Ionic, molecular, and cellular bases of QT-interval prolongation and torsade de pointes

From genes to clinical management: A comprehensive review of long QT syndrome pathogenesis and treatment

Background

Long QT syndrome (LQTS) is a rare cardiac disorder characterized by prolonged ventricular repolarization and increased risk of ventricular arrhythmias. This review summarizes current knowledge of LQTS pathogenesis and treatment strategies.

Objectives

The purpose of this study was to provide an in-depth understanding of LQTS genetic and molecular mechanisms, discuss clinical presentation and diagnosis, evaluate treatment options, and highlight future research directions.

Methods

A systematic search of PubMed, Embase, and Cochrane Library databases was conducted to identify relevant studies published up to April 2024.

Results

LQTS involves mutations in ion channel-related genes encoding cardiac ion channels, regulatory proteins, and other associated factors, leading to altered cellular electrophysiology. Acquired causes can also contribute. Diagnosis relies on clinical history, electrocardiographic findings, and genetic testing. Treatment strategies include lifestyle modifications, β-blockers, potassium channel openers, device therapy, and surgical interventions.

Conclusion

Advances in understanding LQTS have improved diagnosis and personalized treatment approaches. Challenges remain in risk stratification and management of certain patient subgroups. Future research should focus on developing novel pharmacological agents, refining device technologies, and conducting large-scale clinical trials. Increased awareness and education are crucial for early detection and appropriate management of LQTS.

Association of QT interval-prolonging drugs with clinical trial eligibility in patients with advanced cancer

Introduction

Drug-induced prolongation of the heart rate-corrected QT interval (QTc) is associated with increased risk for the potentially fatal arrhythmia torsades de pointes. Due to arrhythmia risk, clinical trials with cancer therapeutics often exclude patients based on thresholds for QTc prolongation. Our objective was to assess associations between prescriptions for QT-prolonging drugs and the odds of meeting cancer trial exclusionary QTc thresholds in a cohort of adults with advanced cancer.

Methods

Electronic health records were retrospectively reviewed for 271 patients seen at our institutional molecular solid tumor clinic. Collected data included demographics, QTc measurements, ventricular arrhythmia-related diagnoses, and all inpatient and outpatient prescriptions. Potential associations were assessed between demographic and clinical variables, including prescriptions for QT-prolonging drugs, and QTc measurements.

Results

Women had longer median QTc measurements than men (p = 0.030) and were prescribed more QT-prolonging drugs during the study (p = 0.010). In all patients, prescriptions for QT-prolonging drugs were associated with longer median and maximum QTc measurements at multiple assessed time points (i.e., for QT-prolonging drugs prescribed within 10, 30, 60, and 90 days of QTc measurements). Similarly, the number of QT-prolonging drugs prescribed was correlated with longer median and maximum QTc measurements at multiple time points. Common QTc-related exclusionary criteria were collected from a review of ClinicalTrials.gov for recent cancer clinical trials. Based on common exclusion criteria, prescriptions for QT-prolonging drugs increased the odds of trial exclusion.

Conclusion

This study demonstrates that prescriptions for QT-prolonging drugs were associated with longer QTc measurements and increased odds of being excluded from cancer clinical trials.

A Comprehensive Evaluation of Sdox, a Promising H2S-Releasing Doxorubicin for the Treatment of Chemoresistant Tumors

Sdox is a hydrogen sulfide (H₂S)-releasing doxorubicin effective in P-glycoprotein-overexpressing/doxorubicin-resistant tumor models and not cytotoxic, as the parental drug, in H9c2 cardiomyocytes. The aim of this study was the assessment of Sdox drug-like features and its absorption, distribution, metabolism, and excretion (ADME)/toxicity properties, by a multi- and transdisciplinary in silico, in vitro, and in vivo approach. Doxorubicin was used as the reference compound. The in silico profiling suggested that Sdox possesses higher lipophilicity and lower solubility compared to doxorubicin, and the off-targets prediction revealed relevant differences between Dox and Sdox towards several cancer targets, suggesting different toxicological profiles. In vitro data showed that Sdox is a substrate with lower affinity for P-glycoprotein, less hepatotoxic, and causes less oxidative damage than doxorubicin. Both anthracyclines inhibited CYP3A4, but not hERG currents. Unlike doxorubicin, the percentage of zebrafish live embryos at 72 hpf was not affected by Sdox treatment. In conclusion, these findings demonstrate that Sdox displays a more favorable drug-like ADME/toxicity profile than doxorubicin, different selectivity towards cancer targets, along with a greater preclinical efficacy in resistant tumors. Therefore, Sdox represents a prototype of innovative anthracyclines, worthy of further investigations in clinical settings.

Flavonoids and hERG channels: Friends or foes?

The cardiac action potential is regulated by several ion channels. Drugs capable to block these channels, in particular the human ether-à-go-go-related gene (hERG) channel, also known as K_V11.1 channel, may lead to a potentially lethal ventricular tachyarrhythmia called "Torsades de Pointes". Thus, evaluation of the hERG channel off-target activity of novel chemical entities is nowadays required to safeguard patients as well as to avoid attrition in drug development. Flavonoids, a large class of natural compounds abundantly present in food, beverages, herbal medicines, and dietary food supplements, generally escape this assessment, though consumed in consistent amounts. Continuously growing evidence indicates that these compounds may interact with the hERG channel and block it. The present review, by examining numerous studies, summarizes the state-of-the-art in this field, describing the most significant examples of direct and indirect inhibition of the hERG channel current operated by flavonoids. A description of the molecular interactions between a few of these natural molecules and the Rattus norvegicus channel protein, achieved by an in silico approach, is also presented.

Centruroides margaritatus scorpion complete venom exerts cardiovascular effects through alpha-1 adrenergic receptors

Centruroides margaritatus scorpion stings are common in Colombia. However, the cardiovascular toxicity of the venom has not been clarified.

AIM

To study the effect and mechanisms of action of the complete venom of C. margaritatus (CmV) on the murine cardiovascular system.

METHODS

We evaluated the in vivo effect of CmV LD50 on the mean arterial pressure (MABP), heart rate, and surface electrocardiogram in male adult normotensive Wistar rats. Ex vivo, we evaluated the vascular reactivity of rat aortic rings to increasing concentrations (1 to 60 μg/mL) of CmV using the blockers L-NAME, indomethacin, seratrodast, and prazosin.

RESULTS

In the first hour of poisoning, CmV increased the MABP. In the second hour after poisoning, the heart rate decreased as the normalized PR interval and QT corrected increased. After that, cardiovascular shock was demonstrated by a drastic fall in the MABP and signs of cardiac conduction system block. In aortic rings, CmV caused a direct vasoconstrictor effect mediated by alpha-1 adrenergic receptors and counteracted by nitric oxide.

CONCLUSION

The direct vascular and probably the cardiac alpha-1 effects likely explain the transient hypertension and the maintenance of cardiac function, while interval lengthening may be due to K+ channel blockage. Afterwards, the effects of both the alpha-1 pathway and the K+ channel pathway converged, resulting in fatal cardiovascular shock. This knowledge could aid in understanding the dynamics of the effects of the venom and in designing treatments to address its cardiovascular effects.

High Level of Real Urban Air Pollution Promotes Cardiac Arrhythmia in Healthy Mice

Background and Objectives

Ambient particulate matter (PM) in real urban air pollution (RUA) is an environmental health risk factor associated with increased cardiac events. This study investigated the threshold level to induce arrhythmia, as well as arrhythmogenic mechanism of RUA that mainly consisted of PM <2.5 μm in aerodynamic diameter close to ultrafine particles.

Methods

RUA was artificially produced by a lately developed pyrolysis based RUA generator. C57BL/6 mice were divided into 4 groups: a control group (control, n=12) and three groups with exposure to RUA with the concentration of 200 µg/m³ (n=12), 400 µg/m³ (n=12), and 800 µg/m³ (n=12). Mice were exposed to RUA at each concentration for 8 hr/day and 5 day/week to mimic ordinary human activity during 3 weeks.

Results

The QRS and QTc intervals, as well as intracellular Ca²⁺ duration, apicobasal action potential duration (APD) gradient, fibrosis, and inflammation of left ventricle of mouse hearts were increased dose-dependently with the increase of RUA concentration, and significantly increased at RUA concentration of 400 µg/m³ compared to control (all p<0.001). In mice exposed to RUA concentration of 800 µg/m³, spontaneous ventricular arrhythmia was observed in 42%, with significant increase of inflammatory markers, phosphorylated Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), and phospholamban (PLB) compared to control.

Conclusions

RUA could induce electrophysiological changes such as APD and QT prolongation, fibrosis, and inflammation dose-dependently, with significant increase of ventricular arrhythmia at the concentration of 400 µg/m³. RUA concentration of 800 µg/m³ increased phosphorylation of CaMKII and PLB.

Opium-associated QT Interval Prolongation: A Cross-sectional Comparative Study

How to cite this article: Sharma A. Opium-associated QT Interval Prolongation: A Cross-sectional Comparative Study. Indian J Crit Care Med 2021;25(1):6-7.

In Silico Classifiers for the Assessment of Drug Proarrhythmicity

Drug-induced torsade de pointes (TdP) is a life-threatening ventricular arrhythmia responsible for the withdrawal of many drugs from the market. Although currently used TdP risk-assessment methods are effective, they are expensive and prone to produce false positives. In recent years, in silico cardiac simulations have proven to be a valuable tool for the prediction of drug effects. The objective of this work is to evaluate different biomarkers of drug-induced proarrhythmic risk and to develop an in silico risk classifier. Cellular simulations were performed using a modified version of the O'Hara et al. ventricular action potential model and existing pharmacological data (IC₅₀ and effective free therapeutic plasma concentration, EFTPC) for 109 drugs of known torsadogenic risk (51 positive). For each compound, four biomarkers were tested: T_x (drug concentration leading to a 10% prolongation of the action potential over the EFTPC), T_qNet (net charge carried by ionic currents when exposed to 10 times the EFTPC with respect to the net charge in control), T_triang (triangulation for a drug concentration of 10 times the EFTPC over triangulation in control), and T_EAD (drug concentration originating early afterdepolarizations over EFTPC). Receiver operating characteristic (ROC) curves were built for each biomarker to evaluate their individual predictive quality. At the optimal cutoff point, accuracies for T_x, T_qNet, T_triang, and T_EAD were 89.9, 91.7, 90.8, and 78.9% respectively. The resulting accuracy of the hERG IC₅₀ test (current biomarker) was 78.9%. When combining T_x, T_qNet and T_triang into a classifier based on decision trees, the prediction improves, achieving an accuracy of 94.5%. The sensitivity analysis revealed that most of the effects on the action potential are mainly due to changes in I_Kr, I_CaL, I_NaL and I_Ks. In fact, considering that drugs affect only these four currents, TdP risk classification can be as accurate as when considering effects on the seven main currents proposed by the CiPA initiative. Finally, we built a ready-to-use tool (based on more than 450 000 simulations), which can be used to quickly assess the proarrhythmic risk of a compound. In conclusion, our in silico tool can be useful for the preclinical assessment of TdP-risk and to reduce costs related with new drug development. The TdP risk-assessment tool and the software used in this work are available at https://riunet.upv.es/handle/10251/136919.

Assay Procedures for Compound Testing of hiPSC-Derived Cardiomyocytes Using Multiwell Microelectrode Arrays

The cardiac action potential requires a precise timing of activation and inactivation of ion channel subtypes. Deviations, for example, due to blockage of specific voltage-gated potassium channels, can result in live-threatening arrhythmias. Due to the limitations of standard cellular assays based on cells which artificially express only single ion channel subtypes, many potentially interesting compounds are discarded during drug development. More predictive functional assays are required. With the upcoming of human stem-cell derived cardiomyocytes (hiPS-CM) these assays are available, supporting even the design of patient-derived disease models. Microelectrode array systems allow to noninvasively record and evaluate cardiac field action potentials. In this chapter we describe how to cultivate hiPS-CM on two parallelized MEA systems and suggest an experimental strategy for compound tests.

Interactions of nanomaterials with ion channels and related mechanisms

The pharmacological potential of nanotechnology, especially in drug delivery and bioengineering, has developed rapidly in recent decades. Ion channels, which are easily targeted by external agents, such as nanomaterials (NMs) and synthetic drugs, due to their unique structures, have attracted increasing attention in the fields of nanotechnology and pharmacology for the treatment of ion channel-related diseases. NMs have significant effects on ion channels, and these effects are manifested in many ways, including changes in ion currents, kinetic characteristics and channel distribution. Subsequently, intracellular ion homeostasis, signalling pathways, and intracellular ion stores are affected, leading to the initiation of a range of biological processes. However, the effect of the interactions of NMs with ion channels is an interesting topic that remains obscure. In this review, we have summarized the recent research progress on the direct and indirect interactions between NMs and ion channels and discussed the related molecular mechanisms, which are crucial to the further development of ion channel-related nanotechnological applications.

A Comparison of the Effect of Sevoflurane and Propofol on Ventricular Repolarisation after Preoperative Cefuroxime Infusion

The aim of this study is to investigate the changes in QT, QTc, and Tp-e intervals and Tp-e/QT ratio on surface electrocardiogram (ECG) signals during anaesthesia induction using propofol or sevoflurane after preoperative cefuroxime infusion. Some 120 cases of gynaecological patients are randomly divided into propofol (P) and sevoflurane (S) groups (n=60). After cefuroxime (1.5 g) was infused in the two groups of patients, propofol target controlled infusion (TCI) was conducted in the P group for 5 min to realise a plasma concentration of 4 μg/ml while patients in the S group inhaled anaesthesia by infusing 1.3 MAC sevoflurane for 6 min. The 12-lead ECGs were separately collected before infusing cefuroxime (T₁), after infusing cefuroxime (T₂), and after infusing propofol or sevoflurane (T₃) to measure QT and Tp-e intervals, calculate QTc and Tp-e/QT, and record MAP and HR. Finally, we demonstrated that QT, QTc, and Tp-e intervals and Tp-e/QT ratio had no change (P > 0.05) after cefuroxime infusion in the two groups of patients compared with that before infusing antibiotics. Moreover, after conducting preoperative cefuroxime infusion, using propofol and sevoflurane had no influence on Tp-e interval, but sevoflurane can significantly prolong QT and QTc intervals (P < 0.05).

Drug-induced prolonged corrected QT interval in patients with methadone and opium overdose

BACKGROUND

Iran is a country with the highest rate of opioid addiction in the world. The most commonly used opioid in Iran is opium, and methadone is in second place. The trend of drug use has changed from opium to methadone from 2006 to 2011. Presence of a large number of addicted people and methadone maintenance therapy clinics make methadone readily available in Iran. Therefore, evaluation of the epidemiological characteristic of methadone toxicity and its effects on the heart is essential.

METHODS

In This cross-sectional, retrospective, descriptive, analytical study all patients with methadone or opium toxicity who had been admitted to Vasei hospital, Sabzevar, Iran, during the years 2015 and 2016 were included, and their records were evaluated. Demographic data, addiction history, underlying diseases, and the outcome of admission were recorded. Then, corrected QT interval (QTc) of the first ECG of the patients after admission was evaluated.

RESULTS

The Majority of toxicities occurred in those above 30 years of age (71.4%), who lived in cities (62.8%), and were married (69.2%). A positive history of addiction was considerably higher in the opium group (72.3% versus 43.3%). There was no significant difference regarding QTc prolongation between patients with methadone and opium toxicity (p = 0.3).

CONCLUSION

QTc prolongation is one of the adverse effects of methadone or opium overdose. It seems that significant QTc prolongation is not uncommon among patients with opium overdose.

NCX-Mediated Subcellular Ca2+ Dynamics Underlying Early Afterdepolarizations in LQT2 Cardiomyocytes

Long QT syndrome type 2 (LQT2) is a congenital disease characterized by loss of function mutations in hERG potassium channels (I_Kr). LQT2 is associated with fatal ventricular arrhythmias promoted by triggered activity in the form of early afterdepolarizations (EADs). We previously demonstrated that intracellular Ca²⁺ handling is remodeled in LQT2 myocytes. Remodeling leads to aberrant late RyR-mediated Ca²⁺ releases that drive forward-mode Na⁺-Ca²⁺ exchanger (NCX) current and slow repolarization to promote reopening of L-type calcium channels and EADs. Forward-mode NCX was found to be enhanced despite the fact that these late releases do not significantly alter the whole-cell cytosolic calcium concentration during a vulnerable period of phase 2 of the action potential corresponding to the onset of EADs. Here, we use a multiscale ventricular myocyte model to explain this finding. We show that because the local NCX current is a saturating nonlinear function of the local submembrane calcium concentration, a larger number of smaller-amplitude discrete Ca²⁺ release events can produce a large increase in whole-cell forward-mode NCX current without increasing significantly the whole-cell cytosolic calcium concentration. Furthermore, we develop novel insights, to our knowledge, into how alterations of stochastic RyR activity at the single-channel level cause late aberrant Ca²⁺ release events. Experimental measurements in transgenic LTQ2 rabbits confirm the critical arrhythmogenic role of NCX and identify this current as a potential target for antiarrhythmic therapies in LQT2.

Inherited primary arrhythmia disorders: cardiac channelopathies and sports activity

Sudden cardiac death (SCD) in an apparently healthy individual is a tragedy. It is important to identify the cause of death and to prevent SCD in potentially at-risk family members. Inherited primary arrhythmia disorders are associated with exercise-related SCD. Despite the well-known benefits of exercise, exercise restriction has been a historical mainstay of therapy for these conditions. However, since familiarity with inherited arrhythmia conditions has increased and patients are often children and young adults, it is necessary to reassess the treatment guidelines regarding exercise constraints. The aim of this review is to analyze the risk of exercise-induced SCD in patients with inherited cardiac conditions and explore the challenges faced when advising patients about exercise limitations. We searched for publications on cardiac channelopathies in PubMed with the following medical subject headings (MeSH): "long QT syndrome"; "short QT syndrome"; "Brugada syndrome"; and "catecholaminergic polymorphic ventricular tachycardia". The abstracts of these articles were scanned, and articles of relevance, along with pertinent references, were read in full. The analysis was restricted to reports published in English. The findings of this analysis suggest that exercise with low-to-moderate cardiovascular demand may be possible under regular clinical follow-up in inherited primary arrhythmia disorders. Recent data show that patients with inherited primary arrhythmia disorders are at low risk for events once a comprehensive treatment program has been established. Recreational activity is likely safe for these individuals, with personalized management based on individual patient preferences and priorities.

Torsade de pointes arrhythmias arise at the site of maximal heterogeneity of repolarization in the chronic complete atrioventricular block dog

Aims

The chronic complete atrioventricular block (CAVB) dog is highly sensitive for drug-induced torsade de pointes (TdP) arrhythmias. Focal mechanisms have been suggested as trigger for TdP onset; however, its exact mechanism remains unclear. In this study, detailed mapping of the ventricles was performed to assess intraventricular heterogeneity of repolarization in relation to the initiation of TdP.

Methods and results

In 8 CAVB animals, 56 needles, each containing 4 electrodes, were inserted in the ventricles. During right ventricular apex pacing (cycle length: 1000-1500 ms), local unipolar electrograms were recorded before and after administration of dofetilide to determine activation and repolarization times (RTs). Maximal RT differences were calculated in the left ventricle (LV) within adjacent electrodes in different orientations (transmural, vertical, and horizontal) and within a square of four needles (cubic dispersion). Dofetilide induced TdP in five out of eight animals. Right ventricle-LV was similar between inducible and non-inducible dogs at baseline (327 ± 30 vs. 345 ± 17 ms) and after dofetilide administration (525 ± 95 vs. 508 ± 15 ms). All measurements of intraventricular dispersion were not different at baseline, but this changed for horizontal (206 ± 20 vs. 142 ± 34 ms) and cubic dispersion (272 ± 29 vs. 176 ± 48 ms) after dofetilide: significantly higher values in inducible animals. Single ectopic beats and the first TdP beat arose consistently from a subendocardially located electrode terminal with the shortest RT in the region with largest RT differences.

Conclusion

Chronic complete atrioventricular block dogs susceptible for TdP demonstrate higher RT differences. Torsade de pointes arises from a region with maximal heterogeneity of repolarization suggesting that a minimal gradient is required in order to initiate TdP.

β-adrenergic stimulation augments transmural dispersion of repolarization via modulation of delayed rectifier currents IKs and IKr in the human ventricle

Long QT syndrome (LQTS) is an inherited or drug induced condition associated with delayed repolarization and sudden cardiac death. The cardiac potassium channel, I_Kr, and the adrenergic-sensitive cardiac potassium current, I_Ks, are two primary contributors to cardiac repolarization. This study aimed to elucidate the role of β-adrenergic (β-AR) stimulation in mediating the contributions of I_Kr and I_Ks to repolarizing the human left ventricle (n = 18). Optical mapping was used to measure action potential durations (APDs) in the presence of the I_Ks blocker JNJ-303 and the I_Kr blocker E-4031. We found that JNJ-303 alone did not increase APD. However, under isoprenaline (ISO), both the application of JNJ-303 and additional E-4031 significantly increased APD. With JNJ-303, ISO decreased APD significantly more in the epicardium as compared to the endocardium, with subsequent application E-4031 increasing mid- and endocardial APD80 more significantly than in the epicardium. We found that β-AR stimulation significantly augmented the effect of I_Ks blocker JNJ-303, in contrast to the reduced effect of I_Kr blocker E-4031. We also observed synergistic augmentation of transmural repolarization gradient by the combination of ISO and E-4031. Our results suggest β-AR-mediated increase of transmural dispersion of repolarization, which could pose arrhythmogenic risk in LQTS patients.

Novel Two-Step Classifier for Torsades de Pointes Risk Stratification from Direct Features

While pre-clinical Torsades de Pointes (TdP) risk classifiers had initially been based on drug-induced block of hERG potassium channels, it is now well established that improved risk prediction can be achieved by considering block of non-hERG ion channels. The current multi-channel TdP classifiers can be categorized into two classes. First, the classifiers that take as input the values of drug-induced block of ion channels (direct features). Second, the classifiers that are built on features extracted from output of the drug-induced multi-channel blockage simulations in the in-silico models (derived features). The classifiers built on derived features have thus far not consistently provided increased prediction accuracies, and hence casts doubt on the value of such approaches given the cost of including biophysical detail. Here, we propose a new two-step method for TdP risk classification, referred to as Multi-Channel Blockage at Early After Depolarization (MCB@EAD). In the first step, we classified the compound that produced insufficient hERG block as non-torsadogenic. In the second step, the role of non-hERG channels to modulate TdP risk are considered by constructing classifiers based on direct or derived features at critical hERG block concentrations that generates EADs in the computational cardiac cell models. MCB@EAD provides comparable or superior TdP risk classification of the drugs from the direct features in tests against published methods. TdP risk for the drugs highly correlated to the propensity to generate EADs in the model. However, the derived features of the biophysical models did not improve the predictive capability for TdP risk assessment.

Normal Ventricular Repolarization and QT Interval: Ionic Background, Modifiers, and Measurements

The QT interval on surface electrocardiogram represents the sum of depolarization and repolarization process of the ventricles. The ventricular recovery process, reflected by ST segment and T wave, mainly depends on the transmembrane outward transport of potassium ions to reestablish the endocellular electronegativity. Outward potassium channels represent a heterogeneous family of ionic carriers, whose global kinetics is modulated by heart rate and autonomic nervous activity. Several cardiac and noncardiac drugs and disease conditions, and several mutations of genes encoding ionic channels, generating distinct genetic channellopathies, may affect the ventricular repolarization, provoke QT interval prolongation and shortening, and increase the susceptibility to ventricular arrhythmias.

Spontaneous initiation of premature ventricular complexes and arrhythmias in type 2 long QT syndrome

The occurrence of early afterdepolarizations (EADs) and increased dispersion of repolarization are two known factors for arrhythmogenesis in long QT syndrome. However, increased dispersion of repolarization tends to suppress EADs due to the source-sink effect, and thus how the two competing factors cause initiation of arrhythmias remains incompletely understood. Here we used optical mapping and computer simulation to investigate the mechanisms underlying spontaneous initiation of arrhythmias in type 2 long QT (LQT2) syndrome. In optical mapping experiments of transgenic LQT2 rabbit hearts under isoproterenol, premature ventricular complexes (PVCs) were observed to originate from the steep spatial repolarization gradient (RG) regions and propagated unidirectionally. The same PVC behaviors were demonstrated in computer simulations of tissue models of rabbits. Depending on the heterogeneities, these PVCs could lead to either repetitive focal excitations or reentry without requiring an additional vulnerable substrate. Systematic simulations showed that cellular phase 2 EADs were either suppressed or confined to the long action potential region due to the source-sink effect. Tissue-scale phase 3 EADs and PVCs occurred due to tissue-scale dynamical instabilities caused by RG and enhanced L-type calcium current (I_Ca,L), occurring under both large and small RG. Presence of cellular EADs was not required but potentiated PVCs when RG was small. We also investigated how other factors affect the dynamical instabilities causing PVCs. Our main conclusion is that tissue-scale dynamical instabilities caused by RG and enhanced I_Ca,L give rise to both the trigger and the vulnerable substrate simultaneously for spontaneous initiation of arrhythmias in LQT2 syndrome.

Quinine and the ABCs of Long QT: A Patient's Misfortune with Arthritis, (Alcoholic) Beverages, and Cramps

A 91-year-old woman presented to the emergency department by ambulance after her family found her minimally responsive. Telemetry monitoring demonstrated episodes of non-sustained polymorphic ventricular tachycardia (PMVT) associated with significantly prolonged repolarization. Her medical history revealed that she was taking quinine or a derivative in three different forms: hydroxychloroquine, quinine sulfate (for leg cramps), and her gin mixed with tonic water (containing quinine). The present case is illustrative of classic etiologies and findings of acquired long QT syndrome, and serves as an important reminder for providers to take a complete medication history, including use of duplicative and alternative medicines and type of alcohol consumption.

Recurrent takotsubo with prolonged QT and torsade de pointes and left ventricular thrombus

Takotsubo cardiomyopathy, also known as “takotsubo syndrome,” refers to transient apical ballooning syndrome, stress cardiomyopathy, or broken heart syndrome and is a recently recognized syndrome typically characterized by transient and reversible left ventricular dysfunction that develops in the setting of acute severe emotional or physical stress. Increased catecholamine levels have been proposed to play a central role in the pathogenesis of the disease, although the specific pathophysiology of this condition remains to be fully determined. At present, there have been very few reports of recurrent takotsubo cardiomyopathy. In this case report, we present a patient with multiple recurrences of takotsubo syndrome triggered by severe emotional stress that presented with recurrent loss of consciousness, QT prolongation, and polymorphic ventricular tachycardia (torsade de pointes) and left ventricular apical thrombus.

Allosteric Modulation of Kv11.1 (hERG) Channels Protects Against Drug-Induced Ventricular Arrhythmias

BACKGROUND

Ventricular arrhythmias as a result of unintentional blockade of the Kv11.1 (hERG [human ether-à-go-go-related gene]) channel are a major safety concern in drug development. In past years, several highly prescribed drugs have been withdrawn for their ability to cause such proarrhythmia. Here, we investigated whether the proarrhythmic risk of existing drugs could be reduced by Kv11.1 allosteric modulators.

METHODS AND RESULTS

Using [(3)H]dofetilide-binding assays with membranes of human Kv11.1-expressing human embryonic kidney 293 cells, 2 existing compounds (VU0405601 and ML-T531) and a newly synthesized compound (LUF7244) were found to be negative allosteric modulators of dofetilide binding to the Kv11.1 channel, with LUF7244 showing the strongest effect at 10 μmol/L. The Kv11.1 affinities of typical blockers (ie, dofetilide, astemizole, sertindole, and cisapride) were significantly decreased by LUF7244. Treatment of confluent neonatal rat ventricular myocyte (NRVM) monolayers with astemizole or sertindole caused heterogeneous prolongation of action potential duration and a high incidence of early afterdepolarizations on 1-Hz electric point stimulation, occasionally leading to unstable, self-terminating tachyarrhythmias. Pretreatment of NRVMs with LUF7244 prevented these proarrhythmic effects. NRVM monolayers treated with LUF7244 alone displayed electrophysiological properties indistinguishable from those of untreated NRVM cultures. Prolonged exposure of NRVMs to LUF7244 or LUF7244 plus astemizole did not affect their viability, excitability, and contractility as assessed by molecular, immunological, and electrophysiological assays.

CONCLUSIONS

Allosteric modulation of the Kv11.1 channel efficiently suppresses drug-induced ventricular arrhythmias in vitro by preventing potentially arrhythmogenic changes in action potential characteristics, raising the possibility to resume the clinical use of unintended Kv11.1 blockers via pharmacological combination therapy.

Sepsis-Induced Takotsubo Cardiomyopathy Leading to Torsades de Pointes

Background. Takotsubo cardiomyopathy (TCM) is sudden and reversible myocardial dysfunction often attributable to physical or emotional triggers. Case Report. We describe a 51-year-old man presented to emergency department with sepsis from urinary tract infection (UTI). He was placed on cefepime for UTI and non-ST-elevation myocardial infarction protocol given elevated troponins with chest pain. Subsequently, patient was pulseless with torsades de pointes (TdP) and then converted to sinus rhythm with cardioversion. An echocardiogram revealed low ejection fraction with hypokinesis of the apical wall. Over 48 hours, the patient was extubated and stable on 3 L/min nasal cannula. He underwent a cardiac catheterization to evaluate coronary artery disease (CAD) and was found to have mild nonobstructive CAD with no further findings. Conclusion. TCM is a rare disorder presenting with symptoms similar to acute coronary syndrome. Though traditionally elicited by physical and emotional triggers leading to transient left ventricular dysfunction, our case suggests that it may also be triggered by a urinary tract infection and lead to severe QT prolongation and a malignant ventricular arrhythmia in TdP.

Heart-specific overexpression of the human short CLC-3 chloride channel isoform limits myocardial ischemia-induced ERP and QT prolongation

INTRODUCTION

Ischemia causes myocardial infarction and arrhythmias. Up-regulation of cardiac CLC-3 chloride channels is important for ischemic preconditioning-induced second-window protection against myocardial infarction. But its consequences in ischemia-induced electrical remodeling are still unknown.

METHODS

The recently-characterized heart-specific overexpression of human short CLC-3 isoform (hsCLC-3(OE)) mice was used to study the effects of CLC-3 up-regulation on cardiac electrophysiology under ischemia/reperfusion conditions. In vivo surface electrocardiography (ECG) and intracardiac electrophysiology (ICEP) were used to compare the electrophysiological properties of age-matched wild-type (Clcn3(+/+)) and hsCLC-3(OE) mice under control and myocardial ischemia-reperfusion conditions.

RESULTS

QT and QTc intervals of hsCLC-3(OE) mice were significantly shorter than those of Clcn3(+/+) mice under control, ischemia and reperfusion conditions. In the ICEP, ventricular effective refractory period (VERP) of hsCLC-3(OE) mice (26.7±1.7ms, n=6) was significantly shorter than that of Clcn3(+/+) mice (36.9±2.8ms, n=8, P<0.05). Under ischemia condition, both VERP (19.8±1.3ms) and atrial effective refractory period (AERP, 34.8±2.5ms) of hsCLC-3(OE) mice were significantly shorter than those of Clcn3(+/+) mice (35.2±3.0ms and 45.8±1.6ms, P<0.01, respectively). Wenckebach atrioventricular nodal block point (AVBP, 91.13±4.08ms) and 2:1 AVBP (71.3±3.8ms) of hsCLC-3(OE) mice were significantly shorter than those of Clcn3(+/+) mice (102.0±2.0ms and 84.1±2.8ms, P<0.05, respectively). However, no differences of ICEP parameters between hsCLC-3(OE) and Clcn3(+/+) mice were observed under reperfusion conditions.

CONCLUSION

Heart-specific overexpression of hsCLC-3 limited the ischemia-induced QT and ERP prolongation and postponed the advancements of Wenckebach and 2:1 AVBP. CLC-3 up-regulation may serve as an important adaptive mechanism against myocardial ischemia.

Computational cardiology and risk stratification for sudden cardiac death: one of the grand challenges for cardiology in the 21st century

Risk stratification in the context of sudden cardiac death has been acknowledged as one of the major challenges facing cardiology for the past four decades. In recent years, the advent of high performance computing has facilitated organ-level simulation of the heart, meaning we can now examine the causes, mechanisms and impact of cardiac dysfunction in silico. As a result, computational cardiology, largely driven by the Physiome project, now stands at the threshold of clinical utility in regards to risk stratification and treatment of patients at risk of sudden cardiac death. In this white paper, we outline a roadmap of what needs to be done to make this translational step, using the relatively well-developed case of acquired or drug-induced long QT syndrome as an exemplar case.

Targeting the late component of the cardiac L-type Ca2+ current to suppress early afterdepolarizations

Early afterdepolarizations (EADs) associated with prolongation of the cardiac action potential (AP) can create heterogeneity of repolarization and premature extrasystoles, triggering focal and reentrant arrhythmias. Because the L-type Ca(2+) current (ICa,L) plays a key role in both AP prolongation and EAD formation, L-type Ca(2+) channels (LTCCs) represent a promising therapeutic target to normalize AP duration (APD) and suppress EADs and their arrhythmogenic consequences. We used the dynamic-clamp technique to systematically explore how the biophysical properties of LTCCs could be modified to normalize APD and suppress EADs without impairing excitation-contraction coupling. Isolated rabbit ventricular myocytes were first exposed to H2O2 or moderate hypokalemia to induce EADs, after which their endogenous ICa,L was replaced by a virtual ICa,L with tunable parameters, in dynamic-clamp mode. We probed the sensitivity of EADs to changes in the (a) amplitude of the noninactivating pedestal current; (b) slope of voltage-dependent activation; (c) slope of voltage-dependent inactivation; (d) time constant of voltage-dependent activation; and (e) time constant of voltage-dependent inactivation. We found that reducing the amplitude of the noninactivating pedestal component of ICa,L effectively suppressed both H2O2- and hypokalemia-induced EADs and restored APD. These results, together with our previous work, demonstrate the potential of this hybrid experimental-computational approach to guide drug discovery or gene therapy strategies by identifying and targeting selective properties of LTCC.

Predictive Power of f99 Repolarization Index for the Occurrence of Ventricular Arrhythmias

BACKGROUND

Defects of cardiac repolarization, noninvasively identifiable by analyzing the electrocardiographic (ECG) ST segment and T wave, are among the major causes of sudden cardiac death. Still, no repolarization-based index has so far shown sufficient sensitivity and specificity to justify preventive treatments. Thus, the aim of this work was to evaluate the predictive power of our recently proposed f99 index for the occurrence of ventricular arrhythmias.

METHODS

Our study populations included 170 patients with implanted cardiac defibrillator (ICD), 44 of which developed ventricular tachycardia and/or fibrillation during the 4-year follow-up (ICD_Cases) and 126 did not (ICD_Controls). The f99 index, defined as the frequency at which the repolarization normalized cumulative energy reaches 99%, was computed in each of the 15 (I to III, aVl, aVr, aVf, V1 -V6 , X, Y, Z) available ECG leads independently, and then maximized over the 6 precordial leads (f99_MaxV1 -V6 ), 12 standard leads (f99_Max12STD) and three orthogonal leads (f99_MaxXYZ) to avoid dispersion-related issues. Each index predictive power was quantified as the area under the receiving operating characteristic curve (AUC).

RESULTS

Median f99_MaxV1 -V6 , f99_Max12STD and f99_MaxXYZ values were significantly higher in the ICD_Cases than in the ICD_Controls (48 Hz vs. 35 Hz, P<0.05; 51 Hz vs. 43 Hz, P<0.05; 45 Hz vs. 31 Hz, P<10(-3) ; respectively), indicating a more fragmented repolarization in the former group. The AUC values were 0.62, 0.63 and 0.68, respectively.

CONCLUSIONS

The f99 represents a promising risk index for the occurrence of ventricular arrhythmias, especially when maximized over the three orthogonal leads.

Late Na(+) current and protracted electrical recovery are critical determinants of the aging myopathy

The aging myopathy manifests itself with diastolic dysfunction and preserved ejection fraction. We raised the possibility that, in a mouse model of physiological aging, defects in electromechanical properties of cardiomyocytes are important determinants of the diastolic characteristics of the myocardium, independently from changes in structural composition of the muscle and collagen framework. Here we show that an increase in the late Na(+) current (INaL) in aging cardiomyocytes prolongs the action potential (AP) and influences temporal kinetics of Ca(2+) cycling and contractility. These alterations increase force development and passive tension. Inhibition of INaL shortens the AP and corrects dynamics of Ca(2+) transient, cell contraction and relaxation. Similarly, repolarization and diastolic tension of the senescent myocardium are partly restored. Thus, INaL offers inotropic support, but negatively interferes with cellular and ventricular compliance, providing a new perspective of the biology of myocardial aging and the aetiology of the defective cardiac performance in the elderly.

Drug-induced proarrhythmia: risk factors and electrophysiological mechanisms

Drug-induced ventricular tachyarrhythmias can be caused by cardiovascular drugs, noncardiovascular drugs, and even nonprescription agents. They can result in arrhythmic emergencies and sudden cardiac death. If a new arrhythmia or aggravation of an existing arrhythmia develops during therapy with a drug at a concentration usually considered not to be toxic, the situation can be defined as proarrhythmia. Various cardiovascular and noncardiovascular drugs can increase the occurrence of polymorphic ventricular tachycardia of the 'torsade de pointes' type. Antiarrhythmic drugs, antimicrobial agents, and antipsychotic and antidepressant drugs are the most important groups. Age, female sex, and structural heart disease are important risk factors for the occurrence of torsade de pointes. Genetic predisposition and individual pharmacodynamic and pharmacokinetic sensitivity also have important roles in the generation of arrhythmias. An increase in spatial or temporal dispersion of repolarization and a triangular action-potential configuration have been identified as crucial predictors of proarrhythmia in experimental models. These studies emphasized that sole consideration of the QT interval is not sufficient to assess the proarrhythmic risk. In this Review, we focus on important triggers of proarrhythmia and the underlying electrophysiological mechanisms that can enhance or prevent the development of torsade de pointes.

Short-term variability in QT interval and ventricular arrhythmias induced by dofetilide are dependent on high-frequency autonomic oscillations

Background and Purpose

The present study was undertaken to investigate an effect of dofetilide, a potent arrhythmic blocker of the voltage-gated K⁺ channel, hERG, on cardiac autonomic control. Combined with effects on ardiomyocytes, these properties could influence its arrhythmic potency.

Experimental Approach

The short-term variability of beat-to-beat QT interval (STV_QT), induced by dofetilide is a strong surrogate of Torsades de pointes liability. Involvement of autonomic modulation in STV_QT was investigated in healthy cynomolgus monkeys and beagle dogs by power spectral analysis under conditions of autonomic blockade with hexamethonium.

Key Results

Increase in STV_QT induced by dofetilide in monkeys and dogs was closely associated with an enhancement of endogenous heart rate and QT interval high-frequency (HF) oscillations. These effects were fully suppressed under conditions of autonomic blockade with hexamethonium. Ventricular arrhythmias, including Torsades de pointes in monkeys, were prevented in both species when HF oscillations were suppressed by autonomic blockade. Similar enhancements of heart rate HF oscillations were found in dogs with other hERG blockers described as causing Torsades de pointes in humans.

Conclusions and Implications

These results demonstrate for the first time that beat-to-beat ventricular repolarization variability and ventricular arrhythmias induced by dofetilide are dependent on endogenous HF autonomic oscillations in heart rate. When combined with evidence of hERG-blocking properties, enhancement of endogenous HF oscillations in heart rate could constitute an earlier and more sensitive biomarker than STV_QT for Torsades de pointes liability, applicable to preclinical regulatory studies conducted in healthy animals.

Hyperphosphorylation of RyRs underlies triggered activity in transgenic rabbit model of LQT2 syndrome

RATIONALE

Loss-of-function mutations in human ether go-go (HERG) potassium channels underlie long QT syndrome type 2 (LQT2) and are associated with fatal ventricular tachyarrhythmia. Previously, most studies focused on plasma membrane-related pathways involved in arrhythmogenesis in long QT syndrome, whereas proarrhythmic changes in intracellular Ca(2+) handling remained unexplored.

OBJECTIVE

We investigated the remodeling of Ca(2+) homeostasis in ventricular cardiomyocytes derived from transgenic rabbit model of LQT2 to determine whether these changes contribute to triggered activity in the form of early after depolarizations (EADs).

METHODS AND RESULTS

Confocal Ca(2+) imaging revealed decrease in amplitude of Ca(2+) transients and sarcoplasmic reticulum Ca(2+) content in LQT2 myocytes. Experiments using sarcoplasmic reticulum-entrapped Ca(2+) indicator demonstrated enhanced ryanodine receptor (RyR)-mediated sarcoplasmic reticulum Ca(2+) leak in LQT2 cells. Western blot analyses showed increased phosphorylation of RyR in LQT2 myocytes versus controls. Coimmunoprecipitation experiments demonstrated loss of protein phosphatases type 1 and type 2 from the RyR complex. Stimulation of LQT2 cells with β-adrenergic agonist isoproterenol resulted in prolongation of the plateau of action potentials accompanied by aberrant Ca(2+) releases and EADs, which were abolished by inhibition of Ca(2+)/calmodulin-dependent protein kinase type 2. Computer simulations showed that late aberrant Ca(2+) releases caused by RyR hyperactivity promote EADs and underlie the enhanced triggered activity through increased forward mode of Na(+)/Ca(2+) exchanger type 1.

CONCLUSIONS

Hyperactive, hyperphosphorylated RyRs because of reduced local phosphatase activity enhance triggered activity in LQT2 syndrome. EADs are promoted by aberrant RyR-mediated Ca(2+) releases that are present despite a reduction of sarcoplasmic reticulum content. Those releases increase forward mode Na(+)/Ca(2+) exchanger type 1, thereby slowing repolarization and enabling L-type Ca(2+) current reactivation.

Increased phosphorylation of Ca(2+) handling proteins as a proarrhythmic mechanism in myocarditis

BACKGROUND

Because fatal arrhythmia is an important cause of death in patients with myocarditis, we investigated the proarrhythmic mechanisms of experimental autoimmune myocarditis. METHODS AND RESULTS: Myocarditis was induced by injection of 2 mg porcine cardiac myosin into the footpads of adult Lewis rats on days 1 and 8 (Myo, n=15) and the results compared with Control rats (Control, n=15). In an additional 15 rats, 6 mg/kg prednisolone was injected into the gluteus muscle before the injection of porcine cardiac myosin on days 1 and 8 (MyoS, n=15). Hearts with myocarditis had longer action potential duration (APD), slower conduction velocity (CV; P<0.01 vs. Control), higher CV heterogeneity, greater fibrosis, higher levels of immunoblotting of high-mobility group protein B1, interleukin 6 and tumor necrosis factor-α proteins. Steroid treatment partially reversed the translations for myocarditis, CV heterogeneity, reduced APD at 90% recovery to baseline, increased CV (P<0.01), and reversed fibrosis (P<0.05). Programmed stimulation triggered sustained ventricular tachycardia in Myo rats (n=4/5), but not in controls (n=0/5) or Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor (KN93) treated Myo rats (n=0/5, P=0.01). CaMKII autophosphorylation at Thr287 (201%), and RyR2 phosphorylation at Ser2808 (protein kinase A/CaMKII site, 126%) and Ser2814 (CaMKII site, 21%) were increased in rats with myocarditis and reversed by steroid.

CONCLUSIONS

The myocarditis group had an increased incidence of arrhythmia caused by increased phosphorylation of Ca(2+)handling proteins. These changes were partially reversed by an antiinflammatory treatment and CaMKII inhibition.

Abnormal repolarization in the acute myocardial infarction patients: a frequency-based characterization

Despite ST elevation having poor sensitivity for acute myocardial infarction (AMI), it remains the main electrocardiographic (ECG) repolarization index for AMI diagnosis. Aim of the present study was to propose a new f99 index, defined as the frequency at which the repolarization normalized cumulative energy reaches 99%, for ECG AMI discrimination from health with good sensitivity and good specificity. Evaluation of such f99 index was performed on 12-standard-lead (I, II, III, aV1, aVr, aVf, V1 to V6) ECG recordings of 47 healthy controls and 108 acute myocardial infarction (AMI) patients. Repolarization dispersion caused f99 distributions to be significantly lead dependent. In most leads (leads I, II, aVl, aVr, V2-V6), f99 median value was lower in the healthy controls (10-17 Hz) than in the AMI patients (12-38 Hz) indicating higher frequency components (i.e. a more fragmented repolarization) in the latter population. AMI patients from healthy controls discrimination by f99, evaluated in terms of sensitivity (Se) and specificity (Sp), was also lead dependent. Single-lead analysis indicated leads I (Se=80%, Sp=77%) and aVl (Se=84%, Sp=74%) as optimal. Instead, lead-system analysis, performed to overcome dispersion issues, provided the best results when averaging over the 6 precordial leads (Se= 81% and Sp=74%). In conclusion, our new f99 index appears as a promising tool for non-invasively and reliably discriminate AMI patients from healthy subjects.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

Evidence for a crucial modulating role of the sodium channel in the QTc prolongation related to antipsychotics

Blockade of the cardiac hERG channel is recognized as the main mechanism underlying the QT prolongation induced by many classes of drugs, including antipsychotics. However, antipsychotics interact with a variety of other pharmacological targets that could also modulate cardiac function. The present study aims to identify those key factors involved in the QT prolongation induced by antipsychotics. The interactions of 28 antipsychotics were measured on a variety of pharmacological targets. Binding affinity (K(i)), functional channel blockade (IC₅₀), and the corresponding ratios to total and free plasma drug concentration were compared with the corrected QT changes (QTc) associated with the therapeutic use of these drugs by multivariable linear regression analysis to determine the best predictors of QTc. Besides confirming hERG as the primary predictor of QTc, all analyses consistently show the concomitant involvement of Na(V)1.5 channel as modulating factor of the QTc related to hERG blockade. In particular, the hERG/Na(V)1.5 ratio explains the 57% of the overall QTc variability associated with antipsychotics. Since it is known that inhibition of late I Na could offset the dysfunctional effects of hERG blockade, we hypothesize the inhibition of late I(Na) as a crucial compensatory mechanism of the QTc associated with antipsychotics and hence an important factor to consider concomitantly with hERG blockade to appraise the arrhythmogenic risk of these drugs more accurately.

Increasing gap junction coupling suppresses ibutilide-induced torsades de pointes

Drug-induced torsades de pointes (TdP) is common with class III antiarrhythmic drugs. Increased transmural dispersion of repolarization (TDR) contributes significantly to the development of TdP. Gap junctions play an important role in maintaining TDR in long QT syndrome. The present study examined the effect of a gap junction enhancer, antiarrhythmic peptide 10 (AAP10), on ibutilide-induced TdP. Coronary-perfused rabbit ventricular wedge preparations were used to evaluate the effect of AAP10 on ibutilide-induced arrhythmia. Transmural electrocardiograms and action potentials were recorded simultaneously. Early afterdepolarizations (EADs), R-on-T extrasystole, TdP and changes in Tpeak-end (Tp-e) and the Tp-e/QT ratio were observed. Changes in the levels of non-phosphorylated connexin 43 (Cx43) were measured by immunoblotting. Compared with those in the control group, the QT interval, Tp-e/QT and incidence rates of EAD and TdP increased with augmented dephosphorylation in the ventricular wedge preparations perfused with ibutilide under conditions of hypokalemia and hypomagnesemia. In the presence of AAP10, the incidence rates of EAD and TdP were reduced and the Tp-e/QT ratio decreased, with a parallel reduction in the level of non-phosphorylated Cx43. The results indicate that AAP10 suppressed ibutilide-induced TdP under conditions of hypokalemia and hypomagnesemia by decreasing TDR. AAP10 reduced TDR, possibly by preventing the dephosphorylation of Cx43 and thereby increasing myocardial cell gap junction coupling.

A study of early afterdepolarizations in a model for human ventricular tissue

Sudden cardiac death is often caused by cardiac arrhythmias. Recently, special attention has been given to a certain arrhythmogenic condition, the long-QT syndrome, which occurs as a result of genetic mutations or drug toxicity. The underlying mechanisms of arrhythmias, caused by the long-QT syndrome, are not fully understood. However, arrhythmias are often connected to special excitations of cardiac cells, called early afterdepolarizations (EADs), which are depolarizations during the repolarizing phase of the action potential. So far, EADs have been studied mainly in isolated cardiac cells. However, the question on how EADs at the single-cell level can result in fibrillation at the tissue level, especially in human cell models, has not been widely studied yet. In this paper, we study wave patterns that result from single-cell EAD dynamics in a mathematical model for human ventricular cardiac tissue. We induce EADs by modeling experimental conditions which have been shown to evoke EADs at a single-cell level: by an increase of L-type Ca currents and a decrease of the delayed rectifier potassium currents. We show that, at the tissue level and depending on these parameters, three types of abnormal wave patterns emerge. We classify them into two types of spiral fibrillation and one type of oscillatory dynamics. Moreover, we find that the emergent wave patterns can be driven by calcium or sodium currents and we find phase waves in the oscillatory excitation regime. From our simulations we predict that arrhythmias caused by EADs can occur during normal wave propagation and do not require tissue heterogeneities. Experimental verification of our results is possible for experiments at the cell-culture level, where EADs can be induced by an increase of the L-type calcium conductance and by the application of I blockers, and the properties of the emergent patterns can be studied by optical mapping of the voltage and calcium.

A century of optocardiography

In the past decade, optical mapping provided crucial mechanistic insight into electromechanical function and the mechanism of ventricular fibrillation. Therefore, to date, optical mapping dominates experimental cardiac electrophysiology. The first cardiac measurements involving optics were done in the early 1900s using the fast cinematograph that later evolved into methods for high-resolution activation and repolarization mapping and stimulation of specific cardiac cell types. The field of "optocardiography," therefore, emerged as the use of light for recording or interfering with cardiac physiology. In this review, we discuss how optocardiography developed into the dominant research technique in experimental cardiology. Furthermore, we envision how optocardiographic methods can be used in clinical cardiology.

Corrected QT interval in white young healthy women: should the norms be updated?

The electrocardiographic QT interval duration is a frequently studied ECG parameter. The aim of this study was to compare the quantitative influence of nine QTc formulae and discuss the duration of QT/QTc normal values. The QTc duration was calculated from manually measured QT intervals and heart rate of the Frank XYZ electrocardiograms in 138 white young healthy Slovak women (18-24 years, 690 primary ECG measurements). Dispersion of the QT/QTc interval values was not Gaussian (Shapiro-Wilk test) in most cases. The course of the measured QT regression lines were -1.9186 HR + 511.13 ms or 0.1504RR + 245.72 ms. Median QTc duration in ms was decreasing in this sequence: Bazett 407.04, Fridericia 394.61, Hodges 393.27, Rautaharju and Arrowood 368.79, Framingham-Sagie 368.78, Lecocq 368.70, Sarma 368.66, and Malik 338.70. No QTc value above 440 ms was found after Hodges' correction only. The differences of median QTc values were significant (P < 0.01) in most possible combinations (Kruskal-Wallis test). We recommend using the QTc formula created from observed persons only and updating the QT interval norms. So-called "ideal" QTc models from other not examined persons are methodically incorrect and may produce the misleading results. The frequently used Bazett formula is appropriate for orientation only.

Short-term effect of percutaneous recanalization of chronic total occlusions on QT dispersion and heart rate variability parameters

Background

QT dispersion (QTd), which is a measure of inhomogeneity of myocardial repolarization, increases following impaired myocardial perfusion. Its prolongation may provide a suitable substrate for life-threatening ventricular arrhythmias. We investigated the changes in QTd and heart rate variability (HRV) parameters after successful coronary artery revascularization in a patient with chronic total occlusions (CTO).

Material/Methods

This study included 139 successfully revascularized CTO patients (118 men, 21 women, mean age 58.3±9.6 years). QTd was measured from a 12-lead electrocardiogram and was defined as the difference between maximum and minimum QT interval. HRV analyses of all subjects were obtained. Frequency domain (LF: HF) and time domain (SDNN, pNN50, and rMSSD) parameters were analyzed. QT intervals were also corrected for heart rate using Bazett’s formula, and the corrected QT interval dispersion (QTcd) was then calculated. All measurements were made before and after percutaneous coronary intervention (PCI).

Results

Both QTd and QTcd showed significant improvement following successful revascularization of CTO (55.83±14.79 to 38.87±11.69; p<0.001 and 61.02±16.28 to 42.92±13.41; p<0.001). The revascularization of LAD (n=38), Cx (n=28) and RCA (n=73) resulted in decrease in HRV indices, including SDDN, rMSSD, and pNN50, but none of the variables reached statistical significance.

Conclusions

Successful revascularization of CTO may result in improvement in regional heterogeneity of myocardial repolarization, evidenced as decreased QTcd after the PCI. The revascularization in CTO lesions does not seem to have a significant impact on HRV.

Promethazine misuse among methadone maintenance patients and community-based injection drug users

OBJECTIVE

Promethazine has been reported to be misused in conjunction with opioids in several settings. Promethazine misuse by itself or in conjunction with opioids may have serious adverse health effects. To date, no prevalence data for the nonmedical use of promethazine have been reported. This study examines the prevalence and correlates of promethazine use in 2 different populations in San Francisco, California: methadone maintenance clinic patients and community-based injection drug users (IDUs).

METHODS

We analyzed urine samples for the presence of promethazine and reviewed the clinical records for 334 methadone maintenance patients at the county methadone clinic. Separately, we used targeted sampling methods to recruit and survey 139 community-based opioid IDUs about their use of promethazine. We assessed prevalence and factors associated with promethazine use with bivariate and multivariate statistics.

RESULTS

The prevalence of promethazine-positive urine samples among the methadone maintenance patients was 26%. Only 15% of promethazine-positive patients had an active prescription for promethazine. Among IDUs reporting injection of opiates in the community-based survey, 17% reported having used promethazine in the past month; 24% of the IDUs who reported being enrolled in methadone treatment reported using promethazine in the past month.

CONCLUSIONS

The finding that one-quarter of methadone maintenance patients in a clinic or recruited in community settings have recently used promethazine provides compelling evidence of significant nonmedical use of promethazine in this patient population. Further research is needed to establish the extent and nature of nonmedical use of promethazine.