Pharmacologic cardioversion of chronic atrial fibrillation in the goat by class IA, IC, and III drugs: a comparison between hydroquinidine, cibenzoline, flecainide, and d-sotalol

Compartmentalization proteomics revealed endolysosomal protein network changes in a goat model of atrial fibrillation

Endolysosomes (EL) are known for their role in regulating both intracellular trafficking and proteostasis. EL facilitate the elimination of damaged membranes, protein aggregates, membranous organelles and play an important role in calcium signaling. The specific role of EL in cardiac atrial fibrillation (AF) is not well understood. We isolated atrial EL organelles from AF goat biopsies and conducted a comprehensive integrated omics analysis to study the EL-specific proteins and pathways. We also performed electron tomography, protein and enzyme assays on these biopsies. Our results revealed the upregulation of the AMPK pathway and the expression of EL-specific proteins that were not found in whole tissue lysates, including GAA, DYNLRB1, CLTB, SIRT3, CCT2, and muscle-specific HSPB2. We also observed structural anomalies, such as autophagic-vacuole formation, irregularly shaped mitochondria, and glycogen deposition. Our results provide molecular information suggesting EL play a role in AF disease process over extended time frames.

Investigational Anti-Atrial Fibrillation Pharmacology and Mechanisms by Which Antiarrhythmics Terminate the Arrhythmia: Where Are We in 2020?

Antiarrhythmic drugs remain the mainstay therapy for patients with atrial fibrillation (AF). A major disadvantage of the currently available anti-AF agents is the risk of induction of ventricular proarrhythmias. Aiming to reduce this risk, several atrial-specific or -selective ion channel block approaches have been introduced for AF suppression, but only the atrial-selective inhibition of the sodium channel has been demonstrated to be valid in both experimental and clinical studies. Among the other pharmacological anti-AF approaches, “upstream therapy” has been prominent but largely disappointing, and pulmonary delivery of anti-AF drugs seems to be promising. Major contradictions exist in the literature about the electrophysiological mechanisms of AF (ie, reentry or focal?) and the mechanisms by which anti-AF drugs terminate AF, making the search for novel anti-AF approaches largely empirical. Drug-induced termination of AF may or may not be associated with prolongation of the atrial effective refractory period. Anti-AF drug research has been largely based on the “suppress reentry” ideology; however, results of the AF mapping studies increasingly indicate that nonreentrant mechanism(s) plays an important role in the maintenance of AF. Also, the analysis of anti-AF drug-induced electrophysiological alterations during AF, conducted in the current study, leans toward the focal source as the prime mechanism of AF maintenance. More effort should be placed on the investigation of pharmacological suppression of the focal mechanisms.

Anti-arrhythmic investigations in large animal models of atrial fibrillation

Atrial fibrillation (AF) constitutes an increasing health problem in the aging population. Animal models reflecting human phenotypes are needed to understand the mechanisms of AF, as well as to test new pharmacological interventions. In recent years, a number of large animal models, primarily pigs, goats, dog and horses have been used in AF research. These animals can to a certain extent recapitulate the human pathophysiological characteristics and serve as valuable tools in investigating new pharmacological interventions for treating AF. This review focuses on anti-arrhythmic investigations in large animals. Initially, spontaneous AF in small and large mammals is discussed. This is followed by a short presentation of frequently used methods for inducing short- and long-term AF. The major focus of the review is on anti-arrhythmic compounds either frequently used in the human clinic (ranolazine, flecainide, vernakalant and amiodarone) or being promising new AF medicine candidates (I_K,Ach , I_SK,Ca and I_K2P blockers).

Electrophysiological effects of ranolazine in a goat model of lone atrial fibrillation

BACKGROUND

There is still an unmet need for pharmacologic treatment of atrial fibrillation (AF) with few effects on ventricular electrophysiology. Ranolazine is an antiarrhythmic drug reported to have strong atrial selectivity.

OBJECTIVE

The purpose of this study was to investigate the electrophysiological effects of ranolazine in atria with AF-induced electrical remodeling in a model of lone AF in awake goats.

METHODS

Electrode patches were implanted on the atrial epicardium of 8 Dutch milk goats. Experiments were performed at baseline and after 2 and 14 days of electrically maintained AF. Several electrophysiological parameters and AF episode duration were measured during infusion of vehicle and different doses of ranolazine (target plasma levels 4, 8, and 16 μM, respectively).

RESULTS

The highest dose of ranolazine significantly prolonged atrial effective refractory period and decreased atrial conduction velocity at baseline and after 2 days of AF. After 2 weeks of AF, ranolazine prolonged the p5 and p50 of AF cycle length distribution in a dose-dependent manner but was not effective in restoring sinus rhythm. No adverse ventricular arrhythmic events (eg, premature ventricular beats or signs of hemodynamic instability) were observed during infusion of ranolazine at any point in the study.

CONCLUSION

The lowest investigated dose of ranolazine, which is expected to block both late I_Na and atrial peak I_Na, had no effect on the investigated electrophysiological parameters. The highest dose affected both atrial and ventricular electrophysiological parameters at different stages of AF-induced remodeling but was not efficacious in cardioverting AF to sinus rhythm in a goat model of lone AF.

Animal Models of Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia encountered in humans and is a significant source of morbidity and mortality. Despite its prevalence, our mechanistic understanding is incomplete, the therapeutic options have limited efficacy, and are often fraught with risks. A better biological understanding of AF is needed to spearhead novel therapeutic avenues. Although "natural" AF is nearly nonexistent in most species, animal models have contributed significantly to our understanding of AF and some therapeutic options. However, the impediments of animal models are also apparent and stem largely from the differences in basic physiology as well as the complexities underlying human AF; these preclude the creation of a "perfect" animal model and have obviated the translation of animal findings. Herein, we review the vast array of AF models available, spanning the mouse heart (weighing 1/1000th of a human heart) to the horse heart (10× heavier than the human heart). We attempt to highlight the features of each model that bring value to our understanding of AF but also the shortcomings and pitfalls. Finally, we borrowed the concept of a SWOT analysis from the business community (which stands for strengths, weaknesses, opportunities, and threats) and applied this introspective type of analysis to animal models for AF. We identify unmet needs and stress that is in the context of rapidly advancing technologies, these present opportunities for the future use of animal models.

Mechanisms by Which Ranolazine Terminates Paroxysmal but Not Persistent Atrial Fibrillation

BACKGROUND

Ranolazine inhibits Na⁺ current (I_Na), but whether it can convert atrial fibrillation (AF) to sinus rhythm remains unclear. We investigated antiarrhythmic mechanisms of ranolazine in sheep models of paroxysmal (PxAF) and persistent AF (PsAF).

METHODS

PxAF was maintained during acute stretch (N=8), and PsAF was induced by long-term atrial tachypacing (N=9). Isolated, Langendorff-perfused sheep hearts were optically mapped.

RESULTS

In PxAF ranolazine (10 μmol/L) reduced dominant frequency from 8.3±0.4 to 6.2±0.5 Hz (P<0.01) before converting to sinus rhythm, decreased singularity point density from 0.070±0.007 to 0.039±0.005 cm^-2 s^-1 (P<0.001) in left atrial epicardium (LA_epi), and prolonged AF cycle length (AFCL); rotor duration, tip trajectory, and variance of AFCL were unaltered. In PsAF, ranolazine reduced dominant frequency (8.3±0.5 to 6.5±0.4 Hz; P<0.01), prolonged AFCL, increased the variance of AFCL, had no effect on singularity point density (0.048±0.011 to 0.042±0.016 cm^-2 s^-1; P=ns) and failed to convert AF to sinus rhythm. Doubling the ranolazine concentration (20 μmol/L) or supplementing with dofetilide (1 μmol/L) failed to convert PsAF to sinus rhythm. In computer simulations of rotors, reducing I_Na decreased dominant frequency, increased tip meandering and produced vortex shedding on wave interaction with unexcitable regions.

CONCLUSIONS

PxAF and PsAF respond differently to ranolazine. Cardioversion in the former can be attributed partly to decreased dominant frequency and singularity point density, and prolongation of AFCL. In the latter, increased dispersion of AFCL and likely vortex shedding contributes to rotor formation, compensating for any rotor loss, and may underlie the inefficacy of ranolazine to terminate PsAF.

The inward rectifier current inhibitor PA-6 terminates atrial fibrillation and does not cause ventricular arrhythmias in goat and dog models

Background and Purpose

The density of the inward rectifier current (I_K1) increases in atrial fibrillation (AF), shortening effective refractory period and thus promoting atrial re‐entry. The synthetic compound pentamidine analogue 6 (PA‐6) is a selective and potent I_K1 inhibitor. We tested PA‐6 for anti‐AF efficacy and potential proarrhythmia, using established models in large animals.

Experimental Approach

PA‐6 was applied i.v. in anaesthetized goats with rapid pacing‐induced AF and anaesthetized dogs with chronic atrio‐ventricular (AV) block. Electrophysiological and pharmacological parameters were determined.

Key Results

PA‐6 (2.5 mg·kg⁻¹·10 min⁻¹) induced cardioversion to sinus rhythm (SR) in 5/6 goats and prolonged AF cycle length. AF complexity decreased significantly before cardioversion. PA‐6 accumulated in cardiac tissue with ratios between skeletal muscle : atrial muscle : ventricular muscle of approximately 1:8:21. In SR dogs, PA‐6 peak plasma levels 10 min post infusion were 5.5 ± 0.9 μM, PA‐6 did not induce significant prolongation of QTc and did not affect heart rate, PQ or QRS duration. In dogs with chronic AV block, PA‐6 did not affect QRS but lengthened QTc during the experiment, but not chronically. PA‐6 did not induce TdP arrhythmias in nine animals (0/9) in contrast to dofetilide (5/9). PA‐6 (200 nM) inhibited I_K1, but not I_K,ACh, in human isolated atrial cardiomyocytes.

Conclusion and Implications

PA‐6 restored SR in goats with persistent AF and, in dogs with chronic AV block, prolonged QT intervals, without inducing TdP arrhythmias. Our results demonstrate cardiac safety and good anti‐AF properties for PA‐6.

Lack of Correlations between Electrophysiological and Anatomical-Mechanical Atrial Remodeling in Patients with Atrial Fibrillation

BACKGROUND

Atrial fibrillation (AF) progressively leads to electrical remodeling (ER) and anatomical-mechanical remodeling (AR), whose relationships in humans remain poorly known.

METHODS

ER and AR were compared in patients undergoing percutaneous radiofrequency (RF) ablation for AF. ER was defined by right and left appendage activation rates as a surrogate for atrial refractory periods. AR was approached by left atrial (LA) diameters and area and left atrial appendage (LAA) area and contractile function (mean emptying flow velocity) (LAAFV) before RF ablation. Mean duration between successive LAA contractions was considered as LAA mechanical rate.

RESULTS

Forty-one patients (31 men, age: 64 ± 9 years) with paroxysmal (27%), persistent (61%), or long-persistent AF (12%) were prospectively included (ejection fraction: 44 ± 16%). Parameters exploring AR were highly correlated to each other: LA area (28 ± 7 cm(2) ), LAA area (5.7 ± 2.25 cm(2) ), LA transverse (49 ± 7 mm), and anteroposterior diameter (59 ± 13 mm) or LAAFV (29 ± 13 cm/s; P < 0.05 for each comparison). Parameters exploring ER were also highly correlated: right atrial appendage (RAA; 181 ± 39 ms) and LAA (176 ± 33 ms) activation rates (P < 0.0001). There was no significant correlation between any ER and AR parameter. Only LAA mechanical rate (174 ± 36 ms) was correlated to LAA or RAA activations rates (P ≤ 0.01).

CONCLUSION

ER and AR are not mutually related, atrial activation rate being not correlated to LA or LAA size or function. Thus, the mechanisms leading to AF-induced atrial remodeling may differ for anatomical and electrophysiological aspects.

Recurrent intradialytic paroxysmal atrial fibrillation: hypotheses on onset mechanisms based on clinical data and computational analysis

Atrial fibrillation (AF) incidence is high in end-stage renal disease (ESRD) patients, and haemodialysis (HD) session may induce paroxysmal AF episodes. Structural atrium remodelling is common in ESRD patients, moreover, HD session induces rapid plasma electrolytes and blood volume changes, possibly favouring arrhythmia onset. Therefore, HD session represents a unique model to study in vivo the mechanisms potentially inducing paroxysmal AF episodes. Here, we present the case report of a patient in which HD regularly induced paroxysmal AF. In four consecutive sessions, heart rate variability analysis showed a progressive reduction of low/high frequency ratio before the AF onset, suggesting a relative increase in vagal activity. Moreover, all AF episodes were preceded by a great increase of supraventricular ectopic beats. We applied computational modelling of cardiac cellular electrophysiology to these clinical findings, using plasma electrolyte concentrations and heart rate to simulate patient conditions at the beginning of HD session (pre-HD) and right before the AF onset (pre-AF), in a human atrial action potential model. Simulation results provided evidence of a slower depolarization and a shortened refractory period in pre-AF vs. pre-HD, and these effects were enhanced when adding acetylcholine effect. Paroxysmal AF episodes are induced by the presence of a trigger that acts upon a favourable substrate on the background of autonomic nervous system changes and in the described case report all these three elements were present. Starting from these findings, here we review the possible mechanisms leading to intradialytic AF onset.

Antiarrhythmic and electrophysiologic effects of flecainide on acutely induced atrial fibrillation in healthy horses

BACKGROUND

Only few pharmacologic compounds have been validated for treatment of atrial fibrillation (AF) in horses. Studies investigating the utility and safety of flecainide to treat AF in horses have produced conflicting results, and the antiarrhythmic mechanisms of flecainide are not fully understood.

OBJECTIVES

To study the potential of flecainide to terminate acutely induced AF of short duration (≥ 15 minutes), to examine flecainide-induced changes in AF duration and AF vulnerability, and to investigate the in vivo effects of flecainide on right atrial effective refractory period, AF cycle length, and ventricular depolarization and repolarization.

ANIMALS

Nine Standardbred horses. Eight received flecainide, 3 were used as time-matched controls, 2 of which also received flecainide.

METHODS

Prospective study. The antiarrhythmic and electrophysiologic effects of flecainide were based on 5 parameters: ability to terminate acute pacing-induced AF (≥ 15 minutes), and drug-induced changes in atrial effective refractory period, AF duration, AF vulnerability, and ventricular depolarization and repolarization times. Parameters were assessed at baseline and after flecainide by programmed electrical stimulation methods.

RESULTS

Flecainide terminated all acutely induced AF episodes (n = 7); (AF duration, 21 ± 5 minutes) and significantly decreased the AF duration, but neither altered atrial effective refractory period nor AF vulnerability significantly. Ventricular repolarization time was prolonged between 8 and 20 minutes after initiation of flecainide infusion, but no ventricular arrhythmias were detected.

CONCLUSIONS AND CLINICAL IMPORTANCE

Flecainide had clear antiarrhythmic properties in terminating acute pacing-induced AF, but showed no protective properties against immediate reinduction of AF. Flecainide caused temporary prolongation in the ventricular repolarization, which may be a proarrhythmic effect.

Tissue velocity imaging of the left atrium predicts response to flecainide in patients with acute atrial fibrillation

BACKGROUND

Acute atrial fibrillation (AF) is often treated with the administration of intravenous flecainide; however, this treatment may not always be successful and is potentially hazardous. Previous studies suggest that electro-echocardiographic tissue velocity imaging (TVI) of the atrial wall may reflect atrial remodeling.

OBJECTIVE

To study whether atrial TVI can be used to identify nonresponders of flecainide administered intravenously in patients with acute AF.

METHODS

We used atrial TVI to measure atrial fibrillatory cycle length determined by using tissue velocity imaging (AFCL-TVI) and atrial fibrillatory wall motion velocity determined by using tissue velocity imaging (AFV-TVI) in the left atrium in 52 (55%) patients presenting with acute AF in the emergency department. These 2 parameters reflect electrical and structural remodeling, respectively. Standard baseline characteristics were recorded.

RESULTS

Patients were predominantly men (76%) and 64 ± 11 years old. Thirty-six (69%) patients had successful cardioversion after flecainide infusion. There were no significant differences in baseline characteristics between responders and nonresponders. Patients with a successful cardioversion had a longer mean AFCL-TVI and higher median (interquartile range) AFV-TVI compared with patients with failed cardioversion: 172 ± 29 ms vs 137 ± 35 ms (P < .001) and 4.2 (3.3-6.2) cm/s vs 2.3 (1.9-3.5) cm/s (P = .001).

CONCLUSIONS

Electro-echocardiographic atrial TVI measurement is a promising noninvasive tool for predicting outcome of pharmacological cardioversion. A short AFCL-TVI and a low AFV-TVI are related to failure of cardioversion of AF using flecainide.

Rotors and the dynamics of cardiac fibrillation

The objective of this article is to present a broad review of the role of cardiac electric rotors and their accompanying spiral waves in the mechanism of cardiac fibrillation. At the outset, we present a brief historical overview regarding reentry and then discuss the basic concepts and terminologies pertaining to rotors and their initiation. Thereafter, the intrinsic properties of rotors and spiral waves, including phase singularities, wavefront curvature, and dominant frequency maps, are discussed. The implications of rotor dynamics for the spatiotemporal organization of fibrillation, independent of the species being studied, are described next. The knowledge gained regarding the role of cardiac structure in the initiation or maintenance of rotors and the ionic bases of spiral waves in the past 2 decades, as well as the significance for drug therapy, is reviewed subsequently. We conclude by examining recent evidence suggesting that rotors are critical in sustaining both atrial and ventricular fibrillation in the human heart and its implications for treatment with radiofrequency ablation.

The intrinsic autonomic nervous system in atrial fibrillation: a review

The procedure of catheter ablation for the treatment of drug resistant atrial fibrillation (AF) has evolved but still relies on lesion sets intended to isolate areas of focal firing, mainly the myocardial sleeves of the pulmonary veins (PVs), from the rest of the atria. However the success rates for this procedure have varied inversely with the type of AF. At best success rates have been 20 to 30% below that of other catheter ablation procedures for Wolff-Parkinson-White syndrome, atrioventricular junctional re-entrant tachycardia and atrial flutter. Basic and clinical evidence has emerged suggesting a critical role of the ganglionated plexi (GP) at the PV-atrial junctions in the initiation and maintenance of the focal form of AF. At present the highest success rates have been obtained with the combination of PV isolation and GP ablation both as catheter ablation or minimally invasive surgical procedures. Various lines of evidence from earlier and more recent reports provide that both neurally based and myocardially based forms of AF can separately dominate or coexist within the context of atrial remodeling. Future studies are focusing on non-pharmacological, non-ablative approaches for the prevention and treatment of AF in order to avoid the substantive complications of both these regimens.

Repetitive evaluation of fibrillation cycle length predicts the efficacy of bepridil for interruption of long-lasting persistent atrial fibrillation

Although bepridil is effective for conversion of long-lasting persistent atrial fibrillation (AF) to sinus rhythm, it sometimes takes a long time to interrupt AF and there is no feasible index to predict its efficacy.In 60 patients with long-lasting persistent AF, bepridil (100-200 mg/day) was administered and continued for 8 weeks while body surface ECG was recorded every 2 weeks. The fibrillation cycle length (FCL) was evaluated using the spectral analysis of the fibrillation waves in each ECG. AF was interrupted in 32 patients receiving bepridil. The conversion was observed at 2 weeks in 4, at 4 weeks in 7, at 6 weeks in 7, and at 8 weeks in 14 patients. When comparing these responders and nonresponders, clinical background characteristics other than the dosage of bepridil did not show any difference and neither did temporal changes in QT parameters and heart rate. In contrast, the FCL and ΔFCL (prolongation in FCL from baseline) became significantly larger in responders than in nonresponders at later observation points (FCL: 177 ± 17 versus 164 ± 19 ms, P = 0.018, and ΔFCL: 38 ± 16 versus 22 ± 12 ms, P < 0.001, at 4-week point; FCL: 188 ± 17 versus 169 ± 19 ms, P = 0.004, and ΔFCL: 49 ± 18 versus 27 ± 14 ms, P < 0.001, at 6-week point).Repetitive evaluation of FCL using spectral analysis of fibrillation waves can be a feasible index to predict the efficacy of bepridil therapy.

Combined blockade of early and late activated atrial potassium currents suppresses atrial fibrillation in a pig model of obstructive apnea

BACKGROUND

Negative tracheal pressure (NTP) during tracheal obstruction in obstructive apnea increases vagal tone and causes pronounced shortening of the atrial effective refractory period (AERP), thereby perpetuating atrial fibrillation (AF). The role of different atrial potassium channels under those conditions has not been investigated.

OBJECTIVE

The purpose of this study was to evaluate the atrial effects of blockade of the late activated potassium current (I(Kr)) by sotalol, of blockade of the early activated potassium currents (I(Kur)/I(to)) by AVE0118, and of the multichannel blocker amiodarone during tracheal occlusions with applied NTP.

METHODS

Twenty-one pigs were anesthetized, and an endotracheal tube was placed to apply NTP (up to -100 mbar) comparable to clinically observed obstructive sleep apnea for 2 minutes. Right AERP and AF inducibility were measured transvenously by a monophasic action potential recording and stimulation catheter.

RESULTS

Tracheal occlusion with applied NTP caused pronounced AERP shortening. AF was inducible during all NTP maneuvers. Blockade of I(Kr) by sotalol, blockade of I(Kur)/I(to) by AVE0118, and amiodarone did not affect NTP-induced AERP shortening, although they prolonged the AERP during normal breathing. Atropine given after amiodarone completely inhibited NTP-induced AERP shortening. The combined blockade of I(Kr) and I(Kur)/I(to) by sotalol plus AVE0118, however, attenuated NTP-induced AERP shortening and AF inducibility independent of the order of administration.

CONCLUSION

The atrial proarrhythmic effect of NTP simulating obstructive apneas is difficult to inhibit by class III antiarrhythmic drugs. Neither amiodarone nor blockade of I(Kr) or I(Kur)/I(to) attenuated NTP-induced AERP shortening. However, the combined blockade of I(Kur)/I(to) and I(Kr) suppressed NTP-induced AERP shortening.

Mechanisms of termination and prevention of atrial fibrillation by drug therapy

Atrial fibrillation (AF) is a disorder of the rhythm of electrical activation of the cardiac atria. It is the most common cardiac arrhythmia, has multiple aetiologies, and increases the risk of death from stroke. Pharmacological therapy is the mainstay of treatment for AF, but currently available anti-arrhythmic drugs have limited efficacy and safety. An improved understanding of how anti-arrhythmic drugs affect the electrophysiological mechanisms of AF initiation and maintenance, in the setting of the different cardiac diseases that predispose to AF, is therefore required. A variety of animal models of AF has been developed, to represent and control the pathophysiological causes and risk factors of AF, and to permit the measurement of detailed and invasive parameters relating to the associated electrophysiological mechanisms of AF. The purpose of this review is to examine, consolidate and compare available relevant data on in-vivo electrophysiological mechanisms of AF suppression by currently approved and investigational anti-arrhythmic drugs in such models. These include the Vaughan Williams class I-IV drugs, namely Na(+) channel blockers, β-adrenoceptor antagonists, action potential prolonging drugs, and Ca(2+) channel blockers; the "upstream therapies", e.g., angiotensin converting enzyme inhibitors, statins and fish oils; and a variety of investigational drugs such as "atrial-selective" multiple ion channel blockers, gap junction-enhancers, and intracellular Ca(2+)-handling modulators. It is hoped that this will help to clarify the main electrophysiological mechanisms of action of different and related drug types in different disease settings, and the likely clinical significance and potential future exploitation of such mechanisms.

The Autonomic Nervous System and Atrial Fibrillation:The Roles of Pulmonary Vein Isolation and Ganglionated Plexi Ablation

After the sequential successes of catheter ablation for the treatment of pre-excitation syndromes (WPW), junctional reentry (AVNRT) atrial flutter (AFL) and ventricular arrhythmias, clinical electrophysiologists have focused on the myocardial basis of atrial fibrillation (AF). Thus, the strategy for ablation of drug and cardioversion refractory AF was to isolate the myocardial connections from the focal firing pulmonary veins (PVs) in addition to altering the atrial substrate maintaining AF. However, the overall success rates have not achieved those of the other types of ablation procedures. In this review we have summarized the favorable aspects and drawbacks of pulmonary vein isolation (PVI). As for the role of the Intrinsic Cardiac Autonomic Nervous System (ICANS), both basic and clinical evidence has shown that ganglionated plexi (GP) stimulation promotes initiation and maintenance of AF, and that GP ablation reduces recurrence of AF following catheter or surgical ablation of these structures. Based on these findings, the GP Hyperactivity Hypothesis has been proposed to explain, at least in part, the mechanistic basis for the focal form of AF. For example, PV isolation may not always be necessary for elimination of AF, as in the early stages of paroxysmal AF. GP ablation alone, in these cases, may suffice for focal AF termination. In the persistent and long standing persistent forms the substrate for AF may be more extensive and therefore require GP ablation plus PV isolation and/or CFAE ablations. Clinical reports, both catheter based as well as minimally invasive surgical procedures, which include PVI plus GP ablation have shown relatively long-term success rates much closer to or equal to those achieved by myocardial ablation procedures in patients with WPW, AVNRT and AFL.

Serial Cardioversion by Class IC Drugs During 4 Months of Persistent Atrial Fibrillation in the Goat

INTRODUCTION

The success rate of pharmacological cardioversion of atrial fibrillation (AF) in patients depends on the duration of AF. It is unknown to what extent AF-induced structural atrial remodeling contributes to this loss of efficacy.

METHODS AND RESULTS

In 10 goats, persistent AF was induced by repetitive burst pacing. During a time period of 16 weeks, the efficacy of flecainide and cibenzoline to cardiovert AF was investigated by serial cardioversion. The drugs were administered intravenously at a rate of 0.1 mg/kg/min. AF cycle length (AFCL) was continuously monitored. Drug infusion was continued until AF was successfully cardioverted or the QRS duration was prolonged about twofold. The average atrial cycle length during persistent AF was 104 +/- 10 msec and did not change during the 16-week period. The success rate of cardioversion by flecainide and cibenzoline decreased with the duration of AF from 60% to 17% and from 80% to 63%. In goats that failed to cardiovert, sinus rhythm was not restored despite a twofold prolongation of the AF cycle length (respectively from 96 +/- 5 msec to 168 +/- 30 msec (flecainide) and 203 +/- 26 msec (cibenzoline)). The sensitivity of AF for Class IC drugs was not altered with time, and the dose-dependent effect on AFCL remained the same (flecainide: 8 +/- 5 vs 7 +/- 2 msec/mg/kg (P = 0.70) and cibenzoline: 13 +/- 3 vs 13 +/- 5 msec/mg/kg (P = 0.95)). In animals in which cardioversion remained possible, the critical AFCL at which cardioversion occurred increased from 96 +/- 5 msec to 211 msec (flecainide) and 189 +/- 24 msec (cibenzoline).

CONCLUSIONS

The progressive loss of efficacy of Class IC drugs to cardiovert AF of longer duration is not due to a decrease in the sensitivity of remodeled atrial myocardium for Class IC drugs. Failure of cardioversion was due to an increase in the critical AF cycle length required for pharmacological cardioversion.

Effects of Na(+) and K(+) channel blockade on vulnerability to and termination of fibrillation in simulated normal cardiac tissue

Na(+) and K(+) channel-blocking drugs have anti- and proarrhythmic effects. Their effects during fibrillation, however, remain poorly understood. We used computer simulation of a two-dimensional (2-D) structurally normal tissue model with phase I of the Luo-Rudy action potential model to study the effects of Na(+) and K(+) channel blockade on vulnerability to and termination of reentry in simulated multiple-wavelet and mother rotor fibrillation. Our main findings are as follows: 1) Na(+) channel blockade decreased, whereas K(+) channel blockade increased, the vulnerable window of reentry in heterogeneous 2-D tissue because of opposing effects on dynamical wave instability. 2) Na(+) channel blockade increased the cycle length of reentry more than it increased refractoriness. In multiple-wavelet fibrillation, Na(+) channel blockade first increased and then decreased the average duration or transient time (<T(s)>) of fibrillation. In mother rotor fibrillation, Na(+) channel blockade caused peripheral fibrillatory conduction block to resolve and the mother rotor to drift, leading to self-termination or sustained tachycardia. 3) K(+) channel blockade increased dynamical instability by steepening action potential duration restitution. In multiple-wavelet fibrillation, this effect shortened <T(s)> because of enhanced wave instability. In mother rotor fibrillation, this effect converted mother rotor fibrillation to multiple-wavelet fibrillation, which then could self-terminate. Our findings help illuminate, from a theoretical perspective, the possible underlying mechanisms of termination of different types of fibrillation by antiarrhythmic drugs.

Mechanisms of Atrial Fibrillation Termination by Pure Sodium Channel Blockade in an Ionically-Realistic Mathematical Model

The mechanisms by which Na + -channel blocking antiarrhythmic drugs terminate atrial fibrillation (AF) remain unclear. Classical “leading-circle” theory suggests that Na + -channel blockade should, if anything, promote re-entry. We used an ionically-based mathematical model of vagotonic AF to evaluate the effects of applying pure Na + -current (I Na ) inhibition during sustained arrhythmia. Under control conditions, AF was maintained by 1 or 2 dominant spiral waves, with fibrillatory propagation at critical levels of action potential duration (APD) dispersion. I Na inhibition terminated AF increasingly with increasing block, terminating all AF at 65% block. During 1:1 conduction, I Na inhibition reduced APD (by 13% at 4 Hz and 60% block), conduction velocity (by 37%), and re-entry wavelength (by 24%). During AF, I Na inhibition increased the size of primary rotors and reduced re-entry rate (eg, dominant frequency decreased by 33% at 60% I Na inhibition) while decreasing generation of secondary wavelets by wavebreak. Three mechanisms contributed to I Na block–induced AF termination in the model: (1) enlargement of the center of rotation beyond the capacity of the computational substrate; (2) decreased anchoring to functional obstacles, increasing meander and extinction at boundaries; and (3) reduction in the number of secondary wavelets that could provide new primary rotors. Optical mapping in isolated sheep hearts confirmed that tetrodotoxin dose-dependently terminates AF while producing effects qualitatively like those of I Na inhibition in the mathematical model. We conclude that pure I Na inhibition terminates AF, producing activation changes consistent with previous clinical and experimental observations. These results provide insights into previously enigmatic mechanisms of class I antiarrhythmic drug-induced AF termination. The full text of this article is available online at http://circres.ahajournals.org

Pharmacological cardioversion of atrial fibrillation: current management and treatment options

Atrial fibrillation (AF) is the most common form of arrhythmia, carrying high social costs. It is usually first seen by general practitioners or in emergency departments. Despite the availability of consensus guidelines, considerable variations exist in treatment practice, especially outside specialised cardiological settings. Cardioversion to sinus rhythm aims to: (i) restore the atrial contribution to ventricular filling/output; (ii) regularise ventricular rate; and (iii) interrupt atrial remodelling. Cardioversion always requires careful assessment of potential proarrhythmic and thromboembolic risks, and this translates into the need to personalise treatment decisions. Among the many clinical variables that affect strategy selection, time from onset is crucial. In selected patients, pharmacological cardioversion of recent-onset AF can be a safely used, feasible and effective approach, even in internal medicine and emergency departments. In most cases of recent-onset AF, pharmacological cardioversion provides an important--and probably more cost effective--alternative to electrical cardioversion, which can then be employed as a second-line therapy for nonresponders. Class IC agents (flecainide or propafenone), which can be safely used in hospitalised patients with recent-onset AF without left ventricular dysfunction, can provide rapid conversion to sinus rhythm after either intravenous administration or oral loading. Although intravenous amiodarone requires longer conversion times, it is still the standard treatment for patients with heart failure. Ibutilide also provides good conversion rates and could be used for AF patients with left ventricular dysfunction (were it not for high costs). For long-lasting AF most pharmacological treatments have only limited efficacy and electrical cardioversion remains the gold standard in this setting. However, a widely used strategy involves pretreatment with amiodarone in the weeks before planned electrical cardioversion: this provides optimal prophylaxis and can sometimes even restore sinus rhythm. Dofetilide may also be capable of restoring sinus rhythm in up to 25-30% of patients and can be used in patients with heart failure. The potential risk of proarrhythmia increases the need for careful therapeutic decision making and management of pharmacological cardioversion. The results of recent trials (AFFIRM [Atrial Fibrillation Follow-up Investigation of Rhythm Management] and RACE [Rate Control versus Electrical Cardioversion for Persistent Atrial Fibrillation]) on rate versus rhythm control strategies in the long term have led to a generalised shift in interest towards rate control. Although carefully designed studies are required to better define the role of pharmacological rhythm control in specific AF settings, this alternative option remains a recommendable strategy for many patients, especially those in acute care.

Sotalol reverses remodeled action potential in patients with chronic atrial fibrillation but does not prevent arrhythmia recurrence

INTRODUCTION

Recurrence of atrial fibrillation (AF) may be related to AF-induced electrical remodeling characterized by shortening of the atrial action potential duration (APD) and loss of its rate adaptation. We investigated the effects of pretreatment with oral d,l-sotalol on rate-dependent changes in atrial monophasic action potential (MAP) duration after cardioversion of chronic AF with reference to the efficacy in preventing the arrhythmia recurrence.

METHODS AND RESULTS

MAPs were recorded from the right atrium at six pacing cycle lengths (CLs) from 300 to 750 ms in 19 chronic AF patients after electrical cardioversion; 9 had been pretreated with oral d,l-sotalol (196 +/- 42 mg/day) for 7 days and 10 were untreated. MAP duration at 90% repolarization (MAPD90) in 11 control patients increased progressively with increases in CLs from 209 +/- 19 ms at CL = 300 ms to 264 +/- 28 ms at CL = 750 ms. In AF patients without sotalol, the CL-MAPD relation was shifted downward and flattened at longer CLs; MAPD90 values were 206 +/- 11 ms and 227 +/- 16 ms at CLs of 300 and 750 ms, respectively. MAPD90 values at CLs > or =500 ms in AF were significantly shorter than controls. In AF patients with sotalol, the normal CL-MAPD relation was preserved; MAPD90 increased from 226 +/- 19 ms to 282 +/- 46 ms in the CL range. AF recurred within 2 weeks after cardioversion in 14 of 24 patients pretreated with d,l-sotalol (216 +/- 51 mg/day) despite of continuation of sotalol treatment.

CONCLUSION

Sotalol reverses AF-induced decrease in MAPD adaptation to rate in the atria of chronic AF patients, but this effect does not lead to prevention of AF recurrence.

Atrial Fibrillation in Rats Induced by Rapid Transesophageal Atrial Pacing during Brief Episodes of Asphyxia: A New In Vivo Model

Non-pharmacological in vivo models of atrial fibrillation (AF) have been developed in large animals only. We aimed to develop and characterize a new small animal non-pharmacological in vivo model of AF. AF was induced by transesophageal atrial burst pacing during 35 seconds periods of asphyxia in anesthetized male Sprague-Dawley rats. AF was reproducibly induced in 81% of the rats. The presence of AF was associated with an increased heart rate, and a decreased blood pressure. Treatment with amiodarone, D,L-sotalol, flecainide, and propranolol all reduced duration of AF, whereas verapamil treatment was associated with a marked profibrillatory effect. Increasing gap junction intracellular communication using the antiarrhythmic peptide analogue AAP10 did not affect AF duration. Basal plasma level of epinephrine and norepinephrine were increased 5- to 20-fold relative to values reported by others, but unchanged following 35 seconds of asphyxia. The results from our study demonstrate that the rat model shares several clinical key characteristics with human AF: (1) hemodynamic response to AF; (2) increased autonomic tone; (3) antiarrhythmic effects of clinically used drugs; (4) profibrillatory effect of verapamil. Relative to existing models of AF in larger animals, this model offers rapid, predictive, and inexpensive testing of antiarrhythmic/profibrillatory effects of new drugs.

Overview of the management of atrial fibrillation: what is the current state of the art?

Management of Atrial Fibrillation. There are three fundamental approaches to the management of atrial fibrillation (AF): rate control, rhythm control, and anticoagulation. Selecting a course of treatment requires a thorough knowledge of these therapeutic alternatives. This article explores treatment options, including the relative benefits of rate control versus rhythm control, which are complicated by the lack of highly effective and safe antiarrhythmic drugs. Anticoagulation is also an important issue in AF management, and warfarin effectively reduces the incidence of thromboembolic events in AF patients. The use of warfarin, however, presents its own complications. We conclude that individualization of therapy is paramount when treating AF.

[Electrophysiologic remodeling and drug treatment of atrial fibrillation]

Since 1995, a number of studies have established and detailed the mechanisms of electrical and structural atrial remodeling induced by atrial fibrillation. Atrial remodeling involves many cellular components, from ionic channels to connexins. The determination of these mechanisms may help to define a new therapeutic targets of atrial fibrillation, a frequent arrhythmia that remains difficult to treat. Atrial remodeling prevention may lead to limit the evolution of the arrhythmia (early recurrences after reduction, AF secondary to atrial tachycardia, permanent AF, decrease in atrial contractility, sinus dysfunction). Except amiodarone, the usual antiarrhythmic drugs have no effect on atrial remodeling. Calcium channel inhibitors prevent early remodeling but have no effect on prolonged remodeling. Digoxin increases remodeling. Angiotensin II receptor inhibitors have been shown to prevent early AF recurrence after reduction and are very promising in such a direction. Other methods such as the one of antioxidant therapy seem to be promising and could define soon a new antiarrhythmic therapeutic class, the antiremodeling drugs.

Recent advances in drug therapy for atrial fibrillation

Despite the major new insights into our knowledge of the mechanisms underlying initiation and perpetuation of atrial fibrillation (AF) gained in the last decade, the treatment of this common arrhythmia remains unsatisfactory in many patients. Although several new treatment modalities (e.g., internal cardioversion, pulmonary vein ablation, preventive pacing) have been developed, pharmacologic therapy remains the first-line therapy in most patients with AF. As illustrated by recent trials comparing rhythm control and rate control, current antifibrillatory drugs are hampered by a relatively low success rate in maintaining long-term sinus rhythm and the occurrence of proarrhythmic and other adverse events. This article discusses currently available antiarrhythmic drugs for rhythm and rate control, with special emphasis on more recently developed drugs and drugs still under development. Selective blockers of atrial ion channels (IKur and IK.ACh), multi-ion channel blockers, and selective A1-adenosine receptor antagonists are examples of the newer antiarrhythmic drugs that are expected to be more effective and safer than those currently available.

Preserved effects of potassium channel blockers in the pacing-induced remodeled canine atrium: a comparison between E4031 and azimilide

This study was designed to evaluate the electrophysiologic effects of E4031 (a pure IKr blocker) and azimilide (AZ: a combined Ikr + IKs blocker) at various stages of atrial electrical remodeling. Twelve dogs underwent continuous rapid atrial pacing (400/min) for 14 days. The electrophysiologic study was performed on the day before as well as after 2, 7, and 14 days of rapid atrial pacing both before and after the administration of either E4031 (n = 6) or AZ (n = 6). In response to rapid atrial pacing, the atrial effective refractory period (ERP), conduction velocity, and wavelength decreased significantly at pacing cycle lengths (PCLs) of 200 and 400 ms (P < 0.05). E4031 prolonged ERP in a reverse use-dependent manner throughout the study period. AZ also prolonged ERP during the 14 days of rapid pacing. ERP prolongation at a PCL of 200 ms was significantly greater with AZ than with E4031 (P < 0.05). The effects of blocking IKr by E4031 and IKr + IKs by AZ were well preserved at various stages of atrial electrical remodeling. However, the effect of prolonging ERP at a shorter PCL was more prominent by AZ than by E4031. Thus, IKs blockade may add a favorable anti-fibrillatory effect to IKr blockade even in the remodeled atrium.

Electrophysiologic actions of dl-sotalol in patients with persistent atrial fibrillation

OBJECTIVES

We sought to determine the electrophysiologic actions of sotalol in the remodeled atrium of humans.

BACKGROUND

In experimental studies, sotalol has limited class III action in the electrically remodeled atrium and did not prevent atrial fibrillation (AF) induction.

METHODS

We determined the effective refractory periods (ERPs) at three pacing cycle lengths (400, 500, and 600 ms) in the high right atrium (HRA) and distal coronary sinus (DCS) before and after intravenous infusion of dl-sotalol in 10 patients with persistent AF who underwent internal cardioversion. The same protocols were performed in 10 control subjects in sinus rhythm.

RESULTS

In the HRA and DCS, the atrial ERPs at different drive cycle lengths were significantly shorter in patients with AF than in control subjects (p < 0.05). In patients with AF, the atrial ERP's adaptation to rate was nearly normal in the HRA, but was poor in the DCS. In both groups, dl-sotalol significantly increased the atrial ERPs at both the HRA and DCS, as compared with baseline (p < 0.05). However, the prolongation of atrial ERPs was significantly less at a drive cycle length of 600 ms in patients with AF versus control subjects (p < 0.05). After infusion of dl-sotalol, the atrial ERP's adaptation to rate at both the HRA and DCS was poor in patients with AF, and AF was still easily inducible in the majority of them, but not in control subjects.

CONCLUSIONS

The results of the present study demonstrate that the electrophysiologic actions of dl-sotalol are significantly attenuated in the chronically remodeled human atrium, and these changes might represent a probable explanation for the low efficacy of dl-sotalol to prevent early AF recurrence after electrical cardioversion.

Effects of acute reduction of temperature on ventricular fibrillation activation patterns

Because of its electrophysiological effects, hypothermia can influence the mechanisms that intervene in the sustaining of ventricular fibrillation. We hypothesized that a rapid and profound reduction of myocardial temperature impedes the maintenance of ventricular fibrillation, leading to termination of the arrhythmia. High-resolution epicardial mapping (series 1; n = 11) and transmural recordings of ventricular activation (series 2; n = 10) were used to analyze ventricular fibrillation modification during rapid myocardial cooling in Langendorff-perfused rabbit hearts. Myocardial cooling was produced by the injection of cold Tyrode into the left ventricle after induction of ventricular fibrillation. Temperature and ventricular fibrillation dominant frequency decay fit an exponential model to arrhythmia termination in all experiments, and both parameters were significantly correlated (r = 0.70, P < 0.0001). Termination of the arrhythmia occurred preferentially in the left ventricle and was associated with a reduction in conduction velocity (-60% in left ventricle and -54% in right ventricle; P < 0.0001) and with activation maps predominantly exhibiting a single wave front, with evidence of wave front extinction. We conclude that a rapid reduction of temperature to <20 degrees C terminates ventricular fibrillation after producing an important depression in myocardial conduction.

Electropharmacologic effects of pilsicainide, a pure sodium channel blocker, on the remodeled atrium subjected to chronic rapid pacing

Clinical experience suggests that sodium channel blockers are effective in converting atrial fibrillation of recent onset but not chronic atrial fibrillation. We investigated changes in the electrophysiologic effects of pilsicainide, a pure sodium channel blocker, on the canine atrium during chronic rapid pacing (400/min). Three pairs of bipolar electrodes were sutured to the right atrial appendage in six dogs. Five days later, rapid atrial pacing was started after baseline measurements of the effective refractory period (ERP), the intra-atrial conduction velocity, the atrial wavelength, and the inducibility of atrial fibrillation. These studies were repeated at 2, 7, and 14 days of pacing, both before and after pilsicainide administration. Before pacing, pilsicainide increased ERP more than it decreased conduction velocity, causing an increase of wavelength, particularly at faster rates. However, this use-dependent prolongation of ERP disappeared after 2 days of pacing. Thus, pilsicainide failed to prolong ERP during chronic pacing, allowing progressive shortening of wavelength in the remodeled atrium. The effect of sodium channel blockers on atrial refractoriness may decline as rapid atrial excitation persists, limiting the usefulness of these agents for the treatment of chronic atrial fibrillation.

Widening of the excitable gap during pharmacological cardioversion of atrial fibrillation in the goat: effects of cibenzoline, hydroquinidine, flecainide, and d-sotalol

BACKGROUND

Previous studies suggest that the antifibrillatory action of class I and III drugs is due to prolongation of the atrial wavelength. The aim of the present study was to directly evaluate the electrophysiological action of antifibrillatory drugs in a goat model of chronic atrial fibrillation (AF).

METHODS AND RESULTS

Six goats were instrumented with multiple atrial electrodes, and sustained AF was induced by electrical remodeling. During sustained AF, the effects of intravenous infusion of cibenzoline, hydroquinidine, flecainide, and d-sotalol on AF cycle length (AFCL), refractory period (RP(AF)), conduction velocity (CV(AF)), pathlength (PL(AF)), wavelength (WL(AF)), temporal (AFCL-RP(AF)), and spatial (PL(AF)-WL(AF)) excitable gap were studied. The RP(AF) was measured by determining the earliest moment at which single stimuli could capture the fibrillating atria. CV(AF) was measured during regional entrainment of AF. Contrary to our expectation, cardioversion of AF could not be attributed to prolongation of WL(AF). Hydroquinidine and d-sotalol did not affect WL(AF) significantly, whereas cibenzoline and flecainide even shortened WL(AF) by 18% and 36%, respectively. PL(AF) was increased by hydroquinidine and d-sotalol by 30%, whereas cibenzoline and flecainide did not prolong PL(AF). The only parameter that correlated consistently with cardioversion of AF was a widening of the temporal excitable gap (cibenzoline 176%, hydroquinidine 105%, flecainide 86%, d-sotalol 88%).

CONCLUSIONS

Pharmacological cardioversion of AF cannot be explained by prolongation of WL(AF). An alternative explanation for the antifibrillatory effect of class I and III drugs may be a widening of the temporal excitable gap.

Contrasting efficacy of dofetilide in differing experimental models of atrial fibrillation

BACKGROUND

Rapid atrial pacing (RAP) and congestive heart failure (CHF) produce different experimental substrates for atrial fibrillation (AF). We tested the hypothesis that AF maintained by different substrates responds differently to antiarrhythmic-drug therapy.

METHODS AND RESULTS

The class III antiarrhythmic agent dofetilide was given intravenously at doses of 10 (D10) and 80 (D80) microg/kg to dogs with AF induced either (1) after 7 days of RAP at 400 bpm or (2) in the presence of CHF induced by rapid ventricular pacing. Dofetilide terminated AF in all CHF dogs, but D10 failed to terminate AF in any RAP dog, and D80 terminated AF in only 1 of 5 RAP dogs (20%) (P<0.01 for efficacy in CHF versus RAP dogs). Dofetilide was highly effective in preventing AF induction by atrial burst pacing in dogs with CHF but was totally ineffective in dogs with RAP. Dofetilide increased atrial effective refractory period and AF cycle length to a greater extent in CHF dogs. Epicardial mapping with 248 bipolar electrodes showed that CHF-related AF was often due to macroreentry, with dofetilide terminating AF by causing block in reentry circuits. RAP-related AF was due to multiple-wave front reentry, with dofetilide slowing reentry and decreasing the number of simultaneous waves, but not sufficiently to stop AF.

CONCLUSIONS

The mechanism underlying AF importantly influences dofetilide efficacy. The dependence of drug efficacy in AF on the underlying mechanism has potentially significant implications for antiarrhythmic drug use and development and may explain the well-known therapeutic resistance of longer-duration AF.

Experimental models of atrial fibrillation/flutter

Animal models that mimic disease states or abnormal physiological events are tools that assist the investigator to understand the mechanism responsible for pathophysiological conditions. Atrial arrhythmias have intrigued physicians and cardiologists for decades. Thus the development of animal models for the study of atrial arrhythmias facilitate the investigation of these abnormal rhythms. Moreover, as our understanding of arrhythmias advances, so does the therapy designed to correct the condition, which ultimately improves the patient's clinical outcome. This manuscript describes a variety of animal models that have been utilized for the exploration of atrial arrhythmia generation and maintenance, as well as models used to evaluate the efficacy of putative antiarrhythmics agents.

The tachycardia-induced dog model of atrial fibrillation. clinical relevance and comparison with other models

In the past, investigators have relied extensively on acute in vivo models of atrial fibrillation (AF), in which AF was induced either pharmacologicly or by vagal stimulation. More recently, there is a need and desire for more clinically relevant models that can only be achieved with the use of chronically instrumented animals. One of these models is the atrial tachycardia-induced AF dog model, which is the main focus of this review. The model produces a persistent AF in 80% of animals paced at 400 beats/min for 6 weeks. Atrial tachycardia also induces various pathophysiologic and ultrastructural changes that often resemble electrical remodeling of atria in patients that have a high susceptibility to AF. This model can also be used to evaluate drug efficacy with respect to attenuation of AF duration or conversion of AF to sinus rhythm. The model may therefore be used to provide further insights into the discovery of new therapeutic approaches to modifying this atrial arrhythmic disorder in man.