Acquired long QT syndromes and the risk of proarrhythmia

A polygenic risk score for the QT interval is an independent predictor of drug-induced QT prolongation

Drug-induced QT prolongation (diLQTS), and subsequent risk of torsade de pointes, is a major concern with use of many medications, including for non-cardiac conditions. The possibility that genetic risk, in the form of polygenic risk scores (PGS), could be integrated into prediction of risk of diLQTS has great potential, although it is unknown how genetic risk is related to clinical risk factors as might be applied in clinical decision-making. In this study, we examined the PGS for QT interval in 2500 subjects exposed to a known QT-prolonging drug on prolongation of the QT interval over 500ms on subsequent ECG using electronic health record data. We found that the normalized QT PGS was higher in cases than controls (0.212±0.954 vs. -0.0270±1.003, P = 0.0002), with an unadjusted odds ratio of 1.34 (95%CI 1.17-1.53, P<0.001) for association with diLQTS. When included with age and clinical predictors of QT prolongation, we found that the PGS for QT interval provided independent risk prediction for diLQTS, in which the interaction for high-risk diagnosis or with certain high-risk medications (amiodarone, sotalol, and dofetilide) was not significant, indicating that genetic risk did not modify the effect of other risk factors on risk of diLQTS. We found that a high-risk cutoff (QT PGS ≥ 2 standard deviations above mean), but not a low-risk cutoff, was associated with risk of diLQTS after adjustment for clinical factors, and provided one method of integration based on the decision-tree framework. In conclusion, we found that PGS for QT interval is an independent predictor of diLQTS, but that in contrast to existing theories about repolarization reserve as a mechanism of increasing risk, the effect is independent of other clinical risk factors. More work is needed for external validation in clinical decision-making, as well as defining the mechanism through which genes that increase QT interval are associated with risk of diLQTS.

Facilitation of hERG Activation by Its Blocker: A Mechanism to Reduce Drug-Induced Proarrhythmic Risk

Modulation of the human Ether-à-go-go-Related Gene (hERG) channel, a crucial voltage-gated potassium channel in the repolarization of action potentials in ventricular myocytes of the heart, has significant implications on cardiac electrophysiology and can be either antiarrhythmic or proarrhythmic. For example, hERG channel blockade is a leading cause of long QT syndrome and potentially life-threatening arrhythmias, such as torsades de pointes. Conversely, hERG channel blockade is the mechanism of action of Class III antiarrhythmic agents in terminating ventricular tachycardia and fibrillation. In recent years, it has been recognized that less proarrhythmic hERG blockers with clinical potential or Class III antiarrhythmic agents exhibit, in addition to their hERG-blocking activity, a second action that facilitates the voltage-dependent activation of the hERG channel. This facilitation is believed to reduce the proarrhythmic potential by supporting the final repolarizing of action potentials. This review covers the pharmacological characteristics of hERG blockers/facilitators, the molecular mechanisms underlying facilitation, and their clinical significance, as well as unresolved issues and requirements for research in the fields of ion channel pharmacology and drug-induced arrhythmias.

Pore opening, not voltage sensor movement, underpins the voltage-dependence of facilitation by a hERG blocker

A drug that blocks the cardiac myocyte voltage-gated K+ channels encoded by the human Ether-à-go-go-Related Gene (hERG) carries a potential risk of long QT syndrome and life-threatening cardiac arrhythmia, including Torsade de Points Interestingly, certain hERG blockers can also facilitate hERG activation to increase hERG currents, which may reduce proarrhythmic potential. However, the molecular mechanism involved in the facilitation effect of hERG blockers remains unclear. The hallmark feature of the facilitation effect by hERG blockers is that a depolarizing preconditioning pulse shifts voltage-dependence of hERG activation to more negative voltages. Here we utilize a D540K hERG mutant to study the mechanism of the facilitation effect. D540K hERG is activated by not only depolarization but also hyperpolarization. This unusual gating property enables tests of the mechanism by which voltage induces facilitation of hERG by blockers. With D540K hERG, we find that nifekalant, a hERG blocker and Class III antiarrhythmic agent, blocks and facilitates not only current activation by depolarization but also current activation by hyperpolarization, suggesting a shared gating process upon depolarization and hyperpolarization. Moreover, in response to hyperpolarizing conditioning pulses, nifekalant facilitates D540K hERG currents but not wild-type currents. Our results indicate that induction of facilitation is coupled to pore opening, not voltage per se We propose that gated access to the hERG central cavity underlies the voltage-dependence of induction of facilitation. This study identifies hERG channel pore gate opening as the conformational change facilitated by nifekalant, a clinically important antiarrhythmic agent. Significance Statement Nifekalant is a clinically important antiarrhythmic agent and a hERG blocker which can also facilitate voltage-dependent activation of hERG channels after a preconditioning pulse. Here we show that the mechanism of action of the preconditioning pulse is to open a conductance gate to enable drug access to a facilitation site. Moreover, we find that facilitation increases hERG currents by altering pore dynamics, rather than acting through voltage sensors.

Assessing Prescriber Behavior with a Clinical Decision Support Tool to Prevent Drug-Induced Long QT Syndrome

OBJECTIVE

Clinical decision support (CDS) alerts built into the electronic health record (EHR) have the potential to reduce the risk of drug-induced long QT syndrome (diLQTS) in susceptible patients. However, the degree to which providers incorporate this information into prescription behavior and the impact on patient outcomes is often unknown.

METHODS

We examined provider response data over a period from October 8, 2016 until November 8, 2018 for a CDS alert deployed within the EHR from a 13-hospital integrated health care system that fires when a patient with a QTc ≥ 500 ms within the past 14 days is prescribed a known QT-prolonging medication. We used multivariate generalized estimating equations to analyze the impact of therapeutic alternatives, relative risk of diLQTS for specific medications, and patient characteristics on provider response to the CDS and overall patient mortality.

RESULTS

The CDS alert fired 15,002 times for 7,510 patients for which the most common response (51.0%) was to override the alert and order the culprit medication. In multivariate models, we found that patient age, relative risk of diLQTS, and presence of alternative agents were significant predictors of adherence to the CDS alerts and that nonadherence itself was a predictor of mortality. Risk of diLQTS and presence of an alternative agent are major factors in provider adherence to a CDS to prevent diLQTS; however, provider nonadherence was associated with a decreased risk of mortality.

CONCLUSION

Surrogate endpoints, such as provider adherence, can be useful measures of CDS value but attention to hard outcomes, such as mortality, is likely needed.

Evaluation of electrocardiographic repolarization parameters after administration of trimethoprim-sulfadiazine, detomidine, or their combination in horses

OBJECTIVE

To determine whether administration of trimethoprim-sulfadiazine (TMS), detomidine (DET), or TMS plus DET would be associated with changes in ECG repolarization parameters in horses.

ANIMALS

9 healthy adult horses.

PROCEDURES

Each horse received 4 treatments in a blinded, randomized, crossover study design as follows: TMS, 16 to 24 mg/kg, IV; DET, 0.015 to 0.02 mg/kg, IV; TMS plus DET; and saline (0.9% NaCl) solution. Surface ECG traces were obtained over 24 hours, and repolarization parameters were measured at predefined time points after each treatment and compared with a 2-way ANOVA for repeated measures.

RESULTS

Heart rate-corrected QT intervals (QTc) were significantly increased after administration of DET (mean ± SD difference in QTc, 36.57 ± 23.07 milliseconds; increase of 7%) and TMS plus DET (44.96 ± 29.16 milliseconds; increase of 9%), compared with baseline (before treatment) values and values after administration of saline solution. Saline solution and TMS alone did not affect QTc.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of DET or TMS plus DET was associated with a significant and possibly clinically relevant prolongation of QTc, with prolongation of 7% to 9%, a range that is considered as a risk factor for the development of cardiac arrhythmias in people. Results were unexpected because DET is considered to be a safe sedative for horses.

An Easy and Reliable Way to Prevent Electrocardiographic Deteriorations of Patients Undergoing Off-Pump Coronary Artery Bypass Surgery: Preoperative Anxiolytic Treatment

OBJECTIVE

To investigate the effects of preoperative anxiety relieving on electrophysiological changes in patients undergoing off-pump coronary artery bypass surgery.

METHODS

A total of 61 patients at ASA III risk group in the age range of 18-65 years were enrolled in the present study. Patients were randomly divided into two groups. Group S (Sedation group) was administered 0.04 mg/kg lorazepam per os (PO) twice before the operation. Group C (control group) was not administered with any anxiolytic premedication. State Trait Anxiety Inventory (STAI-I) and Beck Anxiety Inventory (BAI) were used to evaluate the level of anxiety. Electrocardiography (ECG), pulse oximeter and standard monitoring were performed for each patient. QT and P dispersions in each derivation of all ECGs were calculated.

RESULTS

Preoperative STAI-I scores were significantly lower in sedation group compared to the controls. Mean values of QT dispersion measured before induction, at the 1st minute of induction, 30th second of intubation and 4th minute of intubation in sedation group were significantly reduced compared to controls (P=0.024; P=0.027; P=0.001; P=0.033, respectively). The mean values of P dispersion measured before induction, at the 3rd minute of induction, 30th second of intubation and 4th minute of intubation in sedation group were significantly reduced compared to controls (P=0.001; P=0.020; P=0.023; P=0.005, respectively).

CONCLUSION

Elevated anxiety levels in patients undergoing coronary bypass surgery have a negative effect through prolonged QT and P-wave dispersion times. Anxiolytic treatment before surgery may be useful to prevent ventricular and atrial arrhythmias and associated complications through decreasing the QT and P-wave dispersion duration.

Facilitation of I Kr current by some hERG channel blockers suppresses early afterdepolarizations

Some hERG channel blockers are clinically safe, but others cause fatal cardiac arrhythmias. Furutani et al. show that safe blockers facilitate channel opening in ventricular myocytes and provide a repolarization reserve at precisely the voltages and times needed to suppress arrhythmias.

Drug-induced block of the cardiac rapid delayed rectifying potassium current (I_Kr), carried by the human ether-a-go-go-related gene (hERG) channel, is the most common cause of acquired long QT syndrome. Indeed, some, but not all, drugs that block hERG channels cause fatal cardiac arrhythmias. However, there is no clear method to distinguish between drugs that cause deadly arrhythmias and those that are clinically safe. Here we propose a mechanism that could explain why certain clinically used hERG blockers are less proarrhythmic than others. We demonstrate that several drugs that block hERG channels, but have favorable cardiac safety profiles, also evoke another effect; they facilitate the hERG current amplitude in response to low-voltage depolarization. To investigate how hERG facilitation impacts cardiac safety, we develop computational models of I_Kr block with and without this facilitation. We constrain the models using data from voltage clamp recordings of hERG block and facilitation by nifekalant, a safe class III antiarrhythmic agent. Human ventricular action potential simulations demonstrate the ability of nifekalant to suppress ectopic excitations, with or without facilitation. Without facilitation, excessive I_Kr block evokes early afterdepolarizations, which cause lethal arrhythmias. When facilitation is introduced, early afterdepolarizations are prevented at the same degree of block. Facilitation appears to prevent early afterdepolarizations by increasing I_Kr during the repolarization phase of action potentials. We empirically test this prediction in isolated rabbit ventricular myocytes and find that action potential prolongation with nifekalant is less likely to induce early afterdepolarization than action potential prolongation with dofetilide, a hERG channel blocker that does not induce facilitation. Our data suggest that hERG channel blockers that induce facilitation increase the repolarization reserve of cardiac myocytes, rendering them less likely to trigger lethal ventricular arrhythmias.

Validation of Polygenic Scores for QT Interval in Clinical Populations

BACKGROUND

Polygenic risk scores (PGS) enable rapid estimation of genome-wide susceptibility for traits, which may be useful in clinical settings, such as prediction of QT interval. In this study, we sought to validate PGS for QT interval in 2 real-world cohorts of European ancestry (EA) and African ancestry (AA).

METHODS AND RESULTS

Two thousand nine hundred and fifteen participants of EA and 366 of AA in the MGH CAMP study (Cardiology and Metabolic Patient) were genotyped on a genome-wide array and imputed to the 1000 Genomes reference panel. An additional 820 EA and 57 AA participants in the Partners Biobank were genotyped and used for validation. PGS were created for each individual using effect estimates from association tests with QT interval obtained from prior genome-wide association studies, with variants selected based from multiple significance thresholds in the original study. In regression models, clinical variables explained ≈9% to 10% of total variation in resting QTc in EA individuals and ≈12% to 18% in AA individuals. The PGS significantly increased variation explained at most significance thresholds (P<0.001), with a trend toward increased variation explained at more stringent P value cut points in the CAMP EA cohort (P<0.05). In AA individuals, PGS provided no improvement in variation explained at any significance threshold.

CONCLUSIONS

For individuals of European descent, PGS provided a significant increase in variation in QT interval explained compared with a model with only nongenetic factors at nearly every significance level. There was no apparent benefit gained by relaxing the significance threshold from conventional genome-wide significance (P<5×10^-8).

Cardiopulmonary Resuscitation in Infants and Children With Cardiac Disease: A Scientific Statement From the American Heart Association

Cardiac arrest occurs at a higher rate in children with heart disease than in healthy children. Pediatric basic life support and advanced life support guidelines focus on delivering high-quality resuscitation in children with normal hearts. The complexity and variability in pediatric heart disease pose unique challenges during resuscitation. A writing group appointed by the American Heart Association reviewed the literature addressing resuscitation in children with heart disease. MEDLINE and Google Scholar databases were searched from 1966 to 2015, cross-referencing pediatric heart disease with pertinent resuscitation search terms. The American College of Cardiology/American Heart Association classification of recommendations and levels of evidence for practice guidelines were used. The recommendations in this statement concur with the critical components of the 2015 American Heart Association pediatric basic life support and pediatric advanced life support guidelines and are meant to serve as a resuscitation supplement. This statement is meant for caregivers of children with heart disease in the prehospital and in-hospital settings. Understanding the anatomy and physiology of the high-risk pediatric cardiac population will promote early recognition and treatment of decompensation to prevent cardiac arrest, increase survival from cardiac arrest by providing high-quality resuscitations, and improve outcomes with postresuscitation care.

Basic Cardiac Electrophysiology and Common Drug-induced Arrhythmias

Drugs can be a double-edged sword, providing the benefit of symptom alleviation and disease modification but potentially causing harm from adverse cardiac arrhythmic events. Proarrhythmia is the ability of a drug to cause an arrhythmia, the number one reason for drugs to be withdrawn from the patient. Drug-induced arrhythmias are defined as the production of de novo arrhythmias or aggravation of existing arrhythmias, as a result of previous or concomitant pharmacologic treatment. This review summarizes normal cardiac cell and tissue functioning and provides an overview of drugs that effect cardiac repolarization and the adverse effects of commonly administered antiarrhythmics.

Electrophysiological, hemodynamic, and metabolic response to open procedure or endovascular repair of infrarenal aortic aneurysms

BACKGROUND

The occurrence of cardiovascular diseases in the group of patients undergoing vascular surgery because of abdominal aorta aneurysm is very high. Endovascular procedures are regarded as hemodynamically safer for the patients. The aim of this study was to compare the changes in electrophysiological, hemodynamic, and metabolic parameters depending on the type of operation, using invasive hemodynamic monitoring and Holter electrocardiography recording.

METHODS

A prospective, observational, nonrandomized study was conducted to compare dynamic changes of electrophysiological parameters (dominant rhythm, arrhythmia, corrected QT interval (QTc), invasive blood pressure, electrolytes, and acid-base balance in defined points during perioperative time in 2 groups: vascular prosthesis implantation group (91 patients) and stent-graft implantation group (83 patients).

RESULTS

The study group comprised 174 consecutive adult Caucasian patients (mean age 64.4 ± 8.9 years in stent-graft group and 70.0 ± 7.5 years in vascular prosthesis implantation group). Although patients in the stent-graft implantation group were younger, they were diagnosed with lower limbs vascular atherosclerosis, type 2 diabetes mellitus, and a lower left ventricle ejection fraction more often than patients in the open procedure group. During the open procedure, higher blood pressure amplitudes (P = 0.00009), higher decrease in pH (P = 0.049), increase in the arterial lactate level (P = 0.00002), prolonged QTc values (P = 0.001), more frequent ventricular extrasystoles (P = 0.005), and cardiovascular deaths were observed, when compared with those observed during the endovascular aneurysm repair.

CONCLUSIONS

When compared with the chosen techniques, the one for infrarenal abdominal aneurysm was found to be associated with significant differences in electrophysiological, hemodynamic, and metabolic parameters.

Phosphodiesterase inhibitors, congestive heart failure, and sudden death: time for re-evaluation

A 42-year-old diabetic man was admitted with systolic heart failure and pulmonary hypertension being treated with sildenafil for the previous year. With an increase in creatinine, he experienced 3 episodes of ventricular tachycardia and ventricular fibrillation. Withdrawal of the phosphodiesterase (PDE) inhibitor resulted in no further episodes of dysrhythmias. The basic pharmacology of PDE inhibitors is presented and the use of PDE-3 inhibitors for the treatment of heart failure causing an increase in sudden death is also reviewed. There have been several cases of sudden death associated with sildenafil use and with its increasing use in patients with severe pulmonary hypertension and decompensated heart failure. The authors also reviewed the electrophysiologic effects of PDE-5 inhibitors associated with their use. The crossover between PDE-3 and PDE-5 inhibitors is also discussed and caution is urged when contemplating the use of PDE-5 inhibitors in patients with systolic heart failure and pulmonary hypertension.

Iloperidone (Fanapt®), a novel atypical antipsychotic, is a potent HERG blocker and delays cardiac ventricular repolarization at clinically relevant concentration

QT interval prolongation on the electrocardiogram (ECG) has extensively been reported with iloperidone, a novel antipsychotic drug. The objective of the present study was to evaluate the effects of iloperidone on cardiac ventricular repolarization at three different levels; in vitro, ex vivo and in vivo. (1) In vitro level: whole-cell patch-clamp experiments were performed on HERG-transfected HEK293 cells exposed to iloperidone 0.01-1 μmol/L (n = 35 cells, total) to assess drug effect on HERG current. (2) Ex vivo level: Langendorff retroperfusion experiments were performed on isolated hearts from male Hartley guinea pigs (n = 7) exposed to iloperidone 100 nmol/L with/without chromanol 293B 10 μmol/L to assess drug-induced prolongation of monophasic action potential duration measured at 90% repolarization (MAPD(90)). (3) In vivo level: ECG recordings using wireless cardiac telemetry were performed in guinea pigs (n = 5) implanted with radio transmitters and treated with a single oral gavage dose of iloperidone 3 mg/kg. (1) Patch-clamp experiments revealed an estimated IC50 for iloperidone on HERG current of 161 ± 20 nmol/L. (2) While pacing the hearts at stimulation cycle lengths of 200 or 250 ms, or during natural sinus rhythm (no external pacing), iloperidone 100 nmol/L prolonged MAPD(90) by respectively 9.2 ± 0.9, 11.2 ± 1.6 and 21.4 ± 2.3 ms. After adding chromanol 293B, MAPD(90) was further prolonged by 7.3 ± 3.3, 11.5 ± 2.3 and 29.2 ± 6.7 ms, respectively. (3) Iloperidone 3mg/kg p.o. caused a maximal 42.7 ± 10.2 ms prolongation of corrected QT interval (QTc(F)), 40 min after administration. Iloperidone prolongs the QT interval, the cardiac action potentials and is a potent HERG blocker. Patients are at increased risk of cardiac proarrhythmia during iloperidone treatment, as this drug possesses significant cardiac repolarization-delaying properties at clinically relevant concentration.

Galantamine (Reminyl) delays cardiac ventricular repolarization and prolongs the QT interval by blocking the HERG current

Galantamine is a reversible inhibitor of acetylcholinesterase and an allosteric-potentiating ligand of the nicotinic acetylcholine receptors. It is used for treating mild-to-moderate Alzheimer's disease. Interestingly, QT interval prolongation on the electrocardiogram (ECG), malignant ventricular arrhythmias and syncope have been reported with galantamine. Our objective was to evaluate the effects of galantamine on cardiac ventricular repolarization. Three sets of experiments were undertaken: 1) Whole cell patch-clamp experiments: HERG- or KCNQ1+KCNE1-transfected cells were exposed to galantamine 0.1-1000 μmol/l (n=25 cells, total) to assess drug effect on HERG and KCNQ1+KCNE1 currents. 2) Langendorff perfusion experiments: Isolated hearts from male Hartley guinea pigs (n=9) were exposed to galantamine 1 μmol/l to assess drug-induced prolongation of monophasic action potential duration measured at 90% repolarization (MAPD(90)). 3) Cardiac telemetry experiments: Guinea pigs (n=7) implanted with wireless transmitters were injected a single intraperitoneal (i.p.) dose of galantamine 3mg/kg and 24h ECG recordings were made. 1) The estimated IC(50) for galantamine on HERG current was 760.2 μmol/l. Moreover, galantamine 10 μmol/l had a small inhibiting effect on KCNQ1+KCNE1 current (12.17 ± 2.19% inhibition, n=10 cells). 2) While pacing at cycle lengths of 150, 200 or 250 ms, galantamine 1 μmol/l prolonged MAPD(90) by respectively 5.1 ± 1.6 ms, 9.4 ± 1.9 ms and 12.1 ± 2.1 ms. 3) Galantamine 3 mg/kgi.p. caused a maximal 11.9 ± 2.7 ms prolongation of the corrected QT (QTc). Galantamine is a weak HERG blocker. This contributes to its mild QT-prolonging effect. Patients could be at risk of cardiac proarrhythmia during drug overdosage or interactions involving cytochrome 2D6 drug-metabolizing enzyme.

A mechanism underlying compound-induced voltage shift in the current activation of hERG by antiarrhythmic agents

Nifekalant and azimilide, Class III antiarrhythmic agents, block the human ether-à-go-go-related gene K(+) (hERG) channel. However, when a depolarizing membrane potential is applied, they also increase the current at low potentials by shifting its activation curve towards hyperpolarizing voltages. This phenomenon is called 'facilitation'. In this study, we tried to address the mechanism underlying the facilitation by analyzing the effects of various compounds on hERG expressed in Xenopus oocytes. Like nifekalant, amiodarone, quinidine and carvedilol, but not by dofetilide, caused the current facilitation of hERG, suggesting that the facilitation is a common effect to a subset of hERG blockers. As the concentration of each compound was increased, the total hERG current was suppressed progressively, while the current at low potentials was augmented. Activation curves of the remaining hERG current in the facilitation condition could be described as the sum of two Boltzmann functions reflecting two populations of hERG currents having different activation curves. The voltage shift in the activation curve from control was constant for each compound even at different concentrations; -31 mV in amiodarone, -27 mV in nifekalant, -17 mV in quinidine and -12 mV in carvedilol. Therefore, the facilitation is based on the appearance of hERG whose voltage-dependence for the activation is shifted towards hyperpolarizing voltages.

Prolongation of cardiac ventricular repolarization under paliperidone: how and how much?

INTRODUCTION

Paliperidone (9-hydroxyrisperidone) is a second-generation antipsychotic. As observed with risperidone, QT interval prolongation was reported with paliperidone.

OBJECTIVE

The aim was to evaluate the effects of paliperidone on cardiac ventricular repolarization.

METHODS

(1) Patch-clamp experiments: Human ether-a-go-go-related gene (HERG)- or KCNQ1 + KCNE1-transfected cells were exposed to 0.1-100 μmol/L paliperidone (N = 39 cells, total) to assess the drug effect on HERG and KCNQ1 + KCNE1 currents. (2) Langendorff perfusion experiments: Hearts isolated from male Hartley guinea pigs (N = 9) were exposed to 0.1 μmol/L paliperidone to assess drug-induced prolongation of monophasic action potential duration measured at 90% repolarization. (3) In vivo cardiac telemetry experiments: Guinea pigs (N = 8) implanted with transmitters were injected a single intraperitoneal dose of 1 mg/kg of paliperidone, and 24-hour electrocardiogram recordings were made.

RESULTS

(1) The estimated concentration at which 50% of the maximal inhibitory effect is observed (IC(50)) for paliperidone on HERG current was 0.5276 μmol/L. In contrast, 1 μmol/L paliperidone had hardly any effect on KCNQ1 + KCNE1 current (4.0 ± 1.6% inhibition, N = 5 cells). (2) While pacing the hearts at cycle lengths of 150, 200, or 250 milliseconds, 0.1 μmol/L paliperidone prolonged monophasic action potential duration measured at 90% repolarization by, respectively, 6.1 ± 3.1, 9.8 ± 2.7, and 12.8 ± 2.7 milliseconds. (3) Paliperidone (1 mg/kg) intraperitoneal caused a maximal 15.7 ± 5.3-millisecond prolongation of QTc.

CONCLUSIONS

Paliperidone prolongs the QT interval by blocking HERG current at clinically relevant concentrations and is potentially unsafe.

Antiarrhythmic Drugs: Age, Race, and Gender Effects

Women have a higher risk of developing torsade de pointes when taking QT-prolonging antiarrhythmic drugs. Elderly women with heart failure may have the highest risk of proarrhythmia. Greater caution should be used when treating women with these drugs, especially when additional risk factors for developing proarrhythmia are present. Women with congenital heart disease frequently experience arrhythmias during pregnancy, and use of antiarrhythmic drugs in this setting poses a special challenge. In the elderly population, the risk of antiarrhythmic drug use may outweigh the benefit, especially in individuals with asymptomatic arrhythmias. Limited data suggest potential ethnic differences in arrhythmic substrates and in proarrhythmic response to antiarrhythmic drugs.

Biophysical characterization of KCNQ1 P320 mutations linked to long QT syndrome 1

Hereditary long QT syndrome (LQTS) is a cardiovascular disorder characterized by prolongation of the QT interval on the surface ECG and a high risk for arrhythmia-related sudden death. Mutations in a cardiac voltage-gated potassium channel, KCNQ1, account for the most common form of LQTS, LQTS1. The objective of this study was the characterization of a novel KCNQ1 mutation linked to LQTS. Electrophysiological properties and clinical features were determined and compared to characteristics of a different mutation at the same position. Single-strand conformation polymorphism analysis followed by direct sequencing was performed to screen LQTS genes for mutations. A novel missense mutation in the KCNQ1 gene, KCNQ1 P320H, was identified in the index patient presenting with recurrent syncope and aborted sudden death triggered by physical stress and swimming. Electrophysiological analyses of KCNQ1 P320H and the previously reported KCNQ1 P320A mutation indicate that both channels are non-functional and suppress wild type I(Ks) in a dominant-negative fashion. Based on homology modeling of the KCNQ1 channel pore region, we speculate that the proline residue at position 320 limits flexibility of the outer pore and is required to maintain the functional architecture of the selectivity filter/pore helix arrangement. Our observations on the KCNQ1 P320H mutation are consistent with previous studies indicating that pore mutations in potassium channel alpha-subunits are associated with more severe electrophysiological and clinical phenotypes than mutations in other regions of these proteins. This study emphasizes the significance of mutation screening for diagnosis, risk-assessment, and mutation-site specific management in LQTS patients.

QT as a safety biomarker in drug development

The cardiovascular safety of new drugs is an overarching concern for all stakeholders: the pharmaceutical industry and the US Food and Drug Administration (FDA) prior to approval and doctors and patients during postrelease drug use. Of the many cardiac safety concerns that accompany development of new drugs--including those related to vasculature and valvular tissue, the potential for myopathies, and the possibility of other electrophysiologic perturbations--the most pressing concern is the potential for ventricular arrhythmias causing sudden death.

Tachyarrhythmias and defibrillation

Although cardiac arrhythmias less commonly cause hemodynamic compromise in children than in adults, prompt recognition and treatment of arrhythmias remain an important part of pediatric resuscitation because of the availability of specific, effective therapies. This article summarizes the 2005 American Heart Association Guidelines for Pediatric Advanced Life Support regarding tachyarrhythmias, including treatment with antiarrhythmics and direct current countershock therapy, and provides an update of recent literature since the guidelines were published.

The hERG potassium channel and hERG screening for drug-induced torsades de pointes

Drug-induced torsades de pointes (TdP) arrhythmia is a major safety concern in the process of drug design and development. The incidence of TdP tends to be low, so early pre-clinical screens rely on surrogate markers of TdP to highlight potential problems with new drugs. hERG (human ether-à-go-go-related gene, alternative nomenclature KCNH2) is responsible for channels mediating the 'rapid' delayed rectifier K+ current (IKr) which plays an important role in ventricular repolarization. Pharmacological inhibition of native IKr and of recombinant hERG channels is a shared feature of diverse drugs associated with TdP. In vitro hERG assays therefore form a key element of an integrated assessment of TdP liability, with patch-clamp electrophysiology offering a 'gold standard'. However, whilst clearly necessary, hERG assays cannot be assumed automatically to provide sufficient information, when considered in isolation, to differentiate 'safe' from 'dangerous' drugs. Other relevant factors include therapeutic plasma concentration, drug metabolism and active metabolites, severity of target condition and drug effects on other cardiac ion channels that may mitigate or exacerbate effects of hERG blockade. Increased understanding of the nature of drug-hERG channel interactions may ultimately help eliminate potential hERG blockade early in the design and development process. Currently, for promising drug candidates integration of data from hERG assays with information from other pre-clinical safety screens remains essential.

Pharmacogenetics of antiarrhythmic therapy

Individuals vary widely in their responses to therapy with most drugs. Indeed, responses to antiarrhythmic drugs are so highly variable that study of the underlying mechanisms has elucidated important lessons for understanding variable responses to drug therapy in general. Variability in drug response may reflect variability in the relationship between a drug dose and the concentrations of the drug and metabolite(s) at relevant target sites; this is termed pharmacokinetic variability. Another mechanism is that individuals vary in their response to identical exposures to a drug (pharmacodynamic variability). In this case, there may be variability in the target molecule(s) with which a drug interacts or, more generally, in the broad biological context in which the drug-target interaction occurs. Variants (polymorphisms and mutations) in the genes that encode proteins that are important for pharmacokinetics or for pharmacodynamics have now been described as important contributors to variable drug actions, including proarrhythmia, and these are described in this review. However, the translation of pharmacogenetics into clinical practice has been slow. To this end, the creation of large, well-characterised DNA databases and appropriate control groups, as well as large prospective trials to evaluate the impact of genetic variation on drug therapy, may hasten the impact of pharmacogenetics and pharmacogenomics in terms of delivering personalised drug therapy and to avoid therapeutic failure and serious side effects.

Structural determinants of potassium channel blockade and drug-induced arrhythmias

Cardiac K+ channels play an important role in the regulation of the shape and duration of the action potential. They have been recognized as targets for the actions of neurotransmitters, hormones, and anti-arrhythmic drugs that prolong the action potential duration (APD) and increase refractoriness. However, pharmacological therapy, often for the purpose of treating syndromes unrelated to cardiac disease, can also increase the vul- nerability of some patients to life-threatening rhythm disturbances. This may be due to an underlying propensity stemming from inherited mutations or polymorphisms, or structural abnormalities that provide a substrate allowing for the initiation of arrhythmic triggers. A number of pharmacological agents that have proved useful in the treatment of allergic reactions, gastrointestinal disorders, and psychotic disorders, among others, have been shown to reduce repolarizing K+ currents and prolong the Q-T interval on the electrocardiogram. Understanding the structural determinants of K+ channel blockade might provide new insights into the mechanism and rate-dependent effects of drugs on cellular physiology. Drug-induced disruption of cellular repolarization underlies electrocardiographic abnormalities that are diagnostic indicators of arrhythmia susceptibility.

Drug-induced torsades de pointes: The evolving role of pharmacogenetics

Drug-induced torsades de pointes (TdP) is a rare, but potentially lethal, unwanted effect of drugs, including many commonly prescribed noncardiac drugs. Despite its low frequency, drug-induced TdP has generated a great deal of angst among physicians and pharmaceutical companies as well as tragedy, albeit rare, among patients. Although in retrospect many patients who died suddenly as a result of drug-induced TdP had identifiable risk factors, prediction in individual cases remains problematic. Over the past decade, tremendous progress has been made with respect to elucidating the fundamental pathogenic mechanisms that underlie drug-induced TdP. The vast majority of drugs associated with "QT liability" and the potential for drug-induced TdP, including all of the drugs removed from the market because of this side effect, are "HERG (human ether-á-go-go-related gene) blockers." These drugs inhibit the KCNH2-encoded HERG potassium channel, which is one of the critical repolarizing forces involved in the exquisite orchestration of the heart's action potential. Consequently, myocyte repolarization is potentially delayed as evidenced by prolongation of the QT interval, thus providing the substrate for drug-induced TdP. Rare mutations in KCNH2 provide the pathogenic substrate for type 2 congenital long QT syndrome (LQTS), thus placing this cardiac potassium channel squarely in the intersection between congenital LQTS (the "Rosetta stone" of the heritable channelopathies) and acquired LQTS (drug-induced TdP). In addition, common polymorphisms residing in the LQTS-causing channel genes may confer heightened arrhythmogenic susceptibility and contribute to the makings of a vulnerable host. This review focuses on the present strategy of identifying "at-risk compounds" and the potential future strategy involving pharmacogenetics to pinpoint "at-risk hosts" in an effort to curb this rare, unintended, but potentially life-threatening side effect.

Antagonism by ranolazine of the pro-arrhythmic effects of increasing late INa in guinea pig ventricular myocytes

The new anti-anginal drug ranolazine causes a slight (<10 milliseconds) prolongation of the QT interval, raising the concern that its use may be associated with an increased incidence of torsades de pointes ventricular tachyarrhythmias. The goal of this study was to show that ranolazine inhibits the late component of INa and attenuates prolongation of action potential duration when late INa is increased, both in the absence and presence of IK-blocking drugs. Currents and action potentials of guinea pig isolated ventricular myocytes were measured by whole-cell patch clamp. Sea anemone toxin (ATX)-II was used to increase late INa and mimic the effect of an SCN5A gene mutation. ATX-II (3-5 nmol/L) increased late INa by 5-fold; ranolazine attenuated this increase of late INa by up to 61 +/- 8%. ATX-II (10-20 nmol/L) increased action potential duration (APD) by > 1 seconds, and caused early afterdepolarizations; both actions were attenuated by ranolazine (0.1-30 micromol/L). Ranolazine (10 micromol/L) reduced by 89% the 13.6-fold increase in variability of APD caused by 10 nmol/L ATX-II. The effects of ATX-II (3 nmol/L) in combinations with either the IKr blocker E-4031 or the IKs blocker chromanol 293B to increase APD were attenuated 76 +/- 5% and 71 +/- 4%, respectively, by 10 micromol/L ranolazine. The results demonstrate that ranolazine reduces late INa and has an anti-arrhythmic effect when late INa is increased.

New approaches for identifying antiarrhythmic drug targets

Sudden cardiac death, secondary to ventricular fibrillation (VF), remains the leading cause of death in many developed countries. Substantial experimental and theoretical support exists for the idea that VF is caused by spiral wave re-entry. The initiation and subsequent break-up of spiral waves have been linked to electrical alternans, a phenomenon typically associated with a steeply sloped restitution relationship. Interventions that reduce the slope of the restitution relationship have been shown to prevent the induction of VF and to terminate existing VF in experimental models. These results suggest that electrical restitution may be a promising new target for antiarrhythmic therapies.

Cardiac causes of sudden unexpected death in children and their relationship to seizures and syncope: genetic testing for cardiac electropathies

The sentinel descriptions of congenital long QT syndrome (LQTS) under the eponyms of Jervell and Lange-Nielsen syndrome and Romano-Ward syndrome were provided in 1957 and the early 1960s. In 1995, the discipline of cardiac channelopathies was birthed formally with the landmark discoveries of cardiac channel mutations as the pathogenic basis for LQTS. Over the past decade, the discipline has expanded considerably being comprised of at least a dozen distinct heritable arrhythmia syndromes, several disease-susceptibility genes, and hundreds of implicated mutations. Previously confined to the purview of research testing, diagnostic genetic testing for several channelopathies is now available for routine clinical use.

Ciprofloxacin-induced acquired long QT syndrome

Quinolone antibiotics have potentially serious proarrhythmic effects. The effects on intracardiac potassium channels result in QT interval prolongation, leading to torsades de pointes. Evidence suggests fluoroquinolones cause QT-mediated proarrhythmia, and weak evidence links ciprofloxacin with QT-mediated arrhythmias. Ciprofloxacin may be given to select patients because the agent is believed to be safer than other drugs in its class. We report two cases of unexplained cardiac arrest temporally related to ciprofloxacin administration. Two female patients (ages 44 and 67 years) developed marked QTc prolongation (QTc 590 and 680 ms) within 24 hours of ciprofloxacin administration, with recurrent syncope and documented torsades de pointes requiring defibrillation. The patients previously were stable with sotalol and amiodarone therapy for supraventricular arrhythmia without obvious QTc prolongation prior to ciprofloxacin therapy. Marked QTc prolongation and subsequent proarrhythmia became a clinical concern only after initiation of ciprofloxacin. In both cases, the QTc normalized after cessation of ciprofloxacin. Ciprofloxacin may cause QTc prolongation and rarely torsades de pointes. This effect is of particular concern in patients with predisposing factors, such as concomitant medications or underlying heart disease reflecting decreased repolarization reserve.

Preclinical strategies to assess QT liability and torsadogenic potential of new drugs: The role of experimental models

The recognition of QT prolongation and torsade de pointes (TdP) in humans has resulted in the re-labeling of some drugs and the removal of others from the market in the past decade. Recent regulatory guidelines have recommended a battery of preclinical tests to assess a new drug for the QT liability in humans. The assessment includes the effect of a drug on: 1) the ionic current in stable cell lines expressing hERG channel; 2) action potential duration (APD) measured in isolated ventricular tissues; 3) the QTc interval and TdP in animals in vivo; and 4) APD, the QT interval, transmural dispersion of repolarization (TDR) and TdP potential in the isolated arterially-perfused ventricular wedge preparation. Because a noncardiac drug with an incidence of TdP even less than 0.1% can be potentially removed from the market, the experimental models used for preclinical testing have to be high sensitive and specific to the signals related to TdP. Among available experimental models, the rabbit left ventricle wedge preparation exhibits a high sensitivity and a high specificity in the identification of compounds positive and negative for QT prolongation and TdP. This is attributed to the fact that the preparation demonstrates strong signals related to QT prolongation in response to even a weaker QT prolonging agent. Signals specifically pertinent to the development of TdP, ie, early afterdepolarization (EAD) and an increase in TDR can be detected as well. The preclinical data obtained from the wedge preparation correlate well with clinical outcomes.

Comparison of kinetic properties of quinidine and dofetilide block of HERG channels

Many drugs inhibit human ether-a-go-go related gene (HERG) current and prolong cardiac action potential duration. We examined the kinetic properties of quinidine block of HERG channels expressed in Xenopus oocytes in comparison with those of the block by a class III antiarrhythmic dofetilide. Both of the drugs inhibited HERG currents in a use-dependent and frequency-independent manner. However, the underlying mechanisms were different. Under the steady state, quinidine block was voltage- and time-dependent. At positive membrane potentials, the onset of block was very fast. Thus, quinidine caused frequency-independent block mainly through this fast blocking kinetic. In contrast, dofetilide blocked HERG currents in a voltage- and time-independent manner under the steady state because of very slow unblocking at negative potentials, which also caused frequency-independent block. Therefore, quinidine and dofetilide might cause the reverse frequency-dependent prolongation of action potential duration through distinct mechanisms with regard to blocking and unblocking kinetics.

Contribution of I Kr to Rate-Dependent Action Potential Dynamics in Canine Endocardium

Previous modeling studies have suggested that the rapid component of the delayed rectifier (I(Kr)) may contribute importantly to action potential dynamics during tachycardia. To test this idea experimentally, I(Kr) was measured as the E-4031-sensitive current in isolated canine endocardial myocytes at 37 degrees C using the perforated patch-clamp technique. Command potentials were trains of action potential waveforms recorded at cycle lengths (CLs) of 1000, 500, 320, 170, and 120 ms. Action potential duration (APD) alternans occurred at CLs of 170 and 120 ms. During an action potential, I(Kr) increased gradually to a maximum at -55 to -60 mV. Peak I(Kr) increased initially as CL was shortened from 1000 to 500 ms (from 0.55+/-0.03 to 0.57+/-0.03 pA/pF), but decreased progressively as CL was shortened further (to 0.45+/-0.03 pA/pF at CL=120 ms). Baseline I(Kr) was negligible at CLs of 1000 to 320 ms, but increased to 0.12+/-0.01 pA/pF at a CL of 120 ms. During APD alternans, peak I(Kr) was larger for the short than for the long action potential (0.48+/-0.03 versus 0.46+/-0.03 pA/pF). A computer model of I(Kr) based on these data indicated that increasing I(Kr) suppressed alternans and decreasing I(Kr) increased alternans. In support of the latter result, inhibition of I(Kr) by E-4031 increased the maximal amplitude of alternans. These results indicate that I(Kr) contributes importantly to rate-related alterations of repolarization, including APD alternans. Modifying I(Kr) may be a promising approach to suppressing alternans and thereby preventing ventricular tachyarrhythmias.

Antiarrhythmic effects of ranolazine in a guinea pig in vitro model of long-QT syndrome

Prolongation of the QT interval of the ECG is associated with increased risk of torsades de pointes ventricular tachycardia. Ranolazine, a novel antianginal agent, is reported to decrease the delayed rectifier potassium current, I(Kr), and to increase action potential duration (APD) and the QT interval. However, ranolazine is also reported to reduce late sodium current (late I(Na)), a depolarizing current that contributes to prolongation of the plateau of the ventricular action potential. We hypothesized that ranolazine would decrease APD and the occurrence of arrhythmias when late I(Na) is increased. Therefore, we measured the effects of ranolazine alone and in the presence of anemone toxin (ATX)-II, whose action mimics the sodium channelopathy associated with long-QT3 syndrome, on epicardial monophasic action potentials and ECGs recorded from guinea pig isolated hearts. Ranolazine (0.1-50 microM) prolonged monophasic APD at 90% repolarization (MAPD(90)) by up to 22% but did not cause either early afterdepolarizations (EADs) or ventricular tachycardia (VT). ATX-II (1-20 nM) markedly increased APD and caused EADs and VT. Ranolazine (5-30 microM) significantly attenuated increases in MAPD(90) and reduced episodes of EADs and VT produced by ATX-II. Ranolazine also attenuated the synergistic effect of MAPD(90) increase caused by combinations of ATX-II and blockers of I(K) [E-4031; 1-[2-(6-methyl-2-pyridyl)ethyl]-4-methylsulfonylaminobenzoyl)piperidine]. Thus, although ranolazine alone prolonged APD, it reduced APD and ventricular arrhythmias caused by agents that increased late I(Na) and decreased I(K).

Electrocardiographic Abnormalities and Serum Magnesium in Patients With Subarachnoid Hemorrhage

Background and Purpose— ECG abnormalities and hypomagnesemia frequently occur after aneurysmal subarachnoid hemorrhage (SAH). Because hypomagnesemia is associated with several ECG abnormalities, we studied whether hypomagnesemia mediates ECG abnormalities after SAH.

Methods— We prospectively studied a consecutive series of 62 patients admitted within 72 hours after aneurysmal SAH. A standard 12-lead ECG and serum magnesium measurement were routinely performed at admission. The relationship between serum magnesium and ECG abnormalities was assessed with linear regression analysis and the Mann-Whitney test in case of dichotomized ECG abnormalities.

Results— Hypomagnesemia was present in 23 patients (37%), and 38 patients (61%) had a long QTc duration. Low serum magnesium was related to a long PR interval ( P =0.001) and a shorter QTc interval ( P =0.004). Adjustment for World Federation of Neurological Surgeons score, hydrocephalus, and the amount of cisternal and ventricular blood did not influence these relations.

Conclusions— In patients with SAH, lower serum magnesium levels are related to less pronounced increase in the QTc interval. Although the direction of the relation was unexpected, decreased serum magnesium might be the missing link between SAH and ECG abnormalities.

Genetic polymorphisms, drugs, and proarrhythmia

Humans vary widely in their response to drug therapy. This may reflect variability in the relationship between a drug dose and the concentrations of drug and metabolite(s) at relevant target sites; this is termed pharmacokinetic variability. Another mechanism is that individuals vary in their response to identical exposures to drug (pharmacodynamic variability). In this case, there may be variability in the target molecule(s) with which a drug interacts, or more generally in the broad biologic context in which the drug-target interaction occurs; for example, ischemia, electrolyte disturbances, or hypertrophy can all modulate drug effects. Variants in the genes encoding proteins important for pharmacokinetics or for pharmacodynamics have now been described as important contributors to variable drug actions, including proarrhythmia, and are described here. These increasingly well-recognized examples have two important implications; first, it may be possible to develop drugs devoid of heretofore-unexplained adverse effects and, second, it may become possible to preselect drug for individual patients based on specific genetic factors.

Ethnic differences in cardiac potassium channel variants: implications for genetic susceptibility to sudden cardiac death and genetic testing for congenital long QT syndrome

OBJECTIVE

To determine the spectrum, frequency, and ethnic-specificity of channel variants in the potassium channel genes implicated in congenital long QT syndrome (LQTS) among healthy subjects.

SUBJECTS AND METHODS

Genomic DNA from 744 apparently healthy individuals-305 black, 187 white, 134 Asian, and 118 Hispanic--was subject to a comprehensive mutational analysis of the 4 LQTS-causing potassium channel genes: KCNQ1 (LQT1), KCNH2 (LQT2), KCNE1 (LQT5), and KCNE2 (LQT6).

RESULTS

Overall, 49 distinct amino acid-altering variants (36 novel) were identified: KCNQ1 (n = 16), KCNH2 (n = 25),KCNE1 (n = 5), and KCNE2 (n = 3). More than half of these variants (26/49) were found exclusively in black subjects. The known K897T-HERG and the G38S-min K common polymorphisms were identified in all 4 ethnic groups. Excluding these 2 common polymorphisms, 25% of black subjects had at least 1 nonsynonymous potassium channel variant compared with 14% of white subjects (P < .01).

CONCLUSIONS

To our knowledge, this study represents the first comprehensive determination of the frequency and spectrum of cardiac channel variants found among healthy subjects from 4 major ethnic groups. Defining the population burden of genetic variants in these critical cardiac ion channels is crucial for proper interpretation of genetic test results of individuals at risk for LQTS. This compendium provides a resource for epidemiological and functional investigation of variant effects on the repolarization properties of cardiac tissues, including susceptibility to lethal cardiac arrhythmias.

K+ channel structure-activity relationships and mechanisms of drug-induced QT prolongation

Pharmacological intervention, often for the purpose of treating syndromes unrelated to cardiac disease, can increase the vulnerability of some patients to life-threatening rhythm disturbances. This may be due to an underlying propensity stemming from genetic defects or polymorphisms, or structural abnormalities that provide a substrate allowing for the initiation of arrhythmic triggers. A number of pharmacological agents that have proven useful in the treatment of allergic reactions, gastrointestinal disorders, and psychotic disorders, among others, have been shown to reduce repolarizing K(+) currents and prolong the QT interval on the electrocardiogram. Understanding the structural determinants of K(+) channel blockade may provide new insights into the mechanism and rate-dependent effects of drugs on cellular physiology. Drug-induced disruption of cellular repolarization underlies electrocardiographic abnormalities that are diagnostic indicators of arrhythmia susceptibility.