Some observations on the mechanism of pressure related atrial fibrillation

Mechanisms of stretch-induced electro-anatomical remodeling and atrial arrhythmogenesis

Atrial fibrillation (AF) is the most common cardiac rhythm disorder, often occurring in the setting of atrial distension and elevated myocardialstretch. While various mechano-electrochemical signal transduction pathways have been linked to AF development and progression, the underlying molecular mechanisms remain poorly understood, hampering AF therapies. In this review, we describe different aspects of stretch-induced electro-anatomical remodeling as seen in animal models and in patients with AF. Specifically, we focus on cellular and molecular mechanisms that are responsible for mechano-electrochemical signal transduction and the development of ectopic beats triggering AF from pulmonary veins, the most common source of paroxysmal AF. Furthermore, we describe structural changes caused by stretch occurring before and shortly after the onset of AF as well as during AF progression, contributing to longstanding forms of AF. We also propose mechanical stretch as a new dimension to the concept "AF begets AF", in addition to underlying diseases. Finally, we discuss the mechanisms of these electro-anatomical alterations in a search for potential therapeutic strategies and the development of novel antiarrhythmic drugs targeted at the components of mechano-electrochemical signal transduction not only in cardiac myocytes, but also in cardiac non-myocyte cells.

Electro-Mechanical Alterations in Atrial Fibrillation: Structural, Electrical, and Functional Correlates

Atrial fibrillation is the most common arrhythmia encountered in clinical practice affecting both patients’ survival and well-being. Apart from aging, many cardiovascular risk factors may cause structural remodeling of the atrial myocardium leading to atrial fibrillation development. Structural remodelling refers to the development of atrial fibrosis, as well as to alterations in atrial size and cellular ultrastructure. The latter includes myolysis, the development of glycogen accumulation, altered Connexin expression, subcellular changes, and sinus rhythm alterations. The structural remodeling of the atrial myocardium is commonly associated with the presence of interatrial block. On the other hand, prolongation of the interatrial conduction time is encountered when atrial pressure is acutely increased. Electrical correlates of conduction disturbances include alterations in P wave parameters, such as partial or advanced interatrial block, alterations in P wave axis, voltage, area, morphology, or abnormal electrophysiological characteristics, such as alterations in bipolar or unipolar voltage mapping, electrogram fractionation, endo-epicardial asynchrony of the atrial wall, or slower cardiac conduction velocity. Functional correlates of conduction disturbances may incorporate alterations in left atrial diameter, volume, or strain. Echocardiography or cardiac magnetic resonance imaging (MRI) is commonly used to assess these parameters. Finally, the echocardiography-derived total atrial conduction time (PA-TDI duration) may reflect both atrial electrical and structural alterations.

Comprehensive assessment of left atrial and ventricular remodeling in paroxysmal atrial fibrillation by the cardiovascular magnetic resonance myocardial extracellular volume fraction and feature tracking strain

Atrial fibrillation (AF) is a progressive disease that starts with structural or functional changes in the left atrium and left ventricle, and evolves from paroxysmal toward sustained forms. Early detection of structural or functional changes in the left atrium and left ventricle in the paroxysmal stage could be useful for identifying a higher risk of progression to persistent AF and future cardio-cerebrovascular events. The aim of this study was to test the hypothesis that the feature tracking (FT) left atrial (LA) strain and left ventricular (LV) extracellular volume fraction (ECV) derived from cardiovascular magnetic resonance (CMR) could detect early changes in remodeling of the left atrium and ventricle in the paroxysmal AF (PAF) stage. The participants were comprised of 106 PAF patients (age, 66.1 ± 10.7 years; 66% male) who underwent clinical CMR before pulmonary vein isolation and 20 control subjects (age, 68.3 ± 8.6 years; 55% male). The CMR-FT LA strain/phasic function and LV-ECV were compared between the PAF and control groups. The total and passive LA empty fraction (LAEF) and LA strain (corresponding to LA reservoir and conduit function) were decreased in the PAF group as compared to the control group. However, active LAEF (corresponding to the LA booster pump function) did not differ significantly between the PAF group (33.9 ± 10.9%) and control group (37.9 ± 13.3%, p = 0.15), while the active LA strain (corresponding to the LA booster pump function) was significantly decreased in the PAF group (11.4 ± 4.3 vs. 15.2 ± 5.6%, p = 0.002). The LV-ECV was significantly greater in the PAF group (28.7 ± 2.8%) than control group (26.6 ± 2.0%, p = 0.002). In the PAF group, the LV-ECV correlated significantly with the E/e′ and LA volume index. Regarding the LA strain, correlations were seen between the LV-ECV and both the reservoir function and conduit function. CMR-FT LA strain in combination with the LV-ECV in a single clinical study offers a potential imaging marker that identifies LA/LV remodeling including subtle LA booster pump dysfunction undetectable by the conventional booster pump LAEF in the PAF stage.

The effects of cardiac stretch on atrial fibroblasts: Analysis of the evidence and potential role in atrial fibrillation

Atrial fibrillation (AF) is an important clinical problem. Chronic pressure/volume overload of the atria promotes AF, particularly via enhanced extracellular matrix (ECM) accumulation manifested as tissue fibrosis. Loading of cardiac cells causes cell-stretch that is generally considered to promote fibrosis by directly activating fibroblasts, the key cell-type responsible for ECM-production. The primary purpose of this article is to review the evidence regarding direct effects of stretch on cardiac fibroblasts, specifically: (i) the similarities and differences among studies in observed effects of stretch on cardiac-fibroblast function; (ii) the signaling-pathways implicated; and (iii) the factors that affect stretch-related phenotypes. Our review summarizes the most important findings and limitations in this area and gives an overview of clinical data and animal models related to cardiac stretch, with particular emphasis on the atria. We suggest that the evidence regarding direct fibroblast activation by stretch is weak and inconsistent, in part because of variability among studies in key experimental conditions that govern the results. Further work is needed to clarify whether, in fact, stretch induces direct activation of cardiac fibroblasts and if so, to elucidate the determining factors to ensure reproducible results. If mechanical load on fibroblasts proves not to be clearly profibrotic by direct actions, other mechanisms like paracrine influences, the effects of systemic mediators and/or the direct consequences of myocardial injury or death, might account for the link between cardiac stretch and fibrosis. Clarity in this area is needed to improve our understanding of AF pathophysiology and assist in therapeutic development.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Impact of Transcatheter Device Closure of Atrial Septal Defect on Atrial Arrhythmias Propensity in Young Adults

Atrial septal defect (ASD) is a condition that requires early intervention because of the consequences over the right-side heart. Chronic atrial stretching promotes atrial conduction delay and the imbalance of the conduction homogeneity, which lead to the propensity to atrial arrhythmias (AA). We aim to evaluate the impact of transcatheter closure of ASD on atrial vulnerability markers leading to late AA in young adults. We conducted a prospective, longitudinal study in one hundred patients (mean age 25.2 ± 5.4 years) who underwent transcatheter closure of ASD at Cardiocentro Pediátrico William Soler. P-wave maximum (P_max) and P-wave dispersion (P_d) were analyzed from 12-lead electrocardiogram. Left-side and right-side intraatrial and interatrial electromechanical delay (EMD) were measured with tissue Doppler imaging. Both electrocardiographic and echocardiographic analyses were performed during the study period. Compared to baseline, there was a significant reduction in P max (p ≤ 0.001) and P_d (p ≤ 0.001) after 3 months of procedure. All atrial electromechanical coupling parameters significantly reduced at 6 months of ASD closure and tend to remain at lower values till the last evaluation. Over 9.2 ± 1.6 years of follow-up, 15 subjects (15%) developed AA, of which intraatrial reentrant tachycardia (66.6%) became the main rhythm disturbance. Intra-right atrial EMD ≥ 16 ms (HR 4.08, 95% CI 1.15-14.56; p = 0.03) and P_d 45 ms (HR 1.66, 95% CI 1.06-2.59; p = 0.02) were identified as predictors of late AA. Transcatheter device closure of ASD in young adults promotes a significant reduction of electrocardiographic and echocardiographic markers of AA vulnerability, which persist during the long-term follow-up. Nevertheless, P_d and interatrial EMD were identified as independent risk factors of AA.

Increased thin filament activation enhances alternans in human chronic atrial fibrillation

Action potential duration (APD) alternans (APD-ALT), defined as beat-to-beat oscillations in APD, has been proposed as an important clinical marker for chronic atrial fibrillation (cAF) risk when it occurs at pacing rates of 120-200 beats/min. Although the ionic mechanisms for occurrence of APD-ALT in human cAF at these clinically relevant rates have been investigated, little is known about the effects of myofilament protein kinetics on APD-ALT. Therefore, we used computer simulations of single cell function to explore whether remodeling in myofilament protein kinetics in human cAF alters the occurrence of APD-ALT and to uncover how these mechanisms are affected by sarcomere length and the degree of cAF-induced myofilament remodeling. Mechanistically based, bidirectionally coupled electromechanical models of human right and left atrial myocytes were constructed, incorporating both ionic and myofilament remodeling associated with cAF. By comparing results from our electromechanical model with those from the uncoupled ionic model, we found that intracellular Ca2+ concentration buffering of troponin C has a dampening effect on the magnitude of APD-ALT (APD-ANM) at slower rates (150 beats/min) due to the cooperativity between strongly bound cross-bridges and Ca2+-troponin C binding affinity. We also discovered that cAF-induced enhanced thin filament activation enhanced APD-ANM at these clinically relevant heart rates (150 beats/min). In addition, longer sarcomere lengths increased APD-ANM, suggesting that atrial stretch is an important modulator of APD-ALT. Together, these findings demonstrate that myofilament kinetics mechanisms play an important role in altering APD-ALT in human cAF. NEW & NOTEWORTHY Using a single cell simulation approach, we explored how myofilament protein kinetics alter the formation of alternans in action potential duration (APD) in human myocytes with chronic atrial fibrillation remodeling. We discovered that enhanced thin filament activation and longer sarcomere lengths increased the magnitude of APD alternans at clinically important pacing rates of 120-200 beats/min. Furthermore, we found that altered intracellular Ca2+ concentration buffering of troponin C has a dampening effect on the magnitude of APD alternans.

Progression and reversibility of stretch induced atrial remodeling: Characterization and clinical implications

Atrial fibrillation (AF) is the most common sustained arrhythmia and across the developed nations, it contributes to increasing hospitalizations and healthcare burden. Several comorbidities and risk factors including hypertension, heart failure, obstructive sleep apnoea and obesity are known to play an important role in the initiation and perpetuation of AF and atrial stretch or dilatation may play a central mechanistic role. The impact of atrial stretch in the development of AF can vary dependent on the underlying disease. This review focuses on understanding the substrate for AF in conditions of acute and chronic stretch and in the presence of common co-morbidities or risk factors through the review of findings in both animal and human studies. Additionally, the reversibility of atrial remodeling following stretch release will also be discussed. Identification of clinical conditions associated with increased atrial stretch as well as the treatment or prevention of these conditions may help to prevent AF progression and improve sinus rhythm maintenance.

The Electrophysiologic Effects of Acute Mitral Regurgitation in a Canine Model

BACKGROUND

Atrial fibrillation (AF) occurs in 30% of patients with mitral regurgitation referred for surgical intervention. However, the underlying mechanisms in this population are poorly understood. This study examined the effects of acute left atrial volume overload on atrial electrophysiology and the inducibility of AF.

METHODS

Ten canines underwent insertion of an atrioventricular shunt between the left ventricle and left atrium. Shunt and aortic flows were calculated, and the shunt was titrated to a shunt fraction to 40% to 50% of cardiac output. An epicardial plaque with 250 bipolar electrodes was used to determine activation and refractory periods. Biatrial pressures and volumes, conduction times, and atrial fibrillation inducibility were recorded. Data were collected at baseline and 20 minutes after shunt opening and closure.

RESULTS

Mean shunt flow was 1.3 ± 0.5 L/min with a shunt fraction of 43% ± 6% simulating moderate to severe mitral regurgitation. Compared with baseline, left atrial volumes and maximum pressures increased by 27% and 29%, respectively, after shunt opening. Biatrial effective refractory periods did not change significantly after shunt opening or closure. Conduction times increased by 9% with shunt opening and returned to baseline after closure. AF duration or inducibility did not change with shunt opening.

CONCLUSIONS

This canine model of mitral regurgitation demonstrated that acute left atrial volume overload did not increase the inducibility of atrial arrhythmias in contrast with experimental and clinical findings of chronic left atrial volume overload. This suggests that the substrates for AF in patients with mitral regurgitation are a result of chronic remodeling.

Atrial fibrillation: mechanisms, therapeutics, and future directions

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia, affecting 1% to 2% of the general population. It is characterized by rapid and disorganized atrial activation leading to impaired atrial function, which can be diagnosed on an EKG by lack of a P-wave and irregular QRS complexes. AF is associated with increased morbidity and mortality and is a risk factor for embolic stroke and worsening heart failure. Current research on AF support and explore the hypothesis that initiation and maintenance of AF require pathophysiological remodeling of the atria, either specifically as in lone AF or secondary to other heart disease as in heart failure-associated AF. Remodeling in AF can be grouped into three categories that include: (i) electrical remodeling, which includes modulation of L-type Ca(2+) current, various K(+) currents and gap junction function; (ii) structural remodeling, which includes changes in tissues properties, size, and ultrastructure; and (iii) autonomic remodeling, including altered sympathovagal activity and hyperinnervation. Electrical, structural, and autonomic remodeling all contribute to creating an AF-prone substrate which is able to produce AF-associated electrical phenomena including a rapidly firing focus, complex multiple reentrant circuit or rotors. Although various remodeling events occur in AF, current AF therapies focus on ventricular rate and rhythm control strategies using pharmacotherapy and surgical interventions. Recent progress in the field has started to focus on the underlying substrate that drives and maintains AF (termed upstream therapies); however, much work is needed in this area. Here, we review current knowledge of AF mechanisms, therapies, and new areas of investigation.

Effects of mineralocorticoid receptor antagonist spironolactone on atrial conduction and remodeling in patients with heart failure

BACKGROUND

Spironolactone was shown to reduce mortality in patients with heart failure (HF). However, the effect of spironolactone on the incidence of atrial fibrillation remains unknown. Therefore, we examined the effects of spironolactone on atrial conduction and remodeling in patients with HF.

METHODS AND RESULTS

A total of 21 patients with HF were divided into either spironolactone group (n=11) or control group (n=10). The patients were followed up for 12 months. Blood examination, echocardiogram, and signal-averaged electrocardiogram were performed at study enrollment and after 3 and 12 months of treatment. In the spironolactone group, atrial natriuretic peptide tended to reduce, left atrium dimension was significantly smaller, the ratio of E wave to A wave tended to improve, and P-duration was significantly shortened.

CONCLUSIONS

Spironolactone improves atrial conduction and remodeling in patients with HF.

Contraction-excitation feedback in human atrial fibrillation

BACKGROUND

Contraction-excitation feedback, that is, electrophysiologic changes that are caused or preceded by mechanical changes of the myocardium, has been extensively studied in the ventricles. The role of contraction-excitation feedback in the atria, and more particularly in the genesis and maintenance of atrial fibrillation, has been less adequately investigated.

HYPOTHESIS

The aim of the present study was to determine whether increased right atrial pressure (RAP) facilitates the induction of atrial fibrillation (AF) in patients with a history of lone AF.

METHODS

Sixteen patients with a history of paroxysmal AF but without structural heart disease were included in the study. All patients underwent electrophysiologic study at both a lower (3.1 +/- 2.0 mmHg) and (in 13 cases) a higher (6.4 +/- 2.5 mmHg) RAP. "Higher" was considered the pressure following rapid (in about 30 min) intravenous administration of normal saline or before the administration of a diuretic.

RESULTS

Rapid atrial pacing induced AF in 19 of 29 attempts. At a lower pressure, rapid pacing induced brief (3 s to 3 min) AF in 3 of 16 patients, long-lasting (> 3 min) AF in 3 of 16 patients, and no AF in 10 of 16 patients. At a higher pressure, brief AF was induced in 3 of 10 patients in whom no AF could be induced at a lower pressure, and long-lasting AF in 10 patients in whom either brief AF (3 cases) or no AF (7 cases) was induced at a lower pressure. In 11 patients, in whom Wenckebach periodicity was determined at both higher and lower pressure, the critical cycle length at which atrioventricular block appeared was significantly (p < 0.001, paired t-test) longer (349.1 +/- 44.4 ms, i.e., +15.5 +/- 11.3 ms) at higher than at lower atrial pressure (333.6 +/- 41.0 ms). In nine patients, in whom Wenckebach periodicity was determined and two rhythms occurred at different pressures, the critical cycle length was 332.2 +/- 45.8 ms when associated with sinus rhythm, and significantly (p < 0.01) longer (344.4 +/- 48.0 ms, i.e., +12.2 +/- 8.3 ms) when associated with induction of AF.

CONCLUSION

In patients with lone atrial fibrillation, modest increases in atrial pressure may facilitate the induction of atrial fibrillation.

Atrial Septal Defect and Atrial Fibrillation: The Known and Unknown

Atrial fibrillation (AF) is a common complication in patients with atrial septal defects (ASDs). The link between AF and ASD is fairly complex and entails modifications in electrophysiologic, contractile and structural properties, at the cellular and tissue level, of both atria, mainly due to chronic atrial stretch and dilation. Surgical repair or percutaneous closure of ASDs are equally effective in reducing mortality and symptoms but limited in preventing or curbing AF, unless combined with an arrhythmia-specific procedure. Transesophageal echocardiography (TEE) and intracardiac echocardiography (ICE) have improved the safety and success of the above procedures. Finally, clearer understanding of the pathophysiology of AF in patients with ASD (and CHF, in general) has led to target-specific advances in medical management.

The role of stretch-activated channels in atrial fibrillation and the impact of intracellular acidosis

The incidence of atrial fibrillation correlates with increasing atrial size. The electrical consequences of atrial stretch contribute to both the initiation and maintenance of atrial fibrillation. It is suggested that altered calcium handling and stretch-activated channel activity could explain the experimental findings of stretch-induced depolarisation, shortened refractoriness, slowed conduction and increased heterogeneity of refractoriness and conduction. Stretch-activated channel blocking agents protect against these pro-arrhythmic effects. Gadolinium, GsMTx-4 toxin and streptomycin prevent the stretch-related vulnerability to atrial fibrillation without altering the drop in refractory period associated with stretch. Changes the activity of two-pore K+ channels, which are sensitive to stretch and pH but not gadolinium, could underlie the drop in refractoriness. Intracellular acidosis induced with propionate amplified the change in refractoriness with stretch in the isolated rabbit heart model in keeping with the clinical observation of increased propensity to atrial fibrillation with acidosis. We propose that activation of non-specific cation stretch-activated channels provides the triggers for acute atrial fibrillation with high atrial pressure while activation of atrial two-pore K+ channels shortens atrial refractory period and increases heterogeneity of refractoriness, providing the substrate for atrial fibrillation to be sustained. Stretch-activated channel blockade represents an exciting target for future antiarrhythmic drugs.

Stretch-activated currents in cardiomyocytes isolated from rabbit pulmonary veins

Evidence is growing of a relationship between atrial dilation and atrial fibrillation (AF), the most prevalent type of arrhythmia. Pulmonary veins, which are important ectopic foci for provoking AF, are of increasing interest in relation to the early development of AF. Here, using single cardiomyocytes isolated from rabbit pulmonary veins, we characterised the stretch-activated currents induced by swelling and axial mechanical stretching. Swelling induced both a stretch-activated nonselective cationic current (NSC) and a Cl(-) current. The swelling-induced Cl(-) current (I Cl,swell) was inhibited by DIDS, whereas the swelling-induced NSC (I NSC,swell) was inhibited by Gd3+. The cationic selectivity of the I NSC,swell was K+ >Cs+ >Na+ >Li+, whilst the PK/PNa, PCs/PNa, and PLi/PNa permeability ratios were 2.84, 1.86, and 0.85, respectively. Activation of the I NSC,swell was faster than that of the I Cl,swell. Given a high K+ concentration in the bath solution, the I NSC,swell showed limited amplitude (<-70 mV). Mechanical stretching induced an immediate Gd3+- and streptomycin-sensitive NSC (I NSC,stretch) that was permeable to Na+, K+, Cs+ and NMDG. Persistent stretching activated a DIDS-sensitive current (I Cl,stretch). The I NSC,stretch, but not the I NSC,swell, was completely blocked by 400 microM streptomycin; therefore, the two currents may not be associated with the same channel. In addition, the type of current induced may depend on the type of stretching. Thus, stretch-induced anionic and cationic currents are functionally present in the cardiomyocytes of the main pulmonary veins of rabbits, and they may have pathophysiological roles in the development of AF under stretched conditions.

Mechanoelectric feedback leads to conduction slowing and block in acutely dilated atria: a modeling study of cardiac electromechanics

Atrial fibrillation, a common cardiac arrhythmia, is promoted by atrial dilatation. Acute atrial dilatation may play a role in atrial arrhythmogenesis through mechanoelectric feedback. In experimental studies, conduction slowing and block have been observed in acutely dilated atria. In the present study, the influence of the stretch-activated current ( Isac) on impulse propagation is investigated by means of computer simulations. Homogeneous and inhomogeneous atrial tissues are modeled by cardiac fibers composed of segments that are electrically and mechanically coupled. Active force is related to free Ca2+ concentration and sarcomere length. Simulations of homogeneous and inhomogeneous cardiac fibers have been performed to quantify the relation between conduction velocity and Isac under stretch. In our model, conduction slowing and block are related to the amount of stretch and are enhanced by contraction of early-activated segments. Conduction block can be unidirectional in an inhomogeneous fiber and is promoted by a shorter stimulation interval. Slowing of conduction is explained by inactivation of Na+ channels and a lower maximum upstroke velocity due to a depolarized resting membrane potential. Conduction block at shorter stimulation intervals is explained by a longer effective refractory period under stretch. Our observations are in agreement with experimental results and explain the large differences in intra-atrial conduction, as well as the increased inducibility of atrial fibrillation in acutely dilated atria.

Long-term outcome after cardioversion of atrial fibrillation: Prediction of recurrence with P wave signal averaged ECG and chemoreflexsensitivity

AIM

The recurrence of atrial fibrillation after cardioversion was a general problem in medical practice.

METHODS

We wanted to test the hypothesis that the recurrence of atrial fibrillation could be predicted by measurements of P wave triggered P wave signal averaged ECG and chemoreflexsensitivity (CHRS) in 118 consecutive patients one day after successful electrical cardioversion. We measured the filtered P wave duration (FPD) and the root mean square voltage of the last 20 ms of the P wave (RMS 20) with P wave triggered P wave signal averaged ECG. The CHRS was calculated of a ratio between the difference of RR intervals in ECG and venous pO2 before and after 5-min oxygen inhalation. The pathologic CHRS was predefined as a CHRS below 3.0 ms/mm Hg.

RESULTS

A recurrence of atrial fibrillation was observed in 57 patients (48%) during a 9.3-month follow-up. The left atrial size (41.9+/-4.0 vs. 39.3+/-3.1 mm, P<0.0003) was larger, the FPD (139.6+/-16.0 vs. 118.2+/-14.1 ms, P<0.0001) was longer, the RMS 20 (2.57+/-0.77 vs. 3.90+/-0.99 muV, P<0.0001) and the CHRS (2.66+/-1.18 vs. 4.01+/-1.66 ms/mm Hg, P<0.0001) were lower in patients with recurrence of atrial fibrillation. A cut-off point was defined with FPD > or =126 ms and RMS 20 < or =3.1 muV and could predict atrial fibrillation with a specificity of 77%, a sensitivity of 72%, a positive value of 75%, a negative predictive value of 75% and an accuracy of 75%. Patients with FPD > or =126 ms and RMS 20 < or =3.1 muV had nearly a 7-fold increase risk for recurrence of atrial fibrillation.

CONCLUSION

The recurrence of atrial fibrillation after cardioversion could be detected by P wave signal averaged ECG and an analysis of CHRS. These methods seem to be appropriate to show a delayed atrial conduction and a neurovegetative imbalance which might be possible trigger mechanisms of reinitiating of atrial fibrillation.

Evidence of mechanoelectric feedback in the atria of patients with atrioventricular nodal reentrant tachycardia

OBJECTIVE

Patients with atrioventricular nodal reentrant tachycardia (AVNRT) could serve as a clinical model to study the effects of mechanical stretch in the electrical properties of atrial myocardium.

MATERIALS AND METHODS

We studied 14 patients with AVNRT. Peak, mean and minimal atrial pressures, atrial refractoriness (ERP) in the right atrial appendage and high right atrial lateral wall and monophasic action potential duration at 90% of repolarisation (MAPd90) in the right atrial appendage were assessed during atrial pacing at 500 and 400 ms and after 2 min of pacing at the tachycardia cycle length. Measurements were repeated from the same positions after ventricular pacing at the same cycle lengths and after 2 min of tachycardia. Susceptibility to atrial fibrillation (AF) was assessed by noting whether AF was induced during ERP evaluation.

RESULTS

Atrial pressure showed a statistically significant increase during ventricular pacing compared to baseline. This increase remained substantially unchanged when the tachycardia was induced. A significant reduction in atrial ERP and MAPd90 was also observed during ventricular pacing at all cycle lengths compared to atrial pacing. Two minutes of spontaneous tachycardia were enough to change the atrial ERP and MAPd90 to values significantly lower than those during atrial pacing at the cycle length of tachycardia. During the ERP evaluation AF was induced more often during the tachycardia (28%) than during ventricular (14%) and atrial pacing (0%).

CONCLUSION

In AVNRT patients, ventricular pacing and reentrant tachycardia significantly increase right atrial pressures and subsequently shorten ERP and MAPd90, leading to an enhanced propensity for AF.

Development of a substrate of atrial fibrillation during chronic atrioventricular block in the goat

BACKGROUND

Atrial dilatation is an important risk factor for atrial fibrillation (AF). In the present study, we monitored the electrophysiological changes during progressive atrial dilatation in chronically instrumented goats.

METHODS AND RESULTS

In 8 goats, 2 screw-in leads with piezoelectric crystals were implanted transvenously in the right atrium. After 2 weeks, atrial diameter and effective refractory period were measured. AF paroxysms were induced by burst pacing to determine the baseline AF cycle length and stability of AF. After His-bundle ablation, the above measurements were repeated once a week. After 4 weeks of complete AV block, the free wall of the right atrium was mapped and the atrium was fixed in formalin for histological analysis. After His-bundle ablation, the ventricular rate decreased from 113.8+/-4.8 to 44.6+/-2.5 bpm. Right atrial diameter increased gradually by 13.5+/-3.9% during 4 weeks of AV block (P<0.01). The duration of induced AF paroxysms increased from 4.6 seconds to 6.4 minutes (P<0.05). Atrial effective refractory period and AF cycle length remained constant. Spontaneous paroxysms of AF were not observed. Atrial mapping during rapid pacing revealed that slow conduction (<30 cm/s) was present in 3.7+/-1.0% of the mapped area (control, 0.9+/-0.5%, P<0.05). Histological analysis showed hypertrophy without atrial fibrosis. Connexin40 and connexin43 expression was unchanged.

CONCLUSIONS

Chronic AV block in the goat leads to progressive atrial dilatation, prolongation of induced AF paroxysms, and local conduction delays. The increase in AF stability was not a result of a shortening of atrial refractoriness or atrial fibrosis.

Evaluation of atrial refractoriness immediately after percutaneous mitral balloon commissurotomy in patients with mitral stenosis and sinus rhythm

BACKGROUND

Chronic atrial stretch and rheumatic inflammatory activity leads to atrial dilatation and conduction slowing, and this increases the susceptibility to atrial fibrillation (AF). The aim of this study was to examine the effects of changes in the chronic atrial stretch on atrial refractoriness in the early period after percutaneous mitral balloon commissurotomy (PMBC) in patients with mitral stenosis and sinus rhythm.

METHODS

Twenty-five patients undergoing PMBC were enrolled in this study. We evaluated the changes in pulmonary arterial pressure (PAP), left atrial (LA) pressure, mean mitral diastolic gradient, and mitral valve area in addition to the changes in atrial effective refractory periods (AERPs), AERP dispersion, and intra-atrial and interatrial conduction times after PMBC.

RESULTS

There were significant decreases in mean diastolic gradient, PAP, mean LA pressure, and LA size after PMBC. Accompanying these acute hemodynamic changes after PMBC, AERPs in high right atrium (HRA), distal coronary sinus (DCS), and right posterolateral (RPL) were found to be increased (P <.001), and AERP dispersion, PA(HIS) (an interval between P wave on the surface electrocardiogram and atrial electrogram at the His bundle site), and HRA-DCS intervals were significantly reduced after PMBC (P <.001). It was revealed with linear regression and correlation analysis that only the changes in AERP dispersion were correlated with changes in LA pressure.

CONCLUSIONS

Relief of chronic atrial stretch results in an increase in AERPs and decrease in AERP dispersion, suggesting the potential reversibility of the electrophysiological features of chronic atrial dilatation. Our study emphasizes that an acute reduction of chronic atrial stretch in mitral stenosis resulted in favorable effects on atrial electrophysiological characteristics, and our results provide the first detailed insights into the electrophysiological changes after PMBC in patients with sinus rhythm.

Atrial Refractoriness and Action Potential Duration After Sudden Reversal of Atrioventricular Sequence

To address the potential of atrioventricular (AV) asynchrony to provoke cardiac arrhythmias, atrial electrophysiology was examined during normal and reversed AV interval in anesthetized pigs. A new automatic stimulation technique was adapted to monitor rapid changes in the effective refractory period (ERP), using continuous AV sequential pacing, incremental extrastimulus interval scanning, and automatic detection of capture. Right atrial ERP using 2-8 ms stimulus interval increments and right atrial and ventricular monophasic action potential (MAP) duration were determined simultaneously when the AV interval was changed from normal (+80 ms) to reversed (-40 ms) and back. During reversed AV interval the peak right atrial pressure increased from 8 +/- 3 to 14 +/- 4 mmHg (P < 0.001) and mean arterial pressure decreased from 86 +/- 18 to 65 +/- 21 mmHg (P < 0.001). At new steady state, atrial ERP and MAP duration at 90% level of repolarization were lengthened by 22 +/- 16 and 42 +/- 12 ms respectively (P < 0.001). Ventricular MAP duration did not change. A statistically significant lengthening in atrial ERP could be demonstrated in 5-10 seconds. After reversion of the AV sequence, the ratio of atrial ERP to MAP duration decreased from 1.27 to 0.94 (P < 0.001) on average for 15 seconds, the change being thought to favor reentry. Thus atrial wall stress from contraction during ventricular systole even for a short period of time modifies atrial electrophysiology. Deficient AV synchrony may immediately contribute to the development of atrial arrhythmias.

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.

Mechano-electric feedback and atrial fibrillation

Atrial fibrillation frequently occurs under conditions associated with atrial dilatation suggesting a role of mechano-electric feedback in atrial arrhythmogenesis. Although atrial arrhythmias may be due both to abnormal focal activity and reentrant mechanisms, the majority of sustained atrial arrhythmias have been ascribed to reentrant activity. Atrial stretch may contribute to focal arrhythmias by inducing afterdepolarizations and to reentrant arrhythmias by increasing the atrial surface, by shortening the refractory period and/or slowing the conduction velocity and by increasing their spatial dispersion. Experimental and clinical studies have demonstrated that changes in mechanical loading conditions may modulate the electrophysiological properties of the atria. These studies have, for the most part, involved the effects of acute stretch on atrial refractoriness. While studies in humans and intact animals yield divergent results due to the variety of loading conditions and neurohumoral influences, experimental studies in isolated preparations clearly show that atrial refractory period and action potential duration at early levels of repolarization shorten by acute atrial dilatation. Both experimental and human studies have shown that acute atrial stretch is arrhythmogenic and may induce triggered premature beats and atrial fibrillation.

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.

Gadolinium decreases stretch-induced vulnerability to atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is frequently associated with atrial dilatation caused by pressure or volume overload. Stretch-activated channels (SACs) have been found in myocardial cells and may promote AF in dilated atria. To prove this hypothesis, we investigated the effect of the SAC blocker gadolinium (Gd(3+)) on AF propensity in the isolated rabbit heart during atrial stretch.

METHODS AND RESULTS

In 16 isolated Langendorff-perfused rabbit hearts, the interatrial septum was perforated to equalize biatrial pressures. Caval and pulmonary veins were occluded. Intra-atrial pressure (IAP) was increased in steps of 2 to 3 cm H(2)O by increasing the pulmonary outflow fluid column. Vulnerability to AF was evaluated by 15-second burst pacing at each IAP level. At baseline, IAP needed to be raised to 8.8+/-0.2 cm H(2)O (mean+/-SEM) to induce AF. A dose-dependent decrease in AF vulnerability was observed after Gd(3+) 12.5, 25, and 50 micromol/L was added. AF threshold increased to 19.0+/-0.5 cm H(2)O with Gd(3+) 50 micromol/L (P<0.001 versus baseline). Spontaneous runs of AF occurred in 5 hearts on a rise of IAP to 13.8+/-3.3 cm H(2)O at baseline but never during Gd(3+). Atrial effective refractory period shortened progressively from 78+/-3 ms at 0.5 cm H(2)O to 52+/-3 ms at 20 cm H(2)O (P<0.05). Gd(3+) 50 micromol/L had no significant effect on effective refractory period.

CONCLUSIONS

Acute atrial stretch significantly enhances the vulnerability to AF. Gd(3+) reduces the stretch-induced vulnerability to AF in a dose-dependent manner. Block of SAC might represent a novel antiarrhythmic approach to AF under conditions of elevated atrial pressure or volume.

[Acute changes in wavelength of the process of auricular activation induced by stretching. Experimental study]

OBJECTIVE

An evaluation is made of the acute modifications in the wavelength of the atrial excitation process induced by atrial stretching.

MATERIAL AND METHODS

In 10 isolated Langendorff-perfused rabbit hearts and using a multiple electrode the wavelength of the atrial activation process (functional refractory period x conduction velocity) was determined in the right atrium. An analysis was also made of the inducibility of rapid repetitive atrial responses after 20 episodes of atrial burst pacing. Measurements were made under control conditions, after inducing two degrees of atrial wall stretch (D1 and D2), and following the suppression of atrial dilatation.

RESULTS

Under control conditions the wavelength was 72.6 +/- 7.7 mm (250 ms cycle) and 54.0 +/- 5.1 mm (100 ms cycle). In D1 (mean longitudinal increase in atrial wall length = 24 +/- 3%) the wavelength shortened, with values of 59.8 +/- 6.6 mm (250 ms cycle; p < 0.01) and 44.9 +/- 5.1 mm (100 ms cycle; p < 0.01). In D2 (mean longitudinal increase in atrial wall length = 41 +/- 4%) the wavelength also shortened significantly, with values of 41.6 +/- 2.5 mm (250 ms cycle; p < 0.01 vs control) and 29.6 +/- 2.1 mm (100 ms cycle; p < 0.01 vs control). After suppressing atrial dilatation the wavelength was 65.7 +/- 8.0 mm (250 ms cycle, NS vs control) and 47.9 +/- 5.5 mm (100 ms cycle; NS vs control). The inducibility of rapid repetitive atrial responses increased during dilatation (22 episodes with over 30 consecutive repetitive responses in D1 [p < 0.01], 50 episodes in D2 [p < 0.001] vs 5 episodes under control conditions), and diminished after suppressing atrial dilatation (0 episodes with over 30 consecutive repetitive responses; p < 0.05).

CONCLUSIONS

In the experimental model used, acute atrial dilatation produced a shortening in refractoriness and a decrease in conduction velocity. Both effects shortened the wavelength of the atrial activation process, facilitating the induction of atrial arrhythmias. The effects observed reverted upon suppressing atrial dilatation.

Electrophysiologic characteristics of a dilated atrium in patients with paroxysmal atrial fibrillation and atrial flutter

This study investigated the difference of atrial electrophysiologic characteristics between a normal and dilated atrium and compared them among patients with paroxysmal atrial fibrillation and flutter. Twenty-seven patients with paroxysmal atrial fibrillation and 28 patients with paroxysmal atrial flutter were divided into four subgroups, according to the presence of a normal atrium or bilateral atrial enlargement. Thirty patients without atrial arrhythmia (20 patients with normal atrium and 10 patients with bilateral atrial enlargement) were included in control group. The atrial refractoriness in patients with a dilated atrium was longer than those with normal atrial size. In patients with paroxysmal atrial fibrillation and patients of control group, the P-wave duration and interatrial conduction velocity with or without atrial enlargement were similar. However, in patients with paroxysmal atrial flutter, P-APCS (86 +/- 10 ms vs. 73 +/- 9 ms, p < 0.05) and P-ADCS (109 +/- 9 ms vs. 95 +/- 9 ms, p < 0.05) in patients with a dilated atrium were longer than in patients with a normal atrium. The patients with paroxysmal atrial fibrillation or atrial flutter all demonstrated longer P-wave duration and interatrial conduction time than control group. Among the groups with a normal atrium or a dilated atrium, atrial refractoriness in patients with paroxysmal atrial flutter was shorter than that in control group. Moreover, in the patients with a normal atrium, the potential minimal wavelength in control group (6.6 +/- 1.7) was longer than that of paroxysmal atrial fibrillation (5.3 +/- 1.1), or atrial flutter (5.0 +/- 1.2). These findings suggest that atrial electrophysiologic characteristics of a dilated atrium were different from those of normal atrium, and these changes were different between paroxysmal atrial fibrillation and flutter. Multiple factors are considered to be related to the genesis of atrial tachyarrhythmias.

Effects of atrial dilatation on refractory period and vulnerability to atrial fibrillation in the isolated Langendorff-perfused rabbit heart

BACKGROUND

Atrial fibrillation (AF) is frequently observed under conditions that are associated with atrial dilatation. The aim of this study was to investigate the effects of atrial dilatation on the substrate of AF.

METHODS AND RESULTS

In 15 Langendorff-perfused rabbit hearts, the interatrial septum was perforated, and after occlusion of the caval and pulmonary veins, biatrial pressure was increased by raising the level of an outflow cannula in the pulmonary artery. Right and left atrial effective refractory periods (AERPs), monophasic action potentials (MAPs), and inducibility of AF by single premature stimuli were measured as a function of atrial pressure. Increasing the atrial pressure from 0.5+/-0.7 to 16.2+/-2.2 cm H2O resulted in a progressive shortening of the right AERP from 82.2+/-9.8 to 48.0+/-5.1 ms. In the left atrium, an increase in pressure up to 7.4+/-0.3 cm H2O had no effect on the AERP. At higher pressures, however, the left AERP also shortened, from 67.5+/-7.5 to 49.3+/-2.0 ms. The duration of MAPs also decreased by an increase in atrial pressure, showing a high correlation with the shortening in AERP (r=.94, P<.01). All these changes were completely reversible within 3 minutes after release of the atrial stretch. Dilatation of the atria was a major determinant for the vulnerability to AF. The inducibility of AF increased from 0% at low pressures to 100% when the atrial pressure was >10 cm H2O. Release of the atrial wall stress resulted in prompt cardioversion of AF. The increased vulnerability for AF was highly correlated with the shortening in AERP (logistic regression r=.97). No correlation was found with the spatial dispersion between right and left AERPs.

CONCLUSIONS

Increased atrial pressure in the isolated rabbit heart resulted in a significant increase in vulnerability to AF that was closely correlated to shortening of the AERP. These changes were completely reversible within 3 minutes after release of the atrial stretch, resulting in prompt termination of AF.

Influence of bicaval anastomoses on late occurrence of atrial arrhythmia after heart transplantation

BACKGROUND

The standard technique for orthotopic heart transplantation includes right and left atrial anastomoses, which potentially disturb the integrity of the donor atria. Consequently, electrophysiologic abnormalities such as atrial flutter and fibrillation may occur even late after heart transplanation.

METHODS

Over a 3-year period, 39 heart transplantations were performed using a standard right atrial anastomosis (group A), and 40 were done using bicaval anastomoses (group B). In each group, data of 30 consecutive patients with a minimum follow-up of 9 months were reviewed retrospectively to assess the incidence of atrial arrhythmia after hospital discharge.

RESULTS

Early postoperatively, there was no difference in the duration of temporary pacemaker requirement and incidence of permanent pacemaker implantation (group A, 7%; group B, 7%; not significant) between the two groups. In 12 patients in group A (40%), 16 episodes of atrial flutter and fibrillation were detected 20 to 205 days after heart transplantation. In group B, 1 patient (4%) suffered from atrial fibrillation on day 116 after the operation (p < 0.001).

CONCLUSIONS

Preservation of the integrity of the right donor atrium by construction of bicaval anastomoses results in a significantly decreased incidence of atrial flutter and fibrillation after heart transplantation when compared with the standard technique.