Chronic amiodarone reduces transmural dispersion of repolarization in the canine heart

The ion channel basis of pharmacological effects of amiodarone on myocardial electrophysiological properties, a comprehensive review

Amiodarone is a benzofuran-based class III antiarrhythmic agent frequently used for the treatment of atrial and ventricular arrhythmias. The primary target of class III antiarrhythmic drugs is the cardiac human ether-a-go-go-related gene (hERG) encoded channel, KCNH2, commonly known as HERG, that conducts the rapidly activating delayed rectifier potassium current (IKr). Like other class III antiarrhythmic drugs, amiodarone exerts its physiologic effects mainly through IKr blockade, delaying the repolarization phase of the action potential and extending the effective refractory period. However, while many class III antiarrhythmics, including sotalol and dofetilide, can cause long QT syndrome (LQTS) that can progress to torsade de pointes, amiodarone displays less risk of inducing this fatal arrhythmia. This review article discusses the arrhythmogenesis in LQTS from the aspects of the development of early afterdepolarizations (EADs) associated with Ca2+ current, transmural dispersion of repolarization (TDR), as well as reverse use dependence associated with class III antiarrhythmic drugs to highlight electropharmacological effects of amiodarone on the myocardium.

Argentina's most important contributions in the field of electrophysiology

Latin American electrocardiology emerged internationally thanks to the Argentine School of Electrocardiology. All started when the idea of a different anatomy of the conduction system was not only necessary to change the paradigm of a bifascicular system, but also to question diagnostic electrocardiographic criteria adopted by the scientific community without dispute. Almost every scientific contribution coming from the Argentine School of Electrocardiology represented a significant step forward in the understanding of the electrophysiology of the heart and its electrocardiographic counterpart. There is another reason that increases their value: the noticeable simplicity of the technical facilities with which these studies were done from the modest laboratory in Argentina, whose production was purely and genuinely Latin American. In the following lines we summarize what we consider to be the greatest contributions of the Argentine school to world electrophysiology.

Amiodarone prevents wave front-tail interactions in patients with heart failure: an in silico study

Amiodarone (AM) is an antiarrhythmic drug whose chronic use has proved effective in preventing ventricular arrhythmias in a variety of patient populations, including those with heart failure (HF). AM has both class III [i.e., it prolongs the action potential duration (APD) via blocking potassium channels) and class I (i.e., it affects the rapid sodium channel) properties; however, the specific mechanism(s) by which it prevents reentry formation in patients with HF remains unknown. We tested the hypothesis that AM prevents reentry induction in HF during programmed electrical stimulation (PES) via its ability to induce postrepolarization refractoriness (PRR) via its class I effects on sodium channels. Here we extend our previous human action potential model to represent the effects of both HF and AM separately by calibrating to human tissue and clinical PES data, respectively. We then combine these models (HF + AM) to test our hypothesis. Results from simulations in cells and cables suggest that AM acts to increase PRR and decrease the elevation of takeoff potential. The ability of AM to prevent reentry was studied in silico in two-dimensional sheets in which a variety of APD gradients (ΔAPD) were imposed. Reentrant activity was induced in all HF simulations but was prevented in 23 of 24 HF + AM models. Eliminating the AM-induced slowing of the recovery of inactivation of the sodium channel restored the ability to induce reentry. In conclusion, in silico testing suggests that chronic AM treatment prevents reentry induction in patients with HF during PES via its class I effect to induce PRR.NEW & NOTEWORTHY This work presents a new model of the action potential of the human, which reproduces the complex dynamics during premature stimulation in heart failure patients with and without amiodarone. A specific mechanism of the ability of amiodarone to prevent reentrant arrhythmias is presented.

Different effects of amiodarone and dofetilide on the dispersion of repolarization between well-coupled ventricular and Purkinje fibers1

Increased transmural dispersion of repolarization is an established contributing factor to ventricular tachyarrhythmias. In this study, we evaluated the effect of chronic amiodarone treatment and acute administration of dofetilide in canine cardiac preparations containing electrotonically coupled Purkinje fibers (PFs) and ventricular muscle (VM) and compared the effects to those in uncoupled PF and VM preparations using the conventional microelectrode technique. Dispersion between PFs and VM was inferred from the difference in the respective action potential durations (APDs). In coupled preparations, amiodarone decreased the difference in APDs between PFs and VM, thus decreasing dispersion. In the same preparations, dofetilide increased the dispersion by causing a more pronounced prolongation in PFs. This prolongation was even more emphasized in uncoupled PF preparations, while the effect in VM was the same. In uncoupled preparations, amiodarone elicited no change on the difference in APDs. In conclusion, amiodarone decreased the dispersion between PFs and VM, while dofetilide increased it. The measured difference in APD between cardiac regions may be the affected by electrotonic coupling; thus, studying PFs and VM separately may lead to an over- or underestimation of dispersion.

Ventricular repolarization wave variations during the amiodarone treatment course

The aim of this study was to assess ventricular repolarization wave variations during the amiodarone treatment course for patients with ventricular arrhythmias and atrial fibrillation.Sixty-nine patients with ventricular arrhythmias and 9 patients with persistent atrial fibrillation were treated with intravenous injection of a 150 mg loading dose of amiodarone, followed by 1 mg/minute for 6 hours and 0.5 mg/minute for 48 hours. After the initial 24 hours of intravenous injection, amiodarone was also administered orally at a dose of 0.2 g tid for 1 week; followed by 0.2 g bid for 1 week and 0.2 g qd for maintenance. During the procedure, the heart rate, QT, QTc, QTd, QTcd TpTe, TpTe-c, TpTe-d, TpTe/QT, and QTp were measured on days 1, 3, 7, 10, 14, 17, and 20 of amiodarone treatment.The control rate of arrhythmias was 91.0% (71/78). The heart rate dropped significantly on the 7th day after treatment initiation and reached the minimal value on day 14. The QT interval was prolonged from day 3; TpTe was prolonged from day 7 to day 14; QTp was prolonged from day 1 to day 20. The longest QT interval (441.2 ± 33.9 ms) and TpTe (95.0 ± 18.0 ms) occurred on day 14. QTc, QTd, QTcd, TpTe-c, TpTe-d, and TpTe/QT showed no significant changes throughout the treatment.Amiodarone lowers the heart rate, prolongs QT and QTp intervals, and transiently prolongs TpTe. However, it has no effects on QTc, QTd, QTcd, TpTe-c, TpTe-d or TpTe/QT. Amiodarone prolongs QT interval evenly, showing no effects on repolarization dispersion. TpTe/QT is a better indicator of ventricular transmural repolarization dispersion compared with TpTe.

Traditional and novel electrocardiographic conduction and repolarization markers of sudden cardiac death

Sudden cardiac death, frequently due to ventricular arrhythmias, is a significant problem globally. Most affected individuals do not arrive at hospital in time for medical treatment. Therefore, there is an urgent need to identify the most-at-risk patients for insertion of prophylactic implantable cardioverter defibrillators. Clinical risk markers derived from electrocardiography are important for this purpose. They can be based on repolarization, including corrected QT (QTc) interval, QT dispersion (QTD), interval from the peak to the end of the T-wave (Tpeak - Tend), (Tpeak - Tend)/QT, T-wave alternans (TWA), and microvolt TWA. Abnormal repolarization properties can increase the risk of triggered activity and re-entrant arrhythmias. Other risk markers are based solely on conduction, such as QRS duration (QRSd), which is a surrogate marker of conduction velocity (CV) and QRS dispersion (QRSD) reflecting CV dispersion. Conduction abnormalities in the form of reduced CV, unidirectional block, together with a functional or a structural obstacle, are conditions required for circus-type or spiral wave re-entry. Conduction and repolarization can be represented by a single parameter, excitation wavelength (λ = CV × effective refractory period). λ is an important determinant of arrhythmogenesis in different settings. Novel conduction-repolarization markers incorporating λ include Lu et al.' index of cardiac electrophysiological balance (iCEB: QT/QRSd), [QRSD× (Tpeak - Tend)/QRSd] and [QRSD × (Tpeak - Tend)/(QRSd × QT)] recently proposed by Tse and Yan. The aim of this review is to provide up to date information on traditional and novel markers and discuss their utility and downfalls for risk stratification.

Pro-arrhythmogenic Effects of the V141M KCNQ1 Mutation in Short QT Syndrome and Its Potential Therapeutic Targets: Insights from Modeling

Gain-of-function mutations in the pore-forming subunit of I_Ks channels, KCNQ1, lead to short QT syndrome (SQTS) and lethal arrhythmias. However, how mutant I_Ks channels cause SQTS and the possibility of I_Ks-specific pharmacological treatment remain unclear. V141M KCNQ1 is a SQTS associated mutation. We studied its effect on I_Ks gating properties and changes in the action potentials (AP) of human ventricular myocytes. Xenopus oocytes were used to study the gating mechanisms of expressed V141M KCNQ1/KCNE1 channels. Computational models were used to simulate human APs in endocardial, mid-myocardial, and epicardial ventricular myocytes with and without β-adrenergic stimulation. V141M KCNQ1 caused a gain-of-function in I_Ks characterized by increased current density, faster activation, and slower deactivation leading to I_Ks accumulation. V141M KCNQ1 also caused a leftward shift of the conductance-voltage curve compared to wild type (WT) I_Ks (V_1/2 = 33.6 ± 4.0 mV for WT, and 24.0 ± 1.3 mV for heterozygous V141M). A Markov model of heterozygous V141M mutant I_Ks was developed and incorporated into the O’Hara–Rudy model. Compared to the WT, AP simulations demonstrated marked rate-dependent shortening of AP duration (APD) for V141M, predicting a SQTS phenotype. Transmural electrical heterogeneity was enhanced in heterozygous V141M AP simulations, especially under β-adrenergic stimulation. Computational simulations identified specific I_K1 blockade as a beneficial pharmacologic target for reducing the transmural APD heterogeneity associated with V141M KCNQ1 mutation. V141M KCNQ1 mutation shortens ventricular APs and enhances transmural APD heterogeneity under β-adrenergic stimulation. Computational simulations identified I_K1 blockers as a potential antiarrhythmic drug of choice for SQTS.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Electrophysiological mechanisms of long and short QT syndromes

The QT interval on the human electrocardiogram is normally in the order of 450 ms, and reflects the summated durations of action potential (AP) depolarization and repolarization of ventricular myocytes. Both prolongation and shortening in the QT interval have been associated with ventricular tachy-arrhythmias, which predispose affected individuals to sudden cardiac death. In this article, the molecular determinants of the AP duration and the causes of long and short QT syndromes (LQTS and SQTS) are explored. This is followed by a review of the recent advances on their arrhythmogenic mechanisms involving reentry and/or triggered activity based on experiments conducted in mouse models. Established and novel clinical risk markers based on the QT interval for the prediction of arrhythmic risk and cardiovascular mortality are presented here. It is concluded by a discussion on strategies for the future rational design of anti-arrhythmic agents.

Antiarrhythmic and Proarrhythmic Properties of QT-Prolonging Antianginal Drugs

In recent years there has been a major reorientation of drug therapy for cardiac arrhythmias, its changing role, and above all, a radical change in the class of arrhythmia drugs because of their impact on mortality. The decline in the use of sodium-channel blockers has led to an expanding use of β-blockers and simple or complex class III agents for controlling cardiac arrhythmias. Success with these agents in the context of their side effects has spurred the development of compounds with simpler ion-channel blocking properties that have less complex adverse reactions. The resulting so-called pure class III agents, such as dofetilide or ibutilide, were found to have antifibrillatory effects in atrial fibrillation and flutter and in ventricular tachyarrhythmias. Such agents are effective and have diversity, but they have come into therapeutics with a price: the sometimes-fatal torsades de pointes. The drug amiodarone, a complex compound that was synthesized as an antianginal agent, has been an exception in this regard. Its therapeutic use is associated with a negligibly low incidence of torsades de pointes, even though the drug produces significant bradycardia and QT lengthening to 500 to 700 msec. Recent electrophysiologic studies suggest that this paradox is likely due to the differential block of ion channels in endocardium, epicardium, midmyocardial (M) cells, and Purkinje fibers in the ventricular myocardium. There is also clinical evidence suggesting that amiodarone reduces the “torsadogenic” effects of pure class III agents. Ranolazine was also synthesized for the development of antianginal properties that stem from a partial inhibition of fatty acid oxidation; it too has been found to have electrophysioloigic properties. These are somewhat similar to those of amiodarone on ion channels in endocardium, epicardium, M cells, and Purkinje fibers in the ventricular myocardium, but the drug does not prolong the QT interval to the same extent as amiodarone does. Thus, the drug produces modest increases in repolarization as judged by its effects on the action potential duration (APD) without the potential for the development of torsades de pointes. By virtue of its suppressant action on early afterdepolarizations and triggered activity in Purkinje fibers and M cells, the drug appears to have a powerful potential for reducing the torsadogenic proclivity of conventional class III antiarrhythmic compounds. The rationale for the therapeutic niche for amiodarone, and especially in the case of ranolazine, in the prevention of drug-induced torsades de pointes is discussed.

The role of late I Na in development of cardiac arrhythmias

Late I Na is an integral part of the sodium current, which persists long after the fast-inactivating component. The magnitude of the late I Na is relatively small in all species and in all types of cardiomyocytes as compared with the amplitude of the fast sodium current, but it contributes significantly to the shape and duration of the action potential. This late component had been shown to increase in several acquired or congenital conditions, including hypoxia, oxidative stress, and heart failure, or due to mutations in SCN5A, which encodes the α-subunit of the sodium channel, as well as in channel-interacting proteins, including multiple β subunits and anchoring proteins. Patients with enhanced late I Na exhibit the type-3 long QT syndrome (LQT3) characterized by high propensity for the life-threatening ventricular arrhythmias, such as Torsade de Pointes (TdP), as well as for atrial fibrillation. There are several distinct mechanisms of arrhythmogenesis due to abnormal late I Na, including abnormal automaticity, early and delayed after depolarization-induced triggered activity, and dramatic increase of ventricular dispersion of repolarization. Many local anesthetic and antiarrhythmic agents have a higher potency to block late I Na as compared with fast I Na. Several novel compounds, including ranolazine, GS-458967, and F15845, appear to be the most selective inhibitors of cardiac late I Na reported to date. Selective inhibition of late I Na is expected to be an effective strategy for correcting these acquired and congenital channelopathies.

Antiarrhythmic effects of simvastatin in canine pulmonary vein sleeve preparations

OBJECTIVES

The purpose of this study was to determine the electrophysiologic effects of simvastatin in canine pulmonary vein (PV) sleeve preparations.

BACKGROUND

Ectopic activity arising from the PV plays a prominent role in the development of atrial fibrillation.

METHODS

Transmembrane action potentials were recorded from canine superfused left superior or inferior PV sleeves using standard microelectrode techniques. Acetylcholine (1 μM), isoproterenol (1 μM), high calcium ([Ca(2+)](o) = 5.4 mM), or a combination was used to induce early afterdepolarizations or delayed afterdepolarizations and triggered activity. Voltage clamp experiments were performed in the left atrium measuring fast and late sodium currents.

RESULTS

Under steady-state conditions, simvastatin (10 nM, n = 9) induced a small increase in action potential duration measured at 85% repolarization and a significant decrease in action potential amplitude, take-off potential, and maximum rate of rise of action potential upstroke at the fastest rates. The V(max) decreased from 175.1 ± 34 V/s to 151.7 ± 28 V/s and from 142 ± 47 V/s to 97.4 ± 39 V/s at basic cycle lengths of 300 and 200 ms, respectively. Simvastatin (10 to 20 nM) eliminated delayed afterdepolarizations and delayed afterdepolarization-induced triggered activity in 7 of 7 PV sleeve preparations and eliminated or reduced late-phase 3 early afterdepolarizations in 6 of 6 PV sleeve preparations. Simvastatin (20 nM) did not affect late or fast sodium currents measured using voltage clamp techniques.

CONCLUSIONS

Our data suggest that in addition to its upstream actions to reduce atrial structural remodeling, simvastatin exerts a direct antiarrhythmic effect by suppressing triggers responsible for the genesis of atrial fibrillation.

A new biomarker--index of cardiac electrophysiological balance (iCEB)--plays an important role in drug-induced cardiac arrhythmias: beyond QT-prolongation and Torsades de Pointes (TdPs)

INTRODUCTION

In the present study, we investigated whether a new biomarker - index of cardiac electrophysiological balance (iCEB=QT/QRS) - could predict drug-induced cardiac arrhythmias (CAs), including ventricular tachycardia/ventricular fibrillation (VT/VF) and Torsades de Pointes (TdPs).

METHODS

The rabbit left ventricular arterially-perfused-wedge was used to investigate whether the simple iCEB measured from the ECG is reflective of the more difficult measurement of λ (effective refractory period×conduction velocity) for predicting CAs induced by a number of drugs.

RESULTS

Dofetilide concentration-dependently increased iCEB and λ, predicting potential risk of drug-induced incidence of early afterdepolarizations (EADs) starting at 0.01μM. Digoxin (1 and 5μM), encainide (5 and 20μM) and propoxyphene (10 and 100μM) markedly reduced both iCEB and λ, predicting their ability to induce non-TdP-like VT/VF. At 10μM, both NS1643 and levcromakalim significantly decreased λ and iCEB, which was preceded with presence of non-TdP-like VT/VF. Isoprenaline (0.05 to 0.5μM) significantly reduced both λ and iCEB, which was associated with a high incidence of non-TdP-like VT/VF in most preparations. Other biomarkers (i.e. transmural dispersion of T-wave and instability of the QT interval) predicted only dofetilide-induced long QT and EADs, but did not predict drug-induced risk of non-TdP-like VT/VF.

DISCUSSION

Our data from 7 reference drugs of known pro-arrhythmic effects suggests that 1) this non-invasive iCEB predicts potential risk of drug-induced CAs beyond long QT and TdP; 2) iCEB is more useful than the current biomarkers (i.e. transmural dispersion and instability) in predicting potential risks for drug-induced non-TdP-like VT/VF.

Proarrhythmic potential of dronedarone: emerging evidence from spontaneous adverse event reporting

STUDY OBJECTIVE

To characterize the frequency and type of cardiac events, including torsade de pointes, associated with dronedarone and its structural analog, amiodarone, outside of the clinical trial setting.

DESIGN

Retrospective analysis.

DATA SOURCE

Spontaneous reports in the United States Food and Drug Administration (FDA) Adverse Event Reporting System (AERS) database generated between July 1, 2009, and June 30, 2011.

MEASUREMENTS AND MAIN RESULTS

All reports of adverse events during the study period were reviewed to identify cardiac events associated with any approved drug in the United States. The type and number of cardiac events associated with dronedarone and amiodarone were determined. Active ingredients were identified using the Drugs@FDA database, and the Medical Dictionary for Regulatory Activities (MedDRA) was used to aggregate related adverse events. To avoid redundant reporting, all statistics were generated in reference to unique case identifiers. Dronedarone was associated with more adverse cardiovascular event reports than amiodarone (810 vs 493 reports) during the study period. Dronedarone was also associated with the most reports of torsade de pointes of any approved drug in the United States (37 reports), followed by amiodarone (29 reports). Reports of ventricular arrhythmias and cardiac arrest (138 vs 113 reports) as well as heart failure (179 vs 126 reports) were more common with dronedarone than amiodarone.

CONCLUSION

Dronedarone was associated with reports of ventricular arrhythmia, cardiac arrest, and torsade de pointes in clinical practice. Whether this observation accounts for the increased risk of fatal arrhythmia observed in a recent prospective trial requires further investigation.

Effect of amiodarone on dispersion of ventricular repolarization in a canine congestive heart failure model

The effects of amiodarone on ventricular electrophysiological parameters, especially the dispersion of ventricular repolarization, were investigated in a canine model of congestive heart failure (CHF). Dogs were randomized to either a control, amiodarone, CHF, or CHF+amiodarone group. Dogs in the CHF and CHF+amiodarone groups underwent 4-5 weeks of rapid ventricular pacing; dogs in the control and amiodarone groups underwent sham operation only. Amiodarone (20 mg/kg per day) was administered orally, beginning on postoperative Day 1, in the treatment groups; ventricular electrophysiological variables were evaluated 4-5 weeks after rapid pacing or sham operation. In CHF dogs, the transmural dispersion ventricular repolarization time (TDVRT) increased significantly. Amiodarone significantly decreased the TDVRT in CHF dogs. The ventricular fibrillation threshold (VFT) decreased in the CHF group. Amiodarone increased the VFT in CHF dogs. The TDVRT increased in CHF dogs, but amiodarone decreased TDVRT and increased VFT in these dogs. These results suggest a beneficial effect of amiodarone on malignant arrhythmias and may provide the basis for its use in CHF patients.

Ranolazine versus amiodarone for prevention of postoperative atrial fibrillation

Postoperative atrial fibrillation (AF) is a major complication of cardiothoracic surgery, leading to significant consequences, including a higher rate of stroke, longer hospital stays and increased costs. Amiodarone is among the most widely used agents for prevention of postoperative AF. Ranolazine, a US FDA-approved antianginal agent, has been shown to effectively, safely prevent and terminate nonpostoperative AF in both experimental and clinical studies. In a recent publication, Miles and colleagues directly compared the efficacy and safety of amiodarone and ranolazine for prevention of postoperative AF in 393 patients. The patients were pretreated with amiodarone and ranolaizne for >1 week and 1 day, respectively, and the treatment continued for 10-14 days after surgery. Following coronary artery bypass grafting (CABG), AF occurred in 26.5% of patients taking amiodarone and in 17.5% of patients taking ranolazine (34% reduction; p < 0.035). No differences in adverse events between the two groups of patients were recorded. The results of this retrospective nonrandomized single-center study indicate that ranolazine may be used to effectively and safely prevent postoperative AF. These results need to be confirmed in a larger randomized study. If confirmed, ranolazine may be a good choice for preventing AF in patients undergoing CABG.

Assessment of repolarization heterogeneity for prediction of mortality in cardiovascular disease: peak to the end of the T wave interval and nondipolar repolarization components

BACKGROUND

In the canine wedge preparation, the interval from the peak to the end of the T wave (TpTe) reflects transwedge heterogeneities. Increase of ventricular dispersion of action potential durations has been repeatedly shown to be arrhythmogenic; thus, prolonged TpTe intervals were assumed to reflect increased risk. However, despite attempted extrapolation to clinical electrocardiograms, the appropriateness of this assumption has not been investigated in a large population. In another animal model, nondipolar components of the descending T-wave limb (TWRd) have been shown to correlate with TpTe interval. Although total T-wave nondipolar components (TWRt), believed to reflect heterogeneities during total repolarization, were shown associated with worse outcome of cardiac patients, this has not been investigated for TWRd.

METHODS AND RESULTS

Male cardiovascular patients (n = 813) had digital 12-lead electrocardiograms recorded between 1984 and 1991 and were followed until 2000. Using commercial and previously validated technology, QT intervals, TpTe intervals, TWRd, and TWRt were calculated, heart rate corrected, and compared between survivors and nonsurvivors. Their predictive power was also compared with established markers of mortality risk. In contrast to former reports, TpTe(c) intervals were significantly shorter in nonsurvivors (98.76 ± 20.63 milliseconds vs 103.14 ± 20.87 milliseconds, P = .016) and not predictive of outcome. Although TWRd(c) was significantly higher in nonsurvivors (0.007% ± 0.02% vs 0.005% ± 0.08%, P = .03), it was also not predictive of outcome. Only increased TWRt(c), increased heart rate, and increased age were predictive of death.

CONCLUSIONS

The findings challenge the concept that prolongation of TpTe corresponds to higher risk of death from any cause in every population. Further investigations are needed to confirm that clinically measured TpTe reflects transmural repolarization heterogeneity in all clinical populations and indeed is a useful risk marker.

Phenotypical manifestations of mutations in the genes encoding subunits of the cardiac voltage-dependent L-type calcium channel

The L-type cardiac calcium channel (LTCC) plays a prominent role in the electric and mechanical function of the heart. Mutations in the LTCC have been associated with a number of inherited cardiac arrhythmia syndromes, including Timothy, Brugada, and early repolarization syndromes. Elucidation of the genetic defects associated with these syndromes has led to a better understanding of molecular and cellular mechanisms and the development of novel therapeutic approaches to dealing with the arrhythmic manifestations. This review provides an overview of the molecular structure and function of the LTCC, the genetic defects in these channels known to contribute to inherited disorders, and the underlying molecular and cellular mechanisms contributing to the development of life-threatening arrhythmias.

Ionic mechanisms for electrical heterogeneity between rabbit Purkinje fiber and ventricular cells

The intrinsic heterogeneity of electrical action potential (AP) properties between Purkinje fibers (PFs) and the ventricular wall, as well as within the wall, plays an important role in ensuring successful excitation of the ventricles. It can also be proarrhythmic due to nonuniform repolarization across the Purkinje-ventricular junction. However, the ionic mechanisms that underlie the marked AP differences between PFs and ventricular cells are not fully characterized. We studied such mechanisms by developing a new family of biophysically detailed AP models for rabbit PF cells and three transmural ventricular cell types. The models were based on and validated against experimental data recorded from rabbit at ionic channel, single cell, and tissue levels. They were then used to determine the functional roles of each individual ionic channel current in modulating the AP heterogeneity at the rabbit Purkinje-ventricular junction, and to identify specific currents responsible for the differential response of PFs and ventricular cells to pharmacological interventions.

Electrophysiologic and antiarrhythmic effects of AZD1305 in canine pulmonary vein sleeves

The objective of this study was to examine the electrophysiologic and antiarrhythmic effects of the new antiarrhythmic agent tert-butyl (2-[7-[2-(4-cyano-2-fluorophenoxy)ethyl]-9-oxa-3,7-diazabicyclo[3.3.1]non3-yl]ethyl)carbamate (AZD1305) in canine pulmonary vein (PV) sleeve preparations isolated from untreated and long-term amiodarone-treated animals. Ectopic activity arising from PV sleeves plays a prominent role in the development of atrial fibrillation (AF). Delayed afterdepolarizations (DADs) and late phase 3 early afterdepolarizations (EADs), originating from the PV have been proposed as potential triggers in initiation of AF. Action potentials were recorded from canine superfused PV sleeves using standard microelectrode techniques. Acetylcholine (1 microM), isoproterenol (1 microM), or their combination was used to induce EADs, DADs, and triggered activity (TA). The effects of AZD1305 (0.1-10 microM) were evaluated in PV sleeve preparations isolated from untreated and amiodarone-treated (40 mg/kg daily for 6 weeks) dogs. AZD1305 (0.1-10 microM, 30 min) significantly prolonged action potential duration and reduced excitability. Abbreviating basic cycle length from 1000 to 300 ms resulted in a decrease of V(max) from 314 +/- 79 to 251 +/- 55 V/s (Delta = -20%) in control and from 177 +/- 53 to 76.5 +/- 33 V/s (Delta = -57%) after AZD1305 (n = 6, p < 0.05). AZD1305 markedly attenuated or suppressed DADs and DAD-induced TA, but not late phase 3 EADs. AZD1305-induced attenuation of excitability, leading to activation failure at much longer cycle lengths, was much more pronounced in PV from amiodarone-treated dogs. Potent effects of AZD1305 to depress excitability, prolong action potential duration, and suppress DAD-induced triggered activity in canine PV sleeve preparations may be effective in suppressing triggers responsible for the genesis of AF and other atrial arrhythmias.

Changes in the calcium current among different transmural regions contributes to action potential heterogeneity in rat heart

To clarify the transmural heterogeneity of action potential (AP) time course, we examined the regulation of L-type Ca(2+) current (I(Ca,L)) by voltage and Ca(2+)-dependent mechanisms. Currents were recorded using patch clamp of single rat subepicardial (EPI) and subendocardial (ENDO) of left ventricular, right ventricular (RV) and septal (SEP) cardiomyocytes. Voltage clamp commands were derived from ENDO and EPI APs or rectangular voltage pulses. During rectangular pulses, peak I(Ca,L) was significantly greater in EPI than in other cells. The inactivation of I(Ca,L) by Ca(2+)-dependent mechanisms (suppressed by ryanodine and BAPTA) was present in all cells but greater in extent in ENDO and SEP cells. Activation and inactivation curves for all regions show subtle differences that are Ca(2+) sensitive, with Ca(2+) inactivation shifting the activation variables negative by approximately 7 mV and inactivation variables positive by 2-7 mV (EPI being least, RV greatest). In AP-clamps, the peak I(Ca,L) was significantly smaller in ENDO than in EPI cells, while the integrated current was significantly larger in ENDO than in EPI cells. The results are discussed with regard to the interplay of AP time course and net Ca(2+) influx.

Multipolar QRST isointegral maps and QT dispersion in old myocardial infarction

Chronic myocardial infarction (CMI) may create, due to structural heterogeneity, abnormal electrophysiological substrates which trigger re-entrant life-threatening ventricular arrhythmias.

METHODS

Electrical instability is assessed using body surface mapping (BSM) [multipolar isointegral QRST maps (mp I(QRST))] and 12-lead ECG (QT dispersion: QTd: the difference between maximal and minimal QT interval). The aim was to find the relation between mp I(QRST) and QTd in CMI patients.

RESULTS

The 32 CMI patients, underwent 12-lead ECG and 64-lead BSM. The 80% (25) of the patients had mp I(QRST) maps. QTd was larger in patients with mp than those with dipolar maps (dp): 170 +/- 20 ms in mp vs 94 +/- 19 ms in dp, respectively. The latter, mp I(QRST) was associated with a decrease of maximum and a stronger minimum.

CONCLUSIONS

Multipolar I(QRST) is associated with a loss of maximum values and increased absolute values of the minimum in CMI patients. I(QRST) and QTd provide similar information in predicting postinfarction arrhythmia risk.

Synergistic electrophysiologic and antiarrhythmic effects of the combination of ranolazine and chronic amiodarone in canine atria

BACKGROUND

Amiodarone and ranolazine have been characterized as inactivated- and activated-state blockers of cardiac sodium channel current (I(Na)), respectively, and shown to cause atrial-selective depression of I(Na)-related parameters. This study tests the hypothesis that their combined actions synergistically depress I(Na)-dependent parameters in atria but not ventricles.

METHODS AND RESULTS

The effects of acute ranolazine (5 to 10 micromol/L) were studied in coronary-perfused right atrial and left ventricular wedge preparations and superfused left atrial pulmonary vein sleeves isolated from chronic amiodarone-treated (40 mg/kg daily for 6 weeks) and untreated dogs. Floating and standard microelectrode techniques were used to record transmembrane action potentials. When studied separately, acute ranolazine and chronic amiodarone caused atrial-predominant depression of I(Na)-dependent parameters. Ranolazine produced a much greater reduction in V(max) and much greater increase in diastolic threshold of excitation and effective refractory period in atrial preparations isolated from amiodarone-treated versus untreated dogs, leading to a marked increase in postrepolarization refractoriness. The drug combination effectively suppressed triggered activity in pulmonary vein sleeves but produced relatively small changes in I(Na)-dependent parameters in the ventricle. Acetylcholine (0.5 micromol/L) and burst pacing induced atrial fibrillation in 100% of control atria, 75% of ranolazine-treated (5 micromol/L) atria, 16% of atria from amiodarone-treated dogs, and in 0% of atria from amiodarone-treated dogs exposed to 5 micromol/L ranolazine.

CONCLUSIONS

The combination of chronic amiodarone and acute ranolazine produces a synergistic use-dependent depression of I(Na)-dependent parameters in isolated canine atria, leading to a potent effect of the drug combination to prevent the induction of atrial fibrillation.

Potent antiarrhythmic effects of chronic amiodarone in canine pulmonary vein sleeve preparations

OBJECTIVES

To examine the effects of chronic amiodarone on the electrophysiology of canine pulmonary vein (PV) sleeve preparations and left ventricular wedge preparation.

BACKGROUND

Amiodarone is commonly used for the treatment of ventricular and supraventricular arrhythmias. Ectopic activity arising from the PV plays a prominent role in the development of atrial fibrillation (AF).

METHODS

Standard microelectrode techniques were used to evaluate the electrophysiological characteristics of superfused PV sleeve (left superior or inferior) and arterially perfused left ventricular (LV) wedge preparations isolated from untreated and chronic amiodarone-treated dogs (amiodarone, 40 mg/kg daily for 6 weeks).

RESULTS

In PV sleeves, chronic amiodarone (n = 6) induced a significant increase in action potential duration at 90% repolarization (APD90) and a significant use-dependent reduction in Vmax leading to 1:1 activation failure at long cycle lengths (basic cycle length of 124 +/- 15 ms in control vs 420 +/- 320 ms after chronic amiodarone [P < 0.01]). Diastolic threshold of excitation increased from 0.3 +/- 0.2 to 1.8 +/- 0.7 mA (P < 0.01). Delayed and late phase 3 early afterdepolarizations and triggered activity could be induced in PV sleeve preparations using acetylcholine (ACh, 1 microM), high calcium ([Ca2+]o = 5.4 mM), isoproterenol (Iso, 1 microM), or their combination in 6 of 6 untreated PV sleeves, but in only 1 of 5 chronic amiodarone-treated PV sleeve preparations. Vmax, conduction velocity, and 1:1 activation failure were much more affected in PV sleeves versus LV wedge preparations isolated from amiodarone-treated animals.

CONCLUSIONS

The results point to potent effects of chronic amiodarone to preferentially suppress arrhythmogenic substrates and triggers arising from the PV sleeves of the dog.

Atrial-selective effects of chronic amiodarone in the management of atrial fibrillation

BACKGROUND

Although amiodarone is one of the most effective pharmacologic agents used in clinical management of atrial fibrillation (AF), little is known about its differential effects in atrial and ventricular myocardium.

OBJECTIVES

This study sought to compare the electrophysiological effects of chronic amiodarone in atria and ventricles.

METHODS

We compared the electrophysiological characteristics of coronary-perfused atrial and ventricular wedge preparations isolated from untreated and chronic amiodarone-treated dogs (amiodarone, 40 mg/kg/day for 6 weeks, n = 12).

RESULTS

Chronic amiodarone prolonged action potential duration (APD(90)) predominantly in atria compared to ventricles and prolonged the effective refractory period (ERP) more than APD(90) in both ventricular and atrial preparations (particularly in the latter) due to the development of postrepolarization refractoriness. Amiodarone reduced dispersion of APD(90) in both atria and ventricles. Although the maximum rate of increase of the action potential upstroke (V(max)) was significantly lower in both atria and ventricles of amiodarone-treated hearts versus untreated controls, the reduction of V(max) was much more pronounced in atria. Amiodarone prolonged P-wave duration more significantly than QRS duration, reflecting greater slowing of conduction in atria versus ventricles. These atrioventricular distinctions were significantly accentuated at faster activation rates. Persistent acetylcholine-mediated AF could be induced in only 1 of 6 atria from amiodarone-treated versus 10 of 10 untreated dogs.

CONCLUSION

Our results indicate that under the conditions studied, chronic amiodarone has potent atrial-predominant effects to depress sodium channel-mediated parameters and that this action of the drug is greatly potentiated by its ability to prolong APD predominantly in the atria, thus contributing to its effectiveness to suppress AF.

Sensitive and reliable proarrhythmia in vivo animal models for predicting drug-induced torsades de pointes in patients with remodelled hearts

As an increasing number of non-cardiac drugs have been reported to cause QT interval prolongation and torsades de pointes (TdP), we extensively studied the utility of atrioventricular (AV) block animals as a model to predict their torsadogenic action in human. The present review highlights such in vivo proarrhythmia models. In the case of the canine model, test substances were administered p.o. at conscious state >4 weeks after the induction of AV block, with subsequent Holter ECG monitoring to evaluate drug effects. Control AV block dogs (no pharmacological treatment) survive for several years without TdP attack. For pharmacologically treated dogs, drugs were identified as high, low or no risk. High-risk drugs induced TdP at 1-3 times the therapeutic dose. Low-risk drugs did not induce TdP at this dose range, but induced it at higher doses. No-risk drugs never induced TdP at any dose tested. Electrophysiological, anatomical histological and biochemical adaptations against persistent bradycardia-induced chronic heart failure were observed in AV block dogs. Recently, we have developed another highly sensitive proarrhythmia model using a chronic AV block cynomolgus monkey, which possesses essentially the same pathophysiological adaptations and drug responses as those demonstrated in the canine model. As a common remodelling process leading to a diminished repolarization reserve may present in patients who experience drug-induced TdP and in the AV block animals, the in vivo proarrhythmia models described in this review may be useful for predicting the risk of pharmacologically induced TdP in humans.

Understanding drug-induced torsades de pointes: a genetic stance

Drugs may produce a variety of arrhythmias, but drug-induced QT prolongation and the risk of the polymorphic ventricular tachycardia torsades de pointes (drug-induced long QT syndrome) has garnered the most attention. The wide array of drugs with potential for QT prolongation, the correspondingly large number of patients exposed to such drugs, the difficulty in predicting an individual's risk, and the potentially fatal outcome, make drug-induced long QT syndrome an important public health problem for clinicians, researchers, drug development programs, and regulatory agencies. This review focuses on the genetic risk factors and mechanisms underlying QT prolongation and proarrhythmia. The post-genomic era hints at an improved understanding (and prediction) of how the gene-environment interaction produces this particular adverse drug response.

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

OBJECTIVES

The goal of this study was to identify markers of torsades de pointes (TdP) in patients with drug-associated long QT syndrome (LQTS).

BACKGROUND

Drug-induced LQTS includes individuals developing marked prolongation of ventricular repolarization on exposure to an offending drug. Under these conditions, TdP develops in some but not all patients.

METHODS

This was a case-control study of 123 adults with drug-associated LQTS. Patients were divided into LQTS only (LQTS; n = 40, QT >500 ms on drug) and LQTS + TdP (TdP; n = 83).

RESULTS

Baseline QT intervals were similar in the 2 groups (381 +/- 38 ms [LQTS] vs. 388 +/- 43 ms [TdP]). Clinical variables associated with risk of TdP included hypokalemia and female gender; by contrast, persistent atrial fibrillation (AF) at the time of drug discontinuation for QT prolongation was protective despite similar heart rates in AF and sinus rhythm (n = 20, 71 +/- 13 beats/min vs. 69 +/- 13 beats/min). Electrocardiographic variables that significantly increased the risk for TdP included absolute and rate-corrected QT intervals (QTc) on drug therapy, the magnitude of QT and QTc interval prolongation, and the change in T(peak) to T(end) (DeltaT(p)-T(e)), a relatively new index of transmural dispersion of repolarization and potential arrhythmogenicity. Multivariable logistic regression analysis revealed that only gender was predictive for TdP, whereas persistent AF at the time of drug discontinuation for QT prolongation (odds ratio 0.14, 95% confidence interval 0.03 to 0.63, p = 0.01) was negatively associated with the arrhythmia.

CONCLUSIONS

This study strongly suggests that despite ongoing rate irregularity, AF reduces the likelihood of developing TdP after the administration of drugs that prolong cardiac repolarization.

Acquired long QT syndrome: Unequal regression of Amiodarone-induced repolarization lengthening

We describe full exposure of an acquired Long QT Syndrome (aLQTS) in an elderly female patient on long-term Amiodarone treatment. We propose that the aLQTS is the result of combined block of the rapid (I(Kr)) and delayed (I(Ks)) components of the outward potassium current (I(K)), translated to the ECG in a (more than expected) prolonged QT interval, an augmented transmural dispersion of repolarization (TDR) and an interrupted T wave. The unequal regression of repolarization lengthening made possible to individualize I(K) current components in the inscription of the interrupted T wave, which argues against the U wave as a separate entity. Silent ion channel gene mutations or polymorphisms and downregulation of beta-adrenergic activation of I(Ks) may underlie the unusual repolarization behaviour. The unequal regression over time of Amiodarone-induced repolarization lengthening could have clinical significance.

In vitro effects of acute amiodarone and dronedarone on epicardial, endocardial, and M cells of the canine ventricle

Amiodarone (AM) is an antiarrhythmic agent widely used in the treatment of ventricular and supraventricular arrhythmias. Dronedarone (DR) is a new compound with a pharmacological profile similar to that of AM, but iodine free. We previously demonstrated that chronic AM treatment reduces transmural dispersion of repolarization (TDR) in the canine heart. We used standard microelectrode technique to evaluate the effects of acute AM (100 microM) and DR (30 microM) on epicardial (EPI), endocardial (ENDO), and M region tissues obtained from the left ventricular wall of the canine heart. Amiodarone (100 microM, 120 min of exposure) produced little change in the action potential duration of ENDO and EPI tissues, but it shortened the action potential of M cells, especially at slow rates, leading to a decrease in TDR. Similar results were observed with DR. Acute AM (100 microM) and DR (30 microM) eliminated d-sotalol-induced early afterdepolarizations (EADs) and triggered activity in 3 of 3 and 2 of 6 M cell preparations, respectively. The reduction of TDR and the elimination of EAD-induced triggered activity differentiates AM and DR from other class III agents. These effects may explain the efficacy and low arrhythmogenicity of acute AM and suggest a potential safe use of DR as an antiarrhythmic agent.

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

Torsade de pointes (TdP) is a life-threatening arrhythmia that develops as a consequence of a reduction in the repolarization reserve of cardiac cells leading to amplification of electrical heterogeneities in the ventricular myocardium as well as to the development of early after depolarization-induced triggered activity. Electrical heterogeneities within the ventricles are due to differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization that contributes to the inscription of the electrocardiographic T wave. A number of non-antiarrhythmic drugs and antiarrhythmic agents with class III actions and/or the various mutations and cardiomyopathies associated with the long QT syndrome reduce net repolarizing current and amplify spatial dispersion of repolarization, thus creating the substrate for re-entry. This results in a prolongation of the QT interval, abnormal T waves, and development of TdP. Agents that prolong the QT interval but do not cause an increase in transmural dispersion of repolarization (TDR) do not induce TdP, suggesting that QT prolongation is not the sole or optimal determinant for arrhythmogenesis. This article reviews recent advances in our understanding of these mechanisms, particularly the role of TDR in the genesis of drug-induced TdP, and examines how these may guide us towards development of safer drugs.

The canine virtual ventricular wall: A platform for dissecting pharmacological effects on propagation and arrhythmogenesis

We have constructed computational models of canine ventricular cells and tissues, ultimately combining detailed tissue architecture and heterogeneous transmural electrophysiology. The heterogeneity is introduced by modifying the Hund-Rudy canine cell model in order to reproduce experimentally reported electrophysiological properties of endocardial, midmyocardial (M) and epicardial cells. These models are validated against experimental data for individual ionic current and action potential characteristics, and their rate dependencies. 1D and 3D heterogeneous virtual tissues are constructed, with detailed tissue architecture (anisotropy and orthotropy, due to fibre orientation and sheet structure) of the left ventricular wall wedge extracted from a diffusion tensor imaging data set. The models are used to study the effects of tissue heterogeneity and class III drugs on transmural propagation and tissue vulnerability to re-entry. We have determined relationships between the transmural dispersion of action potential duration (APD) and the vulnerable window in the 1D virtual ventricular wall, and demonstrated how changes in the transmural heterogeneity, and hence tissue vulnerability, can lead to generation of re-entry in the 3D ventricular wedge. Two class III drugs with opposite qualitative effects on transmural APD heterogeneity are considered: d-sotalol that increases transmural APD dispersion, and amiodarone that decreases it. Simulations with the 1D virtual ventricular wall show that under d-sotalol conditions the vulnerable window is substantially wider compared to amiodarone conditions, primarily in the epicardial region where unidirectional conduction block persists until the adjacent M cells are fully repolarised. Further simulations with the 3D ventricular wedge have shown that ectopic stimulation of the epicardial region results in generation of sustained re-entry under d-sotalol conditions, but not under amiodarone conditions or in control. Again, APD increase in M cells was identified as the major contributor to tissue vulnerability--re-entry was initiated primarily due to ectopic excitation propagating around the unidirectional conduction block in the M cell region. This suggests an electrophysiological mechanism for the anti- and proarrhythmic effects of the class III drugs: the relative safety of amiodarone in comparison to d-sotalol can be explained by relatively low transmural APD dispersion, and hence, a narrow vulnerable window and low probability of re-entry in the tissue.

Augmentation of late sodium current unmasks the proarrhythmic effects of amiodarone

AIM

Clinical use of amiodarone is associated with occasional development of torsade de pointes (TdP). However, preclinical models have failed to demonstrate the proarrhythmic potential of amiodarone. The objective of this study was to reveal and explain the pro- and anti-arrhythmic effects of acute exposure to amiodarone in an animal model.

METHODS AND RESULTS

Endo- and epicardial monophasic action potentials (MAPs) and 12-lead electrocardiogram were recorded in female rabbit isolated hearts. Ion channel currents were measured in human embryonic kidney cells expressing SCN5A Na+ and HERG K+ channels. Acute amiodarone alone caused an insignificant increase in duration of MAP (MAPD90) without causing TdP. In the presence of 3 nM sea anemone toxin (ATX-II), amiodarone (1-30 nM) prolonged MAPD90 from 217 +/- 5 to 250 +/- 8 ms (n = 16, P < 0.01), increased transmural dispersion of repolarization (TDR) from 59 +/- 9 to 70 +/- 10 ms and beat-to-beat variability (BVR) of MAPD(90) from 0.75 +/- 0.03 to 1.06 +/- 0.13 ms (P < 0.05). At 30-300 nM, amiodarone induced TdP in 16 out of 17 hearts. A further increase of amiodarone concentration to 1-10 microM abbreviated MAPD(90) to 211 +/- 9 ms, decreased BVR to 0.5 +/- 0.01 ms, decreased TDR (n = 7, P < 0.05), and suppressed TdP. Amiodarone inhibited HERG K+ and late Na+ currents with IC50s of 0.8 +/- 0.1 and 3.0 +/- 0.9 microM, respectively.

CONCLUSION

In hearts in which late INa is augmented to mimic congenital or acquired pathological conditions, amiodarone has a concentration-dependent biphasic effect to induce and then suppress arrhythmic activity, secondary to inhibition of HERG K+ and late Na+ currents. This is the first preclinical model demonstrating the potential for amiodarone to induce TdP.

Relevance of anaesthesia for dofetilide-induced torsades de pointes in alpha1-adrenoceptor-stimulated rabbits

BACKGROUND AND PURPOSE

No information is available concerning the effects of anaesthetics in the most frequently used in vivo pro-arrhythmia model. Accordingly, in this study we examined the effect of pentobarbital, propofol or alpha-chloralose anaesthesia on the pro-arrhythmic activity of the class III anti-arrhythmic dofetilide in alpha(1)-adrenoceptor-stimulated rabbits.

EXPERIMENTAL APPROACH

Rabbits anaesthetized intravenously with pentobarbital, propofol or alpha-chloralose were infused simultaneously with the alpha(1)-adrenoceptor agonist phenylephrine (15 microg kg(-1) min(-1), i.v.) and dofetilide (0.04 mg kg(-1) min(-1), i.v.). The electrocardiographic QT interval, the T (peak)-T (end) interval and certain QT variability parameters were measured. The heart rate variability and the baroreflex sensitivity were utilized to assess the vagal nerve activity. The spectral power of the systolic arterial pressure was calculated in the frequency range 0.15-0.5 Hz to assess the sympathetic activity.

KEY RESULTS

Pentobarbital considerably reduced, whereas propofol did not significantly affect the incidence of dofetilide-induced torsades de pointes (TdP) as compared with the results with alpha-chloralose (40% (P=0.011) and 70% (P=0.211) vs 100%, respectively). In additional experiments, neither doubling of the rate of the dofetilide infusion nor tripling of the rate of phenylephrine infusion elevated the incidence of TdP to the level seen with alpha-chloralose. None of the repolarization-related parameters predicted TdP. The indices of the parasympathetic and sympathetic activity were significantly depressed in the alpha-chloralose and propofol anaesthesia groups.

CONCLUSIONS AND IMPLICATIONS

In rabbits, anaesthetics may affect drug-induced TdP genesis differently, which must be considered when results of different studies are compared.

Dronedarone for maintenance of sinus rhythm in atrial fibrillation or flutter

BACKGROUND

Amiodarone is effective in maintaining sinus rhythm in atrial fibrillation but is associated with potentially serious toxic effects. Dronedarone is a new antiarrhythmic agent pharmacologically related to amiodarone but developed to reduce the risk of side effects.

METHODS

In two identical multicenter, double-blind, randomized trials, one conducted in Europe (ClinicalTrials.gov number, NCT00259428 [ClinicalTrials.gov] ) and one conducted in the United States, Canada, Australia, South Africa, and Argentina (termed the non-European trial, NCT00259376 [ClinicalTrials.gov] ), we evaluated the efficacy of dronedarone, with 828 patients receiving 400 mg of the drug twice daily and 409 patients receiving placebo. Rhythm was monitored transtelephonically on days 2, 3, and 5; at 3, 5, 7, and 10 months; during recurrence of arrhythmia; and at nine scheduled visits during a 12-month period. The primary end point was the time to the first recurrence of atrial fibrillation or flutter.

RESULTS

In the European trial, the median times to the recurrence of arrhythmia were 41 days in the placebo group and 96 days in the dronedarone group (P=0.01). The corresponding durations in the non-European trial were 59 and 158 days (P=0.002). At the recurrence of arrhythmia in the European trial, the mean (+/-SD) ventricular rate was 117.5+/-29.1 beats per minute in the placebo group and 102.3+/-24.7 beats per minute in the dronedarone group (P<0.001); the corresponding rates in the non-European trial were 116.6+/-31.9 and 104.6+/-27.1 beats per minute (P<0.001). Rates of pulmonary toxic effects and of thyroid and liver dysfunction were not significantly increased in the dronedarone group.

CONCLUSIONS

Dronedarone was significantly more effective than placebo in maintaining sinus rhythm and in reducing the ventricular rate during recurrence of arrhythmia.

Monotherapy versus Combination Therapy with Class III Antiarrhythmic Agents to Attenuate Transmural Dispersion of Repolarization: A Potential Risk Factor for Torsade de Pointes

Class III antiarrhythmic agents are used for conversion to and maintenance of sinus rhythm from arrhythmias of atrial or ventricular origin. Monotherapy can be limited by adverse events or recurrent arrhythmias. Sotalol, dofetilide, and ibutilide may induce torsade de pointes in 2-8% of patients, whereas amiodarone induces torsade de pointes in less than 1%. We reviewed the literature regarding the possible combination of class III antiarrhythmics and risk for inducing torsade de pointes. Animal studies using amiodarone plus sotalol or d-sotalol suggest that these drug combinations prolong the QTc interval but do not induce torsade de pointes. Similar data extracted from human studies of ibutilide in patients also receiving amiodarone or sotalol showed greater efficacy with combination therapy than with monotherapy, without increased torsade de pointes induction. Reduced transmural dispersion of repolarization with amiodarone and sotalol combination therapy may serve as a mechanism for reducing the risk of torsade de pointes compared with sotalol monotherapy.

Heterogeneity and cardiac arrhythmias: an overview

This lecture examines the hypothesis that amplification of spatial dispersion of repolarization in the form of transmural dispersion of repolarization (TDR) underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies, including the long QT, short QT, and Brugada syndromes as well as catecholaminergic polymorphic ventricular tachycardia. In the long QT syndrome, amplification of TDR often is secondary to preferential prolongation of the action potential duration of M cells, whereas in Brugada syndrome, it is thought to be due to selective abbreviation of the action potential duration of right ventricular epicardium. In the short QT syndrome, preferential abbreviation of action potential duration of either endocardium or epicardium appears to be responsible for amplification of TDR. In catecholaminergic polymorphic ventricular tachycardia, reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. Thus, the long QT, short QT, Brugada, and catecholaminergic ventricular tachycardia syndromes are pathologies with very different phenotypes and etiologies. However, these syndromes share a common final pathway in their predisposition to sudden cardiac death.