Cellular basis for the Brugada syndrome and other mechanisms of arrhythmogenesis associated with ST-segment elevation

Phenotypic characterization of a large European family with Brugada syndrome displaying a sudden unexpected death syndrome mutation in SCN5A:

INTRODUCTION

Brugada syndrome is characterized by sudden death secondary to malignant arrhythmias and the presence of ST segment elevation in leads V(1) to V(3) of patients with structurally normal hearts. This ECG pattern often is concealed but can be unmasked using potent sodium channel blockers. Like congenital long QT syndrome type 3 (LQT3) and sudden unexpected death syndrome, Brugada syndrome has been linked to mutations in SCN5A.

METHODS AND RESULTS

We screened a large European family with Brugada syndrome. Three members (two female) had suffered malignant ventricular arrhythmias. Ten members showed an ECG pattern characteristic of Brugada syndrome at baseline, and eight showed the pattern only after administration of ajmaline (total 12 female). Haplotype analysis revealed that all individuals with positive ECG at baseline shared the SCN5A locus. Sequencing of SCN5A identified a missense mutation, R367H, previously associated with sudden unexpected death syndrome. Two of the eight individuals who displayed a positive ECG after the administration of ajmaline, but not before, did not have the R367H mutation, and sequencing analysis failed to identify any other mutation in SCN5A. The R367H mutation failed to generate any current when heterologously expressed in HEK cells.

CONCLUSION

Our results support the hypothesis that (1) sudden unexpected death syndrome and Brugada syndrome are the same disease; (2) male predominance of the phenotype observed in sudden unexpected death syndrome does not apply to this family, suggesting that factors other than the specific mutation determine the gender distinction; and (3) ajmaline may provide false-positive results. These findings have broad implications relative to the diagnosis and risk stratification of family members of patients with the Brugada syndrome.

Hydroquinidine therapy in Brugada syndrome

OBJECTIVES

We sought to assess hydroquinidine (HQ) efficacy in selected patients with Brugada syndrome (BrS).

BACKGROUND

Management of asymptomatic patients with BrS and inducible arrhythmias remains a key issue. Effectiveness of class Ia antiarrhythmic drugs, which inhibit the potassium transient outward current of the action potential, has been suggested in BrS.

METHODS

From a cohort of 106 BrS patients, we studied 35 who received HQ (32 men; mean age 48 +/- 11 years). Patients had asymptomatic BrS and inducible arrhythmia (n = 31) or multiple appropriate shocks from an implantable cardioverter-defibrillator (ICD) (n = 4). Asymptomatic patients with inducible arrhythmia underwent electrophysiologic (EP)-guided therapy. When ventricular tachycardia (VT)/ventricular fibrillation (VF) inducibility was not prevented, or in case of HQ intolerance, an ICD was placed.

RESULTS

Hydroquinidine prevented VT/VF inducibility in 76% of asymptomatic patients who underwent EP-guided therapy. Syncope occurred in two of the 21 patients who received long-term (17 +/- 13 months) HQ therapy (1 syncope associated with QT interval prolongation and 1 unexplained syncope associated with probable noncompliance). In asymptomatic patients who received an ICD (n = 10), one appropriate shock occurred during a follow-up period of 13 +/- 8 months. In patients with multiple ICD shocks, HQ prevented VT/VF recurrence in all cases during a mean follow-up of 14 +/- 8 months.

CONCLUSIONS

Hydroquinidine therapy prevented VT/VF inducibility in 76% of asymptomatic patients with BrS and inducible arrhythmia, as well as VT/VF recurrence in all BrS patients with multiple ICD shocks. These preliminary data suggest that preventive treatment by HQ may be an alternative strategy to ICD placement in asymptomatic patients with BrS and inducible arrhythmia.

Basic mechanisms of cardiac impulse propagation and associated arrhythmias

Propagation of excitation in the heart involves action potential (AP) generation by cardiac cells and its propagation in the multicellular tissue. AP conduction is the outcome of complex interactions between cellular electrical activity, electrical cell-to-cell communication, and the cardiac tissue structure. As shown in this review, strong interactions occur among these determinants of electrical impulse propagation. A special form of conduction that underlies many cardiac arrhythmias involves circulating excitation. In this situation, the curvature of the propagating excitation wavefront and the interaction of the wavefront with the repolarization tail of the preceding wave are additional important determinants of impulse propagation. This review attempts to synthesize results from computer simulations and experimental preparations to define mechanisms and biophysical principles that govern normal and abnormal conduction in the heart.

Cellular and ionic basis for the sex-related difference in the manifestation of the Brugada syndrome and progressive conduction disease phenotypes

The Brugada syndrome (BS) has been linked to mutations in SCN5A. Despite equal hereditary transmission of the mutation between the sexes, the syndrome is 8 to 10 times more likely to occur in males. As recently reported, SCN5A mutations such as G1406R lead to development of BS phenotype principally in males and conduction disease phenotype in females. We hypothesized that these differences may be related to a larger transient outward current (Ito)-mediated right ventricular (RV) epicardial (Epi) action potential (AP) notch in males versus females, resulting in a higher incidence of all-or-none repolarization at the end of phase 1 and phase 2 re-entry (P2R) when challenged with sodium and calcium channel block. Using canine RV wedge preparations, we developed an experimental model of the BS using terfenadine to depress the AP dome in RV Epi.

RESULTS

RV Epi AP phase 1 amplitude corrected to phase 2 amplitude was 12% smaller in males (n=18) compared to females (n=8, P<.05) at a cycle length of 2,000 ms. When exposed to 5 microM terfenadine for up to 2 hours, 6 of 7 male but only 2 of 7 female preparations exhibited spontaneous P2R, generating a closely coupled extrasystole. Two of 6 male and 1 of 2 female preparations displaying P2R developed polymorphic VT/VF. Female and male preparations that failed to develop P2R displayed progressive conduction impairment with continued exposure to terfenadine and developed polymorphic and monomorphic VT/VF when paced at rapid rates. Male preparations pretreated with 4-aminopyridine to inhibit Ito displayed progressive conduction impairment but not Brugada syndrome.

CONCLUSION

Our data suggest that the presence of a more prominent Ito-mediated notch in the Epi of males predisposes males to the development of the Brugada phenotype and that a smaller Epi notch in females relegates them to development of progressive conduction problems under conditions in which inward currents are compromised.

Bundle branch reentrant ventricular tachycardia in a patient with the Brugada electrocardiographic pattern

A 55-year-old, previously healthy man presented with an episode of wide QRS tachycardia that had left bundle branch morphology and left superior axis. His electrocardiogram in sinus rhythm showed characteristic Brugada pattern with coved type ST-segment (J-point) elevation in leads V1-V2, mild QRS widening of 110 ms, and left axis deviation. The mechanism of the tachycardia was shown to be bundle branch reentry. Baseline H-V interval of 68 ms additionally lengthened to 119 ms after intravenous procainamide administration indicating significant conduction system disease. The tachycardia was no longer inducible after successful ablation of the right bundle branch.

Mechanisms of genetic arrhythmias: from DNA to ECG

A precise balance of ionic currents underlies normal cardiac excitation and relaxation. Disruption of this equilibrium by genetic defects, polymorphisms, therapeutic intervention, and structural abnormalities can cause arrhythmogenic phenotypes leading to syncope, seizures, and sudden cardiac death. Congenital defects result in an unpredictable expression of phenotypes with variable penetrance, even within single families. Additionally, phenotypically opposite and overlapping cardiac arrhythmogenic syndromes can even stem from the same mutation. Accordingly, the relationship between genetic mutations and clinical syndromes is becoming increasingly complex.

Cellular basis and mechanism underlying normal and abnormal myocardial repolarization and arrhythmogenesis

Regional differences in repolarization characteristics of distinct cell types are responsible for the inscription of the J wave and T wave of the electrocardiogram (ECG). Amplification of these electrical heterogeneities contributes to the development of a variety of cardiac arrhythmias. This brief review examines the ionic and cellular basis for these heterogeneities and their role in the Brugada and long-QT syndromes. Both cases involve an accentuation of transmural dispersion of repolarization (TDR). In the case of the Brugada syndrome. TDR is accentuated as a result of a preferential abbreviation of the right ventricular epicardial action potential, whereas in the long-QT syndrome, accentuation of TDR is secondary to a preferential prolongation of the action potential of the M cell.

Repolarization Abnormality in Idiopathic Ventricular Fibrillation:

INTRODUCTION

We evaluated the characteristics of QT-RR and QaT (apex of T wave)-RR relationships in patients with idiopathic ventricular fibrillation (IVF) compared with control subjects. We hypothesized that IVF patients have unique repolarization dynamics related to a reduced fast Na current and a prominent transient outward current.

METHODS AND RESULTS

The study group consisted of 9 men (age 47 +/- 10 years) with IVF (6 with Brugada type and 3 with non-Brugada type) who had experienced nocturnal episodes of VF. The control group consisted of 28 healthy age-matched men (age 44 +/- 12 years). The relationships between QT and RR intervals and between QaT and RR intervals were analyzed from 24-hour Holter ECG data using an automatic measurement system. Both QT and QaT at RR intervals of 0.6, 1.0, and 1.2 seconds were determined from QT-RR and QaT-RR linear regression lines. Both QT-RR and QaT-RR slopes were lower in the IVF group than in the control group (QT-RR: 0.092 +/- 0.023 vs 0.137 +/- 0.031, P < 0.001; QaT-RR: 0.109 +/- 0.025 vs 0.153 +/- 0.028, P < 0.001). QT at an RR interval of 0.6 second did not differ between two groups, but QT at RR intervals of either 1.0 or 1.2 seconds was significantly shorter in the IVF group than in the control group (RR 1.0 s: 0.384 +/- 0.018 vs 0.399 +/- 0.017, P < 0.05; RR 1.2 s: 0.402 +/- 0.019 vs 0.426 +/- 0.020, P < 0.01). QaT at RR intervals of either 1.0 or 1.2 seconds also was shorter in the IVF group (RR 1.0 s: 0.289 +/- 0.022 vs 0.312 +/- 0.021, P < 0.01; RR 1.2 s: 0.311 +/- 0.024 vs 0.343 +/- 0.024, P < 0.01). In four patients, oral administration of disopyramide (300 mg/day) was effective in suppressing VF episodes and increased slopes of QT-RR and QaT-RR relationships.

CONCLUSION

IVF patients had lower slopes of QT-RR and QaT-RR regression lines and impaired prolongation of QT and QaT at longer RR intervals compared with control subjects. These unique repolarization dynamics may be related to the frequent occurrence of VF episodes at night.

QT-interval prolongation in right precordial leads: an additional electrocardiographic hallmark of Brugada syndrome

OBJECTIVES

The aim of this study was to evaluate whether the occurrence of the Brugada Syndrome typical electrocardiogram (ECG) pattern (i.e., right bundle branch block, coved-type ST-segment elevation, and T-wave inversion in the right precordial leads) is characterized by a concomitant lengthening of QT intervals in the right precordial leads.

BACKGROUND

It has been suggested that the typical ECG pattern of Brugada syndrome is due to a decreased net inward current during phase 1 of the action potential, which also leads to its prolongation in the right epicardium.

METHODS

Thirty-two subjects (19 males) age 37 +/- 15 years with a suspicious baseline ECG, or who were relatives of Brugada syndrome patients, underwent 12-lead ECG before and after the administration of flecainide.

RESULTS

The flecainide test was negative in 14 and positive in 18 subjects. After flecainide administration, the positive ECGs were characterized by a greater QT interval corrected for heart rate (QTc) prolongation in the right precordial leads than that in the negative ECGs (78.2 +/- 35.5 ms vs. 22.0 +/- 28.4 ms in V(1) and 107.1 +/- 43.8 ms vs. 26.7 +/- 30.1 ms in V(2); p < 0.01), whereas there was no difference in the QTc prolongation in the left precordial leads (55.2 +/- 25.3 ms vs. 35.1 +/- 28.1 ms in V(5) and 53.1 +/- 32.8 ms vs. 27.3 +/- 22.4 ms in V(6); p = NS).

CONCLUSIONS

In accordance with the electrophysiological background, the typical ECG pattern of Brugada syndrome is also characterized by a considerable prolongation of the QT interval in right precordial leads.

Ventricular arrhythmia induced by sodium channel blocker in patients with Brugada syndrome

OBJECTIVES

We administered pilsicainide chloride, a class Ic pure sodium channel blocker, to patients with Brugada syndrome (BS) and evaluated the occurrence of ventricular arrhythmia (VA) and T-wave alternans (TWA).

BACKGROUND

Ventricular arrhythmia and TWA are sometimes induced by a sodium channel blocker challenge test in BS patients, but the significance of the induced VA and TWA is not known.

METHODS

Pilsicainide was administered to 65 patients with BS (10 symptomatic and 55 asymptomatic patients), and the occurrence of VA, TWA, and change of electrocardiogram were evaluated. Electrophysiologic study was performed in 57 patients, and the induction of VA by programmed electrical stimulation (PES) was evaluated.

RESULTS

Ventricular arrhythmia was not induced by administration of pilsicainide in 55 patients (no-VA group). Administration of pilsicainide-induced VA in 10 patients (Pil-VA group) and polymorphic ventricular tachycardia in four patients. Pilsicainide-induced VA in 60% of the symptomatic patients but in only 7% of asymptomatic patients (p < 0.01). ST level, QTc, and indexes of cardiac conduction in the Pil-VA group were not different from those in the no-VA group. Ventricular fibrillation was induced by PES in 67% of the patients in the Pil-VA group and in 33% of the patients in the no-VA group. In six cases, macroscopic TWA occurred in association with pilsicainide-induced VA, but TWA occurred in only one patient without pilsicainide-induced arrhythmia.

CONCLUSIONS

Administration of a sodium channel blocker results in induction of not only ST-elevation but also VA and TWA in patients with BS.

Phenotypic differences in transient outward K+ current of human and canine ventricular myocytes: insights into molecular composition of ventricular Ito

The Ca(2+)-independent transient outward K(+) current (I(to)) plays an important electrophysiological role in normal and diseased hearts. However, its contribution to ventricular repolarization remains controversial because of differences in its phenotypic expression and function across species. The dog, a frequently used model of human cardiac disease, exhibits altered functional expression of I(to). To better understand the relevance of electrical remodeling in dogs to humans, we studied the phenotypic differences in ventricular I(to) of both species with electrophysiological, pharmacological, and protein-chemical techniques. Several notable distinctions were elucidated, including slower current decay, more rapid recovery from inactivation, and a depolarizing shift of steady-state inactivation in human vs. canine I(to). Whereas recovery from inactivation of human I(to) followed a monoexponential time course, canine I(to) recovered with biexponential kinetics. Pharmacological sensitivity to flecainide was markedly greater in human than canine I(to), and exposure to oxidative stress did not alter the inactivation kinetics of I(to) in either species. Western blot analysis revealed immunoreactive bands specific for Kv4.3, Kv1.4, and Kv channel-interacting protein (KChIP)2 in dog and human, but with notable differences in band sizes across species. We report for the first time major variations in phenotypic properties of human and canine ventricular I(to) despite the presence of the same subunit proteins in both species. These data suggest that differences in electrophysiological and pharmacological properties of I(to) between humans and dogs are not caused by differential expression of the K channel subunit genes thought to encode I(to), but rather may arise from differences in molecular structure and/or posttranslational modification of these subunits.

Intravenous administration of class I antiarrhythmic drug induced T wave alternans in an asymptomatic Brugada syndrome patient

A 53-year-old man with an abnormal ECG was referred to the Nihon University School of Medicine. The 12-lead ECG showed right bundle branch block and saddleback-type ST elevation in leads V1-V3 (Brugada-type ECG). Signal-averaged ECG showed positive late potentials. Double ventricular extrastimuli (S1: 500 ms, S2: 250 ms, S3: 210 ms) induced VF. Amiodarone (200 mg/day) was administered for 6 months and programmed ventricular stimulation was repeated. VF was induced again by double ventricular stimuli (S1: 600 ms, S2: 240 ms, S3: 170 ms). Intravenous administration of class Ic antiarrhythmic drug, pilsicainide (1 mg/kg), augmented ST-T elevation in leads V1-V3, and visible ST-T alternans that was enhanced by atrial pacing was observed in leads V2 and V3. Visible ST-T wave alternans disappeared in 15 minutes. However, microvolt T wave alternans was present during atrial pacing at a rate of 70/min without visible ST-T alternans.

Ventricular repolarization components on the electrocardiogram: cellular basis and clinical significance

Ventricular repolarization components on the surface electrocardiogram (ECG) include J (Osborn) waves, ST-segments, and T- and U-waves, which dynamically change in morphology under various pathophysiologic conditions and play an important role in the development of ventricular arrhythmias. Our primary objective in this review is to identify the ionic and cellular basis for ventricular repolarization components on the body surface ECG under normal and pathologic conditions, including a discussion of their clinical significance. A specific attempt to combine typical clinical ECG tracings with transmembrane electrical recordings is made to illustrate their logical linkage. A transmural voltage gradient during initial ventricular repolarization, which results from the presence of a prominent transient outward K(+) current (I(to))-mediated action potential (AP) notch in the epicardium, but not endocardium, manifests as a J-wave on the ECG. The J-wave is associated with the early repolarization syndrome and Brugada syndrome. ST-segment elevation, as seen in Brugada syndrome and acute myocardial ischemia, cannot be fully explained by using the classic concept of an "injury current" that flows from injured to uninjured myocardium. Rather, ST-segment elevation may be largely secondary to a loss of the AP dome in the epicardium, but not endocardium. The T-wave is a symbol of transmural dispersion of repolarization. The R-on-T phenomenon (an extrasystole originating on the T-wave of a preceding ventricular beat) is probably due to transmural propagation of phase 2 re-entry or phase 2 early after depolarization that could potentially initiate polymorphic ventricular tachycardia or fibrillation.

Disappearance of the Brugada-type electrocardiogram after surgical castration: a role for testosterone and an explanation for the male preponderance

We describe two cases of asymptomatic Brugada syndrome that displayed a persistent ECG manifestation, but in which the typical ECG pattern disappeared following surgical castration for prostate cancer. These facts suggest a possible association between manifestation of the Brugada-type ECG pattern and testosterone.

Extracellular acidosis modulates drug block of Kv4.3 currents by flecainide and quinidine

INTRODUCTION

As a molecular model of the effect of ischemia on drug block of the transient outward potassium current, the effect of acidosis on the blocking properties of flecainide and quinidine on Kv4.3 currents was studied.

METHODS AND RESULTS

Kv4.3 channels were stably expressed in Chinese hamster ovary cells. Whole-cell, voltage clamp techniques were used to measure the effect of flecainide and quinidine on Kv4.3 currents in solutions of pH 7.4 and 6.0. Extracellular acidosis attenuated flecainide block of Kv4.3 currents, with the IC50 for flecainide (based on current-time integrals) increasing from 7.8 +/- 1.1 microM at pH 7.4 to 125.1 +/- 1.1 microM at pH 6.0. Similar effects were observed for quinidine (IC50 5.2 +/- 1.1 microM at pH 7.4 and 22.1 +/- 1.3 microM at pH 6.0). Following block by either drug, Kv4.3 channels showed a hyperpolarizing shift in the voltage sensitivity of inactivation and a slowing in the time to recover from inactivation/block that was unaffected by acidosis. In contrast, acidosis attenuated the effects on the time course of inactivation and the degree of tonic- and frequency-dependent block for both drugs.

CONCLUSION

Extracellular acidosis significantly decreases the potency of blockade of Kv4.3 by both flecainide and quinidine. This change in potency may be due to allosteric changes in the channel, changes in the proportion of uncharged drug, and/or changes in the kinetics of drug binding or unbinding. These findings are in contrast to the effects of extracellular acidosis on block of the fast sodium channel by these agents and provide a molecular mechanism for divergent modulation of drug block potentially leading to ischemia-associated proarrhythmia.

Brugada syndrome: 1992-2002: a historical perspective

An intriguing new clinical entity characterized by ST-segment elevation in the right precordial electrocardiographic leads and a high incidence of sudden death in individuals with structurally normal hearts was described by Pedro and Josep Brugada in 1992. The past decade has witnessed an exponential rise in the number of reported cases and a dramatic proliferation of papers serving to define the clinical, genetic, cellular, ionic, and molecular aspects of this disease. The purpose of this brief review is to chronicle the historical highlights that have brought us to our present understanding of Brugada syndrome.

Brugada syndrome and "Brugada sign": clinical spectrum with a guide for the clinician

BACKGROUND

Patients with the manifest Brugada syndrome have an inordinate risk of sudden death and are candidates for implantation of a defibrillator. The Brugada type electrocardiogram (ECG) abnormality (the "Brugada sign"), however, is known to be associated with a wide range of conditions, many of which may not pose such a threat. Clinicians need guidance in choosing a rational approach for the evaluation and treatment of patients with a finding of the Brugada sign.

METHODS

A systematic literature search was performed to identify publications on the Brugada syndrome and the Brugada-type ECG abnormality, with special emphasis on analyzing outcomes data. In addition, the ECG database of our institution was reviewed for tracings consistent with the Brugada sign, and, when possible, clinical correlations were made.

RESULTS

Patients with the Brugada sign and a family history of sudden death or a personal history of syncope are at a high risk of sudden death and therefore should be strongly considered for implantation of a defibrillator. In patients who are hospitalized and critically ill, the Brugada sign is frequently the result of severe hyperkalemia, drug toxicity, or right ventricular injury. In most individuals with no symptoms and without a family history of sudden death, the Brugada sign is likely a normal variant.

CONCLUSIONS

Most patients with the Brugada sign can be risk-stratified with simple clinical tools. Specific testing for the Brugada syndrome should be reserved for questionable cases and for the research setting. A provisional diagnostic-therapeutic algorithm is offered as a means of assisting the clinician in the evaluation and treatment of patients with the Brugada sign.

Nucleotide changes in the translated region of SCN5A from Japanese patients with Brugada syndrome and control subjects

The mutations of the SCN5A gene have been implicated to play a pathogenetic role in Brugada syndrome, which causes ventricular fibrillation. To determine the Brugada-associated mutations in Japanese patients, facilitate pre-symptomatic diagnosis, and allow genotype-phenotype studies, we screened unrelated patients with Brugada syndrome for mutations. DNAs from 6 Japanese patients were obtained and the sequence in the translated region of SCN5A was determined. We could not find the mutations reported previously, but found 17 sites of nucleotide change, consisting of 7 synonymous and 10 non-synonymous nucleotide changes in our patients. Among them, two non-synonymous nucleotide changes (G1663A and G5227A) are specific to our patients and these changes were not found in 53 healthy controls. In 4 patients out of 6, no specific nucleotide change for Brugada syndrome could be detected. Our findings demonstrating no patient-specific change in the translated region of the SCN5A gene among two thirds of the small number of patients examined here imply that another gene other than the SCN5A may be associated with this disease, supporting previous investigations in Japan and other countries.

Cardiac sodium channel diseases

In the last few years, a very active line of research took place after the first identification of SCN5A mutations associated with an inherited form of cardiac arrhythmias and sudden death, the LQT3 variant of the long QT syndrome. Subsequently, two allelic diseases additional to LQT3 were shown to be due to mutations in the same gene, the Brugada syndrome (BrS) and the Lev-Lenegre syndrome (progressive cardiac conduction defect). Genotype-phenotype correlation and in vitro expression studies provide evidence that structure-function relationships of the SCN5A protein are much more complex than initially anticipated. The biophysical characterization of the sodium channel defects associated with different phenotypes and the genotype-phenotype correlation studies brought to the attention of the scientific community a plethora of mechanisms by which even a single amino acid substitution may remarkably affect cardiac excitability. Finally, the evidence of patients harboring an SCN5A mutation and overlapping clinical presentations creates a need for a revision of the traditional classification of the above mentioned diseases. It is now appropriate to consider the "sodium channel syndrome" as a unique clinical entity that may manifest itself with a spectrum of possible phenotypes.

A mathematical model of phase 2 reentry: role of L-type Ca current

Phase 2 reentry (P2R) is known to be one of the mechanisms of malignant ventricular arrhythmias, especially those associated with Brugada syndrome. However, little is known about the underlying mechanism for P2R. Our aim in this study was to simulate P2R in a mathematical model to enable us to understand its mechanism and identify a potential therapeutic target. A mathematical model of the L-type Ca current was composed according to whole cell current data from guinea pig ventricular myocytes recorded at 37 degrees C. Our mathematical model was incorporated into the modified Luo-Rudy phase 2 model. We set a dispersion in transient outward current (I(to)) density within the theoretical fiber, composed of 80 serially arranged epicardial cells with gap junctions and then observed the P2R. The dispersion in I(to) density within an only 0.8-cm epicardial theoretical fiber generated P2R with our Ca channel but not with the original model. When the P2R developed in the theoretical fiber, the calculated extracellular field potential showed coved-type ST segment elevation. We succeeded in generating P2R in our model for the first time. The local epicardial P2R may contribute the genesis of coved-type ST segment elevation in the Brugada syndrome.

Relationship between ST-segment morphology and conduction disturbances detected by signal-averaged electrocardiography in Brugada syndrome

BACKGROUND

Although arrhythmogenesis of Brugada syndrome is still unknown, it has been reported to be associated with conduction disturbances. Two ST-segment morphologies (coved and saddle-back patterns) have been described in this syndrome. No study has sought to determine which morphology has stronger conduction disturbances, thereby associating with life-threatening events.

METHODS

Forty-six patients who presented the Brugada-type ECG with either of a characteristic coved (n = 25) or saddle-back (n = 21) pattern of ST-segment morphology underwent signal-averaged ECG (SAECG). SAECG parameters, and the history of life-threatening events defined as syncope or aborted sudden death, were compared between groups.

RESULTS

Although filtered QRS duration did not differ between groups, the incidence of late potentials in the coved group was higher than in the saddle-back group (22 patients (88%) versus 4 patients (19%); P < 0.01), showing lower RMS40 and longer LAS40. Life-threatening events occurred in 17 patients (68%) in the coved group and 7 patients (33%) in the saddle-back group (P = 0.02).

CONCLUSION

The coved pattern of ST segment was more closely related to conduction disturbances than the saddle-back pattern in patients with Brugada-type ECG. Life-threatening events were more common in patients with the coved ST-segment elevation. Conduction disturbances in the coved pattern of ST segment may reflect a substrate of arrhythmogenesis in Brugada syndrome.

Effects of glucose-induced insulin secretion on ST segment elevation in the Brugada syndrome

INTRODUCTION

ST segment elevation in patients with Brugada syndrome is known to fluctuate occasionally, influenced by multiple factors. Insulin has been shown to affect QT dispersion in healthy volunteers, as well as result in abnormality of ventricular repolarization in patients with congenital long QT syndrome.

METHODS AND RESULTS

To assess a possible role of insulin in ST segment elevation in patients with Brugada syndrome, an oral glucose tolerance test (OGTT) was administered to 20 patients with Brugada syndrome and 20 normal patients without ST-T changes as a control group. Plasma glucose and potassium levels, immunoreactive insulin concentration (IRI), and ST segment elevation and ST-T wave changes on 12-lead ECG during OGTT were analyzed. Augmentation (>1 mm) of ST elevation or morphologic changes in ST-T waves were observed frequently in response to increased IRI during OGTT [15/20 cases (75%)] in patients with Brugada syndrome but in none of the patients in the control group [0/20 cases (0%), P < 0.01]. The changes returned to baseline 180 minutes after the glucose load in 9 of 15 patients. Patients who showed coved-type ST elevation before the glucose load exhibited positive ECG changes more frequently than patients with saddleback-type elevation or transiently normalized ST segment [8/8 cases (100%) vs 7/12 (58%), P < 0.05]. There was no significant difference between the two groups in terms of glucose, IRI, and potassium levels during OGTT.

CONCLUSION

The findings suggest that glucose-induced insulin secretion is one of the contributing factors to fluctuation of ST segment elevation in patients with Brugada syndrome.

Novel brugada SCN5A mutation leading to ST segment elevation in the inferior or the right precordial leads

Mutations in the SCN5A gene can lead to the Brugada syndrome, a genetically inherited form of idiopathic ventricular fibrillation that has a characteristic ECG phenotype usually restricted to precordial leads V1-V3. We identified a novel G752R SCN5A missense mutation leading to various degrees of the Brugada ECG phenotype in members of a French family. In the proband, the G752R mutation produced ST segment elevation and prominent J wave in leads II, III, and aVF. In four other relatives, ST segment elevation in the right precordial but not in the inferior leads was observed either spontaneously or under flecainide challenge. Recombinant G752R mutant exhibited a markedly reduced Na+ current amplitude and a voltage shift in both activation and inactivation curves. The mutant was found in all affected but not in nonaffected family members. One additional gene-carrier had an almost normal ECG (silent gene-carrier). We provide genetic demonstration that Brugada ECG anomalies related to a unique SCN5A mutation can be observed either in the inferior or the right precordial leads.

Brugada syndrome with ventricular tachycardia and fibrillation related to hypokalemia

A 60-year-old man with asymptomatic Brugada syndrome and neither a history of syncope nor family history of sudden death was admitted because of bronchial asthma. Serum potassium concentration was 3.8 mmol/L on admission, and decreased to 3.1 mmol/L on the 6th day, probably as a side effect of steroid therapy. The patient was found unconscious on the 7th day, and his serum potassium concentration was 3.4 mmol/L immediately after the episode. On the 8th day, the patient was again found unconscious, and polymorphic ventricular tachycardia and fibrillation (VT/VF) was documented on electrocardiographic (ECG) monitoring. The coved type of ST-segment elevation in leads V(1-3) was observed on the ECG after spontaneous recovery of sinus rhythm, and VT/VF associated with Brugada syndrome was diagnosed. The serum potassium concentration decreased to 2.9 mmol/L immediately after the episode, but QT prolongation was not observed during the clinical course. After the correcting the serum potassium concentration, there was no further recurrence of the malignant ventricular arrhythmia and syncope. An implantable cardioverter defibrillator was inserted to prevent sudden death. Hypokalemia that does not induce QT prolongation may contribute to the occurrence of VT/VF in Brugada syndrome.

Enhancement of J-ST-segment elevation by the glucose and insulin test in Brugada syndrome

The effects of glucose and insulin on J-ST-segment elevation were evaluated in seven men (mean age 45 +/- 10 years) with Brugada syndrome. Six patients had been reanimated from VF and one patient had experienced syncope. The effects of intavenous (1) pilsicainide 50 mg, (2) glucose 50 g, and (3) glucose 50 g plus regular insulin 10 IU on the precordial ECG leads were examined. Pilsicainide significantly enhanced J-ST elevation in all patients and induced VF in 1 patient. A significant accentuation of the abnormal J-ST configuration was observed in all patients at a mean of 51 +/- 40 minutes after glucose and insulin infusion. Changes in blood glucose and serum potassium concentration were 111 +/- 158 mg/dL and -0.30 +/- 0.48 mEq/L, respectively. These changes were not directly related to the ECG changes. Glucose infusion without insulin caused a subtle increase in J-ST elevation. In conclusion, the administration of glucose and insulin safely unmasked or accentuation the J-ST-segment elevation in Brugada syndrome. Blood glucose and insulin concentrations may be factors modulating the circadian or day-to-day ECG variations in this syndrome.

Prolongation of LAS40 (duration of the low amplitude electric potential component (<40 microV) of the terminal portion of the QRS) induced by isoproterenol in 11 patients with Brugada syndrome

The electrophysiological mechanism of Brugada syndrome is unclear, but transmural dispersion of repolarization in the right ventricle is believed to be the most likely mechanism. On the other hand, the presence of a conduction delay region is considered to be related to the occurrence of ventricular fibrillation; that is, a relationship between the presence of a ventricular late potential (LP) and arrhythmogenic right ventricular cardiomyopathy. In this study, the LP from signal-averaged electrocardiography during isoproterenol (ISP) administration in patients with Brugada syndrome is discussed. The subjects were 11 patients with Brugada syndrome and 6 healthy individuals. In all subjects, the total filtered QRS duration (fQRS), root mean square voltage of the 40 ms terminal portion of the QRS (RMS(40)), duration of the low amplitude electric potential component (40 microV) of the terminal portion of the QRS (LAS(40)), and time duration of the fQRS-LAS(40) difference were compared between when ISP was prescribed and when it was not. During ISP administration, a peculiar response, which resulted in an LAS(40) prolongation, was observed in the patients with Brugada syndrome. With ISP, the fQRS remained unchanged, but the RMS(40) and the fQRS-LAS(40) decreased. Consequently another 3 patients with a positive LP were diagnosed using the ordinary standard because of the administration of ISP. We believe that the low-amplitude component was unmasked by shortening of the high-amplitude component. In patients with Brugada syndrome, a conduction delay in the ventricle may be present and may be related to the occurrence of ventricular fibrillation.

ST-segment elevation and ventricular fibrillation without coronary spasm by intracoronary injection of acetylcholine and/or ergonovine maleate in patients with Brugada syndrome

OBJECTIVES

The study examined whether patients with Brugada syndrome are sensitive to vagal stimulation or ischemia.

BACKGROUND

Experimental studies have suggested that a prominent transient outward current (I(to))-mediated action potential notch and a subsequent loss of the action potential dome in the epicardium, but not in the endocardium, give rise to ST-segment elevation and subsequent ventricular fibrillation (VF).

METHODS

We evaluated the frequency of coronary spasm, augmentation (> or =0.1 mV) of ST-segment elevation in leads V(1) to V(3), and induction of VF by intracoronary injection of acetylcholine (ACh) and/or ergonovine maleate (EM) in 27 symptomatic patients with Brugada syndrome and 30 control subjects.

RESULTS

The coronary spasm was induced in 3 (11%) of the 27 patients with Brugada syndrome and in 13 (43%) of the 30 control subjects. ST-segment elevation was augmented by 11 (33%) of the 33 right coronary injections (ACh: 6/11 [55%]; EM: 5/22 [23%]), without coronary spasm, but not by any of the left coronary injections in patients with Brugada syndrome. Ventricular fibrillation was induced by 3 (9%) of the 33 right coronary injections (ACh: 2/11 [18%]; EM: 1/22 [5%]), but not by any of the left coronary injections. In contrast, neither ST-segment elevation nor VF was observed in any of the control subjects.

CONCLUSIONS

Our results support the hypothesis that mild ischemia and vagal influences act additively or synergistically with the substrate responsible for the Brugada syndrome to elevate the ST-segment and precipitate VF. These observations suggest that Brugada patients may be at a higher risk for ischemia-related sudden death.

Long QT syndrome, Brugada syndrome, and conduction system disease are linked to a single sodium channel mutation

The function of the 12 positive charges in the 53-residue III/IV interdomain linker of the cardiac Na(+) channel is unclear. We have identified a four-generation family, including 17 gene carriers with long QT syndrome, Brugada syndrome, and conduction system disease with deletion of lysine 1500 (DeltaK1500) within the linker. Three family members died suddenly. We have examined the functional consequences of this mutation by measuring whole-cell and single-channel currents in 293-EBNA cells expressing the wild-type and DeltaK1500 mutant channel. The mutation shifted V(1/2)h( infinity ) to more negative membrane potentials and increased k(h) consistent with a reduction of inactivation valence of 1. The shift in h( infinity ) was the result of an increase in closed-state inactivation rate (11-fold at -100 mV). V(1/2)m was shifted to more positive potentials, and k(m) was doubled in the DeltaK1500 mutant. To determine whether the positive charge deletion was the basis for the gating changes, we performed the mutations K1500Q and K1500E (change in charge, -1 and -2, respectively). For both mutations, V(1/2)h was shifted back toward control; however, V(1/2)m shifted progressively to more positive potentials. The late component of Na(+) current was increased in the DeltaK1500 mutant channel. These changes can account for the complex phenotype in this kindred and point to an important role of the III/IV linker in channel activation.

The electrophysiologic mechanism of ST-segment elevation in Brugada syndrome

OBJECTIVES

We sought to demonstrate the electrophysiologic (EP) mechanism of the ST-T change in Brugada syndrome.

BACKGROUND

Brugada syndrome is characterized by various electrocardiographic manifestations (e.g., right bundle branch block, ST-segment elevation, and terminal T-wave inversion in the right precordial leads) and sudden cardiac death caused by ventricular fibrillation. Direct evidence in support of the EP mechanism underlying this intriguing syndrome has been lacking.

METHODS

Monophasic action potentials (MAPs) were obtained from three patients with the coved-type ST-segment elevation (Brugada patients) and five control patients using the contact electrode method. Epicardial MAPs were recorded during open-chest surgery in all patients.

RESULTS

A spike-and-dome configuration was documented from epicardial sites of the right ventricular (RV) outflow tract in all Brugada patients but not in control patients. Monophasic action potential recordings from the endocardium with special focus on the RV outflow tract could not demonstrate any morphological abnormalities in three Brugada patients.

CONCLUSIONS

The presence of a deeply notched action potential in the RV epicardium, but not in endocardium, would be expected to induce a transmural current that would contribute to elevation of the ST-segment in the right precordial leads. The spike-and-dome configuration may also prolong the epicardial action potential, thus contributing to a rapid reversal of the transmural gradients and inscription of an inverted T-wave.

Cocaine-induced channelopathies: emerging evidence on the multiple mechanisms of sudden death

Sudden death due to cocaine in the absence of myocardial infarction has been attributed to the precipitation of life-threatening arrhythmias not unlike that due to antiarrhythmic drugs. Cocaine is a slow on-off sodium blocker and a fast on-off potassium blocker. Effects on repolarization are biphasic: At low concentrations, cocaine delays ventricular recovery, whereas at higher levels, cocaine hastens it. Two distinct clinical profiles emerge from case reports of electrocardiographically documented life-threatening arrhythmias attributed to cocaine. The first is monomorphic slow ventricular tachycardia or idioventricular rhythm that occurs in overdose situations and appears to reflect excessive sodium channel block; it may respond to sodium bicarbonate. The second is torsade de pointes that occurs in recreational users who have underlying risks for this tachycardia (such as fully or partially expressed congenital long QT syndrome) and reflects potassium channel blockade. These clinical observations can be explained by recent findings regarding the electrophysiologic effects of cocaine. Other patterns of severe arrhythmias due to cocaine may yet emerge.

Epicardial electrogram of the right ventricular outflow tract in patients with the Brugada syndrome: using the epicardial lead

OBJECTIVES

We tried to record an epicardial electrogram directly, and we examined local electrograms before and after administration of a class IC anti-arrhythmic drug in patients with the Brugada syndrome.

BACKGROUND

Electrical heterogeneity of the epicardium in the right ventricular outflow tract (RVOT) has been thought to be related to the Brugada syndrome. However, an epicardial abnormality has not been demonstrated in patients with the Brugada syndrome.

METHODS

In five patients with a Brugada-type electrocardiogram (ECG), local unipolar electrograms were recorded at the epicardium and endocardium of the RVOT. To record the epicardial electrogram directly, we introduced an electrical guidewire into the conus branch (CB) of the right coronary artery. The duration of the local electrogram after termination of the QRS complex (DP) was measured before and after class IC anti-arrhythmic drug administration. The signal-averaged electrocardiogram (SAECG) was also obtained in all patients.

RESULTS

A definite DP was observed at the epicardium, but not at the endocardium. After administration of a class IC anti-arrhythmic drug, the DP at the epicardium was prolonged from 38 +/- 10 ms to 67 +/- 24 ms. The late potential corresponding to the DP at the epicardium was observed in all patients on the SAECG.

CONCLUSIONS

An epicardial electrogram can be recorded from the CB. Recording from the CB enables identification of an epicardial abnormality in patients with the Brugada syndrome. These abnormal electrograms may be related to a myocardial abnormality in the epicardium of patients with the Brugada syndrome.

Electrophysiologic characteristics and implications of induced ventricular fibrillation in symptomatic patients with Brugada syndrome

OBJECTIVES

The study examined the electrocardiographic and electrophysiologic characteristics in relation to programmed ventricular stimulation (PVS)-induced ventricular fibrillation (VF), as well as the implications of PVS-induced VF on the recurrence of cardiac events in symptomatic Brugada syndrome.

BACKGROUND

Brugada syndrome is characterized by ST-segment elevation in the right precordial leads (V(1)-V(3)) and an episode of VF.

METHODS

Thirty-four symptomatic patients with Brugada syndrome (33 men and 1 woman; 44 +/- 12 years old) were classified into two groups according to the inducibility of VF with PVS: 22 patients with induced VF requiring direct cardioversion for termination (Induced VF group) and 12 patients without induced VF (Noninduced VF group).

RESULTS

The induced VF group showed a longer QRS duration, a higher incidence of right bundle branch block and late potentials detected on the signal-averaged electrocardiogram, longer His-ventricular intervals and a longer conduction time from the RVOT to the left ventricle at extrastimulation than those in the non-induced VF group. However, there was no significant difference in the recurrence of cardiac events (VF documented by an implantable cardioverter-defibrillator and sudden cardiac death) between the two groups (8 [36%] of 22 patients vs. 7 [58%] of 12 patients) during long-term follow-up (range 1 to 149 months; mean 38).

CONCLUSIONS

Our data suggest that induction of VF by PVS depends on the severity of depolarization abnormalities but does not predict the recurrence of cardiac events in symptomatic Brugada syndrome, indicating that both depolarization and repolarization abnormalities are important in the development of VF.

Clinical significance of the dispersion of the activation--recovery interval and recovery time as markers for ventricular fibrillation susceptibility in patients with Brugada syndrome

Brugada syndrome (BS) is associated with sudden cardiac death and the markers for ventricular fibrillation (VF) remain unclear, so the activation-recovery interval (ARI) dispersion and recovery time (RT) dispersion were investigated as possible markers in 20 subjects with BS (BS group) and 22 healthy individuals (H group). The 20 BS subjects were divided into 8 cases with documented VF (BS-VF group), 3 of which had recurrences, and 12 without (BS-N group). The corrected dispersion measurements from the standard 12-lead ECG of the QT interval (QTcd), ARI (ARIcd) and RT (RTcd) were compared among the groups. There were significant differences noted between the BS-VF and BS-N groups for the ARIcd and the RTcd, but not for the QTcd. Further, there were critical differences, 150 ms(1/2), observed for the ARIcd and RTcd, and these were associated with a prolongation of the maximum ARI or RT, shortening of the minimum ARI or RT, and prolongation only of the maximum QT for the QTcd. Susceptibility to VF may be predicted by the ARIcd or RTcd in BS.

Genetics of ventricular tachycardia

Pathogenesis of familial inherited arrhythmias is being progressively clarified thanks to the insights provided by molecular biology and by functional studies. Transmembrane or intracellular ion channel mutations have been identified in genetically determined forms of polymorphic ventricular tachycardia and sudden death such as catecholaminergic ventricular tachycardia, long QT syndrome, and Brugada syndrome. The role of molecular abnormalities in the genesis of monomorphic idiopathic ventricular tachycardias is less well defined, mainly because of the lack of a Mendelian pattern of inheritance. Interestingly, the presence of somatic mutations has been suggested as the mechanism for monomorphic ventricular tachycardia originating from the right ventricular outflow tract. The future goals for the application of molecular genetics to the management of cardiac arrhythmias will be to apply molecular genetics for a better risk stratification of affected individuals and to aim for the identification of gene-specific treatment of idiopathic ventricular tachycardia.

Na(+) channel mutation that causes both Brugada and long-QT syndrome phenotypes: a simulation study of mechanism

BACKGROUND

Complex physiological interactions determine the functional consequences of gene abnormalities and make mechanistic interpretation of phenotypes extremely difficult. A recent example is a single mutation in the C terminus of the cardiac Na(+) channel, 1795insD. The mutation causes two distinct clinical syndromes, long QT (LQT) and Brugada, leading to life-threatening cardiac arrhythmias. Coexistence of these syndromes is seemingly paradoxical; LQT is associated with enhanced Na(+) channel function, and Brugada with reduced function.

METHODS AND RESULTS

Using a computational approach, we demonstrate that the 1795insD mutation exerts variable effects depending on the myocardial substrate. We develop Markov models of the wild-type and 1795insD cardiac Na(+) channels. By incorporating the models into a virtual transgenic cell, we elucidate the mechanism by which 1795insD differentially disrupts cellular electrical behavior in epicardial and midmyocardial cell types. We provide a cellular mechanistic basis for the ECG abnormalities observed in patients carrying the 1795insD gene mutation.

CONCLUSIONS

We demonstrate that the 1795insD mutation can cause both LQT and Brugada syndromes through interaction with the heterogeneous myocardium in a rate-dependent manner. The results highlight the complexity and multiplicity of genotype-phenotype relationships, and the usefulness of computational approaches in establishing a mechanistic link between genetic defects and functional abnormalities.