Persistent atrial fibrillation is associated with reduced risk of torsades de pointes in patients with drug-induced long QT syndrome

Mortality and ventricular arrhythmias in patients on d,l-sotalol for rhythm control of atrial fibrillation: A nationwide cohort study

BACKGROUND

Use of d,l-sotalol for rhythm control in patients with atrial fibrillation (AF) has raised safety concerns. Previous randomized studies are few and not designed for mortality outcome.

OBJECTIVE

The purpose of this study was to compare the incidences of mortality and ventricular arrhythmias in AF patients treated with d,l-sotalol for rhythm control vs matched control patients treated with cardioselective beta-blockers.

METHODS

This population-based cohort study included AF patients from the Swedish National Patient Registry (2006-2017) who underwent rhythm control after a second cardioversion. Incidence rates (IRs) and adjusted hazard ratios (aHRs) for mortality and a composite endpoint of cardiac arrest/death and ventricular arrhythmias were calculated for the overall cohort and a 1:1 propensity score matched cohort of d,l-sotalol vs beta-blocker treatment.

RESULTS

Among patient treated with d,l-sotalol (n = 4987) and beta-blocker (n = 27,078) (mean follow-up 458 days), all-cause mortality was lower in patients treated with d,l-sotalol: IR 1.21; 95% confidence interval 0.95-1.52 vs 2.42 (2.26-2.60) deaths per 100 patient-years; aHR 0.66 (0.52-0.83). The difference in mortality persisted in the propensity score matched comparison (n = 4953 in each group): aHR 0.63 (0.48-0.86). No differences were observed in the composite outcome: IR in propensity cohorts 2.13 (1.78-2.52) vs 2.07 (1.73-2.53) events per 100 years; aHR 1.01 (0.78-1.29).

CONCLUSION

There was no excess mortality with d,l-sotalol compared with cardioselective beta-blockers in patients undergoing rhythm control treatment for AF after a second cardioversion. Our results indicate that the risk associated with d,l-sotalol treatment for AF can be mitigated by careful patient selection and strict adherence to follow-up protocols.

Longitudinal QTc Stability and Impact of Baseline Cardiac Rhythm on Discharge Dose in Dofetilide-treated Patients

BACKGROUND

Dofetilide suppresses AF in a dose-dependent fashion. The protective effect of AF against QT_c prolongation induced torsades de pointe and transient post-cardioversion QT_c prolongation may result in dofetilide under-dosing during initiation. Thus, the optimal timing of cardioversion for AF patients undergoing dofetilide initiation to optimize discharge dose remains unknown as does the longitudinal stability of QT_c .

OBJECTIVE

To evaluate the impact of baseline rhythm on dofetilide dosing during initiation and assess the longitudinal stability of QT_c-all (Bazzett, Fridericia, Framingham, and Hodges) over time.

METHODS

Medical records of patients who underwent pre-planned dofetilide loading at a tertiary care center between January 2016-2019 were reviewed.

RESULTS

A total of 198 patients (66±10 years, 32% female, CHADS₂ -Vasc 3 [2-4]) presented for dofetilide loading in either AF (59%) or SR (41%). Neither presenting rhythm, nor spontaneous conversion to SR impacted discharge dose. The cumulative dofetilide dose prior to cardioversion moderately correlated (r=0.36; p=0.0001) with discharge dose. Post-cardioversion QT_c-all prolongation (p<0.0001) prompted discharge dose reduction (890±224mcg vs 552±199mcg; p<0.0001) in 30% patients. QT_c-all in SR prolonged significantly during loading (p<0.0001). All patients displayed QT_c-all reduction (p<0.0001) from discharge to short-term (46 [34-65] days) that continued at long-term (360 [296-414] days) follow-ups. The extent of QT_c-all reduction over time moderately correlated with discharge QT_c-all (r=0.54-0.65; p<0.0001).

CONCLUSION

Dofetilide initiation prior to cardioversion is equivalent to initiation during SR. Significant QT_c reduction proportional to discharge QT_c is seen over time in all dofetilide-treated patients. QT_c returns to pre-loading baseline during follow-up in patients initiated in SR. This article is protected by copyright. All rights reserved.

Bioengineering approaches to mature induced pluripotent stem cell-derived atrial cardiomyocytes to model atrial fibrillation

Induced pluripotent stem cells (iPSCs) serve as a robust platform to model several human arrhythmia syndromes including atrial fibrillation (AF). However, the structural, molecular, functional, and electrophysiological parameters of patient-specific iPSC-derived atrial cardiomyocytes (iPSC-aCMs) do not fully recapitulate the mature phenotype of their human adult counterparts. The use of physiologically inspired microenvironmental cues, such as postnatal factors, metabolic conditioning, extracellular matrix (ECM) modulation, electrical and mechanical stimulation, co-culture with non-parenchymal cells, and 3D culture techniques can help mimic natural atrial development and induce a more mature adult phenotype in iPSC-aCMs. Such advances will not only elucidate the underlying pathophysiological mechanisms of AF, but also identify and assess novel mechanism-based therapies towards supporting a more 'personalized' (i.e. patient-specific) approach to pharmacologic therapy of AF.

Diurnal QT analysis in patients with sotalol after cardioversion of atrial fibrillation

Background

The risk of ventricular arrhythmias in patients on QT prolonging drugs is indicated to be increased early after cardioversion (CV) of atrial fibrillation (AF) to sinus rhythm (SR). Sotalol, used to prevent AF relapse, prolongs cardiac repolarization and corrected QT interval (QTc). A pronounced QTc prolongation is an established marker of pro‐arrhythmias. Our objective was to use novel technique to quantify and evaluate the diurnal variation of the QTc interval after elective CV to SR in patients on sotalol or metoprolol.

Methods

Fifty patients underwent twelve‐lead Holter recording for 24 hr after elective CV for persistent AF. All patients had the highest tolerable stable dose of sotalol (n = 27) or metoprolol (n = 23). Measurements of QT and RR intervals were performed on all valid beats.

Results

A clear diurnal variation of both HR and QTc was seen in both groups, more pronounced in patients on sotalol, where a high percentage of heartbeats with QTc >500 ms was observed, especially at night. Six patients (22%) on sotalol but none on metoprolol had >20% of all heart beats within the 24‐hour recording with QTc >500 ms.

Conclusion

Twenty‐four‐hour Holter recordings with QT‐measurement immediately after CV demonstrated that one in five patients on sotalol had >20% of all heart beats with prolonged QTc >500 ms, especially during night‐time. The QTc diurnal variation was retained in patients on β‐blockade or a potent class III anti‐arrhythmic drug with β‐blocking properties.

Use of a Smartwatch for Assessment of the QT Interval in Outpatients with Coronavirus Disease 2019

The coronavirus disease 2019 (COVID-19) pandemic has necessitated rapid implementation of innovative strategies to manage patients remotely to help reduce the risk of community and nosocomial transmission. This case demonstrates the use of an Apple Watch (Apple, Cupertino, CA, USA) to monitor for arrhythmias and QT prolongation in a patient with COVID-19 during home isolation.

A case of a late fatal complication after atrial fibrillation ablation related to a prolonged QT interval unmasked by atrial fibrillation ablation

A 79-year-old woman with a history of atrial fibrillation (AF) ablation was referred to our hospital for ventricular fibrillation, which was terminated by an automated external defibrillator. The heart rate corrected QT interval was 489 ms. The electrocardiogram monitoring recorded a polymorphic ventricular tachycardia (VT) reproducibly induced by a single morphology premature ventricular contraction (PVC). Therefore, we performed a trigger PVC ablation and implanted an implantable cardioverter defibrillator. No VT events were observed for at least one year after the ablation. A prolonged QT interval after the AF ablation should be carefully noted because it could introduce fatal complications. <Learning objective: A rare late fatal complication of ventricular tachycardia (VT) after atrial fibrillation (AF) ablation can occur even more than one month after the AF ablation. The AF ablation might have an adverse effect on the masked prolonged QT interval. A trigger ablation of the polymorphic VT was helpful to control lethal VTs.>.

QTc Interval-Prolonging Medications Among Patients With Lung Cancer: Implications for Clinical Trial Eligibility and Clinical Care

BACKGROUND

Concomitant medication use, including agents that prolong the corrected QT (QTc) interval, can result in the exclusion of patients with cancer from clinical trials. To estimate the potential effects on accrual, we determined the prevalence of QTc-prolonging medication prescriptions in a national patient cohort.

PATIENTS AND METHODS

We identified adult patients in the Veterans Affairs system with a diagnosis of lung cancer from 2003 to 2016. The use of QTc interval-prolonging medications and risk category were obtained from CredibleMeds. We calculated the prevalence of prescriptions for QTc-prolonging medications with a known or possible risk of torsade de pointes in the 3 months up to and including the date of cancer diagnosis. The rates across patient groups were compared using χ² test.

RESULTS

A total of 280,068 patients were included in the present study. The mean age was 70 years, 98% were male, and 72% were white. Overall, 28.4% had been prescribed a QTc-prolonging medication, and 7.3% had been prescribed ≥2 in the 3 months before the cancer diagnosis. The most commonly prescribed QTc-prolonging medications were antimicrobial agents (14.0%), psychiatric agents (10.2%), antiemetic agents (2.6%), and cardiac medications (1.7%). Excluding the antimicrobial agents, 18.4% of the patients had been prescribed a QTc-prolonging medication.

CONCLUSIONS

A substantial proportion of individuals with lung cancer will be prescribed QTc-prolonging medications. These prescriptions can limit patients' eligibility for clinical trials and complicate the administration of standard cancer therapies. Further research into the actual clinical risks and optimal management of QTc-prolonging medications in cancer populations is warranted.

Prevalence, predictors and clinical implications of prolonged corrected QT in elderly patients with dementia and suspected syncope

BACKGROUND

Long QT and use of QT-prolonging drugs are common among older patients receiving polytherapies, but real-world evidence on their impact in clinical practice is controversial. We investigated prevalence, variables associated and clinical implications of prolonged corrected QT (QTc) among patients from the Syncope and Dementia study.

METHODS

Observational, prospective, multicenter study. Patients≥65 years with dementia and fall suspected for syncope in the previous three months were enrolled. Several clinical variables and the complete list of medications were recorded for each patient. A 12‑lead ECG was obtained and corrected QT was calculated by the Bazett's formula. One-year followup for death and recurrent syncope was performed.

RESULTS

Prolonged QTc was observed in 25% of the 432 enrolled patients (mean age 83.3), and was significantly associated with male gender (OR 2.09; 95% CI 1.34-3.26) and diuretics use (OR 1.85; 95% CI 1.18-2.90). At one-year 23.3% of patients died and 30.4% reported at least one recurrent event. Variables associated with one-year mortality were: age, male gender, atrial fibrillation (AF), use of calcium channel blockers and prolonged QTc (OR 1.80; 95% CI 1.01-3.20). Among patients with prolonged QTc a significant interaction for mortality was found with AF. Recurrent events were associated with the use of antiplatelets, cholinesterase. inhibitors and antipsychotics, but not with prolonged QTc.

CONCLUSIONS

We documented a high prevalence of prolonged QTc, that was associated with male gender and diuretics but not with psychoactive medications. Patients with prolonged QTc had higher one-year mortality, that was four-fold increased in those with concomitant AF.

Predictive Value of T peak - T end Indices for Adverse Outcomes in Acquired QT Prolongation: A Meta-Analysis

Background: Acquired QT interval prolongation has been linked with malignant ventricular arrhythmias, such as torsade de pointes, in turn predisposing to sudden cardiac death. Increased dispersion of repolarization has been identified as a pro-arrhythmic factor and can be observed as longer T_peak - T_end interval and higher T_peak - T_end/QT ratio on the electrocardiogram. However, the values of these repolarization indices for predicting adverse outcomes in this context have not been systematically evaluated. Method: PubMed, Embase and Cochrane Library databases were searched until 14th February 2018, identifying 232 studies. Results: Five studies on acquired QT prolongation met the inclusion criteria and 308 subjects with drug-induced LQTS patients (mean age: 66 ± 18 years old; 46% male) were included in this meta-analysis. T_peak - T_end intervals were longer [mean difference [MD]: 76 ms, standard error [SE]: 26 ms, P = 0.003; I² = 98%] and T_peak - T_end/QT ratios were higher (MD: 0.14, SE: 0.03, P = 0.000; I² = 29%) in patients with torsade de pointes compared to those without these events. Conclusion: T_peak - T_end interval and T_peak - T_end/QT ratio were higher in patients with acquired QT prolongation suffering from torsade de pointes compared to those who did not. These repolarization indices may provide additional predictive value for identifying high-risk individuals.

Ion channels, long QT syndrome and arrhythmogenesis in ageing

Ageing is associated with increased prevalences of both atrial and ventricular arrhythmias, reflecting disruption of the normal sequence of ion channel activation and inactivation generating the propagated cardiac action potential. Experimental models with specific ion channel genetic modifications have helped clarify the interacting functional roles of ion channels and how their dysregulation contributes to arrhythmogenic processes at the cellular and systems level. They have also investigated interactions between these ion channel abnormalities and age-related processes in producing arrhythmic tendency. Previous reviews have explored the relationships between age and loss-of-function Na_v 1.5 mutations in producing arrhythmogenicity. The present review now explores complementary relationships arising from gain-of-function Na_v 1.5 mutations associated with long QT3 (LQTS3). LQTS3 patients show increased risks of life-threatening ventricular arrhythmias, particularly after 40 years of age, consistent with such interactions between the ion channel abnormailities and ageing. In turn clinical evidence suggests that ageing is accompanied by structural, particularly fibrotic, as well as electrophysiological change. These abnormalities may result from biochemical changes producing low-grade inflammation resulting from increased production of reactive oxygen species and superoxide. Experimental studies offer further insights into the underlying mechanisms underlying these phenotypes. Thus, studies in genetically modified murine models for LQTS implicated action potential recovery processes in arrhythmogenesis resulting from functional ion channel abnormalities. In addition, ageing wild type (WT) murine models demonstrated both ion channel alterations and fibrotic changes with ageing. Murine models then suggested evidence for interactions between ageing and ion channel mutations and provided insights into potential arrhythmic mechanisms inviting future exploration.

Proarrhythmic and Torsadogenic Effects of Potassium Channel Blockers in Patients

The most common arrhythmia requiring drug treatment is atrial fibrillation (AF), which affects 2 to 5 million Americans and continues to be a major cause of morbidity and increased mortality. Despite recent advances in catheter-based and surgical therapies, antiarrhythmic drugs continue to be the mainstay of therapy for most patients with symptomatic AF. However, many antiarrhythmics block the rapid component of the cardiac delayed rectifier potassium current (IKr) as a major mechanism of action, and marked QT prolongation and pause-dependent polymorphic ventricular tachycardia (torsades de pointes) are major class toxicities.

Reduction over time of QTc prolongation in patients with sotalol after cardioversion of atrial fibrillation

BACKGROUND

Sotalol is recommended to prevent relapse of atrial fibrillation after cardioversion (CV). Sotalol prolongs the action potential by blocking the rapid component of the delayed rectifier potassium current, which results in corrected QT (QTc) prolongation on the electrocardiogram. Pronounced QTc prolongation may lead to proarrhythmias and sudden death.

OBJECTIVE

We investigated the dynamics of the QTc interval during the week after CV in patients treated with sotalol compared with patients treated with a β-blocker.

METHODS

Patients who underwent elective CV for persistent atrial fibrillation and maintained sinus rhythm for 1 week were included prospectively. All patients were on the highest tolerable stable dose of metoprolol or sotalol. Twelve-lead electrocardiograms were recorded 1 hour and 1 week after CV.

RESULTS

A total of 104 patients on sotalol and 104 on metoprolol were included; clinical characteristics between groups were comparable. One hour after CV, the QTc interval was significantly longer in sotalol-treated patients than in metoprolol-treated patients (465 ± 25 ms vs 423 ± 30 ms; P ≤ .0001). After 1 week, the QTc interval was reduced by -20.3 ± 24 ms in sotalol-treated patients (P ≤ .001); no such effect was seen in metoprolol-treated patients (-2.5 ± 18 ms; P = 0.28). The heart rate was stable during the week in both groups. In multivariate analysis of sotalol-treated patients, factors contributing to pronounced reduction in the QTc interval were longer QTc interval after CV and renal function.

CONCLUSION

The QTc interval is significantly reduced during the week after CV to sinus rhythm in sotalol-treated patients. This provides insight into the increased risk of proarrhythmias in the immediate time period after CV.

Abnormal repolarization dynamics in a patient with KCNE1(G38S) who presented with torsades de pointes

Risk of G38S, major KCNE1 polymorphism [KCNE1(G38S)], for long QT syndrome (LQTS) remains unclear. A 72-year-old woman was admitted with recurrent torsades de pointes (TdP). She had remarkable QT prolongation (corrected QT interval 568 ms) under conditions of hypokalemia and hypomagnesemia. After correction of this electrolytic imbalance, TdP was suppressed and metoprolol was started. The QT-RR slope in 24-hour Holter electrocardiogram was steep and this enhanced bradycardia-dependent QT prolongation was similar to that in LQTS. She carried KCNE1(G38S). Patients with KCNE1(G38S) could have similar potential risk of ventricular arrhythmia as with LQTS. Analysis of QT-RR relationship could also evaluate the latent arrhythmogenicity of KCNE1(G38S).

Time-Dependent Changes in QT Dynamics after Initiation and Termination of Paroxysmal Atrial Fibrillation

BACKGROUND

Little is known about time-dependent changes in QT dynamics after initiation of atrial fibrillation (AF) and after restoration of sinus rhythm (SR) in patients with paroxysmal AF.

METHODS

Beat-to-beat QT and RR intervals in CM5 lead were measured automatically in 13 patients with both AF and SR on the single 24-hour Holter electrocardiology recording. QT-RR relation was analyzed at six periods of time: 1 hour before AF onset (Pre(0-1h)), 0-1 hour and 4-5 hours after AF onset (AF(0-1h) and AF(4-5h)), and 0-1 hour, 2-3 hours, and 4-5 hours after the restoration of SR (SR(0-1h), SR(2-3h), and SR(4-5h)).

RESULTS

QT-RR slope was gradually decreased after AF onset and gradually returned to the baseline level after restoration of SR. The slope became greater at SR(4-5h) than at AF(4-5h) and AF(0-1h). In patients receiving antiarrhythmic drugs (AADs; n = 5), QT-RR slope was greater at SR(4-5h) than in those not receiving AADs (n = 8).

CONCLUSION

In patients with paroxysmal AF, bradycardia-dependent QT prolongation was attenuated during AF, and was corrected and gradually augmented along with continuation of SR, especially in patients receiving AADs. This could increase the risk of developing torsade de pointes.

Determinants of torsades de pointes in older patients with drug-associated long QT syndrome: a case-control study

OBJECTIVE

Many elderly patients are routinely exposed to drugs that may prolong the cardiac QT interval and cause Torsades de pointes (TdP). However, predictors of TdP in patients with drug-associated long QT syndrome (LQTS) are not fully understood, especially in the geriatric population. The objective of this study was to identify risk factors of TdP in elderly patients with drug-associated LQTS.

METHODS

In this retrospective, case-control study, documented reports of drug-associated LQTS plus TdP (n = 125) and LQTS without TdP (n = 81) in patients ≥65 years of age were retrieved from the French Pharmacovigilance Database over a 10-year period. Available clinical, biological, and drug therapy data were compared in the two groups and logistic regression was performed to identify significant predictors of TdP.

RESULTS

The uncorrected QT interval was significantly longer in patients with TdP than in patients without TdP (577 ± 79 vs. 519 ± 68 ms; p = 0.0001). The number of drugs with a known risk of TdP administered to each patient was not a predictor of arrhythmia, nor was female gender. Logistic regression analysis identified the uncorrected QT interval as the only significant predictor of TdP. The receiver operating characteristic curve analysis was characterized by an area under the curve of 0.77 (95 % confidence interval 0.64-0.88) and a QT cutoff of 550 ms.

CONCLUSION

The uncorrected QT interval was significantly associated with the probability of TdP in elderly patients with acquired, drug-associated LQTS.

Factors affecting the degree of QT prolongation with drug challenge in a large cohort of normal volunteers

BACKGROUND

The degree of QT prolongation by drug is highly variable and related to risk for polymorphic ventricular tachycardia due to drugs.

OBJECTIVE

The purpose of this study was to determine factors that affect the degree of QT prolongation by drugs.

METHODS

QT and QTc were measured before and after administration of the QT-prolonging drug ibutilide in 253 normal volunteers aged 18 to 40 years. Drug effect on QTc prolongation was defined as ΔQTc = QTc after drug minus QTc before drug.

RESULTS

Ibutilide prolonged QT from 396 ± 31 ms to 418 ± 39 ms (P <.001) and QTc from 406 ± 15 ms to 446 ± 33 ms (P <.001). ΔQTc did not correlate with baseline QTc (Pearson correlation 0.016, P = .8). Postdrug QTc was correlated weakly with predrug QTc (Pearson correlation 0.484, P <.001), and strongly with ΔQTc (Pearson correlation 0.882, P <.001). ΔQTc was identical for men and women (39 ± 29 ms vs 39 ± 27 ms, P = .9) but displayed significant differences among body mass index categories (P <.001). Overweight (48 ± 27 ms) and obese (61 ± 31 ms) subjects had significantly more QT prolongation by drug than normal (31 ± 25 ms) or underweight (24 ± 12 ms) subjects.

CONCLUSION

QT prolongation by ibutilide does not correlate to baseline QTc and does not differ between men and women. Overweight and obese subjects have greater drug effect on QTc than subjects with normal or low body mass index. These findings have implications for drug-induced long QT syndrome.

Drug-induced long QT syndrome

The drug-induced long QT syndrome is a distinct clinical entity that has evolved from an electrophysiologic curiosity to a centerpiece in drug regulation and development. This evolution reflects an increasing recognition that a rare adverse drug effect can profoundly upset the balance between benefit and risk that goes into the prescription of a drug by an individual practitioner as well as the approval of a new drug entity by a regulatory agency. This review will outline how defining the central mechanism, block of the cardiac delayed-rectifier potassium current I(Kr), has contributed to defining risk in patients and in populations. Models for studying risk, and understanding the way in which clinical risk factors modulate cardiac repolarization at the molecular level are discussed. Finally, the role of genetic variants in modulating risk is described.

Comparison of QT/RR relation based on a 15-s averaged ECG and a single beat ECG during atrial fibrillation

BACKGROUND

The aim of this study was to compare QT/RR relation based on a 15-s averaged beat ECG with a single beat ECG during atrial fibrillation (AF) and to determine which was better to estimate the QT interval after sinus restoration.

METHODS AND RESULTS

QT and RR intervals were measured using an automatic QT analyzing system in 33 patients who had both AF and sinus rhythm on the same 24-h Holter ECG recording. In 14 patients, antiarrhythmic drugs (AAD) were administered. QT/RR relations were analyzed from ECG waves obtained by the summation of consecutive QRS-T complexes during each 15-s period (QT/RR-average) and a single beat QRS-T (QT/RR-single). During sinus rhythm, the slope of QT/RR-average did not differ from that of QT/RR-single in patients with and without AAD. On the other hand, during AF, the slope of QT/RR-average was significantly greater than that of QT/RR-single (without AAD: 0.12±0.06 vs. 0.06±0.03, P<0.001; with AAD: 0.15±0.05 vs. 0.08±0.04, P<0.001). During AF, the QT interval at an RR interval of 1.2-s (QT-1.2) determined from QT/RR-average was significantly greater than QT-1.2 from QT/RR-single in patients with and without AAD. QT-1.2 in QT/RR-single during AF was significantly smaller than that during sinus rhythm but QT-1.2 in QT/RR-average during AF was not.

CONCLUSIONS

The QT interval after sinus restoration could be estimated better using QT/RR-average than using QT/RR-single during AF.

Knock-in gain-of-function sodium channel mutation prolongs atrial action potentials and alters atrial vulnerability

BACKGROUND

Patients with long QT syndrome (LQTS) are at increased risk not only for ventricular arrhythmias but also for atrial pathology including atrial fibrillation (AF). Some patients with "lone" AF carry Na(+)-channel mutations.

OBJECTIVE

The purpose of this study was to determine the mechanisms underlying atrial pathology in LQTS.

METHODS

In mice with a heterozygous knock-in long QT syndrome type 3 (LQT3) mutant of the cardiac Na(+) channel (ΔKPQ-SCN5A) and wild-type (WT) littermates, atrial size, function, and electrophysiologic parameters were measured in intact Langendorff-perfused hearts, and histologic analysis was performed.

RESULTS

Atrial action potential duration, effective refractory period, cycle length, and PQ interval were prolonged in ΔKPQ-SCN5A hearts (all P < .05). Flecainide (1 μM) reversed atrial action potential duration prolongation and induced postrepolarization refractoriness (P < .05). Arrhythmias were infrequent during regular rapid atrial rate in both WT and ΔKPQ-SCN5A but were inducible in 15 (38%) of 40 ΔKPQ-SCN5A and 8 (29%) of 28 WT mice upon extrastimulation. Pacing protocols generating rapid alterations in rate provoked atrial extrasystoles and arrhythmias in 6 (66%) of 9 ΔKPQ-SCN5A but in 0 (0%) of 6 WT mice (P < .05). Atrial diameter was increased by nearly 10% in ΔKPQ-SCN5A mice > 5 months old without increase in fibrotic tissue.

CONCLUSION

Murine hearts bearing an LQT3 mutation show abnormalities in atrial electrophysiology and subtle changes in atrial dimension, including an atrial arrhythmogenic phenotype on provocation. These results support clinical data suggesting that LQTS mutations can cause atrial pathology and arrhythmogenesis and indicate that murine sodium channel LQTS models may be useful for exploring underlying mechanisms.

Extracellular K+ concentration controls cell surface density of IKr in rabbit hearts and of the HERG channel in human cell lines

Although the modulation of ion channel gating by hormones and drugs has been extensively studied, much less is known about how cell surface ion channel expression levels are regulated. Here, we demonstrate that the cell surface density of both the heterologously expressed K+ channel encoded by the human ether-a-go-go-related gene (HERG) and its native counterpart, the rapidly activating delayed rectifier K+ channel (IKr), in rabbit hearts in vivo is precisely controlled by extracellular K+ concentration ([K+]o) within a physiologically relevant range. Reduction of [K+]o led to accelerated internalization and degradation of HERG channels within hours. Confocal analysis revealed colocalization between HERG and ubiquitin during the process of HERG internalization, and overexpression of ubiquitin facilitated HERG degradation under low [K+]o. The HERG channels colocalized with a marker of multivesicular bodies during internalization, and the internalized HERG channels were targeted to lysosomes. Our results provide the first evidence to our knowledge that the cell surface density of a voltage-gated K+ channel, HERG, is regulated by a biological factor, extracellular K+. Because hypokalemia is known to exacerbate long QT syndrome (LQTS) and Torsades de pointes tachyarrhythmias, our findings provide a potential mechanistic link between hypokalemia and LQTS.

Ibutilide-induced changes in the temporal lability of ventricular repolarization in patients with and without structural heart disease

INTRODUCTION

Ibutilide has been shown to prolong repolarization times and increase the risk of ventricular tachyarrhythmias particularly in patients with structural heart disease. The mechanisms underlying its proarrhythmic effects remain incompletely understood. We sought to define the effects of ibutilide on the temporal lability of ventricular repolarization in patients with and without structural heart disease.

METHODS

Twenty-four patients referred for electrophysiology study underwent monophasic action potential (MAP) recordings in the right ventricle during sinus rhythm and random interval right atrial pacing (RIAP). Ibutilide was subsequently administered and the recordings repeated both in sinus rhythm and with RIAP. Digitized recordings were analyzed offline for calculation of the QT variability index (QTVI) based on surface ECG, and the MAP duration variability index (MAPDVI) based on the intracardiac MAP signal.

RESULTS

Of 24 patients enrolled, analyses were performed in 21 patients (mean age 59 +/- 15 years, 38% women). In three patients, the data were not analyzed due to frequent premature ventricular complexes. Ibutilide resulted in significant changes in heart rate (mean difference: -7.4 +/- 0.91 bpm, P < 0.0001) and the surface QT interval (mean difference: 59.6 +/- 12.2 ms, P = 0.0001) during sinus rhythm. After ibutilide, QTVI remained unchanged from baseline during sinus rhythm but was significantly different in the setting of RIAP (mean difference: 0.345 +/- 0.098, P = 0.0022). With subgroup analyses, these differences remained significant regardless of the presence or absence of heart disease.

CONCLUSION

Ibutilide results in overall prolongation of ventricular repolarization and reductions in baseline sinus rates. Ibutilide increases temporal lability of repolarization only with enriched fluctuations in heart rate.

A mutation in the sodium channel is responsible for the association of long QT syndrome and familial atrial fibrillation

BACKGROUND

Type 3 long-QT syndrome (LQT-3) is caused by gain-of-function mutations in the SCN5A encoding the cardiac sodium channel. Familial atrial fibrillation (AF), previously considered a potassium channelopathy, has recently been related to sodium genetic variants, both in isolated forms and in patients with underlying heart disease.

OBJECTIVE

The purpose of this study was to describe the first family associating LQT-3 and AF due to a gain-of-function mutation in SCN5A and assess the usefulness of the sodium blocker flecainide in individuals with both phenotypes.

METHODS

Complete family screening was performed after identifying a proband showing paroxysmal AF and a long QT interval suggestive of LQT-3. Secondary causes of AF were ruled out in all individuals. Flecainide was used in two patients for LQT-3 diagnosis and therapeutic treatment of AF. Genetic screening was performed by direct sequencing of the exons and exon-intron boundaries of SCN5A.

RESULTS

We identified a three-generation family (eight members), all of them showing long QT intervals. Paroxysmal AF initiated between 20 and 35 years of age in all three adults. The flecainide test led to shortening of the QTc interval. Flecainide was also effective in acutely restoring sinus rhythm. A Y1795C mutation was identified in all members.

CONCLUSION

This is the first report showing an association of familial AF and LQT-3 due to a mutation in SCN5A. This finding provides further evidence of the role of SCN5A in AF. We also confirm the usefulness of flecainide in this particular complex phenotype, both as a diagnostic tool for LQT-3 and as an acute treatment for AF.

The anaesthetised methoxamine-sensitised rabbit model of torsades de pointes

Current guidelines describe strategies on how the potential of non-antiarrhythmic drugs to delay ventricular repolarisation should be assessed. However, the non-clinical guidelines recommend repolarisation assays only and do not advocate experimental models that express the proarrhythmia of concern, torsades de pointes (TdP). Although the repolarisation assays may predict QT interval prolongation in man they cannot alone sufficiently predict proarrhythmia risk. Furthermore, there is also a need for more robust surrogate markers of drug-induced proarrhythmia and such validated markers are on the horizon as a result of the availability of sensitive animal models of TdP. This review will describe the methoxamine-sensitised rabbit model of TdP, one of the most frequently used proarrhythmia models, and present some of it characteristics, its pros and cons and how it historically has been used for assessing proarrhythmia liability of drugs.