Epinephrine unmasks latent mutation carriers with LQT1 form of congenital long-QT syndrome

A novel cardiac ryanodine receptor gene (RyR2) mutation in an athlete with aborted sudden cardiac death: a case of adult-onset catecholaminergic polymorphic ventricular tachycardia

Sudden cardiac death (SCD) in athletes <35 years of age are mostly due to congenital or acquired cardiac malformations or hypertrophic cardiomyopathy. However, ion channelopathies such as catecholaminergic polymorphic ventricular tachycardia (CPVT) or long-QT syndromes, which are less frequently observed, are also potential pathogenesis of SCD in young athletes. CPVT is an inherited arrhythmia that is induced by physical or emotional stress and may lead to ventricular fibrillation syncope or SCD. Here, we report a case of athlete woman with adult-onset CPVT and aborted SCD who has a novel missense mutation (K4392R) in the cardiac RyR2 gene.

Polymorphic ventricular tachycardia--part II: the channelopathies

In this article, we explore the clinical and cellular phenomena of primary electrical diseases of the heart, that is, conditions purely related to ion channel dysfunction and not structural heart disease or reversible acquired causes. This growing classification of conditions, once considered together as "idiopathic ventricular fibrillation," continues to evolve and segregate into diseases that are phenotypically, molecularly, and genetically unique.

Novel calmodulin mutations associated with congenital arrhythmia susceptibility

BACKGROUND

Genetic predisposition to life-threatening cardiac arrhythmias such as congenital long-QT syndrome (LQTS) and catecholaminergic polymorphic ventricular tachycardia (CPVT) represent treatable causes of sudden cardiac death in young adults and children. Recently, mutations in calmodulin (CALM1, CALM2) have been associated with severe forms of LQTS and CPVT, with life-threatening arrhythmias occurring very early in life. Additional mutation-positive cases are needed to discern genotype-phenotype correlations associated with calmodulin mutations.

METHODS AND RESULTS

We used conventional and next-generation sequencing approaches, including exome analysis, in genotype-negative LQTS probands. We identified 5 novel de novo missense mutations in CALM2 in 3 subjects with LQTS (p.N98S, p.N98I, p.D134H) and 2 subjects with clinical features of both LQTS and CPVT (p.D132E, p.Q136P). Age of onset of major symptoms (syncope or cardiac arrest) ranged from 1 to 9 years. Three of 5 probands had cardiac arrest and 1 of these subjects did not survive. The clinical severity among subjects in this series was generally less than that originally reported for CALM1 and CALM2 associated with recurrent cardiac arrest during infancy. Four of 5 probands responded to β-blocker therapy, whereas 1 subject with mutation p.Q136P died suddenly during exertion despite this treatment. Mutations affect conserved residues located within Ca(2+)-binding loops III (p.N98S, p.N98I) or IV (p.D132E, p.D134H, p.Q136P) and caused reduced Ca(2+)-binding affinity.

CONCLUSIONS

CALM2 mutations can be associated with LQTS and with overlapping features of LQTS and CPVT.

In silico screening of the impact of hERG channel kinetic abnormalities on channel block and susceptibility to acquired long QT syndrome

Accurate diagnosis of predisposition to long QT syndrome is crucial for reducing the risk of cardiac arrhythmias. In recent years, drug-induced provocative tests have proved useful to unmask some latent mutations linked to cardiac arrhythmias. In this study we expanded this concept by developing a prototype for a computational provocative screening test to reveal genetic predisposition to acquired long-QT syndrome (aLQTS). We developed a computational approach to reveal the pharmacological properties of IKr blocking drugs that are most likely to cause aLQTS in the setting of subtle alterations in IKr channel gating that would be expected to result from benign genetic variants. We used the model to predict the most potentially lethal combinations of kinetic anomalies and drug properties. In doing so, we also implicitly predicted ideal inverse therapeutic properties of K channel openers that would be expected to remedy a specific defect. We systematically performed "in silico mutagenesis" by altering discrete kinetic transition rates of the Fink et al. Markov model of human IKr channels, corresponding to activation, inactivation, deactivation and recovery from inactivation of IKr channels. We then screened and identified the properties of IKr blockers that caused acquired long QT and therefore unmasked mutant phenotypes for mild, moderate and severe variants. Mutant IKr channels were incorporated into the O'Hara et al. human ventricular action potential (AP) model and subjected to simulated application of a wide variety of IKr-drug interactions in order to identify the characteristics that selectively exacerbate the AP duration (APD) differences between wild-type and IKr mutated cells. Our results show that drugs with disparate affinities to conformation states of the IKr channel are key to amplify variants underlying susceptibility to acquired long QT syndrome, an effect that is especially pronounced at slow frequencies. Finally, we developed a mathematical formulation of the M54T MiRP1 latent mutation and simulated a provocative test. In this setting, application of dofetilide dramatically amplified the predicted QT interval duration in the M54T hMiRP1 mutation compared to wild-type.

Myocardial repolarization dispersion and autonomic nerve activity in a canine experimental acute myocardial infarction model

BACKGROUND

Evidence from a canine experimental acute myocardial infarction (MI) model shows that until the seventh week after MI, the relationship between stellate ganglion nerve activity (SGNA) and vagal nerve activity (VNA) progressively increases.

OBJECTIVE

The purpose of this study was to evaluate how autonomic nervous system activity influences temporal myocardial repolarization dispersion at this period.

METHODS

We analyzed autonomic nerve activity as well as QT and RR variability from recordings previously obtained in nine dogs. From a total of 48 short-term ECG segments, 24 recorded before and 24 recorded 7 weeks after experimentally-induced MI, we obtained three indices of temporal myocardial repolarization dispersion: QTe (from Q-wave to T-wave end), QTp (from Q-wave to T-wave peak), and Te (from T-wave peak to T-wave end) variability index (QTeVI, QTpVI, TeVI). We also performed heart rate variability power spectral analysis on the same segments.

RESULTS

After MI, all the QT variables increased QTeVI (median [interquartile range]) (from -1.76[0.82] to -1.32[0.68]), QTeVI (from -1.90[1.01] to -1.45[0.78]), and TeVI (from -0.72[0.67] to -0.22[1.00]), whereas all RR spectral indices decreased (P <.001 for all). Distinct circadian rhythms in QTeVI (P <.05,) QTpVI (P <.001) and TeVI (P <.05) appeared after MI with circadian variations resembling that of SGNA/VNA. The morning QTpVI and TeVI acrophases approached the SGNA/VNA acrophase. Conversely, the evening QTeVI acrophase coincided with another SGNA/VNA peak. After MI, regression analysis detected a positive relationship between SGNA/VNA and TeVI (R(2): 0.077; β: 0.278; p< 0.001).

CONCLUSION

Temporal myocardial repolarization dispersion shows a circadian variation after MI reaching its peak at a time when sympathetic is highest and vagal activity lowest.

Long QT syndrome: beyond the causal mutation

Congenital long QT syndrome (LQTS) is caused by single autosomal-dominant mutations in a gene encoding for a cardiac ion channel or an accessory ion channel subunit. These single mutations can cause life-threatening arrhythmias and sudden death in heterozygous mutation carriers. This recognition has been the basis for world-wide staggering numbers of subjects and families counselled for LQTS and treated based on finding (putative) disease-causing mutations. However, prophylactic treatment of patients is greatly hampered by the growing awareness that simple carriership of a mutation often fails to predict clinical outcome: many carriers never develop clinically relevant disease while others are severely affected at a young age. It is still largely elusive what determines this large variability in disease severity, where even within one pedigree, an identical mutation can cause life-threatening arrhythmias in some carriers while in other carriers no disease becomes clinically manifested. This suggests that additional factors modify the clinical manifestations of a particular disease-causing mutation. In this article, potential demographic, environmental and genetic factors are reviewed, which, in conjunction with a mutation, may modify the phenotype in LQTS, and thereby determine, at least partially, the large variability in disease severity.

Influence of aging and chronic heart failure on temporal dispersion of myocardial repolarization

Background and purpose:

QT and T_peak-T_end (Te) intervals are associated with sudden cardiac death in patients with chronic heart failure (CHF). We studied age-dependent influence on short-term temporal dispersion of these two variables in patients with postischemic CHF.

Method:

We grouped 75 CHF and 53 healthy control subjects into three age subsets: ≤50 years, >50 years and ≤65 years, and >65 years. We then calculated the following indices: QT and Te variability index (QTVI and TeVI), the ratio between the short-term variability (STV) of QT or Te, and the STV of resting rate (RR) (QT/RR STV and Te/RR STV).

Results:

In all different age subgroups, patients with CHF showed a higher level of QTVI than age-matched control subjects (≤50 years: P < 0.0001; >50 years and ≤65 years: P < 0.05; >65 years: P < 0.05). Patients with CHF < 50 years old also had all repolarization variability indices higher than normal age-matched controls (TeVI, P < 0.05; QT/RR STV, P < 0.05; Te/RR STV, P < 0.05), whereas we did not find any difference between the two older classes of subjects. Both QTVI (r²: 0.178, P < 0.05) and TeVI (r²: 0.433, P < 0.001) were positively related to age in normal subjects, even if the first correlation was weaker than the second one.

Conclusion:

Our data showed that QTVI could be used in all ages to evaluate repolarization temporal liability, whereas the other indices are deeply influenced by age. Probably, the age-dependent increase in QTVI was more influenced by a reduction of RR variability reported in older normal subjects.

A case of Long QT syndrome type 3 aggravated by beta-blockers and alleviated by mexiletine: the role of epinephrine provocation test

Long QT syndrome (LQTs) is an uncommon genetic disease causing sudden cardiac death with Torsade de Pointes (TdP). The first line drug treatment has been known to be β-blocker. We encountered a 15-year-old female student with LQTs who had prolonged QTc and multiple episodes of syncope or agonal respiration during sleep. Although her T wave morphology in surface electrocardiography resembled LQTs type 1, her clinical presentation was unusual. During the epinephrine test, TdP was aggravated during β-blocker medication, but alleviated by sodium channel blocker (mexiletine). Therefore, she underwent implantable cardioverter defibrillator implantation.

Low clinical penetrance in causal mutation carriers for cardiac channelopathies

INTRODUCTION AND OBJECTIVES

Cardiac channelopathies are genetic alterations that can cause sudden death. Long QT syndrome and Brugada syndrome are 2 such conditions. Both are diagnosed according to previously published criteria. Our objective was to determine the sensitivity of these criteria in a consecutive series of patients carrying the mutations that cause them.

METHODS

We enrolled 15 families and 31 causal mutation carriers with a high pathogenic probability of having long QT syndrome and Brugada syndrome. We conducted clinical and electrocardiographic studies to analyze the extent to which these patients fulfilled the diagnostic criteria. Statistical analysis was with SPSS 17.0.

RESULTS

Some 48.3% of the subjects met the criteria indicating a high probability of long QT syndrome or Brugada syndrome. Among those with the mutation for long QT syndrome, only 10 out of 21 had a Schwartz index score ≥ 4. Both the median Schwartz score and the cQT interval were lower in relatives than in probands. Of those with the mutation for Brugada syndrome, 60% failed to meet current diagnostic criteria, which were more frequently fulfilled in relatives. Pharmacological tests with epinephrine and flecainide helped establish the diagnosis in 2 mutation carriers with negative phenotype.

CONCLUSIONS

Current diagnostic criteria for long QT syndrome and Brugada syndrome had low sensitivity in our sample of genetic carriers. Genetic tests supported by pharmacological tests can increase diagnostic sensitivity, especially in asymptomatic relatives.

Assessment of genetic causes of cardiac arrest

Unexplained cardiac arrest is defined as a cardiac arrest in the absence of coronary artery disease and overt structural heart disease, present in 5%-10% of cardiac arrest survivors. A genetic contribution to cardiac arrest is more common in this population, most commonly attributed to an inherited ion channel abnormality leading to familial syncope and sudden death. The common causes are Long QT and Brugada syndrome, catecholaminergic ventricular tachycardia, idiopathic ventricular fibrillation, and early repolarization syndrome. Latent structural causes include inherited cardiomyopathy such as arrhythmogenic right ventricular cardiomyopathy. We review these causes in detail and a structured approach to the investigation of these patients, which provides a diagnosis in approximately half of these patients. This allows for the initiation of disease-specific treatments and enables family screening.

Epinephrine Infusion in the Evaluation of Unexplained Cardiac Arrest and Familial Sudden Death

Background—

Epinephrine infusion may unmask latent genetic conditions associated with cardiac arrest, including long-QT syndrome and catecholaminergic polymorphic ventricular tachycardia (VT).

Methods and Results—

Patients with unexplained cardiac arrest (normal left ventricular function and QT interval) and selected family members from the Cardiac Arrest Survivors with Preserved Ejection Fraction Registry (CASPER) registry underwent epinephrine challenge at doses of 0.05, 0.10, and 0.20 μg/kg per minute. A test was considered positive for long-QT syndrome if the absolute QT interval prolonged by ≥30 ms at 0.10 μg/kg per minute and borderline if QT prolongation was 1 to 29 ms. Catecholaminergic polymorphic VT was diagnosed if epinephrine provoked ≥3 beats of polymorphic or bidirectional VT and borderline if polymorphic couplets, premature ventricular contractions, or nonsustained monomorphic VT was induced. Epinephrine infusion was performed in 170 patients (age, 42±16 years; 49% men), including 98 patients with unexplained cardiac arrest. Testing was positive for long-QT syndrome in 31 patients (18%) and borderline in 24 patients (14%). Exercise testing provoked an abnormal QT response in 42% of tested patients with a positive epinephrine response. Testing for catecholaminergic polymorphic VT was positive in 7% and borderline in 5%. Targeted genetic testing of abnormal patients was positive in 17% of long-QT syndrome patients and 13% of catecholaminergic polymorphic VT patients.

Conclusions—

Epinephrine challenge provoked abnormalities in a substantial proportion of patients, most commonly a prolonged QT interval. Exercise and genetic testing replicated the diagnosis suggested by the epinephrine response in a small proportion of patients. Epinephrine infusion combined with exercise testing and targeted genetic testing is recommended in the workup of suspected familial sudden death syndromes.

Clinical Trial Registration—

URL: http://www.clinicaltrials.gov . Unique identifier: NCT00292032.

Repolarization syndromes

Repolarization syndromes, including early repolarization, Brugada, and short and long QT, have been implicated increasingly as causes of sudden cardiac death (SCD) despite no obvious mechanical cardiac abnormalities. So-called idiopathic ventricular fibrillation is now often reassigned to one of the aforementioned entities. Underlying causes are diverse; genetic mutation has been proven in many but not all cases. Although high-risk individuals generally can be identified, most of the potential victim pool is still unknown and cannot be discovered at this time. Awareness of these entities' existence, knowledge of family history, and 12-lead electrocardiography are the initial steps toward preventing SCD in this population. Underlying mechanisms for ventricular tachycardia/fibrillation in such individuals include phase 2 reentry, early after depolarization, and vortex reentry. For the time-being, although most forms of long QT syndrome can be treated with β-blockers, an implantable cardioverter-defibrillator remains the only definitive therapy for the prevention of arrhythmic death among high-risk populations.

Inherited long QT syndrome: clinical manifestation, genetic diagnostics, and therapy

Inherited long QT syndrome (LQTS) is characterized by a prolonged ventricular repolarization (QTc interval) and symptoms (syncope, sudden cardiac arrest) due to polymorphic ventricular arrhythmias. As of today, 13 different cardiac ion channel genes have been associated with congenital LQTS. The most common ones are due to KCNQ1 (LQT-1), KCNH2 (LQT-2), and SCN5A (LQT-3) gene mutations and account for up to 75 % of cases. Typical clinical findings are an increased QT interval on the surface electrocardiogram, specifically altered T wave morphologies, polymorphic ventricular arrhythmias, or an indicative family history. Recently, in the HRS/EHRA expert consensus statement, comprehensive genetic testing of major LQTS genes was recommended for index patients for whom there is a strong clinical suspicion of LQTS. Overall, antiadrenergic therapy, in particular β-receptor blockers, has been the mainstay of therapy and has significantly reduced cardiac events. For high-risk patients, an implantable cardioverter defibrillator (ICD) is recommended. Importantly, lifestyle modification and avoidance of arrhythmia triggers are additional important approaches.

Autonomic nerve activity and the short-term variability of the Tpeak-Tend interval in dogs with pacing-induced heart failure

BACKGROUND

In congestive heart failure (CHF), autonomic nervous system (ANS) activity is known to modulate arrhythmic risk through its effects on myocardial repolarization. An increased interval between the peak and the end of the T wave (T(peak)-T(end)) has been reported to increase the incidence of sudden cardiac death. However, the ANS influence on the T(peak)-T(end) interval remains unclear.

OBJECTIVE

We directly measured ANS nerve activity in ambulatory dogs with pacing-induced CHF to test the hypothesis that ANS activity modulates the T(peak)-T(end) variability index (T(peak)-T(end)VI), the short-term variability of the T(peak)-T(end) interval obtained on 30 beats (T(peak)-T(end)STV(30)), and the short-term variability of the T(peak)-T(end) interval obtained on 5-minute ECG recording (T(peak)-T(end)STV(T)).

METHODS

By using data previously recorded in 6 ambulatory dogs before and after pacing-induced CHF, we assessed ANS activity recorded with an implanted radiotransmitter that monitored integrated left stellate ganglion nervous activity (iSGNA), integrated vagus nerve activity (iVNA), and electrocardiogram (ECG). We selected for analysis 36 segments recorded at baseline and 36 after pacing-induced CHF with similar iSGNA.

RESULTS

During CHF, T(peak)-T(end)STV(30) (P<.001) and T(peak)-T(end)STV(T) (P<.05) were significantly higher than those at baseline. The multiple linear mixed regression analysis disclosed a significant positive correlation between iSGNA and T(peak)-T(end)STV(T) (baseline: β 2.92, P<.001; CHF: β 1.13, P<.001) and a significant negative correlation between iVNA and T(peak)-T(end)STV(T) (baseline: β-6.74, P<.001; CHF: β-1.42, P< .001).

CONCLUSIONS

In a canine model of pacing-induced CHF, iSGNA correlates positively while iVNA correlates negatively with T(peak)-T(end)STV(T). These findings suggest that SGNA increases while VNA decreases the dispersion of ventricular repolarization in ambulatory dogs with CHF.

Congenital long and short QT syndromes

Congenital long and short QT syndromes are familial arrhythmias characterized by derangement of repolarization and a high risk of sudden cardiac death due to ventricular tachyarrhythmias. With growing understanding of these syndromes in both the medical and lay communities, diagnostic and therapeutic difficulties are increasingly faced by health care providers. Modern genomics has determined the mechanism of arrhythmia induction in these patients, resulting in specific medical therapies and improved risk stratification. This paper reviews the common presentations, genetic etiology, basic evaluation, risk stratification, and therapeutic approach for both syndromes. Particular attention is paid to the effect of the individual syndrome on the cardiac action potential and its correlate the surface 12 lead ECG. In conclusion, patients with long and short QT syndromes are at risk for sudden death, with accurate diagnosis, risk stratification, and resulting appropriate therapy favorably altering their outcome.

The J wave and fragmented QRS complexes in inferior leads associated with sudden cardiac death in patients with chronic heart failure

AIMS

To investigate the relationship between electrocardiogram (ECG) parameters [J wave, fragmented QRS (fQRS), QTc, the peak-to-end interval of T wave (Tp-Te)], and sudden cardiac death (SCD) in chronic heart failure (CHF).

METHODS AND RESULTS

The ECGs of 1570 CHF patients, 572 cases with dilated cardiomyopathy (DCM) and 998 cases with ischaemic cardiomyopathy (ICM) were analysed with the endpoint being an SCD or non-SCD (NSCD). During a median follow-up period of 36 months (0.40-65 months), 438 (27.89%) patients died, of which 158 (35.84%) were SCD. Overall, the occurrence of J wave, fQRS, and long Tp-Te were greater in SCD patients than that of NSCD patients (all P< 0.01). For DCM cases, more SCD patients had J waves observed in the inferior leads than that in the NSCD group (26.78 vs. 13.07%, P<0.001). However, ICM cases with SCD did have more fQRS in the inferior leads than that with NSCD (42.16 vs. 26.67%, P= 0.01). After adjusting for other risk factors, Cox regression analysis revealed that presence of J wave or fQRS in the inferior leads predicted a higher risk for SCD in DCM [hazard ratio (HR), 4.095; 95% confidence interval (CI), 2.132-7.863] and ICM (HR, 2.714; 95% CI, 1.809-4.072) patients. A left ventricular ejection fraction ≤ 30% also predicted SCD and NSCD in DCM and ICM patients. In contrast, the predictive value of QTc and Tp-Te for SCD was not significant.

CONCLUSIONS

Presence of J wave or fQRS in the inferior leads predicted higher risk of SCD in DCM and ICM patients and might serve as independent predictors for SCD in patients with CHF.

The Evaluation of a Borderline Long QT Interval in an Asymptomatic Patient

QT prolongation on resting electrocardiography (ECG) is common, and the clinician is often challenged by the dilemma of excluding acquired causes and recognizing potential congenital long QT syndrome (LQTS). The hallmark of LQTS is an abnormally long QT interval. However, a normal or borderline long QT interval may be observed in up to 50% of patients with LQTS because of the intermittent nature of QT prolongation. This review presents an approach to evaluating the asymptomatic patient with a borderline long QT interval, which incorporates a comprehensive clinical assessment, rest and provocative ECG testing, and genetic testing when appropriate.

Genotype- and Sex-Specific QT-RR Relationship in the Type-1 Long-QT Syndrome

BACKGROUND

Genotype-phenotype investigations have revealed significantly larger risk for cardiac events in patients with type 1 long-QT syndrome (LQT-1), particularly in adult females, with missense mutation in the cytoplasmic loop (C-loop) regions of the α subunit of the KCNQ1 gene associated with an impaired ion channel activation by adrenergic stimulus. We hypothesize that the impaired response to increases in heart rate leads to abnormal QT-RR dynamic profiles and is responsible for the increased cardiac risk for these patients.

METHODS AND RESULTS

We measured the QT-RR slope in 24-hour Holter ECGs from LQT-1 patients with the mutations associated with impaired adrenergic stimulus (C-loop, n=18) and compared to LQT-1 patients with other mutations (non-C-loop, n=48), and to a healthy control group (n=195). The diurnal QT-RR slope was less steep in C-loop mutation patients (0.10±0.05) than in the ECGs from non-C-loop mutation patients (0.17±0.09, P=0.002). For female patients, slower heart rates were associated with prolonged QT and increased QT-RR slope. Male patients with C-loop mutations showed an impaired repolarization for shorter range of heart rates than in females, which is consistent with gender differences in triggers for events in this syndrome.

CONCLUSIONS

Our observations suggest that the C-loop LQT-1 patients have specific impaired adrenergic regulation of the ventricular repolarization. This response to heart rate increases may be useful in identification of high-risk patients with inherited prolonged QT and may help select an optimal antiarrhythmic therapeutic strategy. (J Am Heart Assoc. 2012;1:e000570 doi: 10.1161/JAHA.112.000570.).

Potassium-channel mutations and cardiac arrhythmias--diagnosis and therapy

The coordinated generation and propagation of action potentials within cardiomyocytes creates the intrinsic electrical stimuli that are responsible for maintaining the electromechanical pump function of the human heart. The synchronous opening and closing of cardiac Na(+), Ca(2+), and K(+) channels corresponds with the activation and inactivation of inward depolarizing (Na(+) and Ca(2+)) and outward repolarizing (K(+)) currents that underlie the various phases of the cardiac action potential (resting, depolarization, plateau, and repolarization). Inherited mutations in pore-forming α subunits and accessory β subunits of cardiac K(+) channels can perturb the atrial and ventricular action potential and cause various cardiac arrhythmia syndromes, including long QT syndrome, short QT syndrome, Brugada syndrome, and familial atrial fibrillation. In this Review, we summarize the current understanding of the molecular and cellular mechanisms that underlie K(+)-channel-mediated arrhythmia syndromes. We also describe translational advances that have led to the emerging role of genetic testing and genotype-specific therapy in the diagnosis and clinical management of individuals who harbor pathogenic mutations in genes that encode α or β subunits of cardiac K(+) channels.

The phenomenon of "QT stunning": the abnormal QT prolongation provoked by standing persists even as the heart rate returns to normal in patients with long QT syndrome

BACKGROUND

Patients with long QT syndrome (LQTS) have inadequate shortening of the QT interval in response to the sudden heart rate accelerations provoked by standing-a phenomenon of diagnostic value. We now validate our original observations in a cohort twice as large. We also describe that this abnormal QT-interval response persists as the heart rate acceleration returns to baseline.

OBJECTIVES

To describe a novel observation, termed "QT stunning" and to validate previous observations regarding the "QT-stretching" phenomenon in patients with LQTS by using our recently described "standing test."

METHODS

The electrocardiograms of 108 patients with LQTS and 112 healthy subjects were recorded in the supine position. Subjects were then instructed to stand up quickly and remain standing for 5 minutes during continuous electrocardiographic recording. The corrected QT interval was measured at baseline (QTc(base)), when heart rate acceleration without appropriate QT-interval shortening leads to maximal QT stretching (QTc(stretch)) and upon return of heart rate to baseline (QTc(return)).

RESULTS

QTc(stretch) lengthened significantly more in patients with LQTS (103 ± 80 ms vs 66 ± 40 ms in controls; P <.001) and so did QTc(return) (28 ± 48 ms for patients with LQTS vs -3 ± 32 ms for controls; P <.001). Using a sensitivity cutoff of 90%, the specificity for diagnosing LQTS was 74% for QTc(base), 84% for QTc(return), and 87% for QTc(stretch).

CONCLUSIONS

The present study extends our previous findings on the abnormal response of the QT interval in response to standing in patients with LQTS. Our study also shows that this abnormal response persists even after the heart rate slows back to baseline.

β-blockers protect against dispersion of repolarization during exercise in congenital long-QT syndrome type 1

INTRODUCTION

β-Blocker therapy reduces syncope and sudden death in long-QT syndrome type 1 (LQT1), but the mechanism of protection is incompletely understood. This study tested the hypothesis that β-blockade reduces QT prolongation and dispersion of repolarization, measured as the T peak-to-end interval (T(pe) ), during exercise and recovery in LQT1 patients.

METHODS AND RESULTS

QT and T(pe) were measured in 10 LQT1 patients (33 ± 13 years) and 35 normal subjects (32 ± 12 years) during exercise tests on and off β-blockade. In LQT1 patients, β-blockade reduced QT (391 ± 25 milliseconds vs 375 ± 26 milliseconds, P = 0.04 during exercise; 419 ± 41 milliseconds vs 391 ± 39 milliseconds, P = 0.02 during recovery) and markedly reduced T(pe) (91 ± 26 milliseconds vs 67 ± 19 milliseconds, P = 0.03 during exercise; 103 ± 26 milliseconds vs 78 ± 11 milliseconds, P = 0.02 during recovery). In contrast, in normal subjects, β-blockade had no effect on QT (320 ± 17 milliseconds vs 317 ± 16 milliseconds, P = 0.29 during exercise; 317 ± 13 milliseconds vs 315 ± 14 milliseconds, P = 0.15 during recovery) and mildly reduced T(pe) (69 ± 13 milliseconds vs 61 ± 11 milliseconds, P = 0.01 during exercise; 77 ± 19 milliseconds vs. 68 ± 14 milliseconds, P < 0.001 during recovery).

CONCLUSION

In LQT1 patients, β-blockers reduced QT and T(pe) during exercise and recovery, supporting the theory that β-blocker therapy protects LQT1 patients by reducing dispersion of repolarization during exercise and recovery.

Epinephrine bolus test in detecting long QT syndrome mutation carriers with indeterminable electrocardiographic phenotype

BACKGROUND

In long QT syndrome (LQTS), prolonged and heterogeneous ventricular repolarization predisposes to serious arrhythmias. We examined how QT intervals are modified by epinephrine bolus in mutation carriers of three major LQTS subtypes with indefinite QT interval.

METHODS

Genotyped, asymptomatic subjects with LQTS type 1 (LQT1; n = 10; four different KCNQ1 mutations), type 2 (LQT2; n = 10; three different HERG mutations), and type 3 (LQT3; n = 10; four different SCN5A mutations), and healthy volunteers (n = 15) were examined. Electrocardiogram was recorded with body surface potential mapping system. After an epinephrine 0.04 μg/kg bolus QT end, QT apex, and T-wave peak-to-end (Tpe) intervals were determined automatically as average of 12 precordial leads. Standard deviation (SD) of the 12 channels was calculated.

RESULTS

Heart rate increased 26 ± 10 bpm with epinephrine bolus, and similarly in all groups. QT end interval lengthened, and QT apex interval shortened in LQTS and normals, leading to lengthening of Tpe interval. However, the lengthening in Tpe was larger in LQTS than in normals (mean 32 vs 18 ms; P < 0.05) and SD of QT apex increased more in LQTS than in normals (mean 23 vs 7 ms; P < 0.01). The increase in Tpe was most pronounced in LQT2, and in SD of QT apex in LQT1 and LQT2.

CONCLUSIONS

Abrupt adrenergic stimulation with a moderate dose of exogenous epinephrine affects ventricular repolarization in genotype-specific fashion facilitating distinction from normals. This delicate modification may help in diagnosing electrocardiographically silent mutation carriers when screening LQTS family members.

Phenotypic Manifestations of Mutations in Genes Encoding Subunits of Cardiac Potassium Channels

Since 1995, when a potassium channel gene, hERG (human ether-à-go-go-related gene), now referred to as KCNH2 , encoding the rapid component of cardiac delayed rectifier potassium channels was identified as being responsible for type 2 congenital long-QT syndrome, a number of potassium channel genes have been shown to cause different types of inherited cardiac arrhythmia syndromes. These include congenital long-QT syndrome, short-QT syndrome, Brugada syndrome, early repolarization syndrome, and familial atrial fibrillation. Genotype-phenotype correlations have been investigated in some inherited arrhythmia syndromes, and as a result, gene-specific risk stratification and gene-specific therapy and management have become available, particularly for patients with congenital long-QT syndrome. In this review article, the molecular structure and function of potassium channels, the clinical phenotype due to potassium channel gene mutations, including genotype-phenotype correlations, and the diverse mechanisms underlying the potassium channel gene–related diseases will be discussed.

Effects of head-up tilt-table test on the QT interval

BACKGROUND

The QT interval shortens in response to sympathetic stimulation and its response to epinephrine infusion (in healthy individuals and patients with long QT syndrome) has been thoroughly studied. Head-up tilt-table (HUT) testing is an easy way to achieve brisk sympathetic stimulation. Yet, little is known about the response of the QT interval to HUT.

METHODS

We reviewed the electrocardiograms of HUT tests performed at our institution and compare the heart rate, QT, and QTc obtained immediately after HUT with the rest values.

RESULTS

The study group consisted of 41 patients (27 females and 14 males) aged 23.9 +/- 8.4 years. Head-up tilting led to a significant shortening of the RR interval (from 825 +/- 128 msec at rest phase to 712 +/- 130 msec in the upward tilt phase, P < 0.001) but only to a moderate shortening of the QT interval (from 363.7 +/- 27.9 msec during rest to 355 +/- 30.3 msec during upward tilt, P = 0.001). Since the RR interval shortened more than the QT interval, the QTc actually increased (from 403 +/- 21.5 msec during rest phase to 423.2 +/- 27.4 msec during upward tilt, P < 0.001). The QTc value measured for the upward tilt position was longer than the resting QTc value in 33 of 41 patients. Of those, 4 male patients and 2 female patients developed upward-tilt QTc values above what would be considered abnormal at rest.

CONCLUSIONS

During HUT the QT shortens less than the RR interval. Consequently, the QTc increases during head-up tilt.

The genetic and clinical features of cardiac channelopathies

Sudden cardiac death, secondary to malignant ventricular arrhythmias, has traditionally been associated with structural heart disease. An important exception includes a group of clinical entities referred to as 'channelopathies' that develop secondary to genetic mutations, which alter cardiac ion channel activity. Otherwise healthy individuals affected by these forms of primary electrical disease are vulnerable to fatal arrhythmic events from a very young age. At present, there are four distinct conditions that are classified as cardiac channelopathies, namely congenital long-QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia and short-QT syndrome. Our growing insight into the genetics of these conditions has led to an improved understanding of the molecular pathophysiology responsible for the malignant arrhythmias characterizing these disorders. However, despite our knowledge of these conditions, the success of medical therapy remains modest and the prevention of sudden cardiac death may necessitate insertion of an implantable cardioverter-defibrillator. The young age of affected patients makes this a particularly undesirable treatment strategy and emphasizes the importance of translating our insight into the molecular pathophysiology defining these conditions into more effective forms of therapy.

The role of the epinephrine test in the diagnosis and management of children suspected of having congenital long QT syndrome

The epinephrine test has been shown to be a powerful tool to predict the genotype of congenital long QT syndrome (LQTS). The aim of this study was to evaluate its role in the diagnosis and management of LQTS in children. The test (using the Shimizu protocol) was conducted in patients with some evidence of LQTS but in whom clinical and management decisions were challenging (n = 41, age 9.6 +/- 3.9 years, 19 female). LQT1, LQT2, and negative responses to epinephrine were obtained in 16, 5, and 20 subjects, respectively. LQTS gene positivity was obtained in two subjects. Beta-blocker therapy was started in all subjects with a positive epinephrine response (n = 21) and in some negative responders because of their strong LQTS phenotype (n = 10). No therapy was given to the subset with less convincing features of LQTS who had also responded negatively to epinephrine (n = 10). Follow-up for 3.0 +/- 2 years was uneventful in both management groups. Due to the discordance with genotyping, the epinephrine test cannot be used to diagnose genotype-positive LQTS but when used in combination with phenotype assessment and genetic screening, it could enable better management decisions.

Utility of treadmill testing in identification and genotype prediction in long-QT syndrome

BACKGROUND

The clinical diagnosis of long-QT syndrome (LQTS) remains challenging when ECG abnormalities are borderline or intermittent. Despite issues with access, cost, and heterogeneity of LQTS mutations, genetic testing remains the diagnostic gold standard for diagnosis of LQTS. We sought to develop a provocative testing strategy to unmask the LQTS phenotype and relate this to the results of genetic testing.

METHODS AND RESULTS

From 1995 to 2008, 159 consecutive patients with suspected LQTS underwent provocative testing that consisted of a modified Bruce protocol treadmill exercise test, with ECGs recorded supine at rest, immediately on standing, and at 1-minute intervals during exercise, at peak exercise, and at 1-minute intervals during the recovery phase. Similar testing was carried out on a stationary bike in a gradual and burst exercise fashion. LQTS was confirmed with genotyping in all 95 affected LQTS patients and excluded with negative family screening in 64 control subjects. Patients were studied before and after initiation of beta-blockers. Of 159 patients, 50 had an LQT1 mutation and 45 had an LQT2 mutation. In the LQTS group, 44.3% of patients had a normal-to-borderline resting QTc interval. LQTS patients exhibited a greater prolongation in QTc with postural change than unaffected patients (LQT1: 40 ms [IQR, 42]; LQT2: 35 ms [IQR, 46]; and LQTS-negative: 21 ms [IQR, 37]; P=0.029). During exercise, LQT1 patients had marked QTc prolongation compared with LQT2 and LQTS-negative patients (LQT1: 65 ms [60], LQT2: 3 ms [46], LQTS negative: 5 ms [41]; P<0.0001). QT hysteresis was more pronounced in patients with LQT2 mutations compared with LQT1 and LQT-negative patients (LQT2: 40 ms [10], LQT1: 15 ms [40]; LQTS-negative: 20 ms [20]; P<0.001). beta-Blockade normalized the QTc changes seen with standing and QT hysteresis.

CONCLUSIONS

The presence and genotype of LQTS can be predicted by a combination of postural and exercise changes in the QT/RR relationship. beta-Blockade normalized these changes. Routine exercise testing is useful in predicting and directing genetic testing in LQTS.

Post myocardial ischemia-associated torsades de pointes in a patient carrying a KCNQ1 G643S variant

Polymorphic ventricular tachycardia, which occurs during the subacute phase of myocardial infarction (MI) or ischemia and is not related to ongoing ischemia, has recently been reported. It has characteristics of typical pause-dependent torsades de pointes (TdP) following excessive QT prolongation (post MI/ischemia-associated TdP). We describe a male patient with post MI/ischemia-associated TdP. The patient experienced recurrent TdP with excessive QT prolongation 2 days after transient myocardial ischemia. Genetic screening of the major LQTS-causing genes identified a KCNQ1 G643S variant. This gene variant could be a genetic predisposition to the development of TdP during the subacute phase of MI/ischemia.

Abrupt termination of an ethanol regimen provokes ventricular arrhythmia and enhances susceptibility to the arrhythmogenic effects of epinephrine in rats

BACKGROUND

Pathologists examining victims of sudden unexpected death encounter alcoholics more often than expected; alcohol may play a role in sudden arrhythmic death. Here we determine whether a pattern of alcohol consumption, chronic ethanol intake, and withdrawal increases the incidence of malignant ventricular arrhythmia and modulates susceptibility to the arrhythmogenic potential of sympathetic stimulation from an epinephrine test in rats.

METHODS

Male Wistar rats were treated with a continuous ethanol liquid diet for 7 weeks, and then subjected to 1-day withdrawal or 21-day abstinence. Ventricular ectopy was evaluated by 24-hour electrocardiographic telemetry recording; whole-body sympathetic activation, cardiac sympathovagal balance, and susceptibility to ventricular arrhythmia induced by sympathetic stimulation were evaluated based on blood noradrenalin metabolite concentrations, heart rate variability, and a 3-step epinephrine test.

RESULTS

Ventricular arrhythmia and related death were observed only in rats at 1 day of withdrawal, but not in nonalcoholic, continuous ethanol intake or 21-day abstinence rats. One-day withdrawal after a 7-week continuous ethanol regimen elevated circulating noradrenalin metabolite levels and induced cardiac sympathovagal imbalance. Deaths related to the epinephrine test and ventricular arrhythmia induced by low doses of epinephrine were observed only in 1-day withdrawal rats. However, all anomalies were normalized by 21-day abstinence.

CONCLUSIONS

Abrupt termination of a 7-week continuous ethanol regimen is sufficient to enhance the whole-body sympathetic activation and cardiac sympathovagal imbalance that contribute to ventricular arrhythmia and sudden death in alcoholic rats. Those providing medical care for alcoholics, including in cases of legal imprisonment, should be aware of the possibility of enhanced susceptibility to sudden arrhythmic death due to the abrupt termination of a chronic ethanol regimen.

Tpeak-Tend interval and Tpeak-Tend/QT ratio as markers of ventricular tachycardia inducibility in subjects with Brugada ECG phenotype

AIMS

The present study investigated whether several ECG markers of ventricular repolarization are associated with ventricular tachycardia/fibrillation (VT/VF) inducibility in subjects with type 1 ECG pattern of Brugada syndrome (BS).

METHODS AND RESULTS

The clinical data of 23 individuals (19 males, age 42.69 +/- 14.63) with spontaneous (n = 10) or drug-induced (n = 13) type 1 ECG pattern of BS who underwent programmed ventricular stimulation were analysed. Sustained VT/VF was induced in 17 subjects (74%) and was significantly associated with the presence of spontaneous type 1 ECG of BS (P = 0.012). Among the studied ECG repolarization markers, subjects with inducible VT/VF displayed an increased T(peak)-T(end) interval in leads V(2) (88.82 +/- 15.70 vs. 78.33 +/- 4.08 ms, P = 0.02) and V(6) (76.33 +/- 10.08 vs. 66.66 +/- 5.16 ms, P = 0.04) and a greater T(peak)-T(end)/QT ratio in lead V(6) (0.214 +/- 0.028 vs. 0.180 +/- 0.014, P = 0.009) compared with those without arrhythmias. Ventricular tachycardia/fibrillation inducibility was not associated with arrhythmic events during a mean follow-up period of 4.61 +/- 2.14 years (P = 0.739).

CONCLUSION

The T(peak)-T(end) interval and T(peak)-T(end)/QT ratio were associated with VT/VF inducibility in BS. The utility of T(peak)-T(end)/QT ratio as a new marker of arrhythmogenesis in BS requires further studies, including a large number of patients.

The risk of long QT syndrome in the pediatric population

PURPOSE OF REVIEW

Advances in understanding the biophysical underpinnings of long QT syndrome have provided growing insight into the risk of this syndrome in the pediatric population. This review focuses on developments in this area as reflected in the recent literature.

RECENT FINDINGS

QT interval prolongation on the surface ECG is the hallmark of long QT syndrome. This prolongation reflects protracted ventricular repolarization, primarily due to mutations in genes coding for cardiac ion channels. To date, 12 different genes have been implicated, and current genetic testing methods can provide a specific diagnosis in approximately 70% of patients. Clinical indicators, including age, sex, corrected QT duration, and prior syncope are the most powerful predictors of future life-threatening cardiac events. However, diagnosis, risk assessment, and therapeutic strategies are being guided by genetic analysis to an increasing degree.

SUMMARY

Impressive advancements have been made in understanding the genetic and clinical determinants of this heterogeneous syndrome. As genetic testing techniques become more robust, the ability to assess risk in affected individuals and tailor therapy will improve.

Epinephrine-Induced Polymorphic Ventricular Tachycardia in a Patient With Congenital Long QT Syndrome

A 24-year-old woman presented to the department of plastic surgery for surgical excision of a nevus on her nose. Although her history failed to reveal any cardiac disease, her pre-operative electrocardiogram (ECG) showed an extremely prolonged QT interval of up to 528 msec. Repeated history-taking after admission revealed three syncopal episodes associated with both physical and emotional stress, and because the two-dimensional echocardiography and exercise ECG test were normal except for the prolonged QT interval, an epinephrine test was done to assess QT interval changes after an epinephrine infusion. Immediately after a bolus injection of epinephrine (0.1 µg/kg), marked prolongation of the QT interval developed, followed by polymorphic ventricular tachycardia which was immediately terminated with direct current shock, resulting in the diagnosis of a long QT syndrome (LQTS), probably type 1. Gene studies were recommended, but declined by the patient and her family. She was instructed to avoid competitive sports, and a β-blocker was prescribed after which she remained symptom-free.

Systematic Assessment of Patients With Unexplained Cardiac Arrest

Background— Cardiac arrest without evident cardiac disease may be caused by subclinical genetic conditions. Provocative testing to unmask a phenotype is often necessary to detect primary electrical disease, direct genetic testing, and perform family screening.

Methods and Results— Patients with apparently unexplained cardiac arrest and no evident cardiac disease (normal cardiac function on echocardiogram, no evidence of coronary artery disease, and a normal ECG) underwent systematic evaluation that included cardiac magnetic resonance imaging, signal-averaged ECG, exercise testing, drug challenge, and selective electrophysiological testing. Diagnostic criteria were based on accepted criteria or provocation of the characteristic clinical features for long-QT syndrome, catecholaminergic polymorphic ventricular tachycardia, Brugada syndrome, early repolarization, arrhythmogenic right ventricular cardiomyopathy, coronary spasm, and myocarditis. Sixty-three patients in 9 centers were enrolled (age 43.0±13.4 years, 29 women). A diagnosis was obtained in 35 patients (56%): Long-QT syndrome in 8, catecholaminergic polymorphic ventricular tachycardia in 8, arrhythmogenic right ventricular cardiomyopathy in 6, early repolarization in 5, coronary spasm in 4, Brugada syndrome in 3, and myocarditis in 1. Targeted genetic testing demonstrated evidence of causative mutations in 9 (47%) of 19 patients. Screening of 64 family members of these patients identified 15 affected individuals who were treated (24%). The remaining 28 patients (44%) were considered to have idiopathic ventricular fibrillation.

Conclusions— Systematic clinical testing, including drug provocation and advanced imaging, results in unmasking of the cause of apparently unexplained cardiac arrest in >50% of patients. This approach assists in directing genetic testing to diagnose genetically mediated arrhythmia syndromes, which results in successful family screening.

Relation of increased short-term variability of QT interval to congenital long-QT syndrome

Apart from clinical symptoms the diagnosis and risk stratification in long-QT syndrome (LQTS) is usually based on the surface electrocardiogram. Studies have indicated that not only prolongation of the QT interval but also an increased short-term variability of QT interval (STV(QT)) is a marker for a decreased repolarization reserve in patients with drug-induced LQTS. The aims of this study were to determine if STV(QT) (1) is higher in patients with LQTS compared with controls, (2) if this effect is more pronounced in a high-risk LQTS population, and (3) could increase the diagnostic power of the surface electrocardiogram in identifying mutation carriers. Forty mutation carriers were compared with age- and gender-matched control subjects in the absence of beta-receptor-blocking agents. Lead II or V(5) RR and QT intervals from 30 consecutive beats were manually measured. STV(QT) was determined from Poincaré plots of QT intervals (STV(QT) = Sigma|QTn + 1 - QTn|/[30 x radical2]). Compared with controls, patients with LQTS had a prolonged QTc interval (449 +/- 41 vs 411 +/- 32 ms, p = 0.00049) and increased STV(QT) (6.4 +/- 3.2 vs 4.1 +/- 1.6 ms, p = 0.005). In patients with the highest risk of clinical events, defined as a QTc interval >500 ms or symptoms before beta-blocker therapy, STV(QT) was 9 +/- 4 ms. QTc interval had a sensitivity of 43% and a specificity of 97% in identifying mutation carriers (thresholds 450 ms for men and 460 ms for women). Receiver operator characteristic analysis showed that an STV(QT) of 4.9 ms was the optimal cut-off value to predict mutation carriers. When incorporating an STV(QT) >4.9 ms for those whose QTc interval was within the normal limits, sensitivity to distinguish mutation carriers increased to 83% with a specificity of 68%, so that another 15 mutation carriers could be identified. In conclusion, these are the first results in humans showing that STV(QT) is increased in congenital LQTS, this effect is increased in patients with symptoms before therapy, and, hence, STV(QT) could prove to be a useful noninvasive additive marker for diagnostic screening to bridge the gap before results of genetic testing are available.

Re-evaluating the efficacy of beta-adrenergic agonists and antagonists in long QT-3 syndrome through computational modelling

AIMS

Long QT syndrome (LQTS) is a heterogeneous collection of inherited cardiac ion channelopathies characterized by a prolonged electrocardiogram QT interval and increased risk of sudden cardiac death. Beta-adrenergic blockers are the mainstay of treatment for LQTS. While their efficacy has been demonstrated in LQTS patients harbouring potassium channel mutations, studies of beta-blockers in subtype 3 (LQT3), which is caused by sodium channel mutations, have produced ambiguous results. In this modelling study, we explore the effects of beta-adrenergic drugs on the LQT3 phenotype.

METHODS AND RESULTS

In order to investigate the effects of beta-adrenergic activity and to identify sources of ambiguity in earlier studies, we developed a computational model incorporating the effects of beta-agonists and beta-blockers into an LQT3 mutant guinea pig ventricular myocyte model. Beta-activation suppressed two arrhythmogenic phenomena, transmural dispersion of repolarization and early after depolarizations, in a dose-dependent manner. However, the ability of beta-activation to prevent cardiac conduction block was pacing-rate-dependent. Low-dose beta-blockade by propranolol reversed the beneficial effects of beta-activation, while high dose (which has off-target sodium channel effects) decreased arrhythmia susceptibility.

CONCLUSION

These results demonstrate that beta-activation may be protective in LQT3 and help to reconcile seemingly conflicting results from different experimental models. They also highlight the need for well-controlled clinical investigations re-evaluating the use of beta-blockers in LQT3 patients.