Spatial distribution of conduction disorders during sinus rhythm

BACKGROUND

Length of lines of conduction block (CB) during sinus rhythm (SR) at Bachmann's bundle (BB) is associated with atrial fibrillation (AF). However, it is unknown whether extensiveness of CB at BB represents CB elsewhere in the atria. We aim to investigate during SR 1) the spatial distribution and extensiveness of CB 2) whether there is a predilection site for CB and 3) the association between CB and incidence of post-operative AF.

METHODS

During SR, epicardial mapping of the right atrium (RA), BB and left atrium was performed in 209 patients with coronary artery disease. The amount of conduction delay (CD, Δlocal activation time ≥7ms) and CB (Δ≥12ms) was quantified as % of the mapping area. Atrial regions were compared to identify potential predilection sites for CD/CB. Correlations between CD/CB and clinical characteristics were tested.

RESULTS

Areas with CD or CB were present in all patients, overall prevalence was respectively 1.4(0.2-4.0) % and 1.3(0.1-4.3) %. Extensiveness and spatial distribution of CD/CB varied considerably, however occurred mainly at the superior intercaval RA. Of all clinicalcharacteristics, CD/CB only correlated weakly with age and diabetes (P<0.05). A 1% increase in CD or CB caused a 1.1-1.5ms prolongation of the activation time (P<0.001). There was no correlation between CD/CB and post-operative AF.

CONCLUSION

CD/CB during SR in CABG patients with electrically non-remodeled atria show considerable intra-atrial, but also inter-individual variation. Despite these differences, a predilection site is present at the superior intercaval RA. Extensiveness of CB at the superior intercaval RA or BB does not reflect CB elsewhere in the atria and is not associated with post-operative AF.

Relevance of Conduction Disorders in Bachmann's Bundle During Sinus Rhythm in Humans

BACKGROUND

Bachmann's bundle (BB) is considered to be the main route of interatrial conduction and to play a role in development of atrial fibrillation (AF). The goals of this study are to characterize the presence of conduction disorders in BB during sinus rhythm and to study their relation with AF.

METHODS AND RESULTS

High-resolution epicardial mapping (192 unipolar electrodes, interelectrode distance: 2 mm) of sinus rhythm was performed in 185 patients during coronary artery bypass surgery of whom 13 had a history of paroxysmal AF. Continuous rhythm monitoring was used to detect postoperative AF during the first 5 postoperative days. In 67% of the patients, BB was activated from right to left; in the remaining patients from right and middle (21%), right, central, and left (8%), or central (4%) site. Mean effective conduction velocity was 89 cm/s. Conduction block was present in most patients (75%; median 1.1%, range 0-12.8) and was higher in patients with paroxysmal AF compared with patients without a history of AF (3.2% versus 0.9%; P=0.03). A high amount of conduction block (>4%) was associated with de novo postoperative AF (P=0.02). Longitudinal lines of conduction block >10 mm were also associated with postoperative AF (P=0.04).

CONCLUSIONS

BB may be activated through multiple directions, but the predominant route of conduction is from right to left. Conduction velocity across BB is around 90 cm/s. Conduction is blocked in both longitudinal and transverse direction in the majority of patients. Conduction disorders, particularly long lines of longitudinal conduction block, are more pronounced in patients with AF episodes.

QUest for the Arrhythmogenic Substrate of Atrial fibRillation in Patients Undergoing Cardiac Surgery (QUASAR Study): Rationale and Design

The heterogeneous presentation and progression of atrial fibrillation (AF) implicate the existence of different pathophysiological processes. Individualized diagnosis and therapy of the arrhythmogenic substrate underlying AF may be required to improve treatment outcomes. Therefore, this single-center study aims to identify the arrhythmogenic areas underlying AF by intra-operative, high-resolution, multi-site epicardial mapping in 600 patients with different heart diseases. Participants are divided into 12 groups according to the underlying heart diseases and presence of prior AF episodes. Mapping is performed with a 192-electrode array for 5–10 s during sinus rhythm and (induced) AF of the entire atrial surface. Local activation times are converted into activation and wave maps from which various electrophysiological parameters are derived. Postoperative cardiac rhythm registrations and a 5-year follow-up will show the incidence of postoperative and persistent AF. This project provides the first step in the development of a tool for individual AF diagnosis and treatment.

Dynamics of Focal Fibrillation Waves during Persistent Atrial Fibrillation

The incidence and appearance of focal fibrillation waves on the right and left atrial epicardial surface were visualized during 10 seconds of persistent atrial fibrillation in a 71-year-old woman with valvular heart disease. The frequent, nonrepetitive, widespread, and capricious distribution of focal waves suggests that transmural conduction of fibrillation waves is most likely the mechanism underlying focal fibrillation waves.

HALT & REVERSE: Hsf1 activators lower cardiomyocyt damage; towards a novel approach to REVERSE atrial fibrillation

Background

Atrial fibrillation is a progressive arrhythmia, the exact mechanism underlying the progressive nature of recurrent AF episodes is still unknown. Recently, it was found that key players of the protein quality control system of the cardiomyocyte, i.e. Heat Shock Proteins, protect against atrial fibrillation progression by attenuating atrial electrical and structural remodeling (electropathology). HALT & REVERSE aims to investigate the correlation between electropathology, as defined by endo- or epicardial mapping, Heat Shock Protein levels and development or recurrence of atrial fibrillation following pulmonary vein isolation, or electrical cardioversion or cardiothoracic surgery.

Study design

This study is a prospective observational study. Three separate study groups are defined: (1) cardiothoracic surgery, (2) pulmonary vein isolation and (3) electrical cardioversion. An intra-operative high-resolution epicardial (group 1) or endocardial (group 2) mapping procedure of the atria is performed to study atrial electropathology. Blood samples for Heat Shock Protein determination are obtained at baseline and during the follow-up period at 3 months (group 2), 6 months (groups 1 and 2) and 1 year (group 1 and 2). Tissue samples of the right and left atrial appendages in patients in group 1 are analysed for Heat Shock Protein levels and for tissue characteristics. Early post procedural atrial fibrillation is detected by continuous rhythm monitoring, whereas late post procedural atrial fibrillation is documented by either electrocardiogram or 24-h Holter registration.

Conclusion

HALT & REVERSE aims to identify the correlation between Heat Shock Protein levels and degree of electropathology. The study outcome will contribute to novel diagnostic tools for the early recognition of clinical atrial fibrillation.

Trial Registrations: Rotterdam Medical Ethical Committee MEC-2014-393, Dutch Trial Registration NTR4658

Diagnosis and Therapy of Atrial Fibrillation: The Past, The Present and The Future

Atrial fibrillation (AF) is the most common age-related cardiac arrhythmia. It is a progressive disease, which makes treatment difficult. The progression of AF is caused by the accumulation of damage in cardiomyocytes which makes the atria more vulnerable for AF. Especially structural remodeling and electrical remodeling, together called electropathology are sustainable in the atria and impair functional recovery to sinus rhythm after cardioversion. The exact electropathological mechanisms underlying persistence of AF are at present unknown. High resolution wavemapping studies in patients with different types of AF showed that longitudinal dissociation in conduction and epicardial breakthrough were the key elements of the substrate of longstanding persistent AF. A double layer of electrically dissociated waves propagating transmurally can explain persistence of AF (Double Layer Hypothesis) but the molecular mechanism is unknown. Derailment of proteasis -defined as the homeostasis in protein synthesis, folding, assembly, trafficking, guided by chaperones, and clearance by protein degradation systems - may play an important role in remodeling of the cardiomyocyte. As current therapies are not effective in attenuating AF progression, step-by-step analysis of this process, in order to identify potential targets for drug therapy, is essential. In addition, novel mapping approaches enabling assessment of the degree of electropathology in the individual patient are mandatory to develop patient-tailored therapies. The aims of this review are to 1) summarize current knowledge of the electrical and molecular mechanisms underlying AF 2) discuss the shortcomings of present diagnostic instruments and therapeutic options and 3) to present potential novel diagnostic tools and therapeutic targets.

Tachycardia-induced silencing of subcellular Ca2+ signaling in atrial myocytes

Atrial fibrillation (AF) is characterized by sustained high atrial activation rates and arrhythmogenic cellular Ca2+ signaling instability; however, it is not clear how a high atrial rate and Ca2+ instability may be related. Here, we characterized subcellular Ca2+ signaling after 5 days of high atrial rates in a rabbit model. While some changes were similar to those in persistent AF, we identified a distinct pattern of stabilized subcellular Ca2+ signaling. Ca2+ sparks, arrhythmogenic Ca2+ waves, sarcoplasmic reticulum (SR) Ca2+ leak, and SR Ca2+ content were largely unaltered. Based on computational analysis, these findings were consistent with a higher Ca2+ leak due to PKA-dependent phosphorylation of SR Ca2+ channels (RyR2s), fewer RyR2s, and smaller RyR2 clusters in the SR. We determined that less Ca2+ release per [Ca2+]i transient, increased Ca2+ buffering strength, shortened action potentials, and reduced L-type Ca2+ current contribute to a stunning reduction of intracellular Na+ concentration following rapid atrial pacing. In both patients with AF and in our rabbit model, this silencing led to failed propagation of the [Ca2+]i signal to the myocyte center. We conclude that sustained high atrial rates alone silence Ca2+ signaling and do not produce Ca2+ signaling instability, consistent with an adaptive molecular and cellular response to atrial tachycardia.

Electropathological Substrate of Longstanding Persistent Atrial Fibrillation in Patients With Structural Heart Disease

Background—

During persistent atrial fibrillation (AF), waves with a focal spread of activation are frequently observed. The origin of these waves and their relevance for the persistence of AF are unknown.

Methods and Results—

In 24 patients with longstanding persistent AF and structural heart disease, high-density mapping of the right and left atria was performed during cardiac surgery. In a reference group of 25 patients, AF was induced by rapid pacing. For data analysis, a mapping algorithm was developed that separated the fibrillatory process into its individual wavelets and identified waves with a focal origin. During persistent AF, the incidence of focal fibrillation waves in the right atrium was almost 4-fold higher than during acute AF (median, 0.46 versus 0.12 per cycle per 1 cm 2 (25th to 75th percentile, 0.40 to 0.77 and 0.01 to 0.27; P <0.0001). They were widely distributed over both atria and were recorded at 46±18 of all electrodes. A large majority (90.5) occurred as single events. Repetitive focal activity (>3) happened in only 0.8. The coupling interval was not more than 11 ms shorter than the average AF cycle length ( P =0.04), and they were not preceded by a long interval. Unipolar electrograms at the site of origin showed small but clear R waves. These data favor epicardial breakthrough rather than a cellular focal mechanism as the underlying mechanism. Often, conduction from a site of epicardial breakthrough was blocked in 1 or more directions. This generated separate multiple wave fronts propagating in different directions over the epicardium.

Conclusions—

Focal fibrillation waves are due to epicardial breakthrough of waves propagating in deeper layers of the atrial wall. In patients with longstanding AF, the frequency of epicardial breakthroughs was 4 times higher than during acute AF. Because they provide a constant source of independent fibrillation waves originating over the entire epicardial surface, they offer an adequate explanation for the high persistence of AF in patients with structural heart disease.

Electropathological substrate of long-standing persistent atrial fibrillation in patients with structural heart disease: longitudinal dissociation

BACKGROUND

The electropathological substrate of persistent atrial fibrillation (AF) in humans is largely unknown. The aim of this study was to compare the spatiotemporal characteristics of the fibrillatory process in patients with normal sinus rhythm and long-standing persistent AF.

METHODS AND RESULTS

During cardiac surgery, epicardial mapping (244 electrodes) of the right atrium (RA), the left lateral wall (LA), and the posterior left atrium (PV) was performed in 24 patients with long-standing persistent AF. Twenty-five patients with normal sinus rhythm, in whom AF was induced by rapid pacing, served as a reference group. A mapping algorithm was developed that separated the complex fibrillation process into its individual elements (wave mapping). Parameters used to characterize the substrate of AF were (1) the total length of interwave conduction block, (2) the number of fibrillation waves, and (3) the ratio of block to collision of fibrillation waves (dissociation index). In 4403 maps of persistent AF, no evidence for the presence of stable foci or rotors was found. Instead, many narrow wavelets propagated simultaneously through the atrial wall. The lateral boundaries of these waves were formed by lines of interwave conduction block, predominantly oriented parallel to the atrial musculature. Lines of block were not fixed but continuously changed on a beat-to-beat basis. In patients with persistent AF, the total length of block in the RA was more than 6-fold higher than during acute AF (median, 21.1 versus 3.4 mm/cm(2); P<0.0001). The highest degree of interwave conduction block was found in the PV area (33.0 mm/cm(2)). The number of fibrillation waves during persistent AF was 4.5/cm(2) compared with 2.3 during acute AF, and the dissociation index was 7.3 versus 1.5 (P<0.0001). The interindividual variation of these parameters among patients was high.

CONCLUSIONS

Electric dissociation of neighboring atrial muscle bundles is a key element in the development of the substrate of human AF. The degree of the pathological changes can be measured on an individual basis by electrophysiological parameters in the spatial domain.

Progression from paroxysmal to persistent atrial fibrillation clinical correlates and prognosis

OBJECTIVES

We investigated clinical correlates of atrial fibrillation (AF) progression and evaluated the prognosis of patients demonstrating AF progression in a large population.

BACKGROUND

Progression of paroxysmal AF to more sustained forms is frequently seen. However, not all patients will progress to persistent AF.

METHODS

We included 1,219 patients with paroxysmal AF who participated in the Euro Heart Survey on AF and had a known rhythm status at follow-up. Patients who experienced AF progression after 1 year of follow-up were identified.

RESULTS

Progression of AF occurred in 178 (15%) patients. Multivariate analysis showed that heart failure, age, previous transient ischemic attack or stroke, chronic obstructive pulmonary disease, and hypertension were the only independent predictors of AF progression. Using the regression coefficient as a benchmark, we calculated the HATCH score. Nearly 50% of the patients with a HATCH score >5 progressed to persistent AF compared with only 6% of the patients with a HATCH score of 0. During follow-up, patients with AF progression were more often admitted to the hospital and had more major adverse cardiovascular events.

CONCLUSIONS

A substantial number of patients progress to sustained AF within 1 year. The clinical outcome of these patients regarding hospital admissions and major adverse cardiovascular events was worse compared with patients demonstrating no AF progression. Factors known to cause atrial structural remodeling (age and underlying heart disease) were independent predictors of AF progression. The HATCH score may help to identify patients who are likely to progress to sustained forms of AF in the near future.

Role of progressive widening of the temporal excitable gap for perpetuation of atrial fibrillation in the goat

BACKGROUND

Previous studies suggest that a short temporal excitable gap exists between the fibrillation waves during atrial fibrillation (AF). The aim of this study was to investigate the role of that gap in the development of sustained AF in goats.

METHODS AND RESULTS

Eight female goats were instrumented with left atrium (LA) electrodes, and sustained AF (>24 h) was induced by intermittent rapid atrial pacing for 9.3+/-4.6 days. In the process of sustained AF development, the atrial effective refractory period (AERP), refractory period during AF (RP(AF)), mean AF cycle length (AFCL), temporal excitable gap during AF (EG(AF) = AFCL - RP(AF)) and degree of fractionation of fibrillation electrograms at LA were studied. When the induced AF lasted for 3-10 min, AFCL, RP(AF) and EG(AF) were 98.3+/-11.0 ms, 90.5+/-13.2 ms and 7.8+/-2.4 ms, respectively. During sustained AF, the values were 84.9+/-5.2 ms, 63.0+/-4.8 ms and 21.9+/-3.5 ms, respectively (P<0.05). Percentage of single potentials was 94.2+/-3.9% and 75.6+/-5.5%, respectively (P<0.05).

CONCLUSIONS

In this model progressive shortening of atrial refractoriness and widening of the temporal excitable gap induced by electrical remodeling created an electrophysiologic substrate for the perpetuation of AF.

Low-frequency oscillations of atrial fibrillation cycle length in goats: characterization and potentiation by class III antiarrhythmic almokalant

BACKGROUND

In chronically fibrillating goats, low-frequency oscillations (LFOs) of atrial fibrillation cycle length (AFCL) with a deceleration-acceleration sequence have been observed. The present investigation characterized such oscillations in control conditions and during the infusion of class III antiarrhythmic almokalant, trying to understand their mechanism and possible relevance.

METHODS AND RESULTS

The study was performed on fibrillating goats instrumented with multiple electrodes. LFOs were characterized in 64-s recording samples (1 electrode/atrium) before and during almokalant infusion. Filtering was applied to the raw sequence of AFCL. LFOs were completely random, non-flutterlike and potentiated by almokalant, as evinced by increases in oscillation frequency, duration and amplitude. As compared with nonoscillation periods, the upper part of LFOs displayed an increase in single (84.0 +/- 11.4% vs 72.5 +/- 12.9%) and a reduction in double spikes (12.1 +/- 8.3% vs 20.2 +/- 8.6%), suggesting an improvement of propagation. This was supported by the features of activation maps during LFOs: fast conduction, few wave fronts and many linking beats.

CONCLUSIONS

Chronically fibrillating goats exhibit random LFOs, which are enhanced by almokalant. The improvement of propagation during oscillations suggests an increase in the excitable period/excitable gap. These findings raise the question of LFOs involvement in atrial fibrillation termination.

Management of atrial fibrillation in patients with heart failure

Atrial fibrillation (AF) and chronic heart failure (CHF) are two major and even growing cardiovascular conditions that often coexist. However, few data are available to guide treatment of AF in patients with CHF. This review summarizes current literature concerning the following topics: (i) prognostic relevance of AF in patients with CHF, (ii) relevance and strategies of rhythm and rate control in patients with AF and CHF, and (iii) options for prevention of AF in patients with ventricular dysfunction. In conclusion, AF is associated with increased mortality in CHF patients. However, it is not clear whether there is a causal relationship. Emerging strategies to prevent the occurrence of AF are promising tools that might improve quality of life and survival in patients with CHF.

Cardioversion of persistent atrial fibrillation by a combination of atrial specific and non-specific class III drugs in the goat

OBJECTIVE

In electrically remodeled atria the effect of blockers of the delayed rectifier K+ current I(Kr) on repolarization is reduced, whereas the efficacy of 'early' class III drugs (I(Kur)/I(to)/I(Kach) blockers) is enhanced. We evaluated the electrophysiological and antifibrillatory effects of AVE0118, dofetilide, and ibutilide (alone and in combination) on persistent atrial fibrillation (AF) in the goat.

METHODS AND RESULTS

The effects of separate and combined administration of AVE0118, dofetilide, and ibutilide were determined before and after 48 h of AF. AVE0118 alone markedly prolonged the atrial refractory period (400 ms cycle length) (AERP400) before and after 48 h of AF. The prolongation of AERP(400) by dofetilide and ibutilide, respectively, was reduced by AF from 22+/-2 to 7+/-2 ms (p<0.01) and 25+/-5 to 5+/-2 ms (p=0.01). Pre-treatment with AVE0118 restored the prolongation of AERP400 by dofetilide or ibutilide (to 20+/-3 and 30+/-6 ms; p<0.01). This effect was atrial specific since the QT-interval was not changed. The antifibrillatory action was evaluated in 10 goats that were in persistent AF for 57+/-7 days. Dofetilide (20 microg/kg/h) or ibutilide (4 mg/h) alone restored sinus rhythm in only 20% of the animals. AVE0118 (1, 3 and 10 mg/kg/h) [DOSAGE ERROR CORRECTED] terminated AF in 11, 30, and 60%, respectively. Additional infusion of I(Kr) blockers caused an additional number of cardioversions, resulting in a final cardioversion rate of 56, 80, and 100%, respectively. AVE0118 alone prolonged the AF cycle length (AFCL) while the conduction velocity during AF (CV(AF)) remained unchanged (70+/-1 vs. 68+/-2 cm/s; p=0.3). Addition of dofetilide or ibutilide caused a synergistic increase in AFCL and a slight increase in CV(AF) to 74+/-1 cm/s (p<0.001). The length of the reentrant trajectories increased from 7.6+/-0.3 (control) to 11.6+/-0.5 cm after AVE0118 alone (p<0.001) and 14.8+/-0.8 cm after addition of dofetilide or ibutilide (p<0.001).

CONCLUSIONS

In electrically remodeled atria, blockade of I(Kur)/I(to)/I(KAch) restored the class III action of I(Kr) blockers. Persistent AF could be effectively cardioverted by infusion of a combination of AVE0118 and dofetilide or ibutilide. This antifibrillatory action was associated with an almost twofold lengthening of the intra-atrial pathways for reentry.

Automatic mapping of human atrial fibrillation by template matching

BACKGROUND

The high spatiotemporal variation in morphology of fibrillation electrograms makes mapping of atrial fibrillation (AF) a difficult and burdensome task.

OBJECTIVES

The purpose of this study was to evaluate the results of automatic detection of fibrillation electrograms by a template matching technique.

METHODS

During cardiac surgery in 25 patients without a history of AF, paroxysms of AF were induced by rapid atrial pacing. A mapping array of 244 unipolar electrodes (3.6-cm diameter, 2.25-mm interelectrode distance) was positioned on the free wall of the right atrium. All fibrillation electrograms were correlated with a mathematically constructed library of 128 potentials of different duration, RS ratio, and short double components. The moments of maximal correlation, coinciding with the negative deflection in the fibrillation potentials, were used to create fibrillation maps.

RESULTS

In each patient, a segment of 18.6 +/- 3.8 seconds of AF was analyzed, resulting in 80 to 130 maps per patient. The output of the automatic algorithm was compared with careful manual analysis by an experienced investigator. Of the total database of 398,796 fibrillation potentials, 93.6% +/- 4.2% resulted in a good correlation with one of the templates in the library (correlation coefficient >= 0.7). At a correlation threshold of 0.6, on average template matching yielded slightly more false-positive than false-negative detections (sensitivity 96.6% +/- 2.5%, positive predictive value 94.3% +/- 5.4%). The majority of false-positive detections were due to electrotonic potentials recorded along the lateral boundaries of the fibrillation waves. This led to a slight overlap of fibrillation waves but not to false detection of nonexisting wavefronts. Undersensing was mainly due to the presence of long double and fractionated potentials (2.6%) that were not represented in the template library. Fractionated parts in the electrograms were identified by failure of template matching and can be analyzed separately.

CONCLUSION

Template matching is a useful technique for characterizing unipolar fibrillation electrograms and for visualizing the complex activation patterns during AF. It allows automatic evaluation of the electropathologic substrate of AF on an individual basis.

Minimally invasive mapping guided surgical treatment of atrial fibrillation. Utopia or near future?

Isolation of the pulmonary veins has been used as surgical treatment for atrial fibrillation (AF) from the early 90s, as it was incorporated in the Maze procedure. With the evidence that triggers form this area can induce AF, the Maze III procedure has been adapted and modified towards a single lesion around the pulmonary veins for the treatment of paroxysmal and chronic AF in some centers. New ablation techniques with a diversity of energy sources further paved the way for less invasive procedures. Minimal invasive techniques to prevent major surgery may potentially make the treatment available for a patient population that do not have to undergo cardiac surgery for other reasons. Besides these technical developments, high density mapping can be used to identify the AF substrate in the individual patient and optimization of the treatment by local substrate guided ablation. This review aims to summarize the robotic and thoracoscopic techniques to isolate the pulmonary veins. Furthermore, it is discussed why pulmonary veins isolation may be effective in patients with chronic AF, and whether there is a role for mapping guided minimal invasive surgical treatment of AF in the near future.

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.

Does flecainide regain its antiarrhythmic activity after electrical cardioversion of persistent atrial fibrillation?

OBJECTIVES

The purpose of this study was to evaluate the hypothesis that presumed reversion of electrical remodeling after cardioversion of atrial fibrillation (AF) restores the efficacy of flecainide.

BACKGROUND

Flecainide loses its efficacy to cardiovert when AF has been present for more than 24 hours. Most probably, the loss is caused by atrial electrical remodeling. Studies suggest electrical remodeling is completely reversible within 4 days after restoration of sinus rhythm (SR).

METHODS

One hundred eighty-one patients with persistent AF (median duration 3 months) were included in this prospective study. After failure of pharmacologic cardioversion by flecainide 2 mg/kg IV (maximum 150 mg in 10 minutes) and subsequent successful electrical cardioversion, we performed intense transtelephonic rhythm monitoring three times daily for 1 month. In case of AF recurrence, a second cardioversion by flecainide was attempted as soon as possible.

RESULTS

AF recurred in 123 patients (68%). Successful cardioversion by flecainide occurred only when SR had been maintained for more than 4 days (7/51 patients [14%]). Failure to cardiovert was associated with a prolonged duration of the recurrent AF episode and concurrent digoxin use. Multivariate logistic regression confirmed that successful cardioversion was determined by digoxin use (odds ratio [OR] 0.093, P = .047) and by the interaction between the duration of SR and the (inverse) duration of recurrent AF (OR 6.499, P < .001). When flecainide was administered within 10 hours after AF onset and the duration of SR was greater than 4 days, the success rate was 58%.

CONCLUSIONS

Flecainide recovers its antiarrhythmic action after cardioversion of AF. However, successful pharmacologic cardioversion occurs only after SR has lasted at least 4 days and is expected only for recurrences having duration of a few hours. Immediate pharmacologic cardioversion of AF recurrence may be a worthwhile strategy for management of persistent AF.

Robot-assisted epicardial ablation of the pulmonary veins: is a completed isolation necessary?

AIMS

To study the feasibility and electrophysiological efficacy of minimally invasive beating heart ablation of the pulmonary veins (PVs) via a robot-assisted single-sided approach.

BACKGROUND

PV isolation by minimally invasive epicardial ablation may offer a new treatment for patients with lone atrial fibrillation (AF). However, complete PV isolation has been shown to be difficult to obtain.

METHODS AND RESULTS

In 14 mongrel dogs, robot-assisted epicardial microwave ablation was performed on the beating heart by a single-sided right chest approach. Isolation of all PVs was performed in two steps to study the effect of an incomplete and a complete isolation on AF. AF was studied by random and burst pacing. Incremental pacing was performed to study conduction characteristics across the lesions. Opening of the pericardial reflections, introduction of the catheter and ablation were robotically feasible by a single-sided approach in 11 dogs. The AF duration decreased from 6.6+/-4.1 to 1.3+/-0.8 s (P=0.03) and 1.6+/-1.6 s (P=0.04 compared with control) after incomplete and completed isolation of the PVs. The AF cycle length increased from 134+/-5 to 141+/-5 and 145+/-8 ms (P=0.03) after incomplete and complete isolation, respectively. Several incomplete lesions showed 2:1 exit and/or entrance block during incremental pacing. After complete isolation, AF was no longer inducible from the PVs.

CONCLUSION

Epicardial PV isolation can be successfully performed by a single-sided robot-assisted approach. The effect of PV ablation on AF is not an all or none phenomenon. Incomplete isolation already decreases AF duration and lengthens the AF cycle length. However, complete isolation is necessary to prevent AF induction by triggering from the isolated area.

Synergistic Action of Atrial Dilation and Sodium Channel Blockade on Conduction in Rabbit Atria

INTRODUCTION

The aim of this study was to investigate the interaction of atrial dilation and blockade of the rapid sodium channel on atrial conduction and degree of anisotropy.

METHODS AND RESULTS

The right atrium was acutely dilated by increasing intra-atrial pressure from 2 to 9 cm H2O in 14 isolated rabbit hearts. A rectangular mapping array of 240 electrodes (spatial resolution 0.5 mm) was positioned on the free wall of the right atrium during pacing from four different directions at intervals of 240 and 140 msec. In nondilated atria, 0.5 and 1.0 mg/L of the use-dependent INa blocker flecainide prolonged the total conduction time under the mapping electrode by 15% to 75%. In dilated atria, flecainide depressed conduction by 24% to 89% (P < 0.05). The incidence of intra-atrial conduction block increased from 0.6%-0.8% to 3.3%-7.2% in nondilated atria and from 3.9%-4.6% to 13%-21% in dilated atria (P < 0.05). The direction of activation relative to the crista terminalis and major pectinate muscles was of major importance for occurrence of conduction block. During rapid pacing, the degree of anisotropy in conduction increased by the combination of atrial dilation and flecainide (1.0 mg/L) from 1.7 +/- 0.1 to 2.2 +/- 0.4 (P < 0.05). The effects of dilation and flecainide on conduction were clearly synergistic. The effect of flecainide on the atrial refractory period also was enhanced by atrial dilation.

CONCLUSION

In dilated atria, blockade of the rapid sodium channels caused a higher degree of local conduction delay and intra-atrial conduction block than in nondilated atria.

S-wave predominance of epicardial electrograms during atrial fibrillation in humans: Indirect evidence for a role of the thin subepicardial layer

OBJECTIVES

The purpose of this study was to characterize the morphology of fibrillation electrograms in patients in order to provide insight into the underlying electropathologic substrate of atrial fibrillation (AF).

BACKGROUND

Electrograms recorded during AF show a high degree of spatiotemporal variation.

METHODS

AF was induced by rapid atrial pacing in 25 patients undergoing cardiac surgery. A unipolar mapping array of 244 electrodes was positioned on the free wall of the right atrium to record multiple epicardial fibrillation electrograms. Local anisotropy in conduction and epicardial wavefront curvature during AF were determined by fitting the best quadratic surface on the activation times of rectangular areas of 3 x 3 electrodes.

RESULTS

During AF, unipolar epicardial electrograms revealed a clear predominance of S waves. The average RS difference during type I and II AF was -0.15 +/- 0.08 and -0.22 +/- 0.08. During type III AF, the predominance of S waves was less prominent (-0.07 +/- 0.05; P < .005). In all types of AF, the degree of anisotropy in conduction was remarkably low (anisotropy ratio: 1.24 +/- 0.09), and no clear directional effect on the relative amplitude of R and S waves was found. There was a weak relationship between local curvature of wavefronts and RS difference (r = 0.23; P < .01). Computer simulations showed that the negative RS difference could result from transmural activation in an epicardial to endocardial direction.

CONCLUSIONS

The clear predominance of S waves in epicardial fibrillation electrograms is not due to anisotropy and can only be partly explained by a high curvature of fibrillation waves. Predominant epicardial to endocardial activation seems to be important in producing rS electrograms on the epicardium. This finding provides indirect evidence that the thin epicardial layer of atrial myocardium plays an important role in propagation of fibrillation waves.

Evaluation of Epicardial Microwave Ablation Lesions: Histology Versus Electrophysiology

BACKGROUND

Pulmonary vein isolation is a hallmark in current surgical ablation for atrial fibrillation. However, validation of isolation remains cumbersome. We evaluated electrophysiologic and not histologic means to test isolation.

METHODS

In 16 mongrel dogs, robot-assisted epicardial beating-heart microwave ablation (FLEX 10) was performed around the pulmonary veins. Electrophysiologic isolation was tested by pacing at 4 times threshold values inside and outside the pulmonary veins (exit and entrance block). The histology of lesions was studied for transmurality and continuity of the lesion lines. In 5 dogs, lesions were studied at various time intervals.

RESULTS

Histologic evaluation of the lesions showed incomplete (48% +/- 20%) circumferential myocardial damage in all dogs with acute lesions. Electrophysiologic evaluation showed completion of the box (entrance and exit block) in 8 dogs and in another 5 dogs after repeated ablation (p < 0.01 compared with histologic evaluation). Electrophysiologic evaluation of the dogs with chronic lesions showed completed lesions in 4 of 5 dogs directly after ablation. At follow-up (1 to 3 weeks), the isolations remained electrophysiologically complete. Histologic evaluation of the lesions 1 to 3 weeks after ablation showed complete (100%) circumferential lesions in all 4 dogs (p < 0.001 compared with the histology of dogs with acute lesions).

CONCLUSIONS

Directly after treatment, ablation lesions are best evaluated electrophysiologically, because complete (transmural and circumferential) lesions are not shown by histologic evaluation in the acute stage. After 1 to 3 weeks, the histology is in accordance with the electrophysiology. To obtain a complete isolation, online electrophysiologic evaluation during pulmonary vein microwave ablation is necessary to optimize the results.

“Early” Class III Drugs for the Treatment of Atrial Fibrillation

Background— Currently available antiarrhythmic drugs are only moderately effective against atrial fibrillation (AF) and may cause ventricular proarrhythmia. AVE0118 is a blocker of atrium-specific early K + currents ( I Kur / I to ).

Methods and Results— Effects of intravenous AVE0118 and dofetilide on atrial effective refractory period (AERP) and inducibility of AF were measured before and after 48-hours of AF-induced electrical remodeling in the goat. During persistent AF (53±19 days), the cardioversion efficacy and effects on atrial wavelength of AVE0118, dofetilide, and ibutilide were evaluated. QT durations were measured during atrial pacing and persistent AF. After 48 hours of AF, the effect of dofetilide on AERP was reduced, and induction of AF was not prevented. In contrast, the class III action of AVE0118 was enhanced, and AF inducibility decreased from 100% to 32% ( P <0.001). At 1, 3, and 10 mg · kg −1 · h −1 , AVE0118 terminated persistent AF in 1 of 8, 3 of 8, and 5 of 8 goats, respectively. Dofetilide and ibutilide terminated AF in 1 of 5 and 2 of 7 goats. AVE0118 0.5, 1.5, and 5 mg/kg prolonged the AERP during AF and increased the fibrillation wavelength from 6.7±0.6 to 8.5±0.5, 9.7±0.5, and 11.2±0.9 cm ( P <0.01). Whereas dofetilide and ibutilide prolonged QT duration, AVE0118 had no appreciable effect.

Conclusions— AVE0118 markedly prolongs the AERP during AF without affecting QT duration. Cardioversion of AF was due to an ≈2-fold increase in fibrillation wavelength. Atrium-selective class III drugs like AVE0118 may be a promising new option for safe and effective cardioversion of AF.

Troponin I Isoform Expression in Human and Experimental Atrial Fibrillation

Background— Atrial fibrillation (AF) is accompanied by re-expression of fetal genes and activation of proteolytic enzymes. In this study both aspects were addressed with respect to troponin I (TnI) isoform expression.

Methods and Results— Western blotting and real-time reverse transcription–polymerase chain reaction were used to study TnI isoform expression in patients with paroxysmal or chronic AF and in goats after 1, 2, 4, 8, and 16 weeks of AF. In addition to cardiac TnI (cTnI), low expression of slow-twitch skeletal TnI (ssTnI) protein was found in 60% of patients in sinus rhythm or paroxysmal AF and in 8% of patients with chronic AF. In adult goat atrium, ssTnI protein expression was undetectable. Calcium-dependent degradation of cTnI protein was found in 1 or 2 of 6 animals after 1 to 4 weeks of AF. Although always low, ssTnI mRNA levels were significantly higher in patients who expressed ssTnI protein than in those who did not. Relative ssTnI mRNA expression was significantly lower in patients with paroxysmal AF and chronic AF than in those in sinus rhythm. In goats there was a tendency toward higher relative levels of ssTnI at the onset of AF followed by a normalization when AF had become sustained.

Conclusions— Atrial re-expression of ssTnI during paroxysmal AF in patients and during the first 2 weeks of pacing-induced AF in goats does not seem to be part of the process of AF-associated cardiomyocyte dedifferentiation but seems to result from transient cardiomyocyte stress at the onset of AF.

Fractionation of Electrograms and Linking of Activation During Pharmacologic Cardioversion of Persistent Atrial Fibrillation in the Goat

INTRODUCTION

During atrial fibrillation (AF), there is fractionation of extracellular potentials due to head-to-tail interaction and slow conduction of fibrillation waves. We hypothesized that slowing of the rate of AF by infusion of a Class I drug would increase the degree of organization of AF.

METHODS AND RESULTS

Seven goats were instrumented with 83 epicardial electrodes on the left atrium, left atrial appendage, Bachmann's bundle, right atrium, and right atrial appendage. AF was induced and maintained by an automatic atrial fibrillator. After AF had persisted for 4 weeks, the Class IC drug cibenzoline was infused at a rate of 0.1 mg/kg/min. AF cycle length (AFCL), the percentage of fractionated potentials, conduction velocity (CV), and direction of propagation of the fibrillation waves were measured during baseline, after AFCL was increased by 20, 40, 60, and 80 ms, and shortly before cardioversion. Infusion of cibenzoline increased the mean of the median AFCLs from 96 +/- 6 ms to 207 +/- 43 ms (P < 0.0001). The temporal variation in AFCL in different parts of the atria was 8% to 20% during control and, with the exception of Bachmann's bundle, was not significantly reduced during cibenzoline infusion. CV decreased from 76 +/- 14 ms to 52 +/- 9 cm/s (P < 0.01). Cibenzoline increased the percentage of single potentials from 81%+/- 4% to 91%+/- 4% (P < 0.01) and decreased the incidence of double potentials from 14%+/- 4% to 7 +/- 5% (P < 0.01) and multiple potentials from 5%+/-% to 1%+/- 2% (P < 0.001). Whereas during control, linking (consecutive waves propagating in the same direction) during seven or more beats occurred in 9%+/- 15% of the cycles, after cibenzoline the degree of linking had increased to 40%+/- 33% (P < 0.05). During the last two beats before cardioversion, there was a sudden prolongation in AFCL from 209 +/- 37 ms to 284 +/- 92 ms (P < 0.01) and a strong reduction in fractionated potentials (from 22%+/- 12% to 6%+/- 5%, P < 0.05).

CONCLUSION

The Class IC drug cibenzoline causes a decrease in fractionation of fibrillation electrograms and an increase in the degree of linking during AF. This supports the observation that Class I drugs widen the excitable gap during AF.

Inhibitors of the Na+/H+ Exchanger Cannot Prevent Atrial Electrical Remodeling in the Goat

INTRODUCTION

It has been suggested that blockade of the Na+/H+ exchanger (NHE1) can prevent atrial fibrillation (AF)-induced electrical remodeling and the development of AF.

METHODS AND RESULTS

AF was maintained by burst pacing in 10 chronically instrumented conscious goats. Intravenous and oral dosages of two NHE1 blockers (EMD87580 and EMD125021) resulted in plasma levels several magnitudes higher than required for effective NHE1 blockade. Shortening of atrial refractoriness immediately after 5 minutes of AF was not prevented by NHE1 blockade. In remodeled atria, increasing dosages of EMD87580 and EMD125021 did not reverse shortening of the atrial refractory period or reduce the duration of AF episodes. The cycle length during persistent AF also was not affected. Oral pretreatment with EMD87580 (8 mg/kg bid) starting 3 days before AF could not prevent electrical remodeling. After 24 and 48 hours of remodeling, the duration of AF paroxysms was 47 +/- 32 seconds and 135 +/- 63 seconds compared to 56 +/- 17 seconds and 136 +/- 52 seconds in placebo-treated animals (P > 0.8), respectively.

CONCLUSION

In the goat model of AF, the Na+/H+ exchanger inhibitors EMD87580 and EMD125021 did not prevent or revert AF-induced electrical remodeling. This indicates that activation of the Na+/H+ exchanger is not involved in the intracellular pathways of electrical remodeling. This does not support the suggestion that blockers of the Na+/H+ exchanger may be beneficial for prevention and treatment of AF.

The role of atrial dilatation in the domestication of atrial fibrillation

Numerous clinical investigations as well as recent experimental studies have demonstrated that atrial fibrillation (AF) is a progressive arrhythmia. With time paroxysmal AF becomes persistent and the success rate of cardioversion of persistent AF declines. Electrical remodeling (shortening of atrial refractoriness) develops within the first days of AF and contributes to the increase in stability of the arrhythmia. However, 'domestication of AF' must also depend on other mechanisms since the persistence of AF continues to increase after electrical remodeling has been completed. During the first days of AF in the goat, electrical and contractile remodeling (loss of atrial contractility) followed exactly the same time course suggesting that they are due to the same underlying mechanism. Contractile remodeling not only enhances the risk of atrial thrombus formation, it also enhances atrial dilatation by increasing the compliance of the fibrillating atrium. In goats with chronic AV-block atrial dilatation increased the duration of artificially induced AF-episodes but did not change atrial refractoriness or the AF cycle length. When AF was maintained a couple of days in these animals, a shortening of the atrial refractory period did occur. However, the AF cycle length did not decrease. Long lasting episodes of AF with a long AF cycle length and a wide excitable gap suggest that in this model AF is mainly promoted by conduction disturbances. Chronic atrial stretch induces activation of numerous signaling pathways leading to cellular hypertrophy, fibroblast proliferation and tissue fibrosis. The resulting electroanatomical substrate in dilated atria is characterized by increased non-uniform anisotropy and macroscopic slowing of conduction, promoting reentrant circuits in the atria. Prevention of electroanatomical remodeling by blockade of pathways activated by chronic atrial stretch therefore provides a promising strategy for future treatment of AF.

Reverse structural and gap-junctional remodeling after prolonged atrial fibrillation in the goat

BACKGROUND

Prolonged atrial fibrillation (AF) results in electrical, structural, and gap-junctional remodeling. We examined the reversibility of the changes in (ultra)structure and gap junctions.

METHODS AND RESULTS

Four groups of goats were used: (1) sinus rhythm (SR), (2) 4 months' AF (4 mo AF), (3) 2 months' SR after 4 mo AF (2 mo post-AF), and (4) 4 months' SR after 4 mo AF (4 mo post-AF). Atria were characterized electrophysiologically, (ultra)structure was studied by light and electron microscopy, and structural and gap-junctional protein expression was studied by immunohistochemistry or Western blotting. The atrial effective refractory period had completely returned to normal values 2 mo post-AF. Induced AF episodes still lasted for minutes at 2 and 4 mo post-AF, compared with seconds in the SR group. Structural abnormalities were still present at 2 and 4 mo post-AF, although to a lesser extent. The increased atrial myocyte diameter was back to normal at 4 mo post-AF. The number of myocytes with severe myolysis had almost normalized 4 mo post-AF, whereas myocytes with mild myolysis remained significantly increased. Extracellular matrix area fraction after 4 mo AF was similar to SR. However, the extracellular matrix fraction per myocyte had increased after 4 mo AF and remained higher post-AF. Changes in expression of structural proteins were partially restored post-AF. The reduction of connexin 40 that was observed during AF was completely reversed at 4 mo post-AF.

CONCLUSIONS

Recovery from structural remodeling after 4 mo AF is a slow process and is still incomplete 4 mo post-AF. Several months post-AF, the duration of AF episodes is still prolonged (minutes).

Effects of acute atrial dilation on heterogeneity in conduction in the isolated rabbit heart

INTRODUCTION

Atrial dilation plays an important role in the development and persistence of atrial fibrillation (AF). The mechanisms by which atrial dilation increases the vulnerability to AF are not fully understood.

METHODS AND RESULTS

In 11 isolated rabbit hearts, the right atrium was acutely dilated by increasing the intra-atrial pressure from 2 to 9 and 14 cm H2O. A rectangular mapping array of 240 electrodes (spatial resolution 0.5 mm) was positioned on the free wall of the right atrium. The atrium was paced from four different sites at intervals of 240 and 125 msec. At normal atrial pressure (2 cm H2O), conduction was uniform in all directions with an anisotropy ratio between 1.5 and 1.7. Increasing the pressure to 9 cm H2O decreased the normalized conduction velocity during rapid pacing by 18%. The incidence of areas of slow conduction and conduction block increased from 6.6% and 1.6% to 10.2% and 3.3%. At 14 cm H2O, conduction velocity decreased by 31% and the percentage of slow conduction and block further increased to 11.5% and 6.6% (P < 0.001). The appearance of lines of intra-atrial block was largely dependent on the pacing site. Whereas during pacing at the cranial part of the crista terminalis no increase in conduction delays occurred, pacing from the low right atrium unmasked several lines of block oriented parallel to the major trabeculae and the crista terminalis. In an additional series of six hearts the left atrium also was mapped. The effect of dilation of the left atrium was comparable to that of the right atrium. Increasing the atrial pressure to 14 cm H2O increased the amount of intra-atrial conduction block threefold to fourfold.

CONCLUSION

Acute atrial dilation results in slowing of conduction and an increase of the amount of intra-atrial conduction block. The increase in spatial heterogeneity in conduction was related to the anisotropic properties of the atrial wall.

Non-linear time series analysis: methods and applications to atrial fibrillation

We apply methods from non-linear statistical time series analysis to characterize electrograms of atrial fibrillation. These are based on concepts originating from the theory of non-linear dynamical systems and use the empirical reconstruction density in reconstructed phase space. Application of these methods is not restricted to deterministic chaos but is valid in a general time series context. We illustrate this by applying three recently proposed non-linear time series methods to fibrillation electrograms: 1) a test for time reversibility in atrial electrograms during paroxysmal atrial fibrillation in patients; 2) a test to detect differences in the dynamical behaviour during the pharmacological conversion of sustained atrial fibrillation in instrumented conscious goats; 3) a test for general Granger causality to identify couplings and information transport in the atria during fibrillation. We conclude that a characterization of the dynamics via the reconstruction density offers a useful framework for the non-linear analysis of electrograms of atrial fibrillation.

Mapping of atrial fibrillation

Cardiac mapping has been defined as: "a method by which cardiac signals are recorded from multiple sites of the heart and spatially depicted as a function of time in an integrated manner". It requires determination of the local activation time at each electrode and the creation of activation maps which provide a spatial model of the activation sequence. With respect to atrial fibrillation, mapping is useful to gain insight into the underlying mechanism of atrial fibrillation. In this review, we will discuss the mapping studies of experimental and clinical atrial fibrillation.

Analysis of altered gene expression during sustained atrial fibrillation in the goat

OBJECTIVE

Atrial fibrillation (AF) is characterised by electrical, gap junctional and structural remodelling. However, the underlying molecular mechanisms of these phenomena are largely unknown. To get more insight into atrial remodelling at the molecular level we have analysed changes in gene expression during sustained AF in the goat.

METHODS

The differential display technique (DD) was used to identify genes differentially expressed during sustained AF (13.9 +/- 5.2 weeks) as compared to sinus rhythm (SR). Dot-blot analysis was performed to confirm the altered gene expression and to establish the changes in expression after 1, 2, 4, 8 and 16 weeks of AF. Immunohistochemistry and western blotting were used to validate the DD approach and to further characterise the changed expression of the beta-myosin heavy chain gene at the protein level.

RESULTS

Of the approximately 125 fragments that showed changed expression levels during AF, 34 were cloned and sequenced. Twenty-one of these represented known genes involved in cardiomyocyte structure, metabolism, expression regulation, or differentiation status. The changed expression of 70% of the isolated clones could be confirmed by dot-blot analysis. In addition, time course analysis revealed different profiles of expression as well as transient re-expression of genes, e.g. the gene for hypoxia-inducible factor 1 alpha during the first week of AF. During sustained AF the frequency of cardiomyocytes expressing beta myosin heavy chain (beta MHC) increased from 21.8 +/- 2.1 to 47.9 +/- 2.5% (S.E.M.). The overall expression of MHC (alpha+beta) appeared to be down-regulated during AF.

CONCLUSIONS

AF is accompanied by changes in expression of proteins involved in cellular structure, metabolism, gene expression regulation and (de-)differentiation. Most alterations in expression confirm or support the hypothesis of cardiomyocyte de-differentiation. Furthermore, the results suggest a role for ischemic stress in the early response of cardiomyocytes to AF, possibly via activation of hypoxia-inducible factor 1 alpha.

Importance of right and left atrial dilation and linear ablation for perpetuation of sustained atrial fibrillation

INTRODUCTION

Atrial dilation associated with increasing atrial pressure plays an apparent role in the development of atrial fibrillation (AF). We characterized a new model of separate and biatrial dilation in the Langendorff-perfused rabbit heart. The aim of this study was to examine if sustained AF in this model (1) would be inducible by separate right atrial (RA) and left atrial (LA) dilation; (2) would be reproducibly inducible at the same pressure level; and (3) could be suppressed by RA, LA, or biatrial ablation.

METHODS AND RESULTS

Intra-atrial pressure was increased stepwise in the RA (n = 13), LA (n = 12), or both atria (n = 25) until sustained AF could be induced or a pressure of 20 cm H2O was reached. The stimulation protocol was repeated once in RA and LA dilation (n = 9) and three times in biatrial dilation (n = 7). Then, RA orifices (superior and inferior caval veins, tricuspid valve annulus, and foramen ovale) or LA orifices (pulmonary veins, mitral valve annulus, and foramen ovale) were connected by radiofrequency (RF) lesions. Sustained AF was rendered inducible in 100% of hearts with biatrial dilation, but in only 92% of hearts with RA dilation and 67% with LA dilation. Inducibility of sustained AF was reproducible. Under biatrial dilation, not RA ablation (0/10 hearts; P = NS) but LA ablation (4/11 hearts; P < 0.05) and biatrial ablation (16/21; P < 0.01) reduced the inducibility of sustained AF.

CONCLUSION

The inducibility of sustained AF due to increased intra-atrial pressure differs between the RA and LA. LA and biatrial lesions, not RA RF lesions, reduce the ability to perpetuate sustained AF.

Characterization of a long linear thermolesion in rabbit atria by high density mapping

A major problem in radiofrequency ablation is to evaluate if linear atrial lesions are transmural and continuous. Characteristics of the atrial electrograms recorded from these lesions might be useful to determine the completeness of linear atrial lesions. In seven isolated perfused rabbit atria, a long transmural linear lesion was made with a hot needle (perfused with water of 80 degrees C) (width 2 +/- 0.2 mm). The lesion extended from the orifice of the superior caval vein to the A V ring. High density mapping (240 electrodes, 7.5 x 7.5 mm) of the right atrium was performed before and after the lesion. Complete bidirectional conduction block was confirmed by pacing close at either side of the lesion. No change in conduction or electrogram characteristics occurred outside the lesion. From the center of the lesion, low amplitude double potentials were recorded. The conduction delay around the lesion and the width of the double potentials showed a high correlation (R2 = 0.99) and were both dependent of the site of pacing. From the boundaries of the lesion towards its center, the amplitude and slope of the unipolar electrograms decreased exponentially by 72 +/- 5 and 85 +/- 3%. From the decay of these electrotonic potentials a space constant (lambda of 0.79 +/- 0.04 mm) could be calculated for the lesion. Unipolar electrograms recorded from a complete and transmural linear atrial lesion are electrotonic in nature. Their characteristics could be used to evaluate the width and depth of the lesion.

Electrical remodeling and atrial dilation during atrial tachycardia are influenced by ventricular rate: role of developing tachycardiomyopathy

INTRODUCTION

Atrial fibrillation (AF) and congestive heart failure (CHF) are two clinical entities that often coincide. Our aim was to establish the influence of concomitant high ventricular rate and consequent development of CHF on electrical remodeling and dilation during atrial tachycardia.

METHODS AND RESULTS

A total of 14 goats was studied. Five goats were subjected to 3:1 AV pacing (A-paced group, atrial rate 240 beats/min, ventricular rate 80 beats/min). Nine goats were subjected to rapid 1:1 AV pacing (AV-paced group, atrial and ventricular rates 240 beats/min). During 4 weeks, right atrial (RA) and left ventricular (LV) diameters were measured during sinus rhythm. Atrial effective refractory periods (AERP) and inducibility of AF were assessed at three basic cycle lengths (BCL). After 4 weeks of rapid AV pacing, RA and LV diameters had increased to 151% and 113% of baseline, whereas after rapid atrial pacing alone, these parameters were unchanged. Right AERP (157+/-10 msec vs 144+/-16 msec at baseline with BCL of 400 msec in the A-paced and AV-paced group, respectively) initially decreased in both groups, reaching minimum values within 1 week. Subsequently, AERP partially recovered in AV-paced goats, whereas AERP remained short in A-paced goats (79+/-7 msec vs 102+/-12 msec after 4 weeks; P < 0.05). Left AERP demonstrated a similar time course. Inducibility of AF increased in both groups and reached a maximum during the first week in both groups, being 20% and 48% in the A-paced and AV-paced group, respectively.

CONCLUSION

Nature and time course of atrial electrical remodeling and dilation during atrial tachycardia are influenced by concurrent high ventricular rate and consequent development of CHF.

Flecainide widens the excitable gap at pivot points of premature turning wavefronts in rabbit ventricular myocardium

INTRODUCTION

The mechanisms by which Class IC drugs slow the rate of functional reentrant arrhythmias are not completely understood. We hypothesized that flecainide widens the excitable gap beyond the pivot point of premature turning wavefronts.

METHODS AND RESULTS

In eight perfused subepicardial layers of rabbit left ventricle, a linear lesion was made by radiofrequency (RF) ablation parallel to the fiber orientation. One end of the RF lesion was extended by a short incision. Pacing next to the lesion induced a wavefront propagating with a sharp U-turn around the end of the lesion in either the clockwise or counterclockwise direction. A high-density mapping electrode (240 electrodes, 350-microm resolution) was used to record unipolar electrograms at the pivot point. During control, the shortest V1-V2 interval proximal to the pivot point was 162 +/- 12 msec compared with 173 +/- 13 msec distal to the pivot point (difference 11 +/- 8 msec; P < 0.01). After infusion of flecainide 2 mg/L, the shortest V1-V2 interval proximal and distal to the pivot point were 217 +/- 29 msec and 244 +/- 36 msec (difference 27 +/- 16 msec; P < 0.01). Due to the increase in V1-V2 interval at the pivot point, flecainide widened the temporal excitable gap in the returning limb of the turning wavefront from 30 +/- 11 msec to 55 +/- 22 msec (P < 0.01). High-density mapping at the pivot point revealed that this widening of the excitable gap was due to both macroscopic discontinuous conduction and functional conduction block at the pivot point.

CONCLUSION

Flecainide widens the excitable gap in the returning limb of premature U-turning wavefronts by causing macroscopic discontinuous conduction and functional conduction block at the pivot point.

Cellular mechanisms of depressed atrial contractility in patients with chronic atrial fibrillation

BACKGROUND

After cardioversion of atrial fibrillation (AF), the contractile function of the atria is temporarily impaired. Although this has significant clinical implications, the underlying cellular mechanisms are poorly understood.

METHODS AND RESULTS

Forty-nine consecutive patients submitted for mitral valve surgery were investigated. Twenty-three were in persistent AF (>/=3 months); the others were in sinus rhythm. Before extracorporal circulation, the right atrial appendage was excised. ss-Adrenoceptors were quantified by radioligand binding, and G proteins were quantified by Western blot analysis. The isometric contractile response to Ca(2+), isoproterenol, Bay K8644, and the postrest potentiation of contractile force were investigated in thin atrial trabeculae, which were also examined histologically. The contractile force of the atrial preparations obtained from AF patients was 75% less than that in preparations from patients in sinus rhythm. Also, the positive inotropic effect of isoproterenol was impaired, and Bay K8644 failed to increase atrial contractile force. In contrast, the response to extracellular Ca(2+) was maintained, and the postrest potentiation was preserved. Beta-adrenoceptor density and G-protein expression were unchanged. Histological examination revealed 14% more myolysis in the atria of AF patients.

CONCLUSIONS

After prolonged AF, atrial contractility was reduced by 75%. The impairment of beta-adrenergic modulation of contractile force cannot be explained by downregulation of ss-adrenoceptors or changes in G proteins. Dysfunction of the sarcoplasmic reticulum does not occur after prolonged AF. Failure of Bay K8644 to restore contractility suggests that the L-type Ca(2+) channel is responsible for the contractile dysfunction. The restoration of contractile force by high extracellular Ca(2+) shows that the contractile apparatus itself is nearly completely preserved after prolonged AF.

Profibrillatory effects of verapamil but not of digoxin in the goat model of atrial fibrillation

INTRODUCTION

Verapamil and digoxin have been shown to modulate tachycardia-induced atrial electrical remodeling. The goal of the present study was to determine the direct effects of verapamil and digoxin on atrial fibrillation (AF), before and after electrical remodeling.

METHODS AND RESULTS

In six goats we measured the AF cycle length (AFCL) and duration of AF (DurAF) of 50 consecutive induced paroxysms, before (t = 0) and after 24 hours (t = 24) of electrical remodeling. During AF, conduction velocity (CV(AF)), refractory period (RP(AF)), and type of AF (I, II, III) were determined. Verapamil was administered at a loading dose of 0.1 mg/kg, followed by a continuous (2-hour) infusion of 5 microg/kg/min. Digoxin was given intravenously as a single 0.02 mg/kg bolus. At t = 0 and t = 24, digoxin and verapamil caused a significant slowing of the ventricular rate of >40%. Digoxin had no effect on DurAF, AFCL, CV(AF), or RP(AF). Infusion of verapamil had a direct proarrhythmic effect. Both at t = 0 and t = 24, AFCL and RP(AF) were shortened by about 15%. During acute AF, verapamil prolonged the average duration of AF paroxysms from 7 to 16 seconds. After 24 hours of AF, the proarrhythmic effect was much stronger. Shortly after starting infusion (6 +/- 2 min), verapamil converted paroxysmal AF into sustained AF. As long as verapamil infusion was maintained, AF no longer terminated in any of the goats. This effect was associated with an increase in AF fragmentation from type I to type II-III.

CONCLUSION

Verapamil shortens AFCL and RP(AF) in the presence and absence of electrical remodeling. After 24 hours, it exerted a marked proarrhythmic effect and converted paroxysmal (type I) into sustained (type III) AF. In contrast, digoxin had no effect on the rate or stability of AF.

Anisotropic reentry in a perfused 2-dimensional layer of rabbit ventricular myocardium

BACKGROUND

Anisotropy creates nonuniformity in electrical propagation and may contribute to the occurrence of unidirectional conduction block and reentry. We describe the characteristics of reentrant tachycardia in a 2D layer of anisotropic ventricular myocardium.

METHODS AND RESULTS

A Langendorff-perfused epicardial sheet (1.0+/-0.4 mm, n=35) was created by freezing the intramural layers of the rabbit left ventricle. Epicardial activation maps were constructed by use of different high-resolution mapping arrays connected to a mapping system. In 5 experiments, monophasic action potentials were recorded. In the intact left ventricle, no arrhythmias except VF could be induced. After freezing, programmed electrical stimulation or rapid pacing led to the induction of sustained VT (cycle length 130+/-11 ms). VT was caused by reentry around a functional line of block oriented parallel to the epicardial fiber direction. Action potential recordings demonstrated that the central line of block was kept refractory by electrotonic currents generated by the depolarization waves propagating at either side of the line of block. At the pivot points of the line of block, the pronounced curvature of the turning wave and abrupt loading changes created an excitable gap of 30 ms in the reentrant pathway.

CONCLUSIONS

In uniform anisotropic myocardium, reentry around a functional Z-shaped line of block may occur. The core of the circuit is kept refractory by electrotonic currents. The pronounced wave-front curvature and abrupt loading changes at the pivot points cause local conduction delay and create a small excitable gap.