Amiodarone reduces transmural heterogeneity of repolarization in the human heart

Acute management of atrial fibrillation and atrial flutter in the critical care unit: should it be ibutilide?

BACKGROUND

Ibutilide is currently indicated for the rapid conversion of atrial fibrillation (Afb) or atrial flutter (Afl) of recent onset but limited to patients who are hemodynamically stable and without symptomatic cardiovascular conditions.

HYPOTHESIS

The study was undertaken to assess the efficacy and safety of ibutilide in patients with Afb or Afl associated with acute cardiovascular-medical disorders and in patients receiving prior selective antiarrhythmic drug therapy.

METHODS

The study included 34 patients, mean age 75 +/- 16.3 years, with Afb (n = 25) or Afl (n = 9) having a variety of disorders, for example, congestive heart failure, unstable angina, borderline hypotension, respiratory failure, and chronic renal failure. Prior antiarrhythmic drugs consisted of propafenone (n = 5) or amiodarone (n = 3). Eligibility for cardioversion was established with appropriate anticoagulation or transesophageal echocardiography findings. Ibutilide was given as up to two 10 min infusions of 1 mg separated by 10 min.

RESULTS

The overall conversion rate after ibutilide was 79.4% (27/34 patients): 80% for Afb and 78% for Afl. More than 90% converted within 1 h of treatment. A high conversion rate of 92% resulted in those with an arrhythmia duration of < or = 1 week. All eight patients with prior antiarrhythmic therapy converted to sinus rhythm. The average baseline QTc interval for all patients increased 17.1% (397 +/- 63.3 to 465 +/- 60.2 ms) at 30 min. For eight patients (including four who received prior antiarrhythmic drugs), QTc interval prolongation > or = 500 ms was associated with nearly half the entire incidence of arrhythmic events. Proarrhythmia, the exclusive adverse effect, consisted of ventricular extrasystoles (n = 10) and nonsustained monomorphic ventricular tachycardia (VT) (n = 2) managed with intravenous MgSO4, and sustained polymorphic VT (n = 1) requiring electrical cardioversion.

CONCLUSION

Ibutilide is an effective and well tolerated drug for the rapid termination of Afb or Afl of recent onset associated with symptomatic and/or hemodynamically unstable disorders, and it is most efficacious (> or = 90%) when the atrial arrhythmia is < or = 1 week in duration. Proarrhythmic events are readily manageable in a monitored unit with access to appropriate treatment.

Minimal model for human ventricular action potentials in tissue

Modeling the dynamics of wave propagation in human ventricular tissue and studying wave stability require models that reproduce realistic characteristics in tissue. We present a minimal ventricular (MV) human model that is designed to reproduce important tissue-level characteristics of epicardial, endocardial and midmyocardial cells, including action potential (AP) amplitudes and morphologies, upstroke velocities, steady-state action potential duration (APD) and conduction velocity (CV) restitution curves, minimum APD, and minimum diastolic interval. The model is then compared with three previously published human ventricular cell models, the Priebe and Beuckelmann (PB), the Ten Tusscher-Noble-Noble-Panfilov (TNNP), and the Iyer-Mazhari-Winslow (IMW). For the first time, the stability of reentrant waves for all four models is analyzed, and quantitative comparisons are made among the models in single cells and in tissue. The PB, TNNP, and IMW models exhibit quantitative differences in APD and CV rate adaptation, as well as completely different reentrant wave dynamics of quasi-breakup, stability, and breakup, respectively. All the models have dominant frequencies comparable to clinical values except for the IMW model, which has a large range of frequencies extending beyond the clinical range for both ventricular tachycardia (VT) and ventricular fibrillation (VF). The TNNP and IMW models possess a large degree of short-term memory and we show for the first time the existence of memory in CV restitution. The MV model also can be fitted to reproduce the dynamics of other models and is computationally more efficient: the times required to simulate the MV, TNNP, PB and IMW models follow the ratio 1:31:50:8084.

Mouse models of human arrhythmia syndromes

Sudden cardiac death stemming from ventricular arrhythmogenesis is one of the major causes of mortality in the developed world. Congenital and acquired forms of long QT syndrome (LQTS) are in turn associated with life threatening arrhythmias. Over the past decade our understanding of arrhythmogenic mechanisms in the setting of these diseases has increased greatly due to the creation of a number of animal models. Of these, the genetically amenable mouse has proved to be a particularly powerful tool. This review summarizes the congenital and acquired LQTS and describes the various mouse models that have been created to further probe arrhythmogenic mechanisms.

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.

Current concepts in the mechanisms and management of drug-induced QT prolongation and torsade de pointes

Drug-induced long QT syndrome is characterized by a prolonged corrected QT interval (QTc) and increased risk of a polymorphic ventricular tachycardia known as torsade de pointes (TdP). We review mechanisms, predispositions, culprit agents, and management of this potentially fatal phenomenon. Virtually all drugs that prolong QTc block the rapid component of the delayed rectifier current (I(kr)). Some drugs prolong QTc in a dose-dependent manner, others do so at any dose. Most patients that develop drug-induced TdP have underlying risk factors. Female sex is the most common. Implicated drugs include class 1A and III antiarrhythmics, macrolide antibiotics, pentamidine, antimalarials, antipsychotics, arsenic trioxide, and methadone. Treatment for TdP includes immediate defibrillation for hemodynamic instability and intravenous magnesium sulfate. Potassium levels should be maintained in the high normal range, and all QT prolonging agents must be promptly discontinued.

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.

Influence of Drive Cycle Length on Initiation of Ventricular Fibrillation During Implantable Cardioverter Defibrillator Threshold Testing

BACKGROUND

Programmed electrical stimulation of the heart as a method to induce tachyarrhythmias has been described since the 1960s. To date, no study has examined optimal drive cycle length in the induction of ventricular fibrillation (VF) during defibrillation threshold testing after implantable cardioverter-defibrillator placement. We hypothesized that longer drive cycle length, by means of the longer action potential duration, would promote intramyocardial phase 2 reentry and facilitate induction of VF.

METHODS

Fifty consecutive implants were randomized in a prospective crossover format for this study. The group consisted of 40 men and 10 women, with each patient receiving either a 400 or 600 ms initial drive train prior to 1.2 J internal shock on the T wave with a goal to induce ventricular fibrillation. The timing of the T wave shock was determined by measuring the interval from the beginning of the QRS to the apex of the T wave in lead II. Successful inductions were defibrillated via the cardioverter defibrillator. Patients were then crossed over and the protocol repeated.

RESULTS

Twenty of 23 (87%) patients were successfully induced into VF in the initial 400 ms drive train arm whereas 22 of 27 (81%) were successfully induced in the 600 ms arm. Thus, a total of 44 (88%) patients were successfully induced at 400 ms, 41 (82%) patients were successfully induced at 600 ms, and 2 (4%) patients were not inducible at either cycle length, but were inducible with 50 Hz ventricular stimulation. However, no significant difference was noted between the two groups.

CONCLUSION

No investigation to date has questioned whether a relationship exists between drive cycle length and initiation of ventricular fibrillation. Our study addresses this question, though negative for difference between 400 and 600 ms drive trains. Further research into optimal strategies for inducing ventricular fibrillation will minimize patient sedation time and discomfort while undergoing defibrillator threshold testing.

Alternans and spiral breakup in a human ventricular tissue model

Ventricular fibrillation (VF) is one of the main causes of death in the Western world. According to one hypothesis, the chaotic excitation dynamics during VF are the result of dynamical instabilities in action potential duration (APD) the occurrence of which requires that the slope of the APD restitution curve exceeds 1. Other factors such as electrotonic coupling and cardiac memory also determine whether these instabilities can develop. In this paper we study the conditions for alternans and spiral breakup in human cardiac tissue. Therefore, we develop a new version of our human ventricular cell model, which is based on recent experimental measurements of human APD restitution and includes a more extensive description of intracellular calcium dynamics. We apply this model to study the conditions for electrical instability in single cells, for reentrant waves in a ring of cells, and for reentry in two-dimensional sheets of ventricular tissue. We show that an important determinant for the onset of instability is the recovery dynamics of the fast sodium current. Slower sodium current recovery leads to longer periods of spiral wave rotation and more gradual conduction velocity restitution, both of which suppress restitution-mediated instability. As a result, maximum restitution slopes considerably exceeding 1 (up to 1.5) may be necessary for electrical instability to occur. Although slopes necessary for the onset of instabilities found in our study exceed 1, they are within the range of experimentally measured slopes. Therefore, we conclude that steep APD restitution-mediated instability is a potential mechanism for VF in the human heart.

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.

Recent advances in understanding sex differences in cardiac repolarization

A number of gender differences exist in the human electrocardiogram (ECG): the P-wave and P-R intervals are slightly longer in men than in women, whilst women have higher resting heart rates than do men, but a longer rate-corrected QT (QT(C)) interval. Women with the LQT1 and LQT2 variants of congenital long-QT syndrome (LQTS) are at greater risk of adverse cardiac events. Similarly, many drugs associated with acquired LQTS have a greater risk of inducing torsades de pointes (TdP) arrhythmia in women than in men. There are also male:female differences in Brugada syndrome, early repolarisation syndrome and sudden cardiac death. The differences in the ECG between men and women, and in particular those relating to the QT interval, have been explored experimentally and provide evidence of differences in the processes underlying ventricular repolarization. The data available from rabbit, canine, rat, mouse and guinea pig models are reviewed and highlight involvement of male:female differences in Ca and K currents, although the possible involvement of rapid and persistent Na current and Na-Ca exchange currents cannot yet be excluded. The mechanisms underlying observed differences remain to be elucidated fully, but are likely to involve the influence of gonadal steroids. With respect to the QT interval and risk of TdP, a range of evidence implicates a protective role of testosterone in male hearts, possibly by both genomic and non-genomic pathways. Evidence regarding oestrogen and progesterone is less unequivocal, although the interplay between these two hormones may influence both repolarization and pro-arrhythmic risk.

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.

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.

Epicardial activation of left ventricular wall prolongs QT interval and transmural dispersion of repolarization: implications for biventricular pacing

BACKGROUND

Epicardial pacing of the left ventricle (LV) has been shown to prolong the QT interval and predispose to the development of torsade de pointes arrhythmias. The present study examines the cellular basis for QT prolongation and arrhythmogenesis after reversal of the direction of activation of the LV wall.

METHODS AND RESULTS

A transmural ECG and transmembrane action potentials were simultaneously recorded from epicardial, M, and endocardial cells of arterially perfused canine LV wedge preparations. QT interval increased from 297.6+/-3.9 to 314.0+/-5.7 ms (n=12; P<0.001) and transmural dispersion of repolarization (TDR) increased from 35.5+/-5.2 to 70.3+/-6.2 ms (n=12; P<0.001) as pacing was shifted from endocardium to epicardium. Conduction time between M and epicardial cells increased from 12.1+/-1.2 to 24.2+/-1.5 ms (n=12; P<0.001). Amplification of TDR was further accentuated in the presence of rapidly activating delayed rectifier potassium current blockers (E-4031 and cisapride), increasing from 50.5+/-7.6 to 86.1+/-6.2 ms (n=8; P<0.01). Torsade de pointes arrhythmias could be induced during epicardial, but not endocardial, pacing of LV in the presence of rapidly activating delayed rectifier potassium current blockade.

CONCLUSIONS

Reversal of the direction of activation of the LV wall, as occurs during biventricular pacing, leads to a prominent increase in QT and TDR as a result of earlier repolarization of epicardium and delayed activation and repolarization of the midmyocardial M cells. The increase in TDR creates the substrate for the development of torsade de pointes under long-QT conditions.

A model for human ventricular tissue

The experimental and clinical possibilities for studying cardiac arrhythmias in human ventricular myocardium are very limited. Therefore, the use of alternative methods such as computer simulations is of great importance. In this article we introduce a mathematical model of the action potential of human ventricular cells that, while including a high level of electrophysiological detail, is computationally cost-effective enough to be applied in large-scale spatial simulations for the study of reentrant arrhythmias. The model is based on recent experimental data on most of the major ionic currents: the fast sodium, L-type calcium, transient outward, rapid and slow delayed rectifier, and inward rectifier currents. The model includes a basic calcium dynamics, allowing for the realistic modeling of calcium transients, calcium current inactivation, and the contraction staircase. We are able to reproduce human epicardial, endocardial, and M cell action potentials and show that differences can be explained by differences in the transient outward and slow delayed rectifier currents. Our model reproduces the experimentally observed data on action potential duration restitution, which is an important characteristic for reentrant arrhythmias. The conduction velocity restitution of our model is broader than in other models and agrees better with available data. Finally, we model the dynamics of spiral wave rotation in a two-dimensional sheet of human ventricular tissue and show that the spiral wave follows a complex meandering pattern and has a period of 265 ms. We conclude that the proposed model reproduces a variety of electrophysiological behaviors and provides a basis for studies of reentrant arrhythmias in human ventricular tissue.

Probable case of torsades de pointes induced by fluconazole

A 25-year-old woman who was hospitalized for worsening endocarditis had a prolonged QT interval at baseline and developed monomorphic ventricular arrhythmias, which were managed successfully with pacing and antiarrhythmic therapy. Several days later, the patient started receiving high-dose fluconazole for fungemia and subsequently experienced episodes of torsades de pointes, a polymorphic ventricular arrhythmia associated with a prolonged QT interval or prominent U wave on the electrocardiogram. The arrhythmia developed in the presence of known risk factors. Clinicians should be aware of these risk factors and other relevant structural similarities with drugs that cause torsades de pointes so that they can recognize patients who may be at risk for fluconazole-associated arrhythmia.

Beta-blocker decreases the increase in QT dispersion and transmural dispersion of repolarization induced by bepridil

Bepridil is effective for intractable cardiac arrhythmia, but in rare cases will induce torsades de pointes (TdP) associated with QT interval prolongation. Beta-blockers will effectively prevent TdP in some clinical settings, so the effect of beta-blocker on the change in QT interval, QT dispersion and transmural dispersion of repolarization (TDR) induced by bepridil was investigated in 10 patients (7 male, 3 female; 62+/-6 years old) with intractable paroxysmal atrial fibrillation. The QTc interval, QTc dispersion and TDR were measured before and after 1 month of administration of bepridil, and then a beta-blocker was added and the QTc interval, QTc dispersion and TDR re-measured 1 month later. Bepridil significantly prolonged the QTc interval (0.42+/-0.05 to 0.50+/-0.08; p<0.01), and increased both the QT dispersion (0.07+/-0.05 to 0.14+/-0.08; p<0.01) and TDR (0.10+/-0.04 to 0.16+/-0.05; p<0.01). The addition of a beta-blocker decreased the QTc interval (0.50+/-0.08 to 0.47+/-0.04; p=0.09) and significantly decreased both the QTc dispersion (0.14 +/-0.08 to 0.06+/-0.02; p<0.01) and TDR (0.16+/-0.05 to 0.11+/-0.04; p<0.001). Compared with the control, the combination therapy significantly prolonged the QTc interval, but did not increase either QTc dispersion or TDR, and so was effective in all patients with intractable AF. The findings suggest that beta-blocker reduces the increase in QT dispersion and TDR induced by bepridil, and combined therapy with bepridil and beta-blocker might thus be useful for intractable atrial fibrillation.

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.

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.

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.

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.

Proarrhythmia, a serious complication of antiarrhythmic drugs

Aggravation of arrhythmia with antiarrhythmic drugs is not an infrequent side effect associated with antiarrhythmic drugs. Defined as the provocation of a new arrhythmia or a significant increase in the frequency of a preexisting arrhythmia, it occurs with all antiarrhythmic agents, although the incidence varies from 6% to 23% with the different drugs. In general, arrhythmia aggravation is an early event, occurring within the first several days of initiating drugs therapy. It has been found, however, that this complication can also occur as a late event, particularly in patients who have evidence of ongoing ischemia that may be overt or silent. Although there is no good way to predict the patient who is likely to experience this complication, it has been observed that there several predictors of an increased risk for experiencing arrhythmia aggravation including significant underlying heart disease, congestive heart failure, evidence of active or potentially active ischemia, and electrolyte abnormalities, particularly hypokalemia.