Relationships Between Depolarization Abnormality and Repolarization Abnormality in Patients with Brugada Syndrome:

Zebrafish as a Model System for Brugada Syndrome

Brugada syndrome (BrS) is an inheritable cardiac arrhythmogenic disease, associated with an increased risk of sudden cardiac death. It is most common in males around the age of 40 and the prevalence is higher in Asia than in Europe and the United States. The pathophysiology underlying BrS is not completely understood, but several hypotheses have been proposed. So far, the best effective treatment is the implantation of an implantable cardioverter-defibrillator (ICD), but device-related complications are not uncommon. Therefore, there is an urgent need to improve diagnosis and risk stratification and to find new treatment options. To this end, research should further elucidate the genetic basis and pathophysiological mechanisms of BrS. Several experimental models are being used to gain insight into these aspects. The zebrafish (Danio rerio) is a widely used animal model for the study of cardiac arrhythmias, as its cardiac electrophysiology shows interesting similarities to humans. However, zebrafish have only been used in a limited number of studies on BrS, and the potential role of zebrafish in studying the mechanisms of BrS has not been reviewed. Therefore, the present review aims to evaluate zebrafish as an animal model for BrS. We conclude that zebrafish can be considered as a valuable experimental model for BrS research, not only for gene editing technologies, but also for screening potential BrS drugs.

Brugada Syndrome

Brugada syndrome (BrS) is an "inherited" condition characterized by predisposition to syncope and cardiac arrest, predominantly during sleep. The prevalence is ∼1:2,000, and is more commonly diagnosed in young to middle-aged males, although patient sex does not appear to impact prognosis. Despite the perception of BrS being an inherited arrhythmia syndrome, most cases are not associated with a single causative gene variant. Electrocardiogram (ECG) findings support variable extent of depolarization and repolarization changes, with coved ST-segment elevation ≥2 mm and a negative T-wave in the right precordial leads. These ECG changes are often intermittent, and may be provoked by fever or sodium channel blocker challenge. Growing evidence from cardiac imaging, epicardial ablation, and pathology studies suggests the presence of an epicardial arrhythmic substrate within the right ventricular outflow tract. Risk stratification aims to identify those who are at increased risk of sudden cardiac death, with well-established factors being the presence of spontaneous ECG changes and a history of cardiac arrest or cardiogenic syncope. Current management involves conservative measures in asymptomatic patients, including fever management and drug avoidance. Symptomatic patients typically undergo implantable cardioverter defibrillator insertion, with quinidine and epicardial ablation used for patients with recurrent arrhythmia. This review summarizes our current understanding of BrS and provides clinicians with a practical approach to diagnosis and management.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Arrhythmic substrate, slowed propagation and increased dispersion in conduction direction in the right ventricular outflow tract of murine Scn5a+/- hearts

Aim

To test a hypothesis attributing arrhythmia in Brugada Syndrome to right ventricular (RV) outflow tract (RVOT) conduction abnormalities arising from Na_v1.5 insufficiency and fibrotic change.

Methods

Arrhythmic properties of Langendorff-perfused Scn5a+/− and wild-type mouse hearts were correlated with ventricular effective refractory periods (VERPs), multi-electrode array (MEA) measurements of action potential (AP) conduction velocities and dispersions in conduction direction (CD), Na_v1.5 expression levels, and fibrotic change, as measured at the RVOT and RV. Two-way anova was used to test for both independent and interacting effects of anatomical region and genotype on these parameters.

Results

Scn5a+/− hearts showed greater arrhythmic frequencies during programmed electrical stimulation at the RVOT but not the RV. The Scn5a+/− genotype caused an independent increase of VERP regardless of whether the recording site was the RVOT or RV. Effective AP conduction velocities (CV†s), derived from fitting regression planes to arrays of observed local activation times were reduced in Scn5a+/− hearts and at the RVOT independently. AP conduction velocity magnitudes derived by averaging MEA results from local vector analyses, CV*, were reduced by the Scn5a+/− genotype alone. In contrast, dispersions in conduction direction, were greater in the RVOT than the RV, when the atrioventricular node was used as the pacing site. The observed reductions in Na_v1.5 expression were attributable to Scn5a+/−, whereas increased levels of fibrosis were associated with the RVOT.

Conclusions

The Scn5a+/− RVOT recapitulates clinical findings of increased arrhythmogenicity through reduced CV† reflecting reduced CV* attributable to reduced Na_v1.5 expression and increased CD attributable to fibrosis.

Brugada phenocopy: new terminology and proposed classification

Brugada syndrome is a channelopathy characterized on ECG by coved ST-segment elevation (≥2 mm) in the right precordial leads and is associated with an increased risk of malignant ventricular arrhythmias. The term Brugada phenocopy is proposed to describe conditions that induce Brugada-like ECG manifestations in patients without true Brugada syndrome. An extensive review of the literature identified case reports that were classified according to their suspected etiological mechanism. Future directions to learn more about these intriguing cases is discussed.

Spatial and temporal heterogeneities are localized to the right ventricular outflow tract in a heterozygotic Scn5a mouse model

Ventricular tachycardia (VT) in Brugada Syndrome patients often originates in the right ventricular outflow tract (RVOT). We explore the physiological basis for this observation using murine whole heart preparations. Ventricular bipolar electrograms and monophasic action potentials were recorded from seven epicardial positions in Langendorff-perfused wild-type and Scn5a+/− hearts. VT first appeared in the RVOT, implicating it as an arrhythmogenic focus in Scn5a+/− hearts. RVOTs showed the greatest heterogeneity in refractory periods, response latencies, and action potential durations, and the most fractionated electrograms. However, incidences of concordant alternans in dynamic pacing protocol recordings were unaffected by the Scn5a+/− mutation or pharmacological intervention. Conversely, particularly at the RVOT, Scn5a+/− hearts showed earlier and more frequent transitions into discordant alternans. This was accentuated by flecainide, but reduced by quinidine, in parallel with their respective pro- and anti-arrhythmic effects. Discordant alternans preceded all episodes of VT. The RVOT of Scn5a+/− hearts also showed steeper restitution curves, with the diastolic interval at which the gradient equaled one strongly correlating with the diastolic interval at which discordant alternans commenced. We attribute the arrhythmic tendency within the RVOT to the greater spatial heterogeneities in baseline electrophysiological properties. These, in turn, give rise to a tendency to drive concordant alternans phenomena into an arrhythmogenic discordant alternans. Our findings may contribute to future work investigating possible pharmacological treatments for a disease in which the current mainstay of treatment is implantable cardioverter defibrillator implantation.

The pathophysiological mechanism underlying Brugada syndrome: depolarization versus repolarization

This Point/Counterpoint presents a scholarly debate of the mechanisms underlying the electrocardiographic and arrhythmic manifestations of Brugada syndrome (BrS), exploring in detail the available evidence in support of the repolarization vs. depolarization hypothesis.

Number of electrocardiogram leads displaying the diagnostic coved-type pattern in Brugada syndrome: a diagnostic consensus criterion to be revised

AIMS

According to the diagnostic consensus criteria, the electrocardiographic (ECG) diagnosis of Brugada syndrome requires coved-type > or =2 mm ST-segment elevation in >1 right precordial lead (RPL) V1-V3 in the presence or absence of a sodium-channel blocker. However, this consensus has not been evaluated. We aimed to assess the distribution of coved-type ST-segment elevation on RPLs in a large patient cohort to reevaluate the appropriateness of the diagnostic consensus criteria.

METHODS AND RESULTS

We included 186 individuals with spontaneous and/or drug-induced ECGs of coved-type > or =2 mm ST-segment elevation in at least one RPL. A total of 376 ECGs were analysed for the number, distribution and maximal J-point elevation of diagnostic RPLs. Among all ECGs, 27 (7%) showed a coved-type pattern in 3 RPLs, 205 (55%) in 2 RPLs, and 144 (38%) in only 1 RPL. Leads V1 and V2 were diagnostic in 99% of all ECGs with two diagnostic RPLs. Lead V3 alone was not diagnostic in any ECG. Maximal J-point elevation was significantly higher in lead V2 than V1. Sixty case subjects (32%) had only ECGs with one RPL displaying a coved-type ST-segment elevation. There was no significant difference in clinical presentation and outcome compared with the 126 Brugada patients with ECGs displaying >1 diagnostic RPL. Major arrhythmic events occurred with the same rate (8%) in both groups during a follow-up >5 years.

CONCLUSION

Lead V3 does not yield diagnostic information in Brugada syndrome. Individuals with ECGs displaying only one diagnostic RPL have a similar clinical profile and arrhythmic risk as Brugada patients with ECGs displaying >1 diagnostic RPL. Revision of the consensus criteria should be considered.

Fragmented QRS and other depolarization abnormalities as a predictor of mortality and sudden cardiac death

PURPOSE OF REVIEW

Several invasive and noninvasive tests for risk stratification of sudden cardiac death (SCD) have been studied. Tests such as microwave T wave alternans (repolarization abnormality) and signal-averaged ECG (depolarization abnormality) have high negative predictive values but low positive predictive values in patients with heart disease. The presence of a fragmented QRS (fQRS) complex on a routine 12-lead ECG is another marker of depolarization abnormality. The purpose of this review is to discuss the potential utility of tests to detect depolarization abnormalities of the heart for the risk stratification of mortality and SCD with main emphasis on fQRS.

RECENT FINDINGS

fQRS is associated with increased mortality and arrhythmic events in patients with coronary artery disease. fQRS has also been defined as a marker of arrhythmogenic right ventricular cardiomyopathy and Brugada syndrome. In Brugada syndrome, the presence of fQRS predicts episodes of ventricular fibrillation during follow-up.

SUMMARY

fQRS may be of value in determining the risk for SCD and guiding selection for device therapy in patients with structural heart disease and Brugada syndrome. It is possible that the predictive value of fQRS for SCD can be enhanced further by combining a marker of repolarization abnormality such as microwave T wave alternans.

Abnormal restitution property of action potential duration and conduction delay in Brugada syndrome: both repolarization and depolarization abnormalities

AIMS

This study sought to examine the action potential duration restitution (APDR) property and conduction delay in Brugada syndrome (BrS) patients. A steeply sloped APDR curve and conduction delay are known to be important determinants for the occurrence of ventricular fibrillation (VF).

METHODS AND RESULTS

Endocardial monophasic action potential was obtained from 39 BrS patients and 9 control subjects using the contact electrode method. Maximum slopes of the APDR curve were obtained at both the right ventricular outflow tract (RVOT) and the right ventricular apex (RVA). The onset of activation delay (OAD) after premature stimulation was examined as a marker of conduction delay. Maximum slope of the APDR curve in BrS patients was significantly steeper than that in control subjects at both the RVOT and the RVA (0.77 +/- 0.21 vs. 058 +/- 0.14 at RVOT, P = 0.009; 0.98 +/- 0.23 vs. 0.62 +/- 0.16 at RVA, P = 0.001). The dispersion of maximum slope of the APDR curve between the RVOT and the RVA was also larger in BrS patients than in control subjects. The OAD was significantly longer in BrS patients than in control subjects from the RVOT to RVA and from the RVA to RVOT (from RVOT to RVA: 256 +/- 12 vs. 243 +/- 7 ms, P = 0.003; from RVA to RVOT: 252 +/- 11 vs. 241 +/- 9 ms, P = 0.01).

CONCLUSIONS

Abnormal APDR properties and conduction delay were observed in BrS patients. Both repolarization and depolarization abnormalities are thought to be related to the development of VF in BrS patients.

Brugada syndrome: insights of ST elevation, arrhythmogenicity, and risk stratification from experimental observations

Brugada syndrome (BrS), caused by ion channel abnormalities, is characterized by ST segment elevation and negative T waves in the right precordial electrocardiographic (ECG) leads recorded over the right ventricular outflow tract (RVOT). BrS is sensitive to body temperature and can lead to T-wave alternans (TWA), ventricular tachycardia, and sudden death. Recent studies in an isolated canine RVOT model of BrS demonstrated that reversal of the transmural gradient of repolarization caused the ECG characteristics and that major intraepicardial and transmural dispersion of action potentials (APs) initiated phase 2 reentry, premature ventricular activations, and tachyarrhythmias. Hypothermia enhanced the heterogeneity of the AP and promoted the origination of phase 2 reentry in the epicardium of the RVOT, but the prolonged AP duration frequently blocked reentry. Hyperthermia abbreviated the AP and facilitated the maintenance of reentry and tachyarrhythmias. Bradycardia promoted alternans in the phase 2 dome of the AP within the epicardium of the RVOT, resulting in TWA. The above phenomena were localized in the epicardium of the RVOT. Blockade of the transient outward current, I(to), reduced AP heterogeneity and prevented arrhythmias in the BrS model. In addition, epicardial activation delay led to fragmented QRS, a risk marker of prognosis in BrS. Body surface mapping in patients with BrS supported these experimental findings. In conclusion, the AP heterogeneity within the epicardium of the RVOT contributes to the ECG characteristics, temperature sensitivity, TWA, and arrhythmias in BrS, and body surface mapping and fragmented QRS can be effective predictors of risk in patients with BrS.

Fragmented QRS as a marker of conduction abnormality and a predictor of prognosis of Brugada syndrome

BACKGROUND

Conduction abnormalities serve as a substrate for ventricular fibrillation (VF) in patients with Brugada syndrome (BS). Signal-averaged electrograms can detect late potentials, but the significance of conduction abnormalities within the QRS complex is still unknown. The latter can present as multiple spikes within the QRS complex (fragmented QRS [f-QRS]). We hypothesized that f-QRS could indicate a substrate for VF and might predict a high risk of VF for patients with BS.

METHODS AND RESULTS

In study 1, we analyzed the incidence of f-QRS in 115 patients with BS (13 resuscitated from VF, 28 with syncope, and 74 asymptomatic). f-QRS was observed in 43% of patients, more often in the VF group (incidence of f-QRS: VF 85%, syncope 50%, and asymptomatic 34%, P<0.01). SCN5A mutations occurred more often in patients with f-QRS (33%) than in patients without f-QRS (5%). In patients with syncope or VF, only 6% without f-QRS experienced VF during follow-up (43+/-25 months), but 58% of patients with f-QRS had recurrent syncope due to VF (P<0.01). In study 2, to investigate the mechanism of f-QRS, we studied in vitro models of BS in canine right ventricular tissues (n=4) and optically mapped multisite action potentials. In the experimental model of BS, ST elevation resulted from a large phase 1 notch of the action potential in the epicardium, and local epicardial activation delay reproduced f-QRS in the transmural ECG.

CONCLUSIONS

f-QRS appears to be a marker for the substrate for spontaneous VF in BS and predicts patients at high risk of syncope.

Differences in arrhythmogenicity between the canine right ventricular outflow tract and anteroinferior right ventricle in a model of Brugada syndrome

BACKGROUND

The Brugada syndrome is characterized by ST-segment elevation on the ECG, especially in the right precordial leads sensitive to the right ventricular outflow tract (RVOT).

OBJECTIVES

The purpose of this study was to evaluate the hypothesis that right ventricular electrophysiologic heterogeneity caused arrhythmogenicity in the Brugada syndrome.

METHODS

Action potentials (APs) were mapped on the epicardium of 14 RVOT preparations and on the transmural surfaces of 15 pairs of RVOT and right ventricular anteroinferior (RVAI) preparations isolated from canine hearts. Brugada ECG and arrhythmias were induced with pilsicainide (2.5-12.5 micromol/L), pinacidil (1.25-12.5 micromol/L), and terfenadine (2.0 micromol/L).

RESULTS

Low doses of drugs elevated the J-ST segment and induced APs with both short and long action potential durations (APDs) in contiguous RVOT epicardial regions. In addition, APs in the RVOT had a larger phase 1 notch and longer APD than in RVAI. The longest APDs were in the epicardium in RVOT but in the endocardium in RVAI regions. High doses of drugs eliminated the phase 2 dome of the AP and abbreviated APDs in the epicardium but not in endocardium and reduced the epicardial heterogeneity of APs but increased the transmural gradient of APD in 14 (93%) of the RVOT preparations. In contrast, abbreviations of epicardial APDs occurred in only 4 (27%) of the RVAI preparations. Ventricular tachycardia occurred more frequently in the RVOT (47%) than in paired RVAI preparations (7%). Blocking the transient outward current reduced the heterogeneity of APs and eliminated arrhythmogenicity in all preparations.

CONCLUSION

Compared with the RVAI region, the RVOT has greater electrophysiologic heterogeneity that contributes to arrhythmogenicity in this model of Brugada syndrome.

Endocardial Electrograms From the Right Ventricular Outflow Tract After Induced Ventricular Fibrillation in Patients With Brugada Syndrome

BACKGROUND

The pathogenesis of Brugada syndrome (BS) is reported to be phase 2 reentry resulting from shortening of the action potential duration at the epicardial site of the right ventricular outflow tract (RVOT). However, several reports have shown a high incidence of ventricular late potentials (LPs) and a high rate of induction of ventricular fibrillation (VF) by programmed ventricular stimulation (PVS) among patients with BS. The aim of this study was to investigate the role of slow conduction for the initiation of VF by PVS in these patients.

METHODS AND RESULTS

Endocardial mapping of the RVOT was conducted in 17 patients in whom VF was induced by PVS from the RV apex or RVOT; 11 patients had a positive LP. In 10 patients, RV mapping showed that low-amplitude fragmented and delayed potentials (DPs) were recorded at the RVOT below the pulmonary valve (PV) or between the PV and His bundle electrogram recording site. Electrograms recorded after PVS showed a high incidence of fractionated and disorganized DPs that lead to VF.

CONCLUSIONS

Slow conduction at the RVOT may contribute to the induction of VF by PVS. However, the role of slow conduction in spontaneous VF remains controversial.

Spatial Distribution of Repolarization and Depolarization Abnormalities Evaluated by Body Surface Potential Mapping in Patients with Brugada Syndrome

BACKGROUND

Mutations in sodium channel gene, SCN5A, have been identified in Brugada syndrome, but it is still unclear as to how sodium channel dysfunction relates to arrhythmogenesis. We examined spatial distribution of both repolarization and depolarization abnormalities in patients with Brugada syndrome by using 87-leads body surface potential mapping (BSPM).

METHODS

BSPM was recorded under baseline condition and after pharmacological interventions in 28 patients with Brugada syndrome (27 males, 49 +/- 14 years). The ST-segment amplitude 20 ms after the end of QRS (ST20), QRS duration, and corrected recovery time (RTc) were measured in all 87-leads, and averaged among 6-leads (D-F, 5-6) reflecting right ventricular outflow tract (RVOT) potentials and the other 81-leads.

RESULTS

The ST20 was elevated at baseline, normalized by isoproterenol, and augmented by pilsicainide in only the RVOT. The RTc was longer at baseline and increased by pilsicainide in only the RVOT. On the other hand, the QRS duration was slightly widened at baseline, further increased by pilsicainide, but not changed by isoproterenol in both leads.

CONCLUSIONS

The ST-segment elevation and the RTc prolongation were localized and modulated by agents only in the RVOT region, while the slight QRS widening at baseline and further increase by pilsicainide were observed homogeneously. Our data suggest that depolarization abnormalities are distributed homogeneously, whereas repolarization abnormalities are localized in the RVOT.

High-frequency potentials developed in wavelet-transformed electrocardiogram as a novel indicator for detecting Brugada syndrome

BACKGROUND

A reliable alternative method for detecting Brugada syndrome is desirable because the diagnosis of Brugada syndrome using 12-lead ECG is not optimal.

OBJECTIVES

The purpose of this study was to assess the usefulness of the wavelet-transformed ECG in detecting Brugada syndrome.

METHODS

The study consisted of 15 patients with Brugada syndrome and 15 healthy subjects (control group). The parameters on the signal-averaged ECG and the frequency components recorded from the wavelet-transformed ECG were compared between the two groups. Measurements were repeated after pilsicainide infusion in the two groups of patients, after an isoproterenol infusion following pilsicainide injection, and after administration of cilostazol in the group of patients with Brugada syndrome.

RESULTS

The positive rate of late potentials was 80% in the Brugada syndrome group and 0% in the control group (P <.01). The high-frequency components (80-150 Hz) were developed in the Brugada syndrome group to a greater extent than in the control group, but the low-frequency components (10-50 Hz) did not differ (mean peak power at 80 Hz; 713 +/- 36 vs 488 +/- 60, P <.001). After pilsicainide injection, high-frequency components significantly increased in both groups. However, after isoproterenol and cilostazol administration, high-frequency components significantly decreased but remained higher than in the control group (80 Hz; 655 +/- 40 vs 488 +/- 60, P <.001). The sensitivity of the development of high-frequency components in detecting Brugada syndrome was higher than that of signal-averaged ECG (100% vs 80%), but specificity remained high and similar (100% for both methods).

CONCLUSION

Abnormally high-frequency components recorded from the wavelet-transformed ECG might be a novel factor in detecting Brugada syndrome.

Cellular basis for trigger and maintenance of ventricular fibrillation in the Brugada syndrome model: high-resolution optical mapping study

OBJECTIVES

We examined how repolarization and depolarization abnormalities contribute to the development of extrasystoles and subsequent ventricular fibrillation (VF) in a model of the Brugada syndrome.

BACKGROUND

Repolarization and depolarization abnormalities have been considered to be mechanisms of the coved-type ST-segment elevation (Brugada-electrocardiogram [ECG]) and development of VF in the Brugada syndrome.

METHODS

We used high-resolution (256 x 256) optical mapping techniques to study arterially perfused canine right ventricular wedges (n = 20) in baseline and in the Brugada-ECG produced by administration of terfenadine (5 micromol/l), pinacidil (2 micromol/l), and pilsicainide (5 micromol/l). We recorded spontaneous episodes of phase 2 re-entrant (P2R)-extrasystoles and subsequent self-terminating polymorphic ventricular tachycardia (PVT) or VF under the Brugada-ECG condition and analyzed the epicardial conduction velocity and action potential duration (APD) restitutions in each condition.

RESULTS

Forty-one episodes of spontaneous P2R-extrasystoles in the Brugada-ECG were successfully mapped in 9 of 10 preparations, and 33 of them were originated from the maximum gradient of repolarization (GR(max): 176 +/- 54 ms/mm) area in the epicardium, leading to PVT (n = 12) or VF (n = 5). The epicardial GR(max) was not different between PVT and VF. Wave-break during the first P2R-extrasystole produced multiple wavelets in all VF cases, whereas no wave-break or wave-break followed by wave collision and termination occurred in PVT cases. Moreover, conduction velocity restitution was shifted lower and APD restitution was more variable in VF cases than in PVT cases.

CONCLUSIONS

Steep repolarization gradient in the epicardium but not endocardium develops P2R-extrasystoles in the Brugada-ECG condition, which might degenerate into VF by further depolarization and repolarization abnormalities.