Differential effects of beta-blockade on dispersion of repolarization in the absence and presence of sympathetic stimulation between the LQT1 and LQT2 forms of congenital long QT syndrome

Effects of beta-blockers on ventricular repolarization documented by 24-h electrocardiography in long-QT syndrome type 2

BACKGROUND

Long QT syndrome (LQTS) is an inherited arrhythmia disorder characterized by ventricular repolarization abnormalities and a risk of sudden cardiac death. The electrophysiological components generating the high risk of arrhythmias in LQTS are prolonged repolarization, increased dispersion of repolarization, and early afterdepolarizations, which are clinically estimated as QT interval, T-wave peak to end (TPE) interval, and T2/T1-wave amplitude ratio, respectively. In experimental LQTS type 2 (LQT2) models, beta-blockers decrease dispersion of repolarization and prevent early afterdepolarizations. In clinical studies among LQT2 patients beta-blockers are more effective against exercise-induced than arousal-induced cardiac events.

OBJECTIVES AND METHODS

The aim of the study was to investigate the effects of beta-blocker therapy on QT and TPE intervals, and maximal T2/T1-wave amplitude ratios recorded by 24-h electrocardiograms (ECG) among 25 LQT2 patients.

RESULTS

Beta-blocker therapy decreased the maximal T2/T1-wave amplitude ratio from 2.9±1.1 to 1.8±0.7 (p<0.001), but did not change pause-induced T2/T1-wave amplitude ratio. Under medication abrupt maximal TPE intervals were shorter at heart rates of 75 beats/min or over, and maximal QT intervals were shorter at a heart rate of 100 beats/min.

CONCLUSIONS

Beta-blockers stabilize ventricular repolarization in LQT2 by reducing electrocardiographic early afterdepolarizations, and by reducing abrupt prolongation of electrocardiographic dispersion of repolarization and ventricular repolarization duration at elevated heart rates. The effect of beta-blockers on pause-induced electrocardiographic early afterdepolarizations is weak. The findings provide electrocardiographic explanation for the protective effects of beta-blockers against exercise-induced cardiac events in LQT2.

Differential diagnosis between LQT1 and LQT2 by QT/RR relationships using 24-hour Holter monitoring: A multicenter cross-sectional study

BACKGROUND

The clinical course and therapeutic strategies in the congenital long QT syndrome (LQTS) are genotype-specific. However, accurate estimation of LQTS genotype is often difficult from the standard 12-lead ECG.

OBJECTIVES

This study aims to evaluate the utility of QT/RR slope analysis by the 24-hour Holter monitoring for differential diagnosis of LQTS genotype between LQT1 and LQT2.

METHODS

This cross-sectional study enrolled 54 genetically identified LQTS patients (29 LQT1 and 25 LQT2) recruited from three medical institutions. The QT-apex (QTa) interval and the QT-end (QTe) interval at each 15-second were plotted against the RR intervals, and the linear regression (QTa/RR and QTe/RR slopes, respectively) was calculated from the entire 24-hour and separately during the day or night-time periods of the Holter recordings.

RESULTS

The QTe/RR and QTa/RR slopes at the entire 24-hour were significantly steeper in LQT2 compared to those in LQT1 patients (0.262 ± 0.063 vs. 0.204 ± 0.055, p = .0007; 0.233 ± 0.052 vs. 0.181 ± 0.040, p = .0002, respectively). The QTe interval was significantly longer, and QTe/RR and QTa/RR slopes at daytime were significantly steeper in LQT2 than in LQT1 patients. The receiver operating curve analysis revealed that the QTa/RR slope of 0.211 at the entire 24-hour Holter was the best cutoff value for differential diagnosis between LQT1 and LQT2 (sensitivity: 80.0%, specificity: 75.0%, and area under curve: 0.804 [95%CI = 0.68-0.93]).

CONCLUSION

The continuous 24-hour QT/RR analysis using the Holter monitoring may be useful to predict the genotype of congenital LQTS, particularly for LQT1 and LQT2.

Targeting the β-adrenergic receptor in the clinical management of congenital long QT syndrome

The long QT syndrome (LQTS) is largely treated pharmacologically with β-blockers, despite the role of sympathetic activity in LQTS being poorly understood. Using the trigger-substrate model of cardiac arrhythmias in this review, we amalgamate current experimental and clinical data from both animal and human studies to explain the mechanism of adrenergic stimulation and blockade on LQT arrhythmic risk and hence assess the efficacy of β-adrenoceptor blockade in the management of LQTS. In LQTS1 and LQTS2, sympathetic stimulation increases arrhythmic risk by enhancing early afterdepolarizations and transmural dispersion of repolarization. β-Blockers successfully reduce cardiac events by reducing these triggers and substrates; however, these effects are less marked in LQTS2 compared with LQTS1. In LQTS3, clinical and experimental investigations of the effects of sympathetic stimulation and β-blocker use have produced contradictory findings, resulting in significant clinical uncertainty. We offer explanations for these contradicting results relating to study sample size, the dose of the β-blocker administered associated with its off-target Na⁺ channel effects, as well as the type of β-blocker used. We conclude that the antiarrhythmic efficacy of β-blockers is a genotype-specific phenomenon, and hence the use of β-blockers in clinical practice should be genotype dependent.

German Cardiac Society Working Group on Cellular Electrophysiology state-of-the-art paper: impact of molecular mechanisms on clinical arrhythmia management

Cardiac arrhythmias remain a common challenge and are associated with significant morbidity and mortality. Effective and safe rhythm control strategies are a primary, yet unmet need in everyday clinical practice. Despite significant pharmacological and technological advances, including catheter ablation and device-based therapies, the development of more effective alternatives is of significant interest to increase quality of life and to reduce symptom burden, hospitalizations and mortality. The mechanistic understanding of pathophysiological pathways underlying cardiac arrhythmias has advanced profoundly, opening up novel avenues for mechanism-based therapeutic approaches. Current management of arrhythmias, however, is primarily guided by clinical and demographic characteristics of patient groups as opposed to individual, patient-specific mechanisms and pheno-/genotyping. With this state-of-the-art paper, the Working Group on Cellular Electrophysiology of the German Cardiac Society aims to close the gap between advanced molecular understanding and clinical decision-making in cardiac electrophysiology. The significance of cellular electrophysiological findings for clinical arrhythmia management constitutes the main focus of this document. Clinically relevant knowledge of pathophysiological pathways of arrhythmias and cellular mechanisms of antiarrhythmic interventions are summarized. Furthermore, the specific molecular background for the initiation and perpetuation of atrial and ventricular arrhythmias and mechanism-based strategies for therapeutic interventions are highlighted. Current "hot topics" in atrial fibrillation are critically appraised. Finally, the establishment and support of cellular and translational electrophysiology programs in clinical rhythmology departments is called for to improve basic-science-guided patient management.

Prolonged T-wave peak-end interval in Down syndrome patients with congenitally normal hearts

BACKGROUND

Heterogeneity of ventricular repolarization has been assessed using the QT dispersion in Down syndrome (DS) patients with congenitally normal hearts. Novel repolarization indexes, that is, T-wave peak-end (Tp-e) interval and Tp-e/QT ratio, however, have not previously been evaluated in these patients. The aim of this study was therefore to evaluate the Tp-e interval and Tp-e/QT ratio in DS patients without congenital heart defects.

METHODS

Tp-e interval, Tp-e dispersion, and Tp-e/QT ratio were compared between 160 DS patients and 110 age- and sex-matched healthy controls on 12-lead surface electrocardiogram.

RESULTS

Heart rate, Tp-e interval, Tp-e dispersion, Tp-e/QT and Tp-e/QTc ratios were significantly higher in the DS group than the control group.

CONCLUSION

Myocardial repolarization indexes in DS patients with congenitally normal hearts were found to be prolonged compared with those in normal controls. Further evaluation is warranted to elucidate the relationship between prolonged repolarization indexes and arrhythmic events in these patients.

Effectiveness of beta-blockers depending on the genotype of congenital long-QT syndrome: A meta-analysis

BACKGROUND

Beta-blockers are first-line therapy in patients with congenital long-QT syndrome (LQTS).

OBJECTIVE

This study sought to determine the differences in effectiveness of beta-blockers on risk reduction according to LQTS genotype.

METHODS

We searched MEDLINE, EMBASE, and CENTRAL databases to investigate the use of beta-blockers (atenolol, nadolol, propranolol, and metoprolol) in patients with LQTS. Hazard ratio (HR) and relative risk (RR) were extracted or calculated from studies reporting cardiac events (syncope, aborted cardiac arrest (ACA), or sudden cardiac death (SCD)).

RESULTS

Among 2,113 articles searched, 10 studies (7 registry-based cohort studies (Cohort) and 3 interrupted time series studies (ITS)) involving 9,727 patients were included. In a meta-analysis using a random-effect model, the use of beta-blocker was associated with significant risk reduction of all cardiac events (HR 0.49, p<0.001 in Cohort; RR 0.39, p<0.001 in ITS) and serious cardiac events (ACA or SCD) (HR 0.47, p<0.001 in Cohort). In both LQT1 and LQT2, the risk was reduced with beta-blocker therapy in Cohort (HR 0.59 in LQT1; HR 0.39 in LQT2) as well as ITS (RR 0.29 in LQT1; RR 0.48 in LQT2). Among the beta-blockers, nadolol showed a significant risk reduction in both LQT1 and LQT2 (HR 0.47 and 0.27, respectively), whereas atenolol and propranolol decreased the risk only in LQT1 (HR 0.36 and 0.46, respectively). Metoprolol showed no significant reduction in either genotype. In LQT3, beta-blocker therapy was not as effective as LQT1 or LQT2; however, it was inconclusive due to data insufficiency.

CONCLUSION

This meta-analysis showed that beta-blockers were effective in reducing risk of cardiac events in patients with LQTS. Among them, nadolol was effective in LQT1 and LQT2, whereas other drugs showed different effectiveness depending on LQT genotype.

The safety of modern anesthesia for children with long QT syndrome

BACKGROUND

Patients with long QT syndrome (LQTS) may experience a clinical spectrum of symptoms, ranging from asymptomatic, through presyncope, syncope, and aborted cardiac arrest, to sudden cardiac death. Arrhythmias in LQTS are often precipitated by autonomic changes. This patient population is believed to be at high risk for perioperative arrhythmia, specifically torsades de pointes (TdP), although this perception is largely based on limited literature that predates current anesthetic drugs and standards of perioperative monitoring. We present the largest multicenter review to date of anesthetic management in children with LQTS.

METHODS

We conducted a multicentered retrospective chart review of perioperative management of children with clinically diagnosed LQTS, aged 18 years or younger, who received general anesthesia (GA) between January 2005 and January 2010. Data from 8 institutions were collated in an anonymized database.

RESULTS

One hundred three patients with LQTS underwent a total of 158 episodes of GA. The median (interquartile range) age and weight of the patients at the time of GA was 9 (3-15) years and 30.3 (15.4-54) kg, respectively. Surgery was LQTS-related in 81 (51%) GA episodes (including pacemaker, implantable cardioverter-defibrillator, and loop recorder insertions and revisions and lead extractions) and incidental in 77 (49%). β-blocker therapy was administered to 76% of patients on the day of surgery and 47% received sedative premedication. Nineteen percent of patients received total IV anesthesia, 30% received total inhaled anesthesia, and the remaining 51% received a combination. No patient received droperidol. There were 5 perioperative episodes of TdP, all in neonates or infants, all in surgery that was LQTS-related, and none of which was overtly attributable to anesthetic regimen. Thus the incidence (95% confidence interval) of perioperative TdP in incidental versus LQTS-related surgery was 0/77 (0%; 0%-5%) vs 5/81 (6.2%; 2%-14%).

CONCLUSIONS

With optimized perioperative management, modern anesthesia for incidental surgery in patients with LQTS is safer than anecdotal case report literature might suggest. Our series suggests that the risk of perioperative TdP is concentrated in neonates and infants requiring urgent interventions after failed first-line management of LQTS.

Genotype- and phenotype-guided management of congenital long QT syndrome

Congenital long QT syndrome (LQTS) is a genetically heterogeneous group of heritable disorders of myocardial repolarization linked by the shared clinical phenotype of QT prolongation on electrocardiogram and an increased risk of potentially life-threatening cardiac arrhythmias. At the molecular level, mutations in 15 distinct LQTS-susceptibility genes that encode ion channel pore-forming α-subunits and accessory β-subunits central to the electromechanical function of the heart have been implicated in its pathogenesis. Over the past 2 decades, our evolving understanding of the electrophysiological mechanisms by which specific genetic substrates perturb the cardiac action potential has translated into vastly improved approaches to the diagnosis, risk stratification, and treatment of patients with LQTS. In this review, we describe how our understanding of the molecular underpinnings of LQTS has yielded numerous clinically meaningful genotype-phenotype correlations and how these insights have translated into genotype- and phenotype-guided approaches to the clinical management of LQTS.

The role of acute hyperinsulinemia in the development of cardiac arrhythmias

Patients with perturbed metabolic control are more prone to develop cardiac rhythm disturbances. The main purpose of the present preclinical study was to investigate the possible role of euglycemic hyperinsulinemia in development of cardiac arrhythmias. Euglycemic hyperinsulinemia was induced in conscious rabbits equipped with a right ventricular pacemaker electrode catheter by hyperinsulinemic euglycemic glucose clamp (HEGC) applying two different rates of insulin infusion (5 and 10 mIU/kg/min) and variable rate of glucose infusion to maintain euglycemia (5.5 ± 0.5 mmol/l). The effect of hyperinsulinemia on cardiac electrophysiological parameters was continuously monitored by means of 12-lead surface ECG recording. Arrhythmia incidence was determined by means of programmed electrical stimulation (PES). The possible role of adrenergic activation was investigated by determination of plasma catecholamine levels and intravenous administration of a beta adrenergic blocking agent, metoprolol. All of the measurements were performed during the steady-state period of HEGC and subsequent to metoprolol administration. Both 5 and 10 mIU/kg/min insulin infusion prolonged significantly QTend, QTc, and Tpeak-Tend intervals. The incidence of ventricular arrhythmias generated by PES was increased significantly by euglycemic hyperinsulinemia and exhibited linear relationship to plasma levels of insulin. No alteration on plasma catecholamine levels could be observed; however, metoprolol treatment restored the prolonged QTend, QTc, and Tpeak-Tend intervals and significantly reduced the hyperinsulinemia-induced increase of arrhythmia incidence. Euglycemic hyperinsulinemia can exert proarrhythmic effect presumably due to the enhancement of transmural dispersion of repolarization. Metoprolol treatment may be of benefit in hyperinsulinemia associated with increased incidence of cardiac arrhythmias.

Not all beta-blockers are equal in the management of long QT syndrome types 1 and 2: higher recurrence of events under metoprolol

OBJECTIVES

The purpose of this study was to compare the efficacy of beta-blockers in congenital long QT syndrome (LQTS).

BACKGROUND

Beta-blockers are the mainstay in managing LQTS. Studies comparing the efficacy of commonly used beta-blockers are lacking, and clinicians generally assume they are equally effective.

METHODS

Electrocardiographic and clinical parameters of 382 LQT1/LQT2 patients initiated on propranolol (n = 134), metoprolol (n = 147), and nadolol (n = 101) were analyzed, excluding patients <1 year of age at beta-blocker initiation. Symptoms before therapy and the first breakthrough cardiac events (BCEs) were documented.

RESULTS

Patients (56% female, 27% symptomatic, heart rate 76 ± 16 beats/min, QTc 472 ± 46 ms) were started on beta-blocker therapy at a median age of 14 years (interquartile range: 8 to 32 years). The QTc shortening with propranolol was significantly greater than with other beta-blockers in the total cohort and in the subset with QTc >480 ms. None of the asymptomatic patients had BCEs. Among symptomatic patients (n = 101), 15 had BCEs (all syncopes). The QTc shortening was significantly less pronounced among patients with BCEs. There was a greater risk of BCEs for symptomatic patients initiated on metoprolol compared to users of the other 2 beta-blockers combined, after adjustment for genotype (odds ratio: 3.95, 95% confidence interval: 1.2 to 13.1, p = 0.025). Kaplan-Meier analysis showed a significantly lower event-free survival for symptomatic patients receiving metoprolol compared to propranolol/nadolol.

CONCLUSIONS

Propranolol has a significantly better QTc shortening effect compared to metoprolol and nadolol, especially in patients with prolonged QTc. Propranolol and nadolol are equally effective, whereas symptomatic patients started on metoprolol are at a significantly higher risk for BCEs. Metoprolol should not be used for symptomatic LQT1 and LQT2 patients.

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.

Management of asthma in children with long QT syndrome

The Long QT syndrome (LQTS) is a rare disorder in which patients are prone to life threatening ventricular arrhythmia and is a leading cause of sudden death in childhood. Asthma is common and its management in those with LQTS presents a number of potential difficulties. The mainstay of therapy in LQTS is beta-blockade, which may worsen symptoms of asthma. Conversely, beta-agonist therapy is the mainstay of asthma management; which, in those with LQTS, may provoke ventricular arrhythmias. We review available data regarding the management of coexistent LQTS and asthma, and provide a summary of the necessary considerations in managing these patients.

Arrhythmic risk in congenital long QT syndrome

One of the most important and challenging aspects of caring for patients with congenital long QT syndrome (LQTS) is assessing an individual's risk of sudden cardiac death (SCD) because of torsades de pointes. Current risk assessment integrates clinical and genetic features known to be associated with SCD, but more accurate methods of risk assessment could lead to more appropriate use of therapies, potentially saving lives and avoiding overtreatment. Conventional indices of risk include sex, age, extent of QT prolongation, history of symptoms (syncope or aborted SCD), and genetic subtype. The biophysical properties of specific mutations (eg, those that affect transmembrane segments of the ion channel protein or those that cause a dominant negative effect on ion channel function vs haplotype insufficiency) also contribute to risk. A growing body of basic mechanistic and clinical evidence points to heterogeneity of repolarization as a potent determinant of risk in LQTS patients. Mechanistically, heterogeneities of repolarization provide substrate for reentry, which likely causes perpetuation of torsades de pointes. Clinical markers that reflect heterogeneity of repolarization include abnormal microvolt-level T wave alternans, increased Tpeak-end interval, and dispersion of mechanical contraction time. The optimal methodology for using these indices as risk predictors in LQTS remains under active investigation. Further studies are needed to determine how indices of heterogeneity such as microvolt-level T wave alternans, Tpeak-end interval, and dispersion of mechanical contraction can be incorporated into models of risk prediction in LQTS, both for initial risk stratification and for assessment of efficacy of therapies.

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.

Genetic analysis in cardiovascular disease: a clinical perspective

Many forms of cardiovascular disease (CVD) demonstrate heritability and thus a genetic contribution is likely. This is most evident when considering the "simple" Mendelian traits such as hypertrophic cardiomyopathy. However, family history also influences our assessment of patients with complex traits such as coronary artery disease, hypertension, and common forms of hypercholesterolemia, as observed in clinical practice. Recent research has led to advances in our understanding of the genetic basis of both the simple and complex forms of CVD. This review presents the current state of knowledge regarding major gene disorders, as well as more common, complex forms of CVD such as coronary artery disease. It discusses the fundamental approaches being used to identify the genetic basis of the various disease states, as well as the practical implications of the discoveries to clinicians. It also focuses on our need to assess the extent by which genetic analysis can alter our calculation of an individual's risk of disease, and our ability to successfully target treatment that will modify this process.

Drug evaluation in cardiomyocytes derived from human induced pluripotent stem cells carrying a long QT syndrome type 2 mutation

AIMS

Congenital long QT syndromes (LQTSs) are associated with prolonged ventricular repolarization and sudden cardiac death. Limitations to existing clinical therapeutic management strategies prompted us to develop a novel human in vitro drug-evaluation system for LQTS type 2 (LQT2) that will complement the existing in vitro and in vivo models.

METHODS AND RESULTS

Skin fibroblasts from a patient with a KCNH2 G1681A mutation (encodes I(Kr) potassium ion channel) were reprogrammed to human induced pluripotent stem cells (hiPSCs), which were subsequently differentiated to functional cardiomyocytes. Relative to controls (including the patient's mother), multi-electrode array and patch-clamp electrophysiology of LQT2-hiPSC cardiomyocytes showed prolonged field/action potential duration. When LQT2-hiPSC cardiomyocytes were exposed to E4031 (an I(Kr) blocker), arrhythmias developed and these presented as early after depolarizations (EADs) in the action potentials. In contrast to control cardiomyocytes, LQT2-hiPSC cardiomyocytes also developed EADs when challenged with the clinically used stressor, isoprenaline. This effect was reversed by β-blockers, propranolol, and nadolol, the latter being used for the patient's therapy. Treatment of cardiomyocytes with experimental potassium channel enhancers, nicorandil and PD118057, caused action potential shortening and in some cases could abolish EADs. Notably, combined treatment with isoprenaline (enhancers/isoprenaline) caused EADs, but this effect was reversed by nadolol.

CONCLUSIONS

Findings from this paper demonstrate that patient LQT2-hiPSC cardiomyocytes respond appropriately to clinically relevant pharmacology and will be a valuable human in vitro model for testing experimental drug combinations.

The effects of droperidol and ondansetron on dispersion of myocardial repolarization in children

OBJECTIVES

To compare the effects of droperidol and ondansetron on electrocardiographic indices of myocardial repolarization in children.

AIM

To refine understanding of the torsadogenic risk to children exposed to anti-emetic prophylaxis in the perioperative period.

BACKGROUND

QT interval prolongation is associated with torsades des pointes (TdP), but is a poor predictor of drug torsadogenicity. Susceptibility to TdP arises from increased transmural dispersion of repolarization (TDR) across the myocardial wall, rather than QT interval prolongation per se. TDR can be measured on the electrocardiogram as the time interval between the peak and end of the T wave (Tp-e). Tp-e may therefore provide a readily available, noninvasive assay of drug torsadogenicity. The perioperative period is one of high risk for TdP in children with or at risk of long QT syndromes. Droperidol and ondansetron are two drugs commonly administered perioperatively, for prophylaxis of nausea and vomiting, which can prolong the QT interval. This study investigated their effects on myocardial repolarization.

METHODS

One hundred and eight ASA1-2 children undergoing elective day-case surgery were randomized to receive droperidol, ondansetron, both or neither. Pre- and post-administration 12-lead electrocardiogram (ECGs) were recorded. QT and Tp-e intervals were measured and compared within and between groups, for the primary endpoint of a 25 ms change in Tp-e.

RESULTS

Eighty children completed the study. There were no demographic or baseline ECG differences between groups. QT intervals lengthened by 10-17 ms after allocated treatments, with no between-group differences. Values remained within normal limits for all groups. Tp-e intervals increased by 0-7 ms, with no between-group differences. There were no instances of dysrhythmia.

CONCLUSIONS

Droperidol and ondansetron, in therapeutic anti-emetic doses, produce equivalent, clinically insignificant QT prolongation and negligible Tp-e prolongation, suggesting that neither is torsadogenic in healthy children at these doses.

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 cardiac events and genotype-based management of LQTS patients

This review discusses the risk of cardiac events and genotype-based management of LQTS. We describe here the genetic background of long QT syndrome and the eleven different genes for ion-channels and a structural anchoring protein associated with that disorder. Clinical Background section discusses the risk of cardiac events associated with different LQTS types. Management and Prevention section describes in turn gene-specific therapy, which was based on the identification of the gene defect and the dysfunction of the associated transmembrane ion channel. In patients affected by LQTS, genetic analysis is useful for risk stratification and for making therapeutic decisions. A recent study reported a quite novel pathogenic mechanism for LQTS and suggested that treatments aimed at scaffolding proteins rather than specific ion channels may be an alternative to antiarrhythmic strategy in the future.

Pharmacological approach to the treatment of long and short QT syndromes

Inherited channelopathies have received increasing attention in recent years. The past decade has witnessed impressive progress in our understanding of the molecular and cellular basis of arrhythmogenesis associated with inherited channelopathies. An imbalance in ionic forces induced by these channelopathies affects the duration of ventricular repolarization and amplifies the intrinsic electrical heterogeneity of the myocardium, creating an arrhythmogenic milieu. Today, many of the channelopathies have been linked to mutations in specific genes encoding either components of ion channels or membrane or regulatory proteins. Many of the channelopathies are genetically heterogeneous with a variable degree of expression of the disease. Defining the molecular basis of channelopathies can have a profound impact on patient management, particularly in cases in which genotype-specific pharmacotherapy is available. The long QT syndrome (LQTS) is one of the first identified and most studied channelopathies where abnormal prolongation of ventricular repolarization predisposes an individual to life threatening ventricular arrhythmia called Torsade de Pointes. On the other hand of the spectrum, molecular defects favoring premature repolarization lead to Short QT syndrome (SQTS), a recently described inherited channelopathy. Both of these channelopathies are associated with a high risk of sudden cardiac death due to malignant ventricular arrhythmia. Whereas pharmacological therapy is first line treatment for LQTS, defibrillators are considered as primary treatment for SQTS. This review provides a comprehensive review of the molecular genetics, clinical features, genotype-phenotype correlations and genotype-specific approach to pharmacotherapy of these two mirror-image channelopathies, SQTS and LQTS.

Long QT syndrome: A therapeutic challenge

Congenital long QT syndrome (LQTS) is one of the most common cardiac channelopathies and is characterized by prolonged ventricular repolarization and life-threatening arrhythmias. The mortality is high among untreated patients. The identification of several LQTS genes has had a major impact on the management strategy for both patients and family members. An impressive genotype-phenotype correlation has been noted and genotype identification has enabled genotype specific therapies. Beta blockers continue to be the primary treatment for prevention of life threatening arrhythmias in the majority of patients. Other therapeutic options include pacemakers, implantable cardioverter defibrillators, left cardiac sympathetic denervation, sodium channel blocking medications and lifestyle modification.

A Comparison of the Effect on Dispersion of Repolarization of Age-Adjusted MAC Values of Sevoflurane in Children

BACKGROUND

QT interval prolongation is associated with torsades des pointes (TdP), but is a poor predictor of drug torsadogenicity. Susceptibility to TdP arises from increased transmural dispersion of repolarization (TDR) across the myocardial wall, rather than QT interval prolongation per se. TDR can be measured on the electrocardiogram as the time interval between the peak and end of the T-wave (Tp-e). Thus Tp-e is a readily measured assay of drug torsadogenicity. Several anesthetic drugs prolong the QT interval, but their effect on TDR is largely unknown.

METHODS

We investigated the effects of sevoflurane on corrected QT (QTc) and Tp-e intervals in 54 unpremedicated ASA I-II children, aged 3-10 yr, who were randomized to receive sevoflurane 1, 1.25, or 1.5 MAC, age-adjusted. Twelve-lead electrocardiograms were recorded before and after sevoflurane exposure. QTc and Tp-e were compared within and among groups using 2-way analysis of variance. Change in Tp-e after sevoflurane exposure was the primary outcome measure.

RESULTS

Sevoflurane significantly prolonged preoperative QTc at all doses (P < 0.005), with no dose-response relationship, but had no effect on preoperative Tp-e.

CONCLUSION

Sevoflurane markedly prolongs the QTc in healthy children, but does not increase dispersion of repolarization as measured by the Tp-e interval, indicating low or no torsadogenicity, and making it unlikely to increase predisposition to TdP.

Repolarization morphology in adult LQT2 carriers with borderline prolonged QTc interval

BACKGROUND

At least 50% of LQT2 carriers have borderline QTc (0.42-0.47 s), and they present a diagnostic difficulty to clinicians evaluating patients suspected of having long QT syndrome (LQTS).

OBJECTIVES

Because QTc in this borderline range is nondiagnostic, the purpose of this study was to investigate whether analysis of phenotypic features of T-wave morphology could help identify LQT2 carriers with normal or near-normal QTc-interval duration.

METHODS

Standard 12-lead ECGs recorded without beta-blockers from LQT2 carriers (n = 90, 33 +/- 14 years, 61% female) and noncarriers (n = 69, 38 +/- 17 years, 58% female) were digitized. The following parameters were automatically measured: RR interval, QT/QTc, QT apex, T-wave amplitude, ascending (alpha(L)) and descending slopes (alpha(R)) of the T wave, and T-wave symmetry. We used a linear logistic regression model to identify the most relevant parameters for separating LQT2 carriers from noncarriers, within the overall population and among patients without overt QTc prolongation (390 </= QTc </= 470).

RESULTS

Logistic regression selected three parameters: QT, RR interval, and alpha(L) in all models. In the overall population, the model provided 92.7% sensitivity and 90.0% specificity. In the group of patients without beta-blockers and near-normal QTc interval, 92.0% sensitivity (n = 46) and 81.4% specificity (n = 49) were achieved by the model including alpha(L.)

CONCLUSION

Abnormal T-wave morphology is a phenotypic expression of LQT2, and its quantification could be used to identify patients with suspected LQTS who do not have overt QTc prolongation (QTc >470).

Comparison of HERG channel blocking effects of various beta-blockers-- implication for clinical strategy

beta-Blockers are widely used in the treatment of cardiovascular diseases. However, their effects on HERG channels at comparable conditions remain to be defined. We investigated the direct acute effects of beta-blockers on HERG current and the molecular basis of drug binding to HERG channels with mutations of putative common binding site (Y652A and F656C). beta-Blockers were selected based on the receptor subtype. Wild-type, Y652A and F656C mutants of HERG channel were stably expressed in HEK293 cells, and the current was recorded by using whole-cell patch-clamp technique (23 degrees C). Carvedilol (nonselective), propranolol (nonselective) and ICI 118551 (beta(2)-selective) inhibited HERG current in a concentration-dependent manner (IC(50) 0.51, 3.9 and 9.2 microM, respectively). The IC(50) value for carvedilol was a clinically relevant concentration. High metoprolol (beta(1)-selective) concentrations were required for blockade (IC(50) 145 microM), and atenolol (beta(1)-selective) did not inhibit the HERG current. Inhibition of HERG current by carvedilol, propranolol and ICI 118551 was partially but significantly attenuated in Y652A and F656C mutant channels. Affinities of metoprolol to Y652A and F656C mutant channels were not different compared with the wild-type. HERG current block by all beta-blockers was not frequency-dependent. Drug affinities to HERG channels were different in beta-blockers. Our results provide additional strategies for clinical usage of beta-blockers. Atenolol and metoprolol may be preferable for patients with type 1 and 2 long QT syndrome. Carvedilol has a class III antiarrhythmic effect, which may provide the rationale for a favourable clinical outcome compared with other beta-blockers as suggested in the recent COMET (Carvedilol Or Metoprolol European Trial) substudy.

Effects of beta-blocker therapy on ventricular repolarization documented by 24-h electrocardiography in patients with type 1 long-QT syndrome

OBJECTIVES

We tested the hypothesis that in long-QT syndrome (LQT) type 1 (LQT1), beta-blocker therapy may decrease both the diurnal maximal T-wave peak to T-wave end interval (TPE) and the maximal ratio between late and early T-wave peak amplitude (T2/T1 ratio), which are electrocardiographic counterparts of transmural dispersion of repolarization (TDR) and early afterdepolarizations (EA), respectively.

BACKGROUND

Ventricular repolarization duration and increased TDR and EAs are the three electrophysiological components generating the high risk of ventricular arrhythmias and sudden death in the inherited LQT. In the most prevalent LQT1 form of LQT, treatment with beta-blockers reduces serious arrhythmia events dramatically without a known influence on QT interval duration. In experimental LQT1 models, beta-blockers decrease TDR and prevent EAs.

METHODS

We reviewed 24-h electrocardiographic recordings obtained before and during the treatment with beta-blockers from 24 genotyped LQT1 patients to record maximal TPE intervals and T2/T1 ratios as well as maximal and rate-adapted QT intervals using a computer-assisted program.

RESULTS

Treatment with beta-blockers decreased the maximal diurnal T2/T1 amplitude ratio from 3.0+/- 1.0 to 2.2 +/- 0.6 (p = 0.002). Beta-blockers also decreased both maximal TPE intervals and abrupt maximal QT intervals at heart rates higher than 85 beats/min, whereas QT intervals measured at steady-state conditions remained unchanged.

CONCLUSIONS

Prevention of abrupt increases of electrocardiographic TDR, EA, and ventricular repolarization duration at elevated heart rates may explain the favorable clinical effects of beta-blockers in LQT1.

Potential proarrhythmic effects of biventricular pacing

Resynchronization therapy involving right ventricular endocardial and left ventricular epicardial pacing improves cardiac output, quality of life, and New York Heart Association functional class in patients with congestive heart failure. Although a great deal of attention has been directed at showing the mechanical benefits and in fine-tuning the biventricular pacing configuration and protocol, little attention has been focused on the consequences of reversing the direction of activation of the left ventricular wall. Recent basic science and clinical studies have shown a proarrhythmic effect of reversing the direction of activation of the left ventricular wall. Reversal of the normal activation sequence prolongs the QT interval and increases the existing transmural dispersion of repolarization, creating the substrate and trigger for re-entrant arrhythmias under long QT conditions. A number of case reports of R-on-T extrasystoles and ventricular tachyarrhythmia induction as a result of biventricular pacing support this observation, and raise concern that biventricular pacing may be proarrhythmic in select cases, particularly when associated with a prolonged QT interval. Our focus in this review is on current understanding of transmural heterogeneity of repolarization that exists across the left ventricular wall, how this dispersion of repolarization is amplified as a consequence of reversal of the normal activation sequence, and how these basic experimental findings may apply to patients receiving cardiac resynchronization therapy.

Cellular basis for electrocardiographic and arrhythmic manifestations of Andersen-Tawil syndrome (LQT7)

BACKGROUND

Andersen-Tawil syndrome, a skeletal muscle syndrome associated with periodic paralysis and long QT intervals on the ECG, has been linked to defects in KCNJ2, the gene encoding for the inward rectifier potassium channel (I(K1).)

OBJECTIVES

The purpose of this study was to examine the cellular mechanisms underlying the ECG and arrhythmic manifestations of Andersen-Tawil syndrome.

METHODS

To investigate the effects of KCNJ2 loss-of-function mutations responsible for Andersen-Tawil syndrome, we used barium chloride (BaCl(2)) to inhibit I(K1) in arterially perfused wedge preparation. Transmembrane action potentials (APs) were simultaneously recorded from endocardial, midmyocardial, and epicardial cells, together with a transmural ECG.

RESULTS

BaCl(2) (1 to 30 microM) produced a concentration-dependent prolongation of the QT interval, secondary to a homogeneous prolongation of AP duration of the three cell types. QT interval was prolonged without an increase in transmural dispersion of repolarization (TDR). Low extracellular potassium (2.0 mM), isoproterenol (20-50 nM), and an abrupt increase in temperature (36 degrees C-39 degrees C) in the presence of 10 microM BaCl(2) did not significantly increase TDR but increased ectopic extrasystolic activity. Early afterdepolarizations were not observed under any condition. Spontaneous torsades de pointes arrhythmias were never observed, nor could they be induced with programmed electrical stimulation under any of the conditions studied.

CONCLUSION

Our results provide an understanding of why QT prolongation associated with Andersen-Tawil syndrome is relatively benign in the clinic and provide further support for the hypothesis that the increase in TDR, rather than QT interval, is responsible for development of torsades de pointes.

Refining detection of drug-induced proarrhythmia: QT interval and TRIaD

QT interval prolongation is so frequently associated with torsades de pointes (TdP) that it has come to be recognized as a surrogate marker of this unique tachyarrhythmia. However, not only does TdP not always follow QT interval prolongation, but TdP can occur even in the absence of a prolonged QT interval. Worse still, even shortening of the QT interval may be associated with serious arrhythmias (particularly ventricular tachycardia [VT] and ventricular fibrillation [VF]). It appears increasingly probable that the distinction between various ventricular tachyarrhythmias may be arbitrary, and drug-induced TdP, polymorphic VT, VT, catecholaminergic polymorphic VT, and VF may represent discrete entities within a spectrum of drug-induced proarrhythmia. Although they are differentiated by the coupling interval and the duration of QT interval, they appear to share a common substrate: a set of disturbances of repolarization characterized by Triangulation, Reverse use dependency, electrical Instability of the action potential, and Dispersion (TRIaD). It is becoming increasingly evident that augmentation of TRIaD, rather than changes in the duration of QT interval, provides the proarrhythmic substrate. In contrast, when not associated with an increase of TRIaD, QT interval prolongation can be an antiarrhythmic property. Electrophysiologically, augmentation of TRIaD can be explained by inhibition of hERG (human ether-a-go-go related gene) channel. Because drug-induced disturbances in repolarization commonly result from inhibition of hERG channels or I(Kr), hERG blockade and the resulting prolongation of QT interval are important properties of a drug to be studied. However, these need only be a concern if associated with TRIaD. More significantly, TRIaD so often precedes prolongation of action potential duration or QT interval and ventricular tachyarrhythmias that it should be considered a marker of proarrhythmia until proven otherwise, even in the absence of QT interval prolongation. Detecting drug-induced augmentation of TRIaD may offer an additional, more sensitive, and accurate indicator of the broader proarrhythmic potential of a drug than may QT interval prolongation alone.

Diagnostic value of epinephrine test for genotyping LQT1, LQT2, and LQT3 forms of congenital long QT syndrome

OBJECTIVES

The aim of this study was to test the hypothesis that epinephrine test may have diagnostic value for genotyping LQT1, LQT2, and LQT3 forms of congenital long QT syndrome (LQTS).

BACKGROUND

A differential response of dynamic QT interval to epinephrine infusion between LQT1, LQT2, and LQT3 syndromes has been reported, indicating the potential diagnostic value of the epinephrine test for genotyping the three forms.

METHODS

The responses of 12-lead ECG parameters to epinephrine were retrospectively examined in 15 LQT1, 10 LQT2, 8 LQT3, and 10 healthy volunteers to select the best ECG criteria for separating the four groups. The epinephrine test then was prospectively conducted in 42 probands clinically affected with LQTS, their 67 family members, and 10 new volunteers. The best criteria were applied in a blinded fashion to prospectively separate a different group of 31 LQT1, 23 LQT2, 6 LQT3, and 30 Control patients (10 genotype-negative LQT1, 10 genotype-negative LQT2 family members, and 10 volunteers).

RESULTS

The sensitivity (penetrance) by ECG diagnostic criteria was lower in LQT1 (68%) than in LQT2 (83%) or LQT3 (83%) before epinephrine and was improved with steady-state epinephrine in LQT1 (87%) and LQT2 (91%) but not in LQT3 (83%), without the expense of specificity (100%). The sensitivity and specificity to differentiate LQT1 from LQT2 were 97% and 96%, those from LQT3 were 97% and 100%, and those from Control were 97% and 100%, respectively, when Delta mean corrected Q-Tend >/=35 ms at steady state was used. The sensitivity and specificity to differentiate LQT2 from LQT3 or Control were 100% and 100%, respectively, when Delta mean corrected Q-Tend >/=80 ms at peak was used.

CONCLUSIONS

Epinephrine infusion is a powerful test to predict the genotype of LQT1, LQT2, and LQT3 syndromes as well as to improve the clinical diagnosis of genotype-positive patients, especially those with LQT1 syndrome.

Specific therapy based on the genotype and cellular mechanism in inherited cardiac arrhythmias. Long QT syndrome and Brugada syndrome

Seven forms of congenital long QT syndrome (LQTS) caused by mutations in ion channel genes have been identified. Genotype-phenotype correlation in clinical and experimental studies involving arterially-perfused canine left ventricular wedges suggest that beta-blockers are protective in LQT1, less so in LQT2, but not protective in LQT3. A class IB sodium channel blocker, mexiletine, is most effective in abbreviating QT interval in LQT3, but effectively reduces transmural dispersion of repolarization (TDR) and prevents the development of Torsade de Pointes (TdP) in all 3 models, suggesting its potential as an adjunctive therapy in LQT1 and LQT2. High concentrations of intravenous nicorandil, a potassium channel opener, have been shown to be capable of decreasing QT and TDR, and preventing TdP in LQT1 and LQT2 but not in LQT3. The calcium channel blocker, verapamil, has also been suggested as adjunctive therapy for LQT1, LQT2 and possibly LQT3. Experimental data using right ventricular wedge preparations suggest that a prominent transient outward current (I(to))-mediated action potential (AP) notch and a loss of AP dome in epicardium, but not in endocardium, give rise to a transmural voltage gradient, resulting in ST segment elevation and the induction of ventricular fibrillation (VF), characteristics of the Brugada syndrome. Since the maintenance of the AP dome is determined by the balance of currents active at the end of phase 1 of the AP, any intervention that reduces the outward current or boosts inward current at the end of phase 1 may normalize the ST segment elevation and suppress VF. Such interventions are candidates for pharmacological therapy of the Brugada syndrome. The infusion of isoproterenol, a beta-adrenergic stimulant, strongly augments L-type calcium current (I(Ca-L)), and is the first choice for suppressing electrical storms associated with Brugada syndrome. Quinidine, by virtue of its actions to block I(to), has been proposed as adjunctive therapy, with an implantable cardioverter defibrillator as backup. Oral denopamine, atropine or cilostazol all increase ICa-L, and for this reason may be effective in reducing episodes of VF.

Inherited and acquired vulnerability to ventricular arrhythmias: cardiac Na+ and K+ channels

Mutations in cardiac Na(+) and K(+) channels can disrupt the precise balance of ionic currents that underlies normal cardiac excitation and relaxation. Disruption of this equilibrium can result in 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 stem from one mutation. A number of these defects have been characterized experimentally with the aim of understanding mechanisms of mutation-induced arrhythmia. Improving understanding of abnormalities may provide a basis for the development of therapeutic approaches.

The Effects of Propofol and Sevoflurane on the QT Interval and Transmural Dispersion of Repolarization in Children

Prolongation of the QT interval is associated with torsades de pointes (TdP), especially in children or young adults with long QT syndromes. Susceptibility to TdP arises from increased transmural dispersion of repolarization (TDR) across the myocardial wall. Several anesthetic drugs prolong the QT interval, but their effect on TDR is unknown. TDR can be measured on the electrocardiograph (ECG) as the time interval between the peak and end of the T wave (Tp-e). We investigated the effects of propofol and sevoflurane on the corrected QT (QTc) and Tp-e intervals in 50 unpremedicated ASA physical status I-II children, aged 1-16 yr, who were randomized to receive propofol (group P) or sevoflurane (group S). Twelve-lead ECGs were recorded preoperatively and intraoperatively. Sevoflurane significantly prolonged the preoperative QTc; propofol did not. Neither anesthetic had any significant effect on the preoperative Tp-e. Sevoflurane increases the duration of myocardial repolarization in children to a larger extent than does propofol, but as the dispersion of repolarization appears unaffected, the risk of TdP is likely to be minimal with either anesthetic.

Interval Representative of Transmural Dispersion of Repolarization in Children and Young Adolescents With Congenital Long QT Syndrome

BACKGROUND

It has been shown experimentally that the interval from the nadir of the initial negative T wave to the end of the T wave is representative of transmural dispersion of repolarization (TDR) when complex T waves are present. In the clinical setting, however, the interval representative of TDR in patients with long QT syndrome (LQTS) is a controversial subject.

METHODS AND RESULTS

Five symptomatic patients (3 boys, 2 girls; 3 LQT1, 2 LQT2) were evaluated by a face immersion test before and after treatment to compare the configuration of the T wave. When the notch disappeared after treatment, the single peak of the T wave after treatment coincided with the nadir of the notch before treatment. When the notch remained the same after treatment as before treatment and when the QTc decreased, the corrected interval from the nadir of the notch to the end of the T wave was for the most part shortened.

CONCLUSIONS

The present study showed that the interval representative of the TDR in the clinical surface electrocardiogram can be obtained from the nadir of the notch to the end of the T wave in children and adolescents with LQTS, as was shown in the experimental study.

Sudden Intense Exercise Increases QT Heart Rate Slope and T Wave Complexity in Long QT Syndrome and Normal Subjects

Sudden intense physical activity is a trigger for ventricular arrhythmias in long QT syndrome (LQTS), and beta-blockers (B) reduce the risk of ventricular arrhythmias in LQTS. We compared the effect of graded (gradual intensity) versus burst (sudden intensity) exercise on QT-rate adaptation and T wave complexity in LQTS + B (n = 21), LQTS - B (n = 5), and normal controls (n = 20). Graded exercise consisted of symptom-limited bicycle ergometry (30 W, increment 20 W/min). Burst exercise involved a fixed 200 W load for 1.0 minute. ECGs were digitally recorded every 10 seconds during exercise and a 10 minute recovery period. QT-rate adaptation was quantified using the slope of the QT cycle length relationship fit to a quadratic function. Principle component analysis (PCA) was used to quantify T wave complexity. The QT-rate slope was two-fold greater with burst exercise than graded exercise for LQTS + B (-82E +/- 40E vs -36E +/- 40E, P = 0.0016), LQTS - B (-85E +/- 60E vs -30E +/- 50E, P = 0.011) and controls (-100E +/- 60E vs -48E +/- 100E, P = 0.0011) (E = x10(-5)). For each exercise protocol, there was no difference in QT-rate slope between the three groups. In contrast, the QT-rate slope during the 10 minute recovery period was similar between the burst and graded protocol for LQTS + B (25E +/- 40E vs 30E +/- 50E), LQTS - B (81E +/- 80E vs 85E +/- 70E) and controls (90E +/- 80E vs 82E +/- 80E). The coefficient of variability of PCA (T wave complexity variability) during burst exercise was greater than that during graded exercise for LQTS + B (41 +/- 15 vs 30 +/- 10, P = 0.017), LQTS - B (47 +/- 25 vs 26 +/- 4, P = 0.016), and control (46 +/- 14 vs 33 +/- 19, P = 0.012). For each exercise protocol, no difference in T wave complexity variability was seen between the three groups. In conclusion, QT heart rate slope and T wave complexity variability are greater during sudden intense exercise than graded exercise in LQTS patients (on and off beta-blockers) and normal subjects, with similar findings among the three groups of patients.

Mutation site-specific differences in arrhythmic risk and sensitivity to sympathetic stimulation in the LQT1 form of congenital long QT syndrome: multicenter study in Japan

OBJECTIVES

We sought to compare the arrhythmic risk and sensitivity to sympathetic stimulation of mutations located in transmembrane regions and C-terminal regions of the KCNQ1 channel in the LQT1 form of congenital long QT syndrome (LQTS).

BACKGROUND

The LQT1 syndrome is frequently manifested with variable expressivity and incomplete penetrance and is much more sensitive to sympathetic stimulation than the other forms.

METHODS

Sixty-six LQT1 patients (27 families) with a total of 19 transmembrane mutations and 29 patients (10 families) with 8 C-terminal mutations were enrolled from five Japanese institutes.

RESULTS

Patients with transmembrane mutations were more frequently affected based on electrocardiographic (ECG) diagnostic criteria (82% vs. 24%, p < 0.0001) and had more frequent LQTS-related cardiac events (all cardiac events: 55% vs. 21%, p = 0.002; syncope: 55% vs. 21%, p = 0.002; aborted cardiac arrest or unexpected sudden cardiac death: 15% vs. 0%, p = 0.03) than those with C-terminal mutations. Patients with transmembrane mutations had a greater risk of first cardiac events occurring at an earlier age, with a hazard ratio of 3.4 (p = 0.006) and with an 8% increase in risk per 10-ms increase in corrected Q-Tend. The baseline ECG parameters, including Q-Tend, Q-Tpeak, and Tpeak-end intervals, were significantly greater in patients with transmembrane mutations than in those with C-terminal mutations (p < 0.005). Moreover, the corrected Q-Tend and Tpeak-end were more prominently increased with exercise in patients with transmembrane mutations (p < 0.005).

CONCLUSIONS

In this multicenter Japanese population, LQT1 patients with transmembrane mutations are at higher risk of congenital LQTS-related cardiac events and have greater sensitivity to sympathetic stimulation, as compared with patients with C-terminal mutations.

Excessive Increase in QT Interval and Dispersion of Repolarization Predict Recurrent Ventricular Tachyarrhythmia after Amiodarone

Although chronic amiodarone has been proven to be effective to suppress ventricular tachycardia (VT) and ventricular fibrillation (VF), how we predict the recurrence of VT/VF after chronic amiodarone remains unknown. This study evaluated the predictive value of the QT interval, spatial, and transmural dispersions of repolarization (SDR and TDR) for further arrhythmic events after chronic amiodarone. Eighty-seven leads body surface ECGs were recorded before (pre) and one month after (post) chronic oral amiodarone in 50 patients with sustained monomorphic VT associated with organic heart disease. The Q-Tend (QTe), the Q-Tpeak (QTp), and the interval between Tpeak and Tend (Tp-e) as an index of TDR were measured automatically from 87-lead ECG, corrected Bazett's method (QTce, QTcp, Tcp-e), and averaged among all 87 leads. As an index of SDR, the maximum (max) minus minimum (min) QTce (max-min QTce) and standard deviation of QTce (SD-QTce) was obtained among 87 leads. All patients were prospectively followed (15 +/- 10 months) after starting amiodarone, and 20 patients had arrhythmic events. The univariate analysis revealed that post max QTce, post SD-QTce, post max-min QTce, and post mean Tcp-e from 87-lead but not from 12-lead ECG were the significant predictors for further arrhythmic events. ROC analysis indicated the post max-min QTce > or = 106 ms as the best predictor of events (hazard ratio = 10.4, 95%, CI 2.7 to 40.5, P = 0.0008). Excessive QT prolongation associated with increased spatial and transmural dispersions of repolarization predict the recurrence of VT/VF after amiodarone treatment.

Congenital and Acquired Long QT Syndrome

Congenital long QT syndrome (LQTS) is a rare but potentially lethal disease, characterized by prolongation of QT interval, recurrent syncope, and sudden death. In the pregenomic era (1959-1991), sympathetic imbalance was thought to be responsible for this disease. Since 1991 (postgenomic era), 7 LQTS genes have been discovered and more than 300 mutations have been identified to account for approximately 70% of patients affected. Despite the advancement in molecular genetic knowledge, diagnosis of congenital LQTS is still based on electrocardiographic and clinical characteristics. Beta-blockers remain the mainstay treatment. For high-risk patients, the implantable cardioverter-defibrillator (ICD) offer an effective therapeutic option to reduce mortality. Gene-based specific therapy is still preliminary. Further studies are required to investigate new strategies for targeting the defective genes or mutant channels. For acquired LQTS, it is generally believed that the main issue is the blockade of the slow component of the delayed rectifier K+ current (IKr). These IKr blockers have a "reverse frequency-dependent" effect on the QTc interval and increase the dispersion in repolarization. In the presence of risk factors such as female gender, slow heart rate, and hypokalemia, these IKr blockers have a high propensity to induce torsades de pointes. For patients with a history of drug-induced LQTS, care must be taken to avoid further exposure to QT-prolonging drugs or conditions. Molecular genetic analysis could be useful to unravel subclinical mutations or polymorphisms. Physicians not only need to be aware of the pharmacodynamic and pharmacokinetic interactions of various important drugs, but also need to update their knowledge.