Chronic amiodarone reduces transmural dispersion of repolarization in the canine heart

Role of spatial dispersion of repolarization in inherited and acquired sudden cardiac death syndromes

This review examines the role of spatial electrical heterogeneity within the ventricular myocardium on the function of the heart in health and disease. The cellular basis for transmural dispersion of repolarization (TDR) is reviewed, and the hypothesis that amplification of spatial dispersion of repolarization underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies is evaluated. The role of TDR in long QT, short QT, and Brugada syndromes, as well as catecholaminergic polymorphic ventricular tachycardia (VT), is critically examined. In long QT syndrome, amplification of TDR is often secondary to preferential prolongation of the action potential duration (APD) of M cells; in Brugada syndrome, however, it is thought to be due to selective abbreviation of the APD of the right ventricular epicardium. Preferential abbreviation of APD of the endocardium or epicardium appears to be responsible for the amplification of TDR in short QT syndrome. In catecholaminergic polymorphic VT, reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. In conclusion, long QT, short QT, Brugada, and catecholaminergic polymorphic VT syndromes are pathologies with very different phenotypes and etiologies, but they share a common final pathway in causing sudden cardiac death.

The virtual ventricular wall: a tool for exploring cardiac propagation and arrhythmogenesis

Methods for the experimental and clinical investigation of cardiac arrhythmias are limited to inferring propagation within the myocardium, from surface measurements, or from electrodes at a few sites within the cardiac wall. Biophysically and anatomically detailed computational models of cardiac tissues offer a powerful way for studying the electrical propagation processes and arrhythmias within the virtual heart. We use virtual tissues to study and visualise the effects of patho- and physiological conditions, and pharmacological interventions on transmural propagation in the virtual ventricular walls. Class III drug actions are quantitatively explained by changes induced in the transmural dispersion of action potential duration. We illustrate the automated construction of a virtual anisotropic ventricle from Diffusion Tensor MRI for individual hearts, and use it to explore mechanisms leading to ventricular fibrillation. The virtual ventricular wall provides an effective tool for exploring, evaluating and visualising processes during the initiation and maintenance of ventricular arrhythmias.

Amiodarone: A multifaceted antiarrhythmic drug

Synthesized as an antianginal compound 40 years ago, amiodarone has emerged as a uniquely effective antiarrhythmic compound in recent years. It has numerous properties, the most prominent being the ability to lengthen repolarization in the atria and ventricles associated with bradycardia without the significant potential for torsades de pointes. Amiodarone effectively controls a wide spectrum of atrial and ventricular antiarrhythmic disorders, but its limiting side effects, such as thyroid dysfunction, pulmonary fibrosis, and dermatologic changes, may limit its long-term use in some patients. What aspects of the multiplicity of the properties of amiodarone are relevant to its unusual efficacy is not known. Deiodination and other structural changes in the amiodarone molecule have has led to a the loss of thyroid and pulmonary effects in the resulting derivative, dronedarone, which is in advanced clinical development.

Role of transmural dispersion of repolarization in the genesis of drug-induced torsades de pointes

Torsades de pointes (TdP) is a potentially lethal arrhythmia that develops as a consequence of amplification of electrical heterogeneities intrinsic to the ventricular myocardium. These heterogeneities exist because of 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 responsible for inscription of the ECG T wave. 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 the intrinsic spatial dispersion of repolarization, thus creating the substrate for the development of reentry. The result is prolongation of the QT interval, abnormal T waves, and development of polymorphic reentrant ventricular tachycardia displaying characteristics of TdP. Prolongation of the QT interval apparently is not the sole determinant of a drug's potential to cause TdP. Agents that do not increase transmural dispersion of repolarization have little or no potential to induce TdP despite any ability to prolong the QT interval. In addition, drugs such as amiodarone and sodium pentobarbital can cause large QT prolongations but, by reducing transmural dispersion of repolarization, may reduce the likelihood of TdP. Arterially perfused wedge preparations of canine left ventricle can be used to explore the role of transmural dispersion of repolarization in the genesis of TdP. The purpose of this article is to review recent advances that have improved our understanding of these mechanisms, particularly the role of transmural dispersion of repolarization, in the genesis of drug-induced TdP and to examine how these advances can guide us toward the development of safer and more effective drugs.

Effects of amiodarone on electrical and structural remodeling induced in a canine rapid pacing-induced persistent atrial fibrillation model

Structural in addition to electrical remodeling may be induced by persistent atrial fibrillation per se and make atrial fibrillation refractory to antiarrhythmic drug therapy. Matrix metalloproteinases (MMPs) contribute to structural remodeling in the interstitial space. Amiodarone is effective in treating persistent atrial fibrillation compared with other antiarrhythmic drugs. In mongrel dogs, right atrial pacing at 540 beats/min (bpm) was performed along with ventricular pacing at 100 bpm for 6 weeks after atrioventricular node ablation. Right atrial pacing at 400 bpm was continued for 4 weeks with (n=5) or without (n=5) oral amiodarone (30 mg/kg/day). In sham dogs, only ventricular pacing was done with (n=4) or without (n=6) amiodarone. In atrial pacing without amiodarone group, electrical remodeling characterized by monophasic action potential duration shortening, loss of action potential duration-rate adaptation and depressed conduction velocity and structural remodeling characterized by slightly but significantly increased interstitial fibrosis and enhanced MMP-2 activity compared with sham group were observed, and sustained atrial fibrillation was easily induced. In atrial pacing with amiodarone group, both electrical and structural remodeling were reversed and sustained atrial fibrillation was not induced. In sham group with amiodarone, action potential duration prolongation and depressed conduction velocity compared with sham without amiodarone were observed, but either increased fibrosis or enhanced MMP-2 activity was not observed. Not only electrical but structural remodeling were induced in a canine persistent atrial fibrillation model. Amiodarone reversed both of them, which may be related to its high efficacy in preventing recurrence of persistent atrial fibrillation.

Nifekalant Hydrochloride Administration During Cardiopulmonary Resuscitation Improves the Transmural Dispersion of Myocardial Repolarization Experimental Study in a Canine Model of Cardiopulmonary Arrest

BACKGROUND

Because nifekalant hydrochloride (NIF) displayed a superior defibrillating effect on ventricular tachycardia/fibrillation (VT/VF) in cardiopulmonary arrest (CPA) patients, despite some QT prolongation, its effect on transmural dispersion of repolarization (TDR) in the left ventricle (LV) in an animal model of CPA was investigated.

METHODS AND RESULTS

Eight beagle dogs were created with a myocardial infarction under anesthesia, and then VT/VF induction by continuous stimulation and cardiopulmonary resuscitation (CPR) were repeated. NIF (0.3 mg/kg) was administered under acidotic conditions (pH 7.26). The QTc interval measured by Y-lead ECG showed no significant prolongation before and after NIF. The activation recovery interval (ARI) measured by 64-lead LV surface mapping showed minimum ARI prolongation (40%) by NIF without maximum ARI prolongation, and as a result the ARI dispersion decreased by 67%. The repolarization time (RPT) with the plunge electrode showed 13-19% prolongation in the subendocardium and subepicardium with CPR, but NIF prolonged the RPT in the middle layer alone (17%), and as a result Plunge-TDR decreased by 82% (n=8, p<0.05).

CONCLUSIONS

Administration of NIF during CPR decreased the TDR by RPT prolongation selectively in the middle layer. Because the subendocardial and subepicardial RPTs after CPR were already prolonged before NIF administration, it may have been the reason why the QT-prolonging effect of NIF was not reflected in the body surface ECG.

Efficacy and Safety of Ibutilide for Cardioversion of Atrial Flutter and Fibrillation in Patients Receiving Amiodarone or Propafenone

AIM

The effectiveness and safety of ibutilide (IB) use in patients receiving amiodarone or propafenone for atrial flutter (AFL) and atrial fibrillation (AF) were compared to IB alone.

METHODS AND RESULTS

In 104 consecutive patients with AF (65%) or AFL (35%), receiving amiodarone (n = 46), propafenone (n = 30), or no specific antiarrhythmic drug (n = 28), IB was given for cardioversion. Fifteen patients in amiodarone group were loaded with 1.2 g intravenously before IB administration. The mean duration of arrhythmia episode was 23 +/- 65 days, while 85% of patients had structural heart disease. The left ventricle ejection fraction was 57 +/- 10% and the left atrium size was 4.2 +/- 0.6 cm. The conversion efficacy did not differ among groups (62% for amiodarone vs 55% for propafenone vs 64% for IB alone). The QTc intervals were significantly prolonged, at 10 minutes and 30 minutes after IB administration, in amiodarone group (from 449 +/- 88 to 496 +/- 92 ms, 508 +/- 52 ms; P = 0.001) and in the group where IB was used alone (from 434 +/- 45 to 517 +/- 74 ms, 492 +/- 65 ms; P < 0.001), while it remained unchanged in propafenone group (from 464 +/- 52 to 471 +/- 80 ms, 489 +/- 93 ms; P = 0.536). The only predictor of conversion was the presence of AFL (P = 0.009). Five patients developed ventricular tachycardias after IB administration (two in propafenone, one in amiodarone, and two in IB group).

CONCLUSIONS

The use of IB in patients receiving amiodarone or propafenone for AFL or AF is equally effective and safe as the use of IB alone. The presence of AFL is the stronger predictor factor for cardioversion.

The influence of extracellular acidosis on the effect of IKr blockers

BACKGROUND

Myocardial infarction causes the acidification of the cellular environment and the resultant acidosis maybe arrhythmogenic. The effect of acidosis on the action of antiarrhythmic drugs, an important issue in the antiarrhythmic drug therapy after myocardial infarction, remains to be studied.

METHODS

To evaluate the effect of acidosis on rectifier potassium current (Ikr) blockers, the human ether-a-go-go-related gene (HERG), which encodes IKr, was expressed in Xenopus laevis oocytes. The two electrodes voltage clamp technique was used and the experiments were performed at room temperature.

RESULTS

Quinidine (10 microM) inhibited HERG tail current by 37% +/- 5% at pH7.4. The block decreased to 5% +/- 2% with extracellular pH at 6.2. Dofetilide (0.3 microM) inhibited HERG tail current by 34% +/- 3% and 1% +/- 2% at extracellular pH 7.4 and 6.2, respectively. Azimilide (10 microM) inhibited HERG tail current by 59% +/- 3% and 17% +/- 3% at extracellular pH 7.4 and 6.2. There were significant differences in the HERG inhibition by quinidine, dofetilide, and azimilide between pH 7.4 and pH 6.2 (P < .01). The drug concentration blocking 50% of current (IC50) was 5.8 +/- 0.3 microM for azimilide, 9.9 +/- 1.0 microM for quinidine, and 0.5 +/- 0.02 microM for dofetilide at pH 7.4. When extracellular pH was decreased from 7.4 to 6.2, the IC50 increased to 95.5 +/- 11.3 microM for azimilide, 203.2 +/- 15.7 microM for quinidine, and 12.6 +/- 1.2 microM for dofetilide. Unlike quinidine, dofetilide, and azimilide, there was no significant difference in the percentage of current block by amiodarone between pH 6.2 and 7.4. For amiodarone, the IC50 was 38.3 +/- 8.5 microM at pH 7.4 and 27.3 +/- 1.6 microM at pH 6.2.

CONCLUSION

Our data show that the Ikr blocking effect of azimilide, dofetilide, and quinidine was attenuated at acid pH, whereas this was not the case for amiodarone. These observations may explain the efficacy of amiodarone in reducing arrhythmic death in patients after a myocardial infarction compared with other IKr blockers.

Electrophysiologic properties and antiarrhythmic actions of a novel antianginal agent

Ranolazine is a novel antianginal agent capable of producing anti-ischemic effects at plasma concentrations of 2 to 6 microM without a significant reduction of heart rate or blood pressure. This review summarizes the electrophysiologic properties of ranolazine. Ranolazine significantly blocks I(Kr) (IC(50) = 12 microM), late I(Na), late I(Ca), peak I(Ca), I(Na-Ca) (IC(50) = 5.9, 50, 296, and 91 microM, respectively) and I(Ks) (17% at 30 microM), but causes little or no inhibition of I(to) or I(K1). In left ventricular tissue and wedge preparations, ranolazine produces a concentration-dependent prolongation of action potential duration (APD) in epicardium, but abbreviation of APD of M cells, leading to either no change or a reduction in transmural dispersion of repolarization (TDR). The result is a modest prolongation of the QT interval. Prolongation of APD and QT by ranolazine is fundamentally different from that of other drugs that block I(Kr) and induce torsade de pointes in that APD prolongation is rate-independent (ie, does not display reverse rate-dependent prolongation of APD) and is not associated with early after depolarizations, triggered activity, increased spatial dispersion of repolarization, or polymorphic ventricular tachycardia. Torsade de pointes arrhythmias were not observed spontaneously nor could they be induced with programmed electrical stimulation in the presence of ranolazine at concentrations as high as 100 microM. Indeed, ranolazine was found to possess significant antiarrhythmic activity, acting to suppress the arrhythmogenic effects of other QT-prolonging drugs. Ranolazine produces ion channel effects similar to those observed after chronic exposure to amiodarone (reduced late I(Na), I(Kr), I(Ks), and I(Ca)). Ranolazine's actions to reduce TDR and suppress early after depolarization suggest that in addition to its anti-anginal actions, the drug possesses antiarrhythmic activity.

Arrhythmogenic mechanisms of QT prolonging drugs: is QT prolongation really the problem?

Torsade de Pointes (TdP) is an atypical ventricular arrhythmias associated with the acquired and congenital forms of the long QT syndrome. The substrate for the arrhythmia develops as a consequence of the amplification of electrical heterogeneities intrinsic to the ventricular myocardium. These heterogeneities exist because of 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 responsible for the inscription of the electrocardiographic T wave. A wide variety of drugs are capable of reducing net repolarizing current and thus amplifying the intrinsic spatial dispersion of repolarization, so as to create the substrate for the development of re-entry. The result is a prolongation of the QT interval, abnormal T waves, and development of polymorphic re-entrant ventricular tachycardia displaying characteristics of TdP. Recent studies demonstrate that prolongation of the QT interval is not the sole determinant of the potential of a drug to cause TdP; agents that do not increase transmural dispersion of repolarization have little or no potential to induce the arrhythmia despite their ability to prolong the QT interval. Moreover, drugs such as sodium pentobarbital, which reduce transmural dispersion of repolarization, can diminish the likelihood of TdP, despite their ability to prolong the QT interval.

Comparison of the in vitro electrophysiologic and proarrhythmic effects of amiodarone and sotalol in a rabbit model of acute atrioventricular block

The mechanisms for the different proarrhythmic potential of antiarrhythmic drugs in the presence of comparable QT prolongation are not completely understood. The reasons for the lower proarrhythmic potential of amiodarone as compared with other class-III antiarrhythmic drugs such as sotalol, a fact that has been well established for years, is insufficiently known. Therefore, the aim of our study was to assess the different electrophysiologic effects of amiodarone and sotalol in a previously developed experimental model of proarrhythmia. In eight male rabbits, amiodarone (280-340 mg/d) was fed over a period of six weeks. Hearts were excised and retrogradely perfused. Up to eight simultaneous epi- and endocardial monophasic action potentials (MAP) were recorded. Results were compared with sotalol-treated (10-50-100 microM) hearts (n = 13). Amiodarone and sotalol (50 microM and 100 microM) led to a significant increase in QT interval (mean increase: amiodarone: 31 +/- 6 ms; sotalol: 41 +/- 4 ms and 61 +/- 9 ms) and MAP-duration (mean increase-MAP90: amiodarone: 20 +/- 5 ms; sotalol: 17 +/- 5 ms and 25 +/- 8 ms) (P < 0.01). In bradycardic (AV-blocked) hearts, MAP-recordings demonstrated reverse-use dependence and a significant increase in dispersion of repolarization (MAP90) in the presence of sotalol (P < 0.01), but not in amiodarone-treated hearts (10%; p = ns). Sotalol led to early afterdepolarizations (EAD) and torsade de pointes (TdP) after lowering of potassium concentration (6 of 13 hearts). In amiodarone-treated, hypokalemic hearts, no EAD or TdP occurred. Sotalol changed the MAP configuration to a triangular pattern (ratio-MAP90/50: 1.52 as compared with 1.36 at baseline) whereas amiodarone caused a rectangular pattern of MAP prolongation (ratio-MAP90/50: 1.36). In conclusion, these results show no direct correlation between the occurrence of TdP and the degree of QT prolongation. Several factors including reverse-use dependence, dispersion of repolarization, and the propensity to induce early afterdepolarizations but also differences in the action potential configuration may help to understand proarrhythmic side effects of drugs.

Drug-induced QT dispersion: does it predict the risk of torsade de pointes?

Drug-induced delay in ventricular repolarization and proarrhythmias have attracted considerable regulatory attention. The measure of delayed ventricular repolarization most frequently used clinically is the ability of the new chemical entity (NCE) to prolong the QTc interval on surface electrocardiogram. Before they can be approved, new chemical entities with systemic bioavailability require characterization for their potential to prolong the QTc interval. Inevitably, QTc interval prolongation has come to be recognized as a surrogate marker of the risk of torsade de pointes (TdP)--a unique form of potentially fatal polymorphic ventricular tachycardia. Although it is the best and the simplest clinical measure that is available at present, QTc interval is not a reliable surrogate of TdP. Intramyocardial dispersion of repolarization appears to play a more important role both in electrical stability of the ventricles and in arrhythmogenesis. The potential importance of myocardial dispersion of refractoriness in arrhythmogenesis has led to a number of attempts to assess it from the surface electrocardiogram. This review summarizes the evidence for and against the predictive value of one of these attempts-measurement of the so-called QT dispersion. Although the concept of QT dispersion is the best known and most widely investigated, it has also proved to be the least successful in predicting the risks of drug-induced TdP.

Individual Patterns of Dynamic QT/RR Relationship in Survivors of Acute Myocardial Infarction and Their Relationship to Antiarrhythmic Efficacy of Amiodarone

INTRODUCTION

Amiodarone is an effective antiarrhythmic drug, but it has serious side effects and conducted trials did not support its prophylactic use in survivors of acute myocardial infarction. It is possible that the prophylactic use of the drug has not been tested effectively. To optimize therapy outcome, markers of drug efficacy might be developed to identify patients who, although at arrhythmic risk, would not benefit from amiodarone treatment. We investigated descriptors of QT/RR relationship for their potential value in predicting inefficient amiodarone treatment.

METHODS AND RESULTS

The study used 866 Holter recordings (462 amiodarone, 404 placebo) obtained 1 month after randomization in the European Myocardial Infarct Amiodarone Trial (EMIAT). A commercial Holter system was used to measure RR and QT intervals. Subject-specific descriptors of QT/RR relationship were calculated. Comparison was performed in amiodarone- and placebo-treated patients, distinguishing patients who did and did not suffer from arrhythmic death. QT/RR relationship and individually corrected QTc interval differed significantly, not only between amiodarone- and placebo-treated postmyocardial infarction patients but also between patients with and without arrhythmic death on amiodarone (QTc with vs without arrhythmic death 426.30 +/- 33.93 ms vs 444.23 +/- 36.65 ms, P = 6.5 x 10(-3)). In a multivariate analysis, reduced optimum regression residuum (14.33 +/- 7.08 vs 20.11 +/- 9.39, P = 4.4 x 10(-3)) and flatter slope (0.44 +/- 0.19 vs 0.55 +/- 0.24, P = 4.0 x 10(-2)) of the QT/RR relationship independently predicted arrhythmic death during follow-up.

CONCLUSION

Chronic amiodarone treatment markedly affects the QT/RR relationship. The lack of treatment-related QT/RR changes predicts arrhythmic death. Descriptors of complexity of QT/RR relation seem to be potent markers of treatment efficiency.

Electrophysiological effects of ranolazine, a novel antianginal agent with antiarrhythmic properties

BACKGROUND

Ranolazine is a novel antianginal agent capable of producing antiischemic effects at plasma concentrations of 2 to 6 micromol/L without reducing heart rate or blood pressure. The present study examines its electrophysiological effects in isolated canine ventricular myocytes, tissues, and arterially perfused left ventricular wedge preparations.

METHODS AND RESULTS

Transmembrane action potentials (APs) from epicardial and midmyocardial (M) regions and a pseudo-ECG were recorded simultaneously from wedge preparations. APs were also recorded from epicardial and M tissues. Whole-cell currents were recorded from epicardial and M myocytes. Ranolazine inhibited I(Kr) (IC50=11.5 micromol/L), late I(Na), late I(Ca), peak I(Ca), and I(Na-Ca) (IC50=5.9, 50, 296, and 91 micromol/L, respectively) and I(Ks) (17% at 30 micromol/L), but caused little or no inhibition of I(to) or I(K1). In tissues and wedge preparations, ranolazine produced a concentration-dependent prolongation of AP duration of epicardial but abbreviation of that of M cells, leading to reduction or no change in transmural dispersion of repolarization (TDR). At [K+]o=4 mmol/L, 10 micromol/L ranolazine prolonged QT interval by 20 ms but did not increase TDR. Extrasystolic activity and spontaneous torsade de pointes (TdP) were never observed, and stimulation-induced TdP could not be induced at any concentration of ranolazine, either in normal or low [K+]o. Ranolazine (5 to 20 micromol/L) suppressed early afterdepolarizations (EADs) and reduced the increase in TDR induced by the selective I(Kr) blocker d-sotalol.

CONCLUSIONS

Ranolazine produces ion channel effects similar to those observed after chronic amiodarone (reduced I(Kr), I(Ks), late I(Na), and I(Ca)). The actions of ranolazine to suppress EADs and reduce TDR suggest that, in addition to its antianginal actions, the drug may possess antiarrhythmic activity.

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

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

Drug-induced torsades de pointes and implications for drug development

Torsades de pointes is a potentially lethal arrhythmia that occasionally appears as an adverse effect of pharmacotherapy. Recently developed understanding of the underlying electrophysiology allows better estimation of the drug-induced risks and explains the failures of older approaches through the surface ECG. This article expresses a consensus reached by an independent academic task force on the physiologic understanding of drug-induced repolarization changes, their preclinical and clinical evaluation, and the risk-to-benefit interpretation of drug-induced torsades de pointes. The consensus of the task force includes suggestions on how to evaluate the risk of torsades within drug development programs. Individual sections of the text discuss the techniques and limitations of methods directed at drug-related ion channel phenomena, investigations aimed at action potentials changes, preclinical studies of phenomena seen only in the whole (or nearly whole) heart, and interpretation of human ECGs obtained in clinical studies. The final section of the text discusses drug-induced torsades within the larger evaluation of drug-related risks and benefits.

KB130015, a new amiodarone derivative with multiple effects on cardiac ion channels

KB130015 (KB015), a new drug structurally related to amiodarone, has been proposed to have antiarrhythmic properties. In contrast to amiodarone, KB015 markedly slows the kinetics of inactivation of Na(+) channels by enhancing concentration-dependently (K(0.5) asymptotically equal to 2 microM) a slow-inactivating I(Na) component (tau(slow) asymptotically equal to 50 ms) at the expense of the normal, fast-inactivating component (tau(fast) asymptotically equal to 2 to 3 ms). However, like amiodarone, KB015 slows the recovery from inactivation and causes a shift (K(0.5) asymptotically equal to 6.9 microM) of the steady-state voltage-dependent inactivation to more negative potentials. Despite prolonging the opening of Na(+) channels KB015 does not lengthen but often shortens the action potential duration (APD) in pig myocytes or in multicellular preparations. Only short APDs in mouse are markedly prolonged by KB015, which frequently induces early afterdepolarizations. KB015 has also an effect on other ion channels. It decreases the amplitude of the L-type Ca(2+) current (I(Ca-L)) without changing its time course, and it inhibits G-protein gated and ATP-gated K(+) channels. Both the receptor-activated I(K(ACh)) (induced in atrial myocytes by either ACh, adenosine or sphingosylphosphorylcholine) and the receptor-independent (GTPgammaS-induced or background) I(K(ACh)) are concentration-dependently (K(0.5) asymptotically equal to 0.6 - 0.9 microM) inhibited by KB015. I(K(ATP)), induced in atrial myocytes during metabolic inhibition with 2,4-dinitrophenol (DNP), is equally suppressed. However, KB015 has no effect on I(K1) or on I(to). Consistent with the effects in K(+) currents, KB015 does not depolarize the resting potential but antagonizes the APD shortening by muscarinic receptor activation or by DNP. Intracellular cell dialysis with KB015 has marginal or no effect on Na(+) or K(+) channels and does not prevent the effect of extracellularly applied drug, suggesting that KB015 interacts directly with channels at sites more easily accessible from the extracellular than the intracellular side of the membrane. At high concentrations KB015 exerts a positive inotropic action. It also interacts with thyroid hormone nuclear receptors. Its toxic effects remain largely unexplored, but it is well tolerated during chronic administration.

Effect of amiodarone on the descending limb of the T wave

Comparing patients treated after myocardial infarction with amiodarone or with placebo, we found a significant rate-dependent prolongation of TpTe interval in patients who received amiodarone. Patients who had arrhythmic death had significantly longer TpTe intervals than others on placebo but not on amiodarone. Assuming that TpTe reflects transmural repolarization heterogeneity, our findings suggest that heterogeneity and arrhythmic risk are increased by amiodarone. This contradicts the finding of decreased transmural repolarization heterogeneity by amiodarone and the appreciated antiarrhythmic efficacy of this drug.

Cisapride-induced transmural dispersion of repolarization and torsade de pointes in the canine left ventricular wedge preparation during epicardial stimulation

BACKGROUND

Cisapride, a gastrointestinal prokinetic agent, was recently withdrawn from the market because of its propensity to induce torsade de pointes (TdP) arrhythmias. The present study examines the electrophysiological actions of cisapride in the isolated arterially perfused canine left ventricular wedge preparation.

METHODS AND RESULTS

Transmembrane action potentials from epicardial and M regions and a pseudo-ECG were simultaneously recorded. Cisapride (0.1 to 5 micromol/L) was added to the coronary perfusate. Cisapride prolonged the QT interval and increased transmural dispersion of repolarization (TDR) at relatively low but not at high concentrations. TdP could be induced with programmed electrical stimulation only at a low concentration of drug (0.2 micromol/L), when TDR was maximally prolonged. Moreover, TdP could only be induced during epicardial (but not endocardial) activation of the wedge, which was found to augment TDR. At higher concentrations of cisapride, QT was further prolonged, TDR was diminished, and TdP could no longer be induced. Tpeak-Tend interval and Tpeak-Tend area provided reasonable electrocardiographic indices of TDR.

CONCLUSIONS

Our data (1) demonstrate a biphasic concentration/response relationship for the effect of cisapride to induce long-QT syndrome and TdP, (2) show the value of the left ventricular wedge preparation in identifying drugs that pose an arrhythmic risk, (3) support the hypothesis that risk for development of TdP is related to the increase in TDR rather than to prolongation of the QT interval, and (4) indicate that epicardial activation of the left ventricle, as occurs during biventricular pacing, can facilitate the development of TdP under long-QT conditions.

Atrial fibrillation: epidemiologic considerations and rationale for conversion and maintenance of sinus rhythm

Atrial fibrillation is now the most common cardiac arrhythmia for which a patient is hospitalized. Clinically, it presents in a form that is paroxysmal, persistent, or permanent and may be symptomatic or asymptomatic, occurring in the setting of either no cardiac disease ("lone atrial fibrillation") or, most often, in association with an underlying disease. Atrial fibrillation is associated with a 2-fold increase in mortality and, in the United States alone, causes over 75,000 cases of stroke per year. The annual prevalence of stroke is 5% to 7%, but the use of adequate anticoagulation can reduce this to less than 1%. Atrial fibrillation is a disorder of the elderly, with almost equal prevalence in men and women. In the United States, 80% of atrial fibrillation occurs in patients over the age of 65 years, and its prevalence tracks that of heart failure, which may be the cause, as well as the result, of the arrhythmia. Both conditions are increasing in epidemic proportions in the aging population. The most common causes of atrial fibrillation are hypertensive heart disease, coronary artery disease, and heart failure with a miscellany of lesser conditions, with about 10% lacking structural heart disease. Unlike other supraventricular arrhythmias, cure by the use of catheter ablation and surgical techniques has not been a reality except in a relatively small number of cases. However, restoration and maintenance of sinus rhythm remain the initial goal of therapy for most patients. Pharmacologic approaches remain the mainstay of therapy for rate control and anticoagulation as well as for maintenance of sinus rhythm following pharmacological or electrical conversion. The changing epidemiology of atrial fibrillation is highlighted, with the focus on its conversion by the use of newer and novel antifibrillatory agents relative to the mechanisms of the arrhythmia, to restore the stability of sinus rhythm.

Amiodarone-induced postrepolarization refractoriness suppresses induction of ventricular fibrillation

It is still incompletely understood why amiodarone is such a potent antiarrhythmic drug. We hypothesized that chronic amiodarone treatment produces postrepolarization refractoriness (PRR) without conduction slowing and that PRR modifies the induction of ventricular arrhythmias. In this study, the hearts of 15 amiodarone-pretreated (50 mg/kg p.o. for 6 weeks) rabbits and 13 controls were isolated and eight monophasic action potentials were simultaneously recorded from the epicardium and endocardium of both ventricles. Steady-state action potential duration (APD), conduction times, refractory periods, and dispersion of action potential durations were determined during programmed stimulation and during 50-Hz burst stimuli, and related to arrhythmia inducibility. Amiodarone prolonged APD by 12 to 15 ms at pacing cycle lengths of 300 to 600 ms (p < 0.05) but did not significantly increase conduction times or dispersion of APD. Amiodarone prolonged refractoriness more than action potential duration, resulting in PRR (refractory period - APD at 90% repolarization, 14 +/- 10 ms, p < 0.05 versus controls). PRR curtailed the initial sloped part of the APD restitution curve by 20%. During burst stimulation, pronounced amiodarone-induced PRR (40 +/- 15 ms, p < 0.05 versus controls) reduced the inducibility of ventricular arrhythmias (p < 0.05 versus controls). Furthermore, in 35% of bursts only monomorphic ventricular tachycardias and no longer ventricular fibrillation were inducible in amiodarone-treated hearts (p < 0.05 versus controls). Chronic amiodarone treatment prevents ventricular tachycardias by inducing PRR without much conduction slowing, thereby curtailing the initial part of APD restitution. PRR without conduction slowing is a desirable feature of drugs designed to prevent ventricular arrhythmias.

Mechanisms of antiarrhythmic drug actions and their clinical relevance for controlling disorders of cardiac rhythm

This review on antiarrhythmic drugs traces the evolution of the fundamental mechanisms of action of drugs that have been used to control disorders of cardiac rhythm. It describes the very earliest data from experimental studies that dealt with the effects of acute and chronic administration of drugs in whole animals combined with the measurements of the action potential duration and the effective refractory period in isolated tissues. Antiarrhythmic drugs were found to have properties consistent with the block of fast sodium channel conduction, adrenergic blockade, repolarization block, and the block of slow-channel mediated conduction especially in the atrioventricular node. Over the past 15 years, the attention has focused on atrial tissue with atrial fibrillation emerging as the most common arrhythmia in clinical practice. Drug-induced increases in refractoriness as a function rate and in wavelength (product of refractoriness and conduction velocity), and a reduction in numbers of wavelets have been found to be critical in the conversion of atrial fibrillation and maintenance of sinus rhythm. The continued development of newer pharmacologic agents is likely to lead to the resolution of the controversy regarding rhythm versus rate control in various clinical subsets of the arrhythmia by controlled clinical trials.

The effects of acute and chronic amiodarone on activation patterns and defibrillation threshold during ventricular fibrillation in dogs

OBJECTIVES

The goal of this study was to evaluate the effects of acute and chronic amiodarone on activation patterns during ventricular fibrillation (VF), ventricular effective refractory period (VERP) and defibrillation threshold (DFT).

BACKGROUND

Acute and chronic amiodarone may act through different mechanisms.

METHODS

The VERP, VF activation patterns and DFT were determined in 24 dogs. Twelve dogs received acute intravenous amiodarone (10 mg/kg, n = 6) or saline (n = 6), and 12 dogs received chronic oral amiodarone (20 mg/kg/day, n = 6) or placebo (n = 6). Epicardial VF activation patterns were recorded with 504 electrodes. Quantitative descriptors of VF were calculated.

RESULTS

The DFT was unchanged by acute or chronic amiodarone. Although chronic amiodarone significantly extended the VERP, acute amiodarone did not. In the mapped region, acute and chronic amiodarone decreased the number of VF wavefronts by 42% and 60%. Acute amiodarone decreased conduction block by 22%, while chronic amiodarone increased block by 41% but decreased wave fractionation by 50%. Both chronic and acute amiodarone increased the size of the core of re-entrant circuits and decreased the incidence of re-entry by 44% and 57%; however, chronic amiodarone increased wavelength, while acute amiodarone did not.

CONCLUSIONS

Neither acute nor chronic amiodarone change the DFT. While both acute and chronic amiodarone decrease the number of wavefronts, decrease the incidence of re-entry and increase the size of re-entrant cores in the mapped region during VF, they achieve these antiarrhythmic effects through different electrophysiologic mechanisms. Chronic amiodarone prolonged the VF cycle length and slowed conduction velocity, indicating it increased the wavelength and/or the excitable gap.

A Benefit-Risk Assessment of Class III Antiarrhythmic Agents

With beta-blockers as the exception, increasing doubt is emerging on the value of antiarrhythmic drug therapy following a series of trials that have either shown no mortality benefit or even an excess mortality. Vaughan Williams class I drugs are generally avoided in patients with structural heart disease, and class IV drugs are avoided in heart failure. Unfortunately, arrhythmias are a growing problem due to an increase in the incidence of atrial fibrillation and sudden death. The population is becoming older and more patients survive for a longer time period with congestive heart failure, which again increases the frequency of both supraventricular as well as ventricular arrhythmias. Class III antiarrhythmic drugs act by blocking repolarising currents and thereby prolong the effective refractory period of the myocardium. This is believed to facilitate termination of re-entry tachyarrhythmias. This class of drugs is developed for treatment of both supraventricular and ventricular arrhythmias. Amiodarone, sotalol, dofetilide, and ibutilide are examples of class III drugs that are currently available. Amiodarone and sotalol have other antiarrhythmic properties in addition to pure class III action, which differentiates them from the others. However, all have potential serious adverse events. Proarrhythmia, especially torsade de pointes, is a common problem making the benefit-risk ratio of these drugs a key question. Class III drugs have been evaluated in different settings: primary and secondary prevention of ventricular arrhythmias and in treatment of atrial fibrillation or flutter. Based on existing evidence there is no routine indication for antiarrhythmic drug therapy other than beta-blockers in patients at high risk of sudden death. Subgroup analyses of trials with amiodarone and dofetilide suggest that patients with atrial fibrillation may have a mortality reduction with these drugs. However, this needs to be tested in a prospective trial. Similarly, subgroups that will benefit from prophylactic treatment with class III antiarrhythmic drugs may be found based on QT-intervals or - in the future - from genetic testing. Class III drugs are effective in converting atrial fibrillation to sinus rhythm and for the maintenance of sinus rhythm after conversion. This is currently by far the most important indication for this class of drugs. As defined by recent guidelines, amiodarone and dofetilide have their place as second-line therapy except for patients with heart failure where they are first line therapy being the only drugs where the safety has been documented for this group of high risk patients.

Cellular mechanisms underlying the long QT syndrome

QT prolongation is commonly associated with life-threatening torsade de pointes arrhythmias that develop as a consequence of the amplification of electrical heterogeneities intrinsic to the ventricular myocardium. These heterogeneities exist because of 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 responsible for the inscription of the electrocardiographic T wave. Agents and conditions that reduce net repolarizing current amplify the intrinsic spatial dispersion of repolarization, thus creating the substrate for the development of re-entry. The result is a prolongation of the QT interval, abnormal T waves, and development of polymorphic re-entrant ventricular tachycardia displaying characteristics of torsades de pointes. These conditions also predispose M cells and Purkinje fibers to develop early afterdepolarization-induced extrasystoles, which are thought to trigger episodes of torsades de pointes. Agents that prolong the QT interval but do not increase transmural dispersion of repolarization are not capable of inducing torsades de pointes. The available data suggest that that the principal problem with the long QT syndrome is not long QT intervals but rather the dispersion of repolarization that often accompanies prolongation of the QT interval.

Chronic amiodarone evokes no torsade de pointes arrhythmias despite QT lengthening in an animal model of acquired long-QT syndrome

BACKGROUND

Amiodarone is an effective antiarrhythmic drug rarely associated with torsade de pointes arrhythmias (TdP). The noniodinated compound dronedarone could resemble amiodarone and be devoid of the adverse effects. In the dog with chronic complete atrioventricular (AV) block (CAVB) and acquired long-QT syndrome, the electrophysiological and proarrhythmic properties of the drugs were compared after 4 weeks of oral treatment.

METHODS AND RESULTS

Amiodarone (n=7, 40 mg. kg(-1). d(-1)) and dronedarone (n=8, 20 mg/kg BID) were started at 6 weeks of CAVB (baseline). Six dogs served as controls. Surface ECGs and endocardially placed monophasic action potential catheters in the left (LV) and right (RV) ventricles were recorded to assess QTc time, action potential duration (APD), interventricular dispersion (DeltaAPD=LV APD minus RV APD), early afterdepolarizations (EADs), ectopic beats, and TdP. Both amiodarone (+21%) and dronedarone (+31%) increased QTc time. Amiodarone showed no increase in DeltaAPD in 4 of 7 dogs, whereas dronedarone augmented DeltaAPD in 7 of 8 animals. After dronedarone, TdP occurred in 4 of 8 dogs with the highest DeltaAPD (105+/-20 ms). TdP was never seen with amiodarone, not even in the dogs that had DeltaAPD values comparable to those with dronedarone. Furthermore, a difference existed in EADs and ectopic activity incidence (dronedarone 3 of 8; amiodarone 0 of 7), which was also seen during an epinephrine challenge.

CONCLUSIONS

In the CAVB dog model, both amiodarone and dronedarone prolong QT time (class III effect). The absence of TdP with amiodarone seems to be related to homogeneous APD lengthening in the majority of dogs and the lack of EADs and/or ventricular ectopic beats in all.

Beneficial effects of amiodarone and dronedarone (SR 33589b), when applied during low-flow ischemia, on arrhythmia and functional parameters asssessed during reperfusion in isolated rat hearts

The effects of short-term amiodarone and dronedarone treatments on action potential characteristics and arrhythmia (ventricular tachycardia ) induced by reperfusion after global low-flow ischemia were studied in rat hearts. The actions of amiodarone and SR on recovery of coronary flow and contractile function were also determined. Isolated hearts were stabilized for 40 min and were then submitted to 25-min global low-flow ischemia (constant coronary flow, 0.3 ml/min) followed by 30 min of reperfusion at constant pressure. Drugs were applied only during ischemia: consequently, action potential duration (APD) tended to widen. During reperfusion, APD tended to recover or shorten, and the more complete the recovery, the less the arrhythmia. Despite its ability to widen APD during ischemia, amiodarone facilitated APD recovery during reperfusion. Moreover, APD shortening and ventricular tachycardia suppression exhibit a bell-shaped concentration-response relation, implying that the drugs affect ventricular tachycardia by a class III-independent action. These results point to an anti-ischemic action supported by improvement in function and inhibition of reactive hyperemia.

Electrophysiological effects of dronedarone (SR 33589), a noniodinated amiodarone derivative in the canine heart: comparison with amiodarone

The electrophysiological effects of dronedarone, a new nonionidated analogue of amiodarone were studied after chronic and acute administration in dog Purkinje fibres, papillary muscle and isolated ventricular myocytes, and compared with those of amiodarone by applying conventional microelectrode and patch-clamp techniques. Chronic treatment with dronedarone (2x25 mg(-1) kg(-1) day p.o. for 4 weeks), unlike chronic administration of amiodarone (50 mg(-1) kg(-1) day p.o. for 4 weeks), did not lengthen significantly the QTc interval of the electrocardiogram or the action potential duration (APD) in papillary muscle. After chronic oral treatment with dronedarone a small, but significant use-dependent V(max) block was noticed, while after chronic amiodarone administration a strong use-dependent V(max) depression was observed. Acute superfusion of dronedarone (10 microM), similar to that of amiodarone (10 microM), moderately lengthened APD in papillary muscle (at 1 Hz from 239.6+/-5.3 to 248.6+/-5.3 ms, n=13, P<0.05), but shortened it in Purkinje fibres (at 1 Hz from 309.6+/-11.8 to 287.1+/-10.8 ms, n=7, P<0.05). Both dronedarone (10 microM) and amiodarone (10 microM) superfusion reduced the incidence of early and delayed afterdepolarizations evoked by 1 microM dofetilide and 0.2 microM strophantidine in Purkinje fibres. In patch-clamp experiments 10 microM dronedarone markedly reduced the L-type calcium current (76.5+/-0.7 %, n=6, P<0.05) and the rapid component of the delayed rectifier potassium current (97+/-1.2 %, n=5, P<0.05) in ventricular myocytes. It is concluded that after acute administration dronedarone exhibits effects on cardiac electrical activity similar to those of amiodarone, but it lacks the 'amiodarone like' chronic electrophysiological characteristics.

Azimilide and dofetilide produce similar electrophysiological and proarrhythmic effects in a canine model of Torsade de Pointes arrhythmias

Torsade de Pointes arrhythmias are a feared proarrhythmic effect of (antiarrhythmic) drugs. In dogs with chronic complete AV-block bradycardia-induced volume overload leads to electrical remodeling, which includes increased susceptibility to drug-induced Torsade de Pointes arrhythmias. The IKr channel blocker, dofetilide (Tikosyn, 0.025 mg/kg/5 min), and the less specific ion channel blocker, azimilide (5 mg/kg/5 min), were compared in nine anesthetized dogs at 4 and 6 weeks of AV-block in a randomized cross-over design. Dosages were based on our own dose-dependence studies and on anti-arrhythmic dosages reported in the literature. Monophasic action potential catheters were placed endocardially in both the left and right ventricle to measure action potential duration, visualize early afterdepolarizations, and to assess interventricular dispersion of repolarization (i.e. left ventricular monophasic action potential duration (at 100%) minus right ventricular monophasic action potential duration (at 100%). Cycle length of idioventricular rhythm, QT-time and the occurrence of drug-induced Torsade de Pointes arrhythmias were determined using the surface electrocardiogram (ECG). Before drug administration, the electrophysiological parameters were identical at 4 and 6 weeks. Both azimilide and dofetilide increased monophasic action potential duration, cycle length of idioventricular rhythm, and QT-time. Dissimilar lengthening of left ventricular and right ventricular monophasic action potential duration increased the interventricular dispersion significantly from 55 to 110 ms for both drugs. All dogs had early afterdepolarizations, while, in the majority, ectopic ventricular beats developed (dofetilide 8/9 and azimilide 7/9). Torsade de Pointes arrhythmias incidence was comparable for dofetilide (6/9) and azimilide (5/9). In conclusion, azimilide and dofetilide show similar electrophysiological and proarrhythmic effects in our canine model with a high incidence of Torsade de Pointes arrhythmias.

Chemical cardioversion of atrial fibrillation or flutter with ibutilide in patients receiving amiodarone therapy

BACKGROUND

Ibutilide is a class III drug that is used for the cardioversion of atrial arrhythmias, but it can cause torsade de pointes. Amiodarone also prolongs the QT interval but rarely causes torsade de pointes. There are no studies in which the concomitant use of the 2 agents was examined. The purpose of the present study was to assess the efficacy and safety of cardioversion with combination therapy in patients with atrial fibrillation or flutter.

METHODS AND RESULTS

The study included 70 patients who were treated with long-term oral amiodarone and were referred for elective cardioversion of atrial fibrillation (57 of 70, 81%) or flutter (13 of 70, 19%). Patients were taking amiodarone (153+/-259 days, mean+/-SD) and were administered 2 mg intravenous ibutilide. Left ventricular ejection fraction was measured with echocardiography. The QT intervals were measured on 12-lead ECG. Fifty-five patients (79%) had structural heart disease. Patients were in arrhythmia for 196+/-508 days before cardioversion, with a left ventricular ejection fraction of 50+/-11%. In patients with atrial fibrillation, 22 (39%) of 57 and 7 (54%) of 13 patients with flutter converted within 30 minutes of infusion. Thirty-nine patients who did not convert after ibutilide were treated with electrical cardioversion, and 35 (90%) of 39 patients were successfully converted. The QT intervals were further prolonged after ibutilide for the group from 371+/-61 to 479+/-92 ms (P:<0.001). There was 1 episode of nonsustained torsade de pointes (1 of 70, 1.4%) after ibutilide.

CONCLUSIONS

The use of ibutilide converted 54% of patients with atrial flutter and 39% of patients with atrial fibrillation who were treated with long-term amiodarone. Despite QT-interval prolongation after ibutilide, only 1 episode of torsade de pointes occurred. Our observations suggest that combination therapy may be a useful cardioversion method for chronic atrial fibrillation or flutter.

Antiarrhythmic and electrophysiological effects of GYKI-16638, a novel N-(phenoxyalkyl)-N-phenylalkylamine, in rabbits

The effect of N-[4-[2-N-methyl-N-[1-methyl-2-(2, 6-dimethylphenoxy)ethylamino]-ethyl]-phenyl]-methanesulfonamide. hydrochloride (GYKI-16638; 0.03 and 0.1 mg/kg, i.v.), a novel antiarrhythmic compound, was assessed and compared to that of D-sotalol (1 and 3 mg/kg, i.v.) on arrhythmias induced by 10 min of coronary artery occlusion and 10 min of reperfusion in anaesthetized rabbits. Also, its cellular electrophysiological effects were studied in rabbit right ventricular papillary muscle preparations and in rabbit single isolated ventricular myocytes. In anaesthetized rabbits, intravenous administration of 0.03 and 0.1 mg/kg GYKI-16638 and 1 and 3 mg/kg D-sotalol significantly increased survival during reperfusion (GYKI-16638: 82% and 77%, D-sotalol: 75% and 83% vs. 18% in controls, P<0.05, respectively). GYKI-16638 (0.1 mg/kg) significantly increased the number of animals that did not develop arrhythmias during reperfusion (46% vs. 0% in controls, P<0.05). In isolated rabbit right ventricular papillary muscle, 2 microM GYKI-16638, at 1 Hz stimulation frequency, lengthened the action potential duration at 50% and 90% repolarization (APD(50-90)) without influencing the resting membrane potential and action potential amplitude (APA). It decreased the maximal rate of depolarization (V(max)) in a use-dependent manner. This effect was statistically significant only at stimulation cycle lengths shorter than 700 ms. The offset kinetics of this V(max) block were relatively rapid, the corresponding time constant for recovery of V(max) was 328.2+/-65.0 ms. In patch-clamp experiments, performed in rabbit ventricular myocytes, 2 microM GYKI-16638 markedly depressed the rapid component of the delayed rectifier outward and moderately decreased the inward rectifier K(+) current without significantly altering the slow component of the delayed rectifier and transient outward K(+) currents. These results suggest that in rabbits, GYKI-16638 has an in vivo antiarrhythmic effect, comparable to that of D-sotalol, which can be best explained by its combined Class I/B and Class III actions.

Chronic amiodarone effects on epicardial conduction and repolarization in the isolated porcine heart

Amiodarone is a potent antiarrhythmic agent with complex chronic effects, notably on repolarization and conduction, that are not fully understood. Its low arrhythmogenic potential has been related to a lack of increase in repolarization dispersion. Since its effects are not documented in pigs we conducted a mapping study of activation and repolarization in isolated perfused porcine hearts. Amio20 female pigs (n = 7) received amiodarone 20 mg/kg per day over 4 weeks while Amio50 female pigs (n = 7) received 50 mg/kg per day over 4 weeks. Concentrations of the drug encompassed values found in clinical studies. Then, activation patterns and activation-to-recovery intervals (ARI) were mapped epicardially from 128 unipolar electrograms in isolated perfused hearts in corroboration of epicardial action potential recordings. Mean ARI was longer in Amio20 experiments compared to the seven control hearts (325 +/- 11 ms vs 288 +/- 5 ms at 1,000 ms), whereas ARI dispersion was not different, being comprised between 7 and 11 ms and generating smooth gradients. In Amio50 experiments, mean ARI was further prolonged (390 +/- 10 ms at 1,500 ms) with an exaggerated reverse rate dependence concomitant with a depressant effect on the plateau of the action potential. Again, ARI dispersion did not differ from controls. Finally, the drug depressed the maximal rate of depolarization (Vmax) and slowed conduction in a rate dependent and concentration dependent fashion. In conclusion, chronic amiodarone induces Class I and Class III antiarrhythmic effects in ventricular porcine epicardium that are concentration dependent but does not affect dispersion of repolarization. This may partly explain its low arrhythmogenic potential.

Differential expression of KvLQT1 isoforms across the human ventricular wall

Long Q-T mutant (KvLQT1) K(+) channels associate with their regulatory subunit IsK to produce the slow component of the delayed rectifier potassium (I(Ks)) cardiac current. The amplitude of KvLQT1 current depends on the expression of a KvLQT1 splice variant (isoform 2) that exerts strong dominant negative effects on the full-length KvLQT1 protein (isoform 1). We used RNase protection assays to determine the relative expression of KvLQT1 isoforms 1 and 2 and IsK mRNAs in human ventricular layers. Overall expression of KvLQT1 and IsK genes was similar in the three layers. However, there was a significant difference in the ratio between KvLQT1 isoforms 1 and 2. Isoform 2 represented 25.2 +/- 2.3%, 31.7 +/- 1.2%, and 24.9 +/- 1.7% of total KvLQT1 expression in left ventricular endocardial, midmyocardial, and epicardial tissues, respectively. Similar data were obtained from right ventricular samples. COS-7 cells were intranuclearly injected with KvLQT1 isoforms 1 or 2 plus IsK cDNAs, using two different isoform 2-to-isoform 1 ratios. Cells injected with an isoform 2-to-isoform 1 ratio mimicking that in the midmyocardium showed a K(+) current with approximately 75% reduced amplitude compared with those injected with a ratio mimicking that in the epicardium. Our results suggest that differential expression of KvLQT1 isoform 2 in endocardial, midmyocardial, and epicardial tissues is responsible for differential I(Ks) amplitude and contributes to the regional action potential heterogeneity observed across the ventricular wall.

Effects of antiarrhythmic drugs on QT interval dispersion--relationship to antiarrhythmic action and proarrhythmia

Class IA, IC, and III antiarrhythmic drugs prolong ventricular repolarization (VR) which is manifest as QT interval prolongation on the surface electrocardiogram. These drugs may prolong VR in a spatially heterogeneous manner which results in increased dispersion of VR. This may be manifest as increased QT interval dispersion. Antiarrhythmic drug-induced decreases in QT interval dispersion are associated with antiarrhythmic efficacy in patients with the long QT syndrome and in patients with sustained ventricular tachycardia. Antiarrhythmic drug-induced increases in QT interval dispersion are associated with ventricular proarrhythmia secondary to torsades de points ventricular tachycardia. A number of factors may modulate the effects of antiarrhythmic drugs on dispersion of VR, including the disease state, transient ischemia, electrolyte abnormalities, changes in autonomic tone, and hemodynamic stress.

Analysis of the electrophysiological effects of ambasilide, a new antiarrhythmic agent, in canine isolated ventricular muscle and purkinje fibers

The aim of the study was to determine the in vitro rate-dependent cellular electrophysiological effects of ambasilide (10 and 20 microM/l), a new investigational antiarrhythmic agent, in canine isolated ventricular muscle and Purkinje fibers by applying the standard microelectrode technique. At the cycle length (CL) of 1000 ms, ambasilide significantly prolonged the action potential duration measured at 90% repolarization (APD(90)) in both ventricular muscle and Purkinje fibers. Ambasilide (10 microM/l) produced a more marked prolongation of APD(90) at lower stimulation frequencies in Purkinje fibers (at CL of 2000 ms = 56.0 +/- 16.1%, n = 6, versus CL of 400 ms = 15.1 +/- 3.7%, n = 6; p < 0.05), but, in 20 microM/l, this effect was considerably diminished (15.2 +/- 3.6%, n = 6, versus 7.3 +/- 5.1%, n = 6, p < 0.05). In ventricular muscle, however, both concentrations of the drug induced an almost frequency-independent lengthening of APD(90) in response to a slowing of the stimulation rate (in 20 microM/l at CL of 5000 ms = 19.0 +/- 1.5%, n = 9, versus CL of 400 ms = 16.9 +/- 1.4%, n = 9). Ambasilide induced a marked rate-dependent depression of the maximal rate of rise of the action potential upstroke (V(max)) (in 20 microM/l at CL of 300 ms = -45.1 +/- 3.9%, n = 6, versus CL of 5000 ms = -8.5 +/- 3.9%, n = 6, p < 0. 05, in ventricular muscle) and the corresponding recovery of V(max) time constant was tau = 1082.5 +/- 205.1 ms (n = 6). These data suggest that ambasilide, in addition to its Class III antiarrhythmic action, which is presumably due to its inhibitory effect on the delayed rectifier potassium current, possesses I/B type antiarrhythmic properties as a result of the inhibition of the fast sodium channels at high frequency rate with relatively fast kinetics. This latter effect may play an important role in its known less-pronounced proarrhythmic ("torsadogenic") potential.

Inherited long QT syndromes: a paradigm for understanding arrhythmogenesis

The inherited long QT syndrome (LQTS) is a familial disease characterized by QT interval changes that often are labile, syncope, and sudden death due to arrhythmias, predominantly in young people. Multiple mutations in five genes encoding structural subunits of cardiac ion channels now have been identified in families with LQTS. Correlations are being described between genotype and specific clinical features in LQTS. However, increasing screening of affected families and sporadic cases has identified incomplete penetrance with highly variable clinical manifestations, even among individuals carrying the same mutations. The identification of LQTS disease genes represents a crucial first step in developing an understanding of the molecular basis for normal cardiac repolarization. This information will be important not only for identifying new therapies in LQTS, but also in further understanding arrhythmias, and their potential therapies, in situations such as heart failure, cardiac hypertrophy, myocardial infarction, or sudden infant death syndrome, where abnormal repolarization has been linked to sudden death. LQTS thus presents a new paradigm to cardiac electrophysiology, in which new molecular information is being brought to bear both on clinical management of patients and on development of a new framework to study the fundamental causes of arrhythmias and new approaches to therapy.

Effects of the gap junction uncoupler palmitoleic acid on the activation and repolarization wavefronts in isolated rabbit hearts

1. The heart normally acts as an electrical syncytium coupled via gap junctional channels. Since closure of these channels has been considered arrhythmogenic, we wanted to elucidate, how activation and repolarization wavefronts are altered during progressive pharmacological gap junctional uncoupling. 2. We used the well known gap junction uncoupler palmitoleic acid (PA). The specificity of PA was tested in rabbit papillary muscles, which exhibited slowed conduction without affecting action potential morphology. We submitted isolated rabbit hearts (Langendorff-technique) to increasing concentrations of palmitoleic acid (0.2, 1, 2, 5, 10, 20 microM), while 256 channel epicardial potential mapping was carried out. 3. In presence of PA activation recovery intervals (ARI) at the 256 electrodes became highly inhomogeneous with a dramatic increase in the dispersion of activation recovery intervals (from 6 to 35 ms, P>0.01; EC50=7 microM), while the mean ARI-duration at 256 sites remained stable. PA led to marked alterations of the activation pattern, expressed as percentage of unchanged activation vectors (reduction from 32 to 10%, P<0.01, EC50=3.3 microM), to prolongation of atrioventricular conduction time (from 58 to 107 ms, P<0.01; EC50=8 microM) of total activation time (from 7 to 14 ms, P<0.05, EC50=11 microM) and of QRS-complex-duration. 4. In additional experiments the ventricle was paced via a bipolar electrode during the mapping procedure. From the isochrones longitudinal and transversal velocities were assessed showing that PA reduced transversal conduction velocity more distinctly than longitudinal. 5. With regard to maximum effects and EC50 values we conclude that gap junction uncoupling by PA mainly affects atrioventricular conduction, ARI-dispersion and ventricular activation pattern. As important arrhythmogenic effects of uncoupling enhancement of dispersion with concomitant disturbation of the normal activation pattern and slowing of conduction might be considered.

Mortality reduction by antiadrenergic modulation of arrhythmogenic substrate: significance of combining beta blockers and amiodarone

Over the last 3 decades, there have been numerous experimental and clinical studies that utilized beta blockers for acute as well as chronic myocardial syndromes, especially in the setting of myocardial infarction in which the focus has been on mortality reduction. The results of these studies demonstrated the benefits of these agents at all stages of coronary artery disease. Although these data have always indicated that beta blockade per se is an antiarrhythmic as well as an antifibrillatory mechanism, the recognition of this phenomenon has been slow in finding universal appreciation. More recent studies have evaluated the additive role of beta blockers to newer therapies. A number of investigations have now established that this class of drugs does exert antifibrillatory action in preventing the occurrence of ventricular tachycardia (VT) and ventricular fibrillation (VF), thereby reducing sudden arrhythmic death and prolonging survival. It is of interest that 2 of the leading antiarrhythmic drugs, amiodarone and sotalol, also have antiadrenergic properties. This article reviews the expanding role of beta-blocking drugs in the control and prevention of life-threatening ventricular tachyarrhythmias with a particular focus on the evidence for synergistic benefits when they are combined with other interventions, especially amiodarone.

Antiarrhythmic agents for atrial fibrillation: focus on prolonging atrial repolarization

Atrial fibrillation (AF) has been the subject of considerable attention and intensive clinical research in recent years. Current opinion among physicians on the management of AF favors the restoration and maintenance of normal sinus rhythm. This has several potential benefits, including the alleviation of arrhythmia-associated symptoms, hemodynamic improvements, and possibly a reduced risk of thromboembolic events. After normal sinus rhythm has been restored, antiarrhythmic therapy is necessary to reduce the frequency of AF recurrence. In the selection of an antiarrhythmic agent, both efficacy and safety should be taken into consideration. Many antiarrhythmic agents have the capacity to provoke proarrhythmia, which may result in an increase in mortality. This is of particular concern with sodium-channel blockers in the context of patients with structural heart disease. Flecainide and propafenone are well tolerated and effective in maintaining sinus rhythm in patients without significant cardiac disease but with AF. Recent interest has focused on the use of class III antiarrhythmic agents, such as amiodarone, sotalol, dofetilide (recently approved), ibutilide (approved for chemical conversion of AF and atrial flutter), and azimilide (still to be approved) in patients with AF and structural heart disease. To date, amiodarone and sotalol still hold the greatest interest, and although controlled clinical trials with these agents have been few, a number are in progress and some have been recently completed. These agents are effective in maintaining normal sinus rhythm in patients with paroxysmal and persistent AF and are associated with a low incidence of proarrhythmia when used appropriately. Because of the relative paucity of placebo-controlled trials of antiarrhythmic agents in patients with AF, experience until recently has tended to dictate treatment decisions. Increasingly, selection of drug therapy is being based on a careful and individualized benefit-risk evaluation by means of controlled clinical trials, an approach that is likely to dominate the overall approach to the control of atrial fibrillation in the largest numbers of cases of the arrhythmia. Pharmacologic therapy is likely to be dominated by compounds that exert their predominant effect by prolonging atrial repolarization.

Electropharmacologic characteristics of ventricular proarrhythmia induced by ibutilide

The purpose of this study was to evaluate in vivo the proarrhythmic effects of ibutilide in dogs with or without ventricular hypertrophy. Fourteen dogs received repeated experiments both during the acute and chronic phases (8 weeks, with ventricular hypertrophy) of complete atrioventricular (AV) block. Twelve-lead ECG, monophasic action potentials in the left and right ventricle were recorded before and after each dose of ibutilide (0.01-0.08 mg/kg) during different ventricular rates. In these dogs, ibutilide increased QT interval, biventricular APD90, interventricular deltaAPD90 (difference between the left and right ventricular APD90), and QT dispersion, and induced early afterdepolarizations in a dose-dependent manner. The interventricular deltaAPD90, QT dispersion, and increases of QT interval were more pronounced during slower ventricular rates. There were greater QT interval, biventricular APD90 interventricular deltaAPD90, and QT dispersion values during chronic AV block than during acute AV block. Moreover, ibutilide can induce higher incidences of early afterdepolarizations and torsades de pointes [six (43%) of 14 versus 0 of 14; p < 0.05] during chronic AV block than during acute AV block. In conclusion, ibutilide can prolong ventricular repolarization and increase dispersion of ventricular repolarization in a dose-dependent and reverse rate-dependent manner. The high incidence of torsades de pointes in the dogs during chronic AV block suggests the importance of ventricular hypertrophy in the occurrence of ibutilide-induced proarrhythmia.

The M cell: its contribution to the ECG and to normal and abnormal electrical function of the heart

The discovery and characterization of the M cell, a unique cell type residing in the deep layers of the ventricular myocardium, has opened a new door in our understanding of the electrophysiology and pharmacology of the heart in both health and disease. The hallmark of the M cell is the ability of its action potential to prolong much more than that of other ventricular myocardial cells in response to a slowing of rate and/or in response to agents that act to prolong action potential duration. Our goal in this review is to provide a comprehensive characterization of the M cell, its contribution to transmural heterogeneity, and its role in the normal electrical function of the heart, in the inscription of the ECG (particularly the T wave), and in the development of QT dispersion, T wave alternans, long QT intervals, and cardiac arrhythmias, such as torsades de pointes. Our secondary goal is to address the controversy that has arisen relative to the functional importance of the M cell in the normal heart. The controversy derives largely from the failure of some investigators to demonstrate transmural heterogeneity of repolarization in the dog in vivo under control conditions and after administration of quinidine. The inability to demonstrate transmural heterogeneity under these conditions may be due to the use of bipolar recording techniques that, in our experience, seriously underestimate transmural dispersion of repolarization (TDR). The use of sodium pentobarbital and alpha-chloralose as anesthesia also is problematic, because these agents reduce or eliminate TDR by affecting a variety of ion channel currents. Finally, attempts to amplify transmural dispersion of repolarization with an agent such as quinidine must take into account that relatively high concentrations can result in effects opposite to those desired due to drug inhibition of multiple ion channels. These observations may explain the inability of earlier studies to detect the M cell.

Sodium pentobarbital reduces transmural dispersion of repolarization and prevents torsades de Pointes in models of acquired and congenital long QT syndrome

INTRODUCTION

Sodium pentobarbital is widely used for anesthesia in experimental studies as well as in clinics, and it is known to prevent the development of torsades de pointes (TdP) in in vivo models of the long QT syndrome (LQTS).

METHODS AND RESULTS

This study examines the effects of pentobarbital on transmural dispersion of repolarization (TDR) and induction of TdP in arterially perfused canine left ventricular wedge preparations in which transmembrane action potentials were simultaneously recorded from epicardial, M, and endocardial regions using floating glass microelectrodes together with a transmural ECG. d-Sotalol and ATX-II were used to mimic the LQT2 and LQT3 forms of congenital LQTS. Both d-sotalol (100 micromol/L, n = 6) and ATX-II (20 nmol/L, n = 6) preferentially prolonged the action potential duration (APD90) of the M cell, thus increasing in the QT interval and TDR, and leading to the development of spontaneous and stimulation-induced TdP. In the absence and presence of d-sotalol, pentobarbital (10, 20, and 50 microg/mL) prolonged the APD90 of epicardial and endocardial cells, and, to a lesser extent, that of the M cell, thus prolonging the QT interval but reducing TDR. In the ATX-II model, the effects of pentobarbital on the QT interval and APD90 were biphasic: 10 microg/mL pentobarbital further prolonged APD90 of epicardial and endocardial cells more than that of the M cell; 20 to 50 microg/mL pentobarbital abbreviated the APD90 of epicardial and endocardial cells less than that of the M cell, thus abbreviating the QT interval and markedly reducing TDR. Twenty to 50 microg/mL pentobarbital totally suppressed spontaneous as well as stimulation-induced TdP in both models

CONCLUSION

Our data indicate that pentobarbital reduces TDR in control and under conditions of congenital and acquired LQTS, and suggest that this mechanism may contribute to the ability of the anesthetic to prevent the development of spontaneous as well as stimulation-induced TdP under conditions mimicking LQT2, LQT3, and acquired (drug-induced) forms of the LQTS. The data also serve to illustrate that there are circumstances under which QT prolongation may not be arrhythmogenic.

Current antiarrhythmic drugs: an overview of mechanisms of action and potential clinical utility

Reorientation in drug therapy to control cardiac arrhythmias continues to evolve in the wake of ongoing refinements in techniques and indications for radiofrequency ablation and the use of implantable devices for atrial and ventricular arrhythmias. The role of sodium channel blockers continues to be questioned, and data from clinical trials indicate that the use of this class of drugs should be limited to control symptoms in patients who have arrhythmias and either no or minimal heart disease. The decline in the use of sodium channel blockers has led to greater use of beta blockers and complex Class III agents, such as sotalol and amiodarone, as both primary therapy and adjunctive therapy with implantable defibrillators in patients with cardiac disease of varying degrees of ventricular dysfunction. Success with these Class III agents in the context of their side effects has led to the synthesis and characterization of compounds with simpler ion channel-blocking properties. The need for such compounds stemmed from the observation that atrial fibrillation (AF) as an arrhythmia is, for the most part, still not amenable to curative therapy by interventional procedures. The isolated block of the rapid component of the delayed rectifier current (IKr) has been found to have either a neutral (e.g., dofetilide) or deleterious (e.g., d-sotalol) effect on mortality in survivors of myocardial infarction. Thus, the objective of drug development should be the appropriate match between the substrate and an antiarrhythmic drug. The so-called pure Class III agents have been shown to have beneficial antifibrillatory effects in patients with AF. They are effective in inducing acute chemical conversion, preventing paroxysmal AF, and maintaining sinus rhythm in patients with persistent AF restored to sinus rhythm with DC cardioversion. AF is a complex arrhythmia, undoubtedly a result of multifaceted derangement of atrial ionic currents. Attention has therefore focused on newer compounds that have the propensity to block more than one ion channel. Examples of such agents are tedisamil and azimilide, the latter having been studied extensively in humans. It is the first of the Class III agents that block both components (IKr and IKs) of the delayed rectifier current, which results in a spectrum of electrophysiologic properties that includes lack of rate or use dependency in terms of effect on repolarization and refractoriness of atrial and ventricular myocardium. Available but unpublished clinical data indicate that azimilide may be effective over a wide range of tachycardia cycle lengths with a low incidence of torsades de pointes. In these respects, its properties, at least in terms of its use in AF, resemble those of amiodarone. However, the drug has little or no effect on AV conduction, which precludes the modulation of ventricular response in patients relapsing to AF.