Body surface electrocardiographic imaging and endocardial and epicardial electroanatomical mapping of both ventricles were performed during sinus rhythm and VF for localization of triggers, substrates, and drivers. Ablations were performed on VF substrates, defined as areas that had late depolarization abnormalities characterized by low-voltage fractionated late potentials, and VF triggers.

RESULTS

Fifty-one of the 52 patients had detailed mapping that revealed 2 phenotypes: group 1 had late depolarization abnormalities predominantly at the right ventricular (RV) epicardium (n=40), and group 2 had no depolarization abnormalities (n=11). Group 1 can be subcategorized into 2 groups: Group 1A included 33 patients with ERS with Brugada electrocardiographic pattern, and group 1B included 7 patients with ERS without Brugada electrocardiographic pattern. Late depolarization areas colocalize with VF driver areas. The anterior RV outflow tract/RV epicardium and the RV inferior epicardium are the major substrate sites for group 1. The Purkinje network is the leading underlying VF trigger in group 2 that had no substrates. Ablations were performed in 43 patients: 31 and 5 group 1 patients had only VF substrate ablation and VF substrates plus VF trigger, respectively (mean, 1.4±0.6 sessions); 6 group 2 patients and 1 patient without group classification had only Purkinje VF trigger ablation (mean, 1.2±0.4 sessions). Ablations were successful in reducing VF recurrences (P<0.0001). After follow-up of 31±26 months, 39 (91%) had no VF recurrences.

CONCLUSIONS

There are 2 phenotypes of ERS/J-wave syndrome: one with late depolarization abnormality as the underlying mechanism of high-amplitude J-wave elevation that predominantly resides in the RV outflow tract and RV inferolateral epicardium, serving as an excellent target for ablation, and the other with pure ERS devoid of VF substrates but with VF triggers that are associated with Purkinje sites. Ablation is effective in treating symptomatic patients with ERS/J-wave syndrome with frequent VF episodes.

Clinical Differences in Japanese Patients Between Brugada Syndrome and Arrhythmogenic Right Ventricular Cardiomyopathy With Long-Term Follow-Up

Some Brugada syndrome (BrS) patients have been suspected of being in the initial state of arrhythmogenic right ventricular cardiomyopathy (ARVC). This study aimed to clarify the electrocardiographic (ECG) and clinical differences between BrS and ARVC in long-term follow-up (mean 11.9 ± 6.3 years). A total of 50 BrS and 65 ARVC patients with fatal ventricular tachyarrhythmia (VTA) were evaluated according to the revised Task Force Criteria for ARVC. Based on the current diagnostic criteria concerning electrocardiographic, repolarization abnormality was positive in 2.0% and 2.6% of BrS patients at baseline and follow-up, and depolarization abnormality was positive in 6.0% and 12.8% of BrS patients at baseline and follow-up, respectively. At baseline, none of the BrS patients were definitively diagnosed with ARVC. Considering patients' lives since birth, Kaplan-Meier analysis revealed that age at first VTA attack showed the same tendency between the groups (BrS: mean 42.2 ± 12.5 years old vs ARVC: mean 44.8 ± 13.7 years old, log-rank p = 0.123). Moreover, the incidence of VTA recurrence was similar between the groups during follow-up (log-rank p = 0.906). Incidence of sustained monomorphic ventricular tachycardia was significantly higher in ARVC than in BrS whereas the opposite was true for ventricular fibrillation (log-rank p <0.001 and p <0.001, respectively). None of the diagnoses of BrS patients were changed to ARVC during follow-up. During long-term follow-up, although age at first VTA attack and VTA recurrence were similar, BrS consistently exhibited features that differed from those of ARVC.

Higher Dispersion Measures of Conduction and Repolarization in Type 1 Compared to Non-type 1 Brugada Syndrome Patients: An Electrocardiographic Study From a Single Center

Background: Brugada syndrome (BrS) is a cardiac ion channelopathy that predisposes affected individuals to sudden cardiac death (SCD). Type 1 BrS is thought to take a more malignant clinical course than non-type 1 BrS. We hypothesized that the degrees of abnormal repolarization and conduction are greater in type 1 subjects and these differences can be detected by electrocardiography (ECG). Methods: Electrocardiographic data from spontaneous type 1 and non-type 1 BrS patients were analyzed. ECG parameters were measured from leads V1 to V3. Values were expressed as median [lower quartile-upper quartile] and compared using Kruskal-Wallis ANOVA. Results: Compared to non-type 1 BrS patients (n = 29), patients with spontaneous type 1 patterns (n = 22) showed similar (P > 0.05) heart rate (73 [64-77] vs. 68 [62-80] bpm), QRS duration (136 [124-161] vs. 127 [117-144] ms), uncorrected QT (418 [393-443] vs. 402 [386-424] ms) and corrected QT intervals (457 [414-474] vs. 430 [417-457] ms), JTpeak intervals (174 [144-183] vs. 174 [150-188] ms), Tpeak- Tend intervals (101 [93-120] vs. 99 [90-105] ms), Tpeak- Tend/QT ratios (0.25 [0.23-0.27] vs. 0.24 [0.22-0.27]), Tpeak- Tend/QRS (0.77 [0.62-0.87] vs. 0.77 [0.69-0.86]), Tpeak- Tend/(QRS × QT) (0.00074 [0.00034-0.00096] vs. 0.00073 [0.00048-0.00012] ms-1), index of Cardiac Electrophysiological Balance (iCEB, QT/QRS, marker of wavelength: 3.14 [2.56-3.35] vs. 3.21 [2.85-3.46]) and corrected iCEB (QTc/QRS: 3.25 [2.91-3.73] vs. 3.49 [2.99-3.78]). Higher QRS dispersion was seen in type 1 subjects (QRSd: 34 [24-66] vs. 24 [12-34] ms) but QT dispersion (QTd: 48 [39-71] vs. 43 [22-94] ms), QTc dispersion (QTcd: 52 [41-79] vs. 46 [23-104] ms), JTpeak dispersion (44 [23-62] vs. 45 [30-62] ms), Tpeak- Tend dispersion (28 [15-34] vs. 29 [22-53] ms) or Tpeak- Tend/QT dispersion (0.06 [0.03-0.08] vs. 0.08 [0.04-0.12]) did not differ between the two groups. Type 1 subjects showed higher (QRSd × Tpeak- Tend)/QRS (25 [19-44] vs. 19 [9-30] ms) but similar iCEB dispersion (0.83 [0.49-1.14] vs. 0.61 [0.34-0.92]) and iCEBc dispersion (0.93 [0.51-1.15] vs. 0.65 [0.39-0.96]). Conclusion: Higher levels of dispersion in conduction and repolarization are found in type 1 than non-type 1 BrS patients, potentially explaining the higher incidence of ventricular arrhythmias in the former group.