The effect of atrial dilatation on the genesis of atrial arrhythmias

The Mechanisms of an Increased Dominant Frequency in the Left Atrial Posterior Wall During Atrial Fibrillation in Acute Atrial Dilatation

BACKGROUND

Previous studies have shown that the highest dominant frequency (DF) is located in the left atrium (LA) during atrial fibrillation (AF) in pacing-induced AF. However, there have been few studies on the mechanisms of the increased DF of AF during acute atrial dilatation. The purpose of this study was to investigate the mechanisms of the increased maximal DF (max DF) in pacing-induced AF during acute atrial dilatation.

METHODS

In eight Langendorff-perfused canine hearts (26 +/- 2 kg), noncontact balloon catheters were placed into the right atrium (RA) and LA, respectively. AF was induced by extrastimulation pre- and postdilatation in the atrium (0 and 15 cm H(2)O, respectively). Fast Fourier transformation analysis was performed to analyze the max DF and harmonic index (HI) from the bi-atrial unipolar virtual electrograms during AF. The fibrillation cycle lengths were obtained from different atrial sites. The number of wavefronts was analyzed during AF. The frequency of regional splitting was defined as the number of wavefront splits per second in different atrial regions during AF. The percentage of the low-voltage zones (<0.5 mV) was defined as the ratio of the area of the low-voltage zones to the total atrial surface area.

RESULTS

The DF was measured during AF. The shortest fibrillation cycle length was located in the LA posterior wall and became shorter during acute atrial dilatation. The max DF was located in the LA posterior wall and increased during acute atrial dilatation (7.1 +/- 0.8 vs 8.8 +/- 2.1, P = 0.02). The max DF of the LA correlated with the wavefront number (r = 0.797, P < 0.001 predilatation; r = 0.860, P < 0.001 postdilatation). The splitting of wavefronts facilitated the formation of new wavefronts. During acute atrial dilatation, the frequency of regional splitting was closely correlated with the percentage of the low-voltage zones (r = 0.876, P < 0.001). Furthermore, the LA posterior wall had a higher percentage of the low-voltage zones than the other sites.

CONCLUSION

In acute atrial dilatation, the percentage of the low-voltage zones increased, especially in the LA posterior wall, which correlated with the regional splitting of the AF wavefronts. The increase in the splitting facilitated the formation of new wavefronts and resulted in a higher max DF during acute atrial dilatation.

Electrical and Structural Remodeling: Role in the Genesis and Maintenance of Atrial Fibrillation

Atrial fibrillation (AF) and congestive heart failure (CHF) are 2 frequently encountered conditions in clinical practice. Both lead to changes in atrial function and structure, an array of processes known as atrial remodeling. This review provides an overview of ionic, electrical, contractile, neurohumoral, and structural atrial changes responsible for initiation and maintenance of AF. In the last decade, many studies have evaluated atrial remodeling due to AF or CHF. Both conditions often coexist, which makes it difficult to distinguish the contribution of each. Because of atrial stretch in the setting of hypertension or CHF, atrial remodeling frequently occurs long before AF arises. Alternatively, AF may lead to electrical remodeling, that is, shortening of refractoriness due to the high atrial rate itself. In many experimental AF or rapid atrial pacing studies, the ventricular rate was uncontrolled. In those studies, atrial stretch due to CHF may have interfered with the high atrial rate to produce a mixed type of electrical and structural remodeling. Other studies have dissected the individual role of AF or atrial tachycardia from the role CHF plays in atrial remodeling. Atrial fibrillation itself does not lead to structural remodeling, whereas this is frequently produced by hypertension or CHF, even in the absence of AF. Primary and secondary prevention programs should tailor treatment to the various types of remodeling.

Targeting the renin-angiotensin-aldosterone-system in atrial fibrillation: a shift from electrical to structural therapy?

Despite its increasing incidence and prevalence, treatment options in atrial fibrillation (AF) are far from ideal and often limited. After decades of focus on the electrical aspects of AF with unsatisfactory results, recent research is focusing increasingly on the atrial structural remodelling that underlies the development of AF in different pathological conditions, such as hypertension, heart failure, diabetes mellitus and coronary artery disease. The aim of this review is to provide a comprehensive overview of the role of the renin-angiotensin-aldosterone-system in AF and to highlight the clinical evidence on renin-angiotensin-aldosterone-system blockade as a therapeutic option in AF.

Congestive heart failure and atrial fibrillation: rhythm versus rate control

BACKGROUND

The incidence both of heart failure and atrial fibrillation is steadily increasing in the United States' population, and these conditions frequently coexist in the same patient. It is likely that the onset of one of these disorders leads to the onset and propagation of the other through multiple mechanisms. Several studies have investigated the prognosis of patients with both conditions, but a definitive conclusion regarding outcomes such as mortality and quality of life has yet to be determined.

METHODS AND RESULTS

Evidence demonstrating the improvement of left ventricular function and other hemodynamic parameters with the restoration and maintenance of sinus rhythm does exist. beta-blockade, angiotensin-converting enzyme inhibition, and aldosterone antagonism have been shown to improve survival in patients with heart failure. However, the efficacy of these therapies in patients with coexisting atrial fibrillation has not been adequately assessed. Furthermore, these therapies do not directly address the issue of rhythm management. The use of several antiarrhythmic medications and device therapy is becoming more frequent in the management of this subset of patients. Recent investigations of antiarrhythmic treatment have assessed outcomes such as survival, quality of life, exercise tolerance, and maintenance of sinus rhythm. Data from these studies suggest that antiarrhythmic therapy may be efficacious in such patients. Device therapy is another alternative which has been demonstrated to be at least as beneficial as medical therapy.

CONCLUSION

Both retrospective and prospective studies of antiarrhythmic therapy and device therapy have demonstrated promising results. Several studies are ongoing and will provide more insight into the management of such patients.

Association of stage of left ventricular diastolic dysfunction with P wave dispersion and occurrence of atrial fibrillation after first acute anterior myocardial infarction

OBJECTIVES

The aim of this study was to investigate the association of stage of left ventricular diastolic dysfunction after acute myocardial infarction (AMI) with P maximum, P dispersion, and atrial fibrillation (AF) occurrence rate.

BACKGROUND

The occurrence of AF following AMI is frequently associated with a left ventricle restrictive filling pattern. Increased P dispersion is also associated with the occurrence of AF after AMI. But, the relation between the stage of left ventricular diastolic dysfunction and the P wave measurements after AMI has not yet been investigated.

METHODS

Electrocardiograms of 90 patients with first anterior AMI were recorded on admission, and P wave measurements were performed. The left ventricular diastolic functions were evaluated by transthoracic echocardiography. On the basis of mitral inflow, subjects were stratified into three left ventricular diastolic filling patterns. All patients were monitored continuously for the detection of AF in the Coronary Care Unit.

RESULTS

Thirty patients had a normal filling pattern (33.3%) (NF group), 37 had impaired relaxation (41.1%) (IR group), and 23 had pseudonormal/restrictive filling pattern (25.6%) (PN/R group). P maximum was longer in the PN/R group (103 +/- 12 ms) compared with the NF group (94 +/- 9 ms, P = 0.019), but no significant difference was found between PN/R and IR (96 +/- 13 ms, P > 0.05) groups, and between NF and IR groups (P > 0.05). There was no significant difference for P minimum among the groups (P > 0.05). P dispersion was longer in the PN/R group (35 +/- 6 ms) than in the NF (26 +/- 7 ms, P < 0.001) and IR groups (26 +/- 6 ms, P < 0.001), but not different between the NF and IR groups (P > 0.05). Occurrence of AF was significantly more frequent in the PN/R group (52.2%) than in the NF (16.7%, P = 0.007) and IR groups (10.8%, P = 0.001). Frequency of AF was not different between the NF and IR groups (P > 0.05). In multivariate analyses, the stage of diastolic dysfunction was independently associated with P maximum, P minimum, P dispersion, and the occurrence of AF (P < 0.001, P = 0.035, P < 0.001, and P = 0.002, respectively).

CONCLUSIONS

P maximum and P dispersion are increased, and AF occurrence risk is higher in patients with pseudonormal/restrictive filling pattern after first anterior AMI. The stage of diastolic dysfunction is an independent predictor of P wave measurements and AF occurrence.

Moderate and chronic hemodynamic overload of sheep atria induces reversible cellular electrophysiologic abnormalities and atrial vulnerability

OBJECTIVES

The aim of this study was to evaluate the myocardial consequences of a chronic volume overload of the left atrium (LA).

BACKGROUND

Atrial dilation is a major risk factor for atrial fibrillation (AF), but the underlying mechanisms are poorly understood.

METHODS

A left-right aorto-pulmonary artery shunt (APS) was created in sheep. The cardiopathy was characterized by echocardiography, electrophysiologic testing, and histologic analysis. Cellular action potential (AP) and calcium current (I(Ca)) were recorded by means of microelectrode and patch clamp techniques.

RESULTS

Three to four months after surgery, all animals in the APS state had a dilated LA (146.2 +/- 35.4 cm(2)/m(2) vs. 91.7 +/- 10.4 cm(2)/m(2) in the control state; p = 0.0024) but remained in sinus rhythm. Repetitive atrial firing was triggered by a single extra beat in five of six animals in the APS state and in two of six animals in the control state. Moreover, in two animals in the APS state, a single extra beat triggered sustained AF. Myocytes were enlarged and 39.8% showed some degree of myolysis. In animals in the APS state, the AP had no plateau phase or small amplitude and numerous myocytes were unexcitable. The I(Ca) density was 45.2% lower in APS animals than in control animals. Beta-adrenergic stimulation normalized I(Ca) and restored the plateau phase of the AP. After shunt suppression, the electrophysiologic properties of the atria returned to normal.

CONCLUSIONS

The APS induced moderate, isolated LA dilation, which was sufficient to cause major changes in cellular electrophysiologic properties and to render the atria vulnerable to fibrillation. These effects were reversed by shunt suppression.

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.

Angiotensin-converting enzyme inhibition in hypertensive patients is associated with a reduction in the occurrence of atrial fibrillation

OBJECTIVE

The objective of this study was to determine the effects of angiotensin-converting enzyme inhibition (ACEI) versus long-acting calcium-channel blockade (CCB) on atrial fibrillation (AF) in patients with hypertension.

BACKGROUND

Atrial fibrillation is the most common significant cardiac arrhythmia, and angiotensin II has been implicated in its pathophysiology.

METHODS

This was a retrospective, longitudinal cohort study from a database of 8 million people in the U.S. Patients age > or =18 years with hypertension were eligible if they filled a prescription for either an ACEI or a CCB between January 1995 and June 1999. The use of all other antihypertensive medications was permitted. Patient chronic disease burden was assessed using a modified Charlson index. Patients were matched on a propensity score generated from a logistic regression model. A survival analysis approach was used to compare the incidence of AF between groups. The final cohorts were evaluated until June 2002, and the average follow-up was 4.5 years.

RESULTS

After cohort matching, 10,926 patients were included in the analysis and divided equally into the ACEI and CCB groups. Mean patient age was 65 years. The adjusted hazards ratio (95% confidence interval [CI]) in the ACEI versus CCB groups for the entire follow-up period was 0.85 (95% CI: 0.74 to 0.97) for new-onset AF, and the adjusted incidence ratio for AF-related hospitalizations was 0.74 (95% CI: 0.62 to 0.89).

CONCLUSIONS

Angiotensin-converting enzyme inhibition was associated with a reduced incidence of AF for patients with hypertension in a usual care setting. These results need to be confirmed in a large-scale randomized clinical trial.

Enalapril treatment and hospitalization with atrial tachyarrhythmias in patients with left ventricular dysfunction

BACKGROUND

Experimental and clinical evidence suggests a preventive role for agiotensin-coverting enzyme (ACE) inhibitors on the development of atrial fibrillation. However, the effect of ACE inhibition on hospitalization with atrial tachyarrhythmias in patients with left ventricular (LV) dysfunction is not known. We sought to determine whether enalapril treatment reduced hospitalizations with atrial tachyarrhythmias in patients with LV dysfunction.

METHODS

We performed a retrospective analysis of the Studies of Left Ventricular Dysfunction (SOLVD) trial. Hospitalizations with atrial tachyarrhythmias were noted.

RESULTS

A total of 192 hospitalizations with atrial tachyarrhythmias occurred in 158 patients during a follow-up period of 34 months. The time to first hospitalization with atrial tachyarrhythmias or death was significantly lower in the enalapril group (P =.005). In a multivariate analysis adjusting for the presence of atrial fibrillation at study entry, enalapril treatment was associated with a reduction in the rate of hospitalization with atrial tachyarrhythmias or death (RR, 0.87; 95% CI, 0.79-0.96; P =.007). The incidence of hospitalization with atrial tachyarrhythmias was 7.9 hospitalizations per 1000 patient-years of follow-up in the enalapril group, compared with 12.4 per 1000 patient-years in the placebo group (RR, 0.64; 95% CI, 0.48-0.85; P =.002).

CONCLUSION

Enalapril is associated with a decreased incidence of hospitalization with atrial tachyarrhythmias in patients with LV dysfunction.

Left Septal Atrial Flutter

BACKGROUND

We describe the clinical and electrophysiological characteristics of a novel macroreentrant form of left atrial flutter circuit.

METHODS AND RESULTS

A total of 11 patients were included in the study. The mean tachycardia cycle length was 278+/-41 ms. Nine of the 11 patients were treated with antiarrhythmic drugs at the time of the study for concomitant atrial fibrillation. With the use of entrainment pacing and either the CARTO Biosense mapping system (9 patients) or conventional mapping (2 patients), the flutter circuit was found to rotate around the left septum primum with a critical isthmus located between the pulmonary veins posteriorly and/or mitral annulus anteriorly and the septum primum. In 5 patients, radiofrequency ablation was performed from the septum primum to the right inferior pulmonary vein (group 1), and in 6 patients, a lesion was made from the septum primum to the mitral annulus (group 2). After a follow-up of 13+/-6 months, 2 patients in group 1 and all patients in group 2 remained in sinus rhythm without recurrence.

CONCLUSIONS

Slowing of electric conduction in the left atrial septum due to antiarrhythmic drugs and/or atrial myopathy seems to promote left septal atrial flutter. Radiofrequency ablation of this arrhythmia is usually effective and safe. A line of block between the septum primum and the mitral annulus proved to be effective for cure of tachycardia.

Direction-dependent conduction abnormalities in a canine model of atrial fibrillation due to chronic atrial dilatation

Chronic rapid atrial pacing (RAP) leads to changes that perpetuate atrial fibrillation (AF). Chronic atrial dilatation due to mitral regurgitation (MR) also increases AF inducibility, but it is not clear whether the underlying mechanism is similar. Therefore, we have investigated atrial electrophysiology in a canine MR model (mitral valve avulsion, 1 mo) using high-resolution optical mapping and compared it with control dogs and with the canine RAP model (6-8 wk of atrial pacing at 600 beats/min, atrioventricular block, and ventricular pacing at 100 beats/min). At followup, optical action potentials were recorded using a 16 x 16 photodiode array from 2 x 2-cm left atrial (LA) and right atrial (RA) areas in perfused preparations, with pacing electrodes around the field of view to study direction dependency of conduction. Action potential duration at 80% repolarization (APD(80)) was not different between control and MR but was reduced in RAP atria. Conduction velocities during normal pacing were not different between groups. However, the MR LA showed increased conduction heterogeneity during pacing at short cycle lengths and during premature extrastimuli, which frequently caused pronounced regional conduction slowing. Conduction in the MR LA during extrastimulation also displayed a marked dependence on propagation direction. These phenomena were not observed in the MR RA and in control and RAP atria. Thus both models form distinctly different AF substrates; in RAP dogs, the decrease in APD(80) may stabilize reentry. In MR dogs, regional LA conduction slowing and increased directional dependency, allowing unidirectional conduction block and preferential paths of conduction, may account for increased AF inducibility.

Atrial Fibrillation in Heart Failure:

AF in Heart Failure. Atrial fibrillation and congestive heart failure are commonly occurring cardiac disorders that often exist concomitantly. The prognostic significance of the presence or absence of atrial fibrillation, as an independent risk factor, in patients with heart failure remains controversial. Antiarrhythmic drugs with good hemodynamic profiles and neutral effects on survival are preferred treatments for converting atrial fibrillation and maintaining sinus rhythm. Other standard therapies for congestive heart failure, such as angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and beta-blockers also have a role in the treatment of these coexisting disease states. The article presents an overview of atrial fibrillation in patients with heart failure and reviews the prevalence, prognostic significance, and efficacy of various antiarrhythmic agents for the conversion and maintenance of sinus rhythm.

Enalapril decreases the incidence of atrial fibrillation in patients with left ventricular dysfunction: insight from the Studies Of Left Ventricular Dysfunction (SOLVD) trials

BACKGROUND

Atrial fibrillation (AF) is frequently encountered in patients with heart failure (HF) and is also a predictor of morbidity and mortality in this population. Recent experimental studies have shown electrical and structural atrial remodeling with increased fibrosis in animals with HF and have suggested a preventive effect of ACE inhibitors (ACEi) on the development of AF. To verify the hypothesis that ACEi prevent the development of AF in patients with HF, we conducted a retrospective analysis of the patients from the Montreal Heart Institute (MHI) included in the Studies Of Left Ventricular Dysfunction (SOLVD).

METHODS AND RESULTS

Clinical charts were reviewed and serial ECGs interpreted by a single cardiologist blinded to drug allocation. Patients with AF or flutter on the baseline ECG were excluded. Baseline characteristics were obtained from the SOLVD databases. The mean follow-up was 2.9+/-1.0 years. Of the 391 patients randomly assigned at MHI, 374 were in sinus rhythm at the time of random assignment, with 186 taking enalapril and 188 taking placebo. Baseline characteristics were similar in the two groups except for a higher incidence of previous myocardial infarction in the enalapril group. Fifty-five patients had AF during the follow-up: 10 (5.4%) in the enalapril group and 45 (24%) in the placebo group (P<0.0001). By Cox multivariate analysis, enalapril was the most powerful predictor for risk reduction of AF (hazard ratio, 0.22; 95% CI, 0.11 to 0.44; P<0.0001).

CONCLUSIONS

Treatment with the ACEi enalapril markedly reduces the risk of development of atrial fibrillation in patients with left ventricular dysfunction.

Atrial fibrillation: hypertension as a causative agent, risk factor for complications, and potential therapeutic target

Atrial fibrillation and hypertension are 2 prevalent, and often coexistent, conditions in the North American population. Their incidence increases with advancing age, and they are responsible for considerable morbidity and mortality. Although the relation between the 2 conditions has long been known, the treatment of hypertension is not currently a focus in the clinical management of atrial fibrillation. Hypertension is associated with left ventricular hypertrophy, impaired ventricular filling, left atrial enlargement, and slowing of atrial conduction velocity. These changes in cardiac structure and physiology favor the development of atrial fibrillation, and they increase the risk of thromboembolic complications. Conventional therapy of atrial fibrillation has focused on interventions to control heart rate and rhythm and the prevention of stroke through the use of anticoagulant medications. In patients with atrial fibrillation, aggressive treatment of hypertension may reverse the structural changes in the heart, reduce thromboembolic complications, and retard or prevent the occurrence of atrial fibrillation. Specific pharmacotherapy could potentially play a major role in the primary and secondary prevention of atrial fibrillation and its complications.

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.

Atrial pacing lead location alters the hemodynamic effects of atrial-ventricular delay in dogs with pacing induced cardiomyopathy

The role of atrial lead location in cardiovascular function in the presence of impaired ventricular dysfunction is unknown. We tested the hypothesis that left atrial (LA) and left ventricular (LV) hemodynamics are affected by alterations in AV delay and are influenced by atrial pacing site in dogs with dilated cardiomyopathy. Dogs (n = 7) were chronically paced at 220 beats/min for 3 weeks to produce cardiomyopathy and then instrumented for measurement of LA, LV end diastolic pressure (LVEDP) and mean arterial pressure (MAP), LA volume, LV short-axis diameter, and aortic and pulmonary venous blood flow. Hemodynamics were measured after instrumentation and during atrial overdrive pacing from the right atrial appendage (RAA), coronary sinus ostium (CSO) and lower LA lateral wall (LAW). The AV node was then ablated, and hemodynamics were compared during dual chamber AV pacing (right ventricular apex) from each atrial lead location at several AV delays between 20 and 350 ms. Atrial overdrive pacing from different sites did not alter hemodynamics. Cardiac output (CO), stroke volume, LVEDP, MAP and +dLVP/dt demonstrated significant (P < 0.05) variation with AV delay during dual chamber pacing. CO was higher during LAW pacing than RAA and CSO pacing (2.3 +/- 0.4 vs 2.1 +/- 0.3 vs 2.0 +/- 0.3 l/min, respectively) at an AV delay of 120 ms. Also, MAP was higher in the LAW than RAA and CSO (65 +/- 9 vs 59 +/- 9 vs 54 +/- 11 mmHg, respectively) at an AV delay of 350 ms. Atrial lead location affects indices of LV performance independent of AV delay during dual chamber pacing in dogs with cardiomyopathy.

Atrial fibrillation in heart failure: epidemiology, pathophysiology, and rationale for therapy

Heart failure (HF) affects almost 5 million patients in the United States and is a leading cause of morbidity and mortality. Atrial fibrillation (AF), like HF, affects millions of patients and markedly increases in prevalence with age. As the US population ages, the number of patients afflicted with HF and AF will continue to grow. HF with preserved ejection fraction is particularly common in the elderly population. The prevalence of AF in patients with HF increases from <10% in those with New York Heart Association (NYHA) functional class I HF to approximately 50% in those with NYHA functional class IV HF. The pathophysiologic changes that occur in patients with HF and AF are complex and incompletely understood. Alterations in neurohormonal activation, electrophysiologic parameters, and mechanical factors conspire to create an environment in which HF predisposes to AF and AF exacerbates HF. Mechanisms include atrial remodeling and tachycardia-induced myopathy. The development of AF in HF appears to independently predict death resulting from pump failure and total mortality. Although the currently available therapeutic options for AF in patients with HF are varied, their effect on prognosis remains unknown and is the subject of ongoing clinical trials. It will be critical to define and plan therapies specifically for those patients with AF, HF, and preserved ejection fraction in addition to the population with low ejection fraction that has dominated previous investigations.

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.

Bigeminy pacing: a new protocol to unmask atrial vulnerability

INTRODUCTION

One of the most exciting developments in our understanding of atrial fibrillation (AF) mechanisms has been the recognition that "AF begets AF" in a process termed atrial remodeling. Little information is available about the events that mediate short-term remodeling. In a bigeminy atrial pacing protocol that produces a continuous extrasystole-postextrasystole cycle length, we sought to evaluate the electrophysiologic consequences of irregular atrial pacing.

METHODS AND RESULTS

This study included 22 consecutive patients with documented paroxysmal AF and 10 control subjects. After evaluating the effective refractory period (ERP) and functional refractory period (FRP), bigeminy atrial pacing was performed for 5 minutes. The S1-S2 coupling interval during bigeminy pacing was programmed to a mean value of 275 +/- 45 msec, i.e., 45 msec longer than the basic ERP measured at 100 beats/min. During bigeminy pacing, AF that lasted longer than 1 minute occurred in 12 AF patients and in none of the control subjects (group I). Short salvos of AF occurred in 5 patients and 3 controls (group II). No arrhythmia occurred in 5 patients and 7 controls (group III). Sensitivity, specificity, and negative and positive predictive values of sustained AF induced by bigeminy pacing were 54%, 100%, 50%, and 100%, respectively. No differences were observed between different pacing rates during bigeminy, the premature coupling interval S1-S2, or the conduction parameters S2-A2 and A2. Group I had the shortest basic ERP (222 +/- 38 msec) and group III the longest ERP (242 +/- 21 msec, P < 0.05); group II was intermediate. Atrial ERPs and FRPs measured immediately after termination of 5 minutes of bigeminy pacing were shorter than during baseline. The degree of shortening was similar in AF patients and in controls. The locoregional conduction delay A2 did not change after the bigeminy protocol.

CONCLUSION

This study demonstrates that atrial bigeminy pacing highly increases atrial vulnerability. This protocol appears interesting because its sensitivity and specificity are higher than those of the conventional extrastimulation test. This makes it attractive for routine diagnosis of undocumented paroxysmal AF. Because it may induce atrial arrhythmias independently of the classic mechanisms of wavelength shortening, this study emphasizes the need for new modalities in the prevention of atrial arrhythmias.

[Effects of myocardial stretching on excitation frequencies determined by spectral analysis during ventricular fibrillation]

INTRODUCTION AND OBJECTIVES

The aim of this study was to analyze the effects of myocardial stretching on excitation frequencies, as determined by spectral analysis, during ventricular fibrillation.

METHODS

In 12 isolated rabbit heart preparations, ventricular activation during ventricular fibrillation was recorded with multiple electrodes. Recordings were obtained before, during and after ventricular dilatation produced with an intraventricular balloon. The dominant frequency of the signals obtained with each of the electrodes was determined by spectral analysis.

RESULTS

During the control phase, the mean, minimum and maximum dominant frequencies were, respectively, 14.3 1.7, 12.5 1.7, and 16.2 1.4 Hz, and the average difference between the maximum and minimum frequencies was 3.6 2.1 Hz. This difference was over 4 Hz in four cases, and in no case did it exceed 8 Hz. During ventricular stretching, the mean dominant frequency increased significantly (21.1 6.1 Hz; p < 0.0001), as did the minimum values (14 2.6 Hz; p < 0.05) and especially the maximum values (26.6 7.7 Hz; p < 0.0001). The difference between the maximum and minimum frequencies (12.6 6.4 Hz; p < 0.001) was over 4 Hz in all cases except one, and over 8 Hz in 9 cases. The maximum values were distributed heterogeneously during ventricular stretching. Upon suppressing ventricular stretching, the dominant frequency did not differ from controls.

CONCLUSIONS

Myocardial frequency maps during ventricular fibrillation show limited variations in the dominant frequency of the signals recorded in the lateral wall of the left ventricle. During stretching, the patterns were heterogeneous, due mainly to the marked increase in the maximum dominant frequency. In the experimental model used, the effects of stretching remitted after suppressing ventricular dilatation.

Atrial pacing for the prevention and termination of atrial fibrillation

Atrial fibrillation (AF) affects about 2% of the general population and 8%-11% of those older than 65 years. The demand for effective therapeutic strategies for AF is anticipated to increase substantially as the proportion of the elderly population increases. Atrioventricular nodal ablation accompanied by permanent pacemaker implantation is an established option in elderly patients with intractable arrhythmia and poor ventricular rate control. However, it renders most patients pacemaker dependent and does not eliminate symptoms associated with loss of atrial transport or reduce the risk of stroke. The considerable limitations of rhythm or rate control strategies prompted interest in preventative atrial pacing, which may reduce the incidence of AF by either eliminating the triggers and/or by modifying the substrate of AF. Atrial or dual-chamber pacing has been proven to prevent or delay progression to permanent AF in elderly patients with sinus node dysfunction as compared with ventricular pacing. Patients with advanced atrial conduction delay may benefit from atrial resynchronization pacing. There may be additional benefits associated with the use of particular sites of pacing, specific pacing algorithms designed to target potential triggers of AF, and pace-termination of atrial tachycardia. Preventive and antitachycardia pacing algorithms incorporated in implantable cardioverter-defibrillators and pacemakers are currently under investigation and may offer a valuable alternative to antiarrhythmic drug therapy in elderly patients with left ventricular dysfunction at high risk of proarrhythmia or worsening heart failure. The evolution of hybrid therapy, in which two or more different strategies are employed in the same patient, may be the most effective approach to management of AF.

Prevention of and medical therapy for atrial arrhythmias in heart failure

A large proportion of heart failure patients suffer from atrial arrhythmias, prime amongst them being atrial fibrillation (AF). Ventricular dysfunction and the syndrome of heart failure can also be a concomitant pathology in up to 50% of patients with AF. However this association is more than just due to shared risk factors, research from animal and human studies suggest a causal relationship between AF and heart failure. There are numerous reports of tachycardia-induced heart failure where uncontrolled ventricular rate in AF results in heart failure, which is reversible with cardioversion to sinus rhythm or ventricular rate control. However the relationship extends beyond tachycardia-induced cardiomyopathy. Optimal treatment of AF may delay progressive ventricular dysfunction and the onset of heart failure whilst improved management of heart failure can prevent AF or improve ventricular rate control. Prevention and treatment of atrial arrhythmias, and in particular atrial fibrillation, is therefore an important aspect of the management of patients with heart failure. This review describes the incidence and possible predictors of AF and other atrial arrhythmias in patients with heart failure and discusses the feasibility of primary prevention. The evidence for the management of atrial fibrillation in heart failure is systematically reviewed and the strategies of rate versus rhythm control discussed in light of the prevailing evidence.

Dynamic nature of atrial fibrillation substrate during development and reversal of heart failure in dogs

BACKGROUND

Clinical atrial fibrillation (AF) often results from pathologies that cause atrial structural remodeling. The reversibility of arrhythmogenic structural remodeling on removal of the underlying stimulus has not been studied systematically.

METHODS AND RESULTS

Chronically instrumented dogs were subjected to 4 to 6 weeks of ventricular tachypacing (VTP; 220 to 240 bpm) to induce congestive heart failure (CHF), followed by a 5-week recovery period leading to hemodynamic normalization at 5-week recovery (Wk5(rec)). The duration of burst pacing-induced AF under ketamine/diazepam/isoflurane anesthesia increased progressively during VTP and recovered toward baseline during the recovery period, paralleling changes in atrial dimensions. However, even at full recovery, sustained AF could still be induced under relatively vagotonic morphine/chloralose anesthesia. Wk5(rec) dogs showed no recovery of CHF-induced atrial fibrosis (3.1+/-0.3% for controls versus 10.7+/-1.0% for CHF and 12.0+/-0.8% for Wk5(rec) dogs) or local conduction abnormalities (conduction heterogeneity index 1.8+/-0.1 in controls versus 2.3+/-0.1 in CHF and 2.2+/-0.2 in Wk5(rec) dogs). One week of atrial tachypacing failed to affect the right atrial effective refractory period significantly in CHF dogs but caused highly significant effective refractory period reductions and atrial vulnerability increases in Wk5(rec) dogs.

CONCLUSIONS

Reversal of CHF is followed by normalized atrial function and decreased duration of AF; however, fibrosis and conduction abnormalities are not reversible, and a substrate that can support prolonged AF remains. Early intervention to prevent fixed structural abnormalities may be important in patients with conditions that predispose to the arrhythmia.

Atrial fibrillation after coronary artery bypass graft surgery is unrelated to cardiac abnormalities detected by transesophageal echocardiography

Atrial fibrillation is a common complication of coronary artery bypass graft (CABG) surgery that is associated with adverse patient outcomes. We evaluated whether preexisting abnormalities of cardiac structure or function detected with transesophageal echocardiography (TEE) are prevalent in patients later developing atrial fibrillation after CABG surgery. TEE imaging was performed after induction of general anesthesia, but before primary CABG surgery, in 62 consecutive patients without cardiac valvular disease or preexisting atrial fibrillation. Measurements included left atrial diameter, left ventricular wall thickness, left ventricular end-systolic and end-diastolic dimensions and fractional area change. Pulsed-wave Doppler measurements of pulmonary venous and trans-mitral blood flow velocity were obtained. Continuous monitoring with telemetry electrocardiography for the development of atrial fibrillation was performed. Eighteen patients (29%) developed postoperative atrial fibrillation. There were no significant differences in left atrial or left ventricular TEE variables or pulsed-wave Doppler pulmonary venous flow measurements between patients with and without postoperative atrial fibrillation. After adjusting for age and duration of aortic cross-clamping, there were no differences in the transmitral Doppler diastolic filling variables between these same groups. These data suggest that atrial fibrillation commonly occurs after CABG surgery in the absence of atrial enlargement or Doppler-derived cardiac functional abnormalities. The data imply that the use of TEE immediately before surgery would be an insensitive means for routine identification of patients susceptible to this arrhythmia.

IMPLICATIONS

Transesophageal echocardiography performed immediately before coronary artery bypass graft (CABG) surgery is not useful for prediction of susceptibility to develop atrial fibrillation postoperatively. Postoperative atrial fibrillation commonly occurs after CABG surgery in the absence of preoperative atrial enlargement or Doppler derived functional abnormalities.

Epidemiology and natural history of atrial fibrillation: clinical implications

With a substantial impact on morbidity and mortality, the growing "epidemic" of atrial fibrillation (AF) intersects with a number of conditions, including aging, thromboembolism, hemorrhage, hypertension and left ventricular dysfunction. Currently, the epidemiology and natural history of AF govern all aspects of its clinical management. The ongoing global investigative efforts toward understanding AF are also driven by epidemiologic findings. New developments, by affecting the natural history of the disease, could eventually alter the nature of decision making in patients with AF. The crucial issue of rate versus rhythm control awaits completion of the AF Follow-up Investigation of Rhythm Management trial. The processes of electrical and structural remodeling that perpetuate AF appear to be reversible. In the era of functional genomics, the molecular basis of this ubiquitous arrhythmia is in the process of being defined. Unraveling the molecular genetics of AF might provide new insights into the structural and electrical phenotypes resulting from genetic mutations and, as such, new approaches to treatment of this arrhythmia at the ion channel and cellular levels. Thus, current adverse trends are superimposed on a background of a rapidly developing knowledge base and potentially exciting new therapeutic options. Consequently, an understanding of the epidemiology and natural history of AF is crucial to the future allocation of resources and the utilization of an expanding range of therapies aimed at reducing the impact of this disease on a changing patient population.

Atrial fibrillation in patients with heart failure: pharmacologic options

Atrial fibrillation is a common arrhythmia in patients with heart failure. The presence of atrial fibrillation deteriorates cardiac function and increases the risk of thromboembolic events. The management of patients with atrial fibrillation in association with heart failure should consist of ventricular rate control, prevention of thromboembolic events, and conversion to normal sinus rhythm. Traditionally, digoxin has been widely used in patients with heart failure and atrial fibrillation; however, it does very little to restore sinus rhythm and requires the addition of another rate-limiting agent to control ventricular rate. The likelihood of successful cardioversion is dependent on the duration of heart failure and the degree of neurohormonal activation. The initiation of antiarrhythmic drug therapy in patients with heart failure should be guided by safety issues as well as consideration of potential benefits vs. risks associated with therapy. Amiodarone has been evaluated in numerous clinical trials and appears to be safe and effective when used in low dosage. Treatment with dofetilide is another option. Comparative studies with oral dofetilide vs. amiodarone are needed to evaluate their efficacy in restoration and maintenance of sinus rhythm in patients with heart failure. Such trials will clearly define the role of dofetilide in the treatment of atrial fibrillation. Routine prophylactic use of antiarrhythmic drug therapy for chronic atrial fibrillation in the setting of heart failure is not recommended due to a low efficacy rate and high proarrhythmic risk. Anticoagulation with warfarin and rate control remain the standard therapy. (c)2001 by CHF, Inc.

Ablate and pace for drug refractory paroxysmal atrial fibrillation. Is ablation necessary?

BACKGROUND

Atrio-ventricular junctional ablation with pacemaker insertion has been shown to improve quality of life in patients with drug refractory paroxysmal atrial fibrillation. It is unknown whether this improvement is secondary to the ablation procedure or to the pacemaker mode utilised. To investigate this we reviewed our experience of implanting a dual chamber rate responsive pacemaker with mode switching (DDDR/MS) alone on quality of life in this patient group.

METHODS AND RESULTS

Over a 1-year period, 19 patients (mean age 62+/-9 years, 13 female) with drug refractory paroxysmal atrial fibrillation (mean duration of symptoms 8.7+/-7 years, failed 3.1+/-0.9 anti-arrhythmic drugs, amiodarone in 15) were recruited. Quality of life was assessed at baseline and after 1 month using a cardiac specific questionnaire, the modified Karolinska questionnaire. The mean score for all patients significantly improved by 39% at follow up (baseline 59+/-24, 1 month 36+/-24, P=0.001). Individually 15 patients (79%) had an improvement in their score, whilst for 13 patients (68%) their symptoms were sufficiently improved after pacing that ablation was not required. The benefit was maintained to a mean follow up of 12+/-5 months (score 31+/-20, P<0.001). Six patients remained symptomatic after pacing and requested further treatment. Benefit was unrelated to symptoms at baseline or the number and total duration of paroxysmal atrial fibrillation episodes recorded on pacemaker Holter.

CONCLUSIONS

Patients with drug refractory paroxysmal atrial fibrillation, DDDR/MS pacing alone can improve quality of life without concurrent atrio-ventricular junctional ablation in a significant proportion of patients.

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.

Prediction of paroxysmal atrial fibrillation in patients with congestive heart failure: a prospective study

OBJECTIVES

We sought to prospectively determine whether patients with congestive heart failure (CHF) at risk for paroxysmal atrial fibrillation (PAF) could be identified by clinical and study variables including the P-wave signal-averaged electrocardiogram (P-SAECG).

BACKGROUND

Although it is important to assess the risk of developing PAF in patients with CHF, it still remains difficult to predict the PAF appearance in patients with CHF clinically.

METHODS

The study group consisted of 75 patients in sinus rhythm without a history of PAF, whose left ventricular ejection fraction, as measured by radionuclide angiography, was <40%. These patients underwent P-SAECG, echocardiography and 24-h Holter monitoring; in addition, the plasma concentration of atrial natriuretic peptide (ANP) was measured at study entry.

RESULTS

An abnormal P-SAECG was found at study entry in 29 of 75 patients. In the follow-up period of 21 +/- 9 months, the PAF attacks documented on the ECG significantly more frequently occurred in patients with (32%) rather than without an abnormal P-SAECG (2%) (p = 0.0002). The plasma ANP level was significantly higher in patients with rather than without PAF attacks (75 +/- 41 vs. 54 +/- 60 pg/ml, p = 0.01), although there were no significant differences in age, left atrial dimension or high grade atrial premature beats between the groups. The multivariate Cox analysis identified that the variables significantly associated with PAF development were an abnormal P-SAECG (hazard ratio 19.1, p = 0.0069) and elevated ANP level > or =60 pg/ml (hazard ratio 8.6, p = 0.018).

CONCLUSIONS

An abnormal P-SAECG and elevated ANP level could be predictors of PAF development in patients with CHF.

Trandolapril reduces the incidence of atrial fibrillation after acute myocardial infarction in patients with left ventricular dysfunction

BACKGROUND

Studies have suggested that ACE inhibitors have an antiarrhythmic effect on ventricular arrhythmias. Whether they have an effect on atrial fibrillation is unknown.

METHODS AND RESULTS

We investigated the effect of ACE inhibition with trandolapril on the incidence of atrial fibrillation in patients with reduced left ventricular function secondary to acute myocardial infarction. The patients in this study were those who qualified for inclusion into the TRAndolapril Cardiac Evaluation (TRACE) study, a randomized double-blind placebo-controlled study and who had sinus rhythm on the ECG obtained at randomization. Patients who fulfilled the criteria for inclusion were randomized to treatment with the ACE inhibitor trandolapril or placebo and were followed up for 2 to 4 years. Development and time to occurrence of atrial fibrillation in one 12-lead ECG recorded at the outpatient visits was the primary end point of this investigation. Of the 1749 patients included in the TRACE study, 1577 had sinus rhythm on the ECG recorded at randomization. Of these patients, 790 were randomized to trandolapril treatment and 787 to placebo treatment. The groups differed only slightly with respect to baseline characteristics. A total of 64 patients developed atrial fibrillation during the 2- to 4-year follow-up period. Significantly more patients developed atrial fibrillation in the placebo group than in the trandolapril group, 5.3% (n=42) versus 2.8% (n=22), respectively, P<0.05. Cox multivariable regression analysis, adjusting for important baseline characteristics, revealed that trandolapril treatment significantly reduced the risk of developing atrial fibrillation (RR, 0.45; 95% CI, 0.26 to 0.76; P<0.01).

CONCLUSIONS

The results from the present study demonstrate that trandolapril treatment reduces the incidence of atrial fibrillation in patients with left ventricular dysfunction after acute myocardial infarction.

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

INTRODUCTION

Recently, we reported that repetitive induction of atrial fibrillation (AF) in the goat causes electrical remodeling of the atria leading to the development of sustained AF. The aim of the present study was to compare Class IA, IC, and III drugs in their ability to cardiovert chronic AF in remodeled atria.

METHODS AND RESULTS

In 16 goats with sustained AF, hydroquinidine (HQ), cibenzoline (Ci), flecainide (FI), and d-sotalol (dS) were infused. HQ, Ci, Fl, and dS restored sinus rhythm (SR) in 83%, 91%, 67%, and 92% of the cases, while adverse drug effects occurred in 17%, 36%, 56%, and 8%. Prior to restoration of SR, AF cycle length prolonged by 68%, 103%, 53%, and 20%, respectively. The QRS width increased by 14%, 64%, and 58% (HQ, Ci, and Fl), and remained unchanged by administration of dS. RR intervals were slightly prolonged by HQ, Ci, and Fl, and markedly prolonged by dS (48%). The QT interval was moderately prolonged by HQ, Ci, and Fl, and considerably by dS (34%). QTc was only slightly prolonged by each of the drugs. Directly after cardioversion of AF, the atrial refractory period was 87+/-29 (HQ), 119+/-32 (Ci), 66+/-10 (Fl), and 73+/-18 msec (dS) (control: 146+/-18 msec). Atrial conduction velocity was 85+/-6, 71+/-11, 86+/-12, and 110+/-11 cm/sec compared with a control value of 116+/-10 cm/sec. Because directly after cardioversion the atrial wavelength was still very short (5.7 to 8.4 cm), the vulnerability for AF was still very high, and a single premature beat reinduced AF in 71% (Ci) to 100% (HQ, Fl, and dS) of the cases.

CONCLUSION

In a goat model of sustained AF, Class IA, IC, and III drugs restored sinus rhythm in 67% to 92% of the cases. However, after cardioversion, the atrial wavelength was still abnormally short, and AF was readily inducible in 71% to 100% of the cases.

Atrial electrophysiologic remodeling: another vicious circle?

INTRODUCTION

With few exceptions, acquired heart disease is the result of gradual changes in the heart, progressing during several months or years. This also includes certain cardiac arrhythmias, as for instance atrial fibrillation (AF). In spite of the important role of slowly progressing pathologic processes, most of our knowledge about mechanisms of cardiac arrhythmias is based on acute experiments. Only recently, the attention also is more focused on long-term adaptation processes like cardiac memory, electrical remodeling, and tachycardia-induced cardiomyopathy. In experimental animal models, it has been shown that AF induces a vicious circle of electrophysiologic and structural changes that inevitably leads to "domestication" of the arrhythmia ("AF begets AF"). In this article, the studies on AF-induced electrophysiologic and cellular remodeling are discussed.

[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.

The new onset of atrial arrhythmias following major noncardiothoracic surgery is associated with increased mortality

STUDY OBJECTIVES

To examine the incidence and consequences of atrial arrhythmias in surgical ICU patients following major noncardiac, nonthoracic surgery.

DESIGN

Prospective observational study.

SETTING

University hospital surgical ICU.

PATIENTS

Four hundred sixty-two consecutive patients after noncardiothoracic surgery.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Patients were assigned to one of three groups: group 1-new-onset atrial arrhythmias (n=47); group 2-history of atrial arrhythmias (n=58); and group 3-no atrial arrhythmias (n=357). New arrhythmias occurred in 10.2% of patients. Most began within the first 2 postoperative days. These patients had a higher mortality rate (23.4%), longer ICU stay (8.5+/-17.4 [SD] days), and extended hospital stay (23.3+/-23.6 days) than patients without atrial arrhythmias (mortality, 4.3%; ICU stay, 2.0+/-4.5 days; hospital stay; 13.3+/-17.7 days; p<0.02). Thirteen percent of patients had a history of atrial arrhythmias. They had a higher mortality rate (8.6%) and longer ICU stays (2.9+/-4.9 days; p<0.02) than patients without arrhythmias. Most deaths in the two arrhythmia groups were not due to cardiac problems, but to sepsis or cancer.

CONCLUSIONS

Patients admitted to a surgical ICU after noncardiothoracic surgery with a history of or who developed new atrial arrhythmias had greater mortality and longer ICU stays than patients without arrhythmias. The incidence of new-onset arrhythmias was lower than reported after cardiac and thoracic surgery, but higher than in the general population. Atrial arrhythmias were not the cause of death and appear to be markers of increased mortality and morbidity.

Cardioversion of atrial fibrillation in the setting of mild to moderate heart failure

We investigated the effect of electrical cardioversion of atrial fibrillation in patients with heart failure. The study group consisted of 24 patients with mild to moderate heart failure [13 men, mean age 67+/-7 years, mean peak oxygen consumption (peak VO2) 16.3+/-2.8 ml/min/kg] and chronic atrial fibrillation (median duration 19 (1-228) months). Patients were stable on digoxin, diuretics, nitrates and angiotensin converting enzyme inhibitors; no prophylaxis with antiarrhythmics was started after cardioversion. Cardioversion was unsuccessful in 6 patients; of the 18 patients in whom sinus rhythm was obtained 9 had a relapse of atrial fibrillation within 6 weeks after cardioversion. The remaining 9 patients with maintenance of sinus rhythm and the 15 (6+9) patients with atrial fibrillation at follow-up after 6 weeks did not differ with respect to any baseline characteristic, including age, peak VO2, duration of atrial fibrillation, echocardiographic left ventricular and left atrial dimensions, plasma atrial natriuretic peptide and norepinephrine. In the patients with maintenance of sinus rhythm, baseline measurements were repeated at follow-up. Peak VO2 did not change significantly (16.7+/-2.8 to 17.6+/-3.3 ml/min/kg, P=0.29); also, echo parameters, atrial natriuretic peptide and norepinephrine were not significantly affected. These results indicate that it is difficult to achieve lasting sinus rhythm through electrical cardioversion in patients with atrial fibrillation and mild to moderate heart failure. Moreover, in patients with maintenance of sinus rhythm after cardioversion no significant benefit in terms of peak VO2, cardiac dimensions, and neurohumoral status is to be expected. Hence, indiscriminate cardioversion of atrial fibrillation in the setting of heart failure does not appear to be useful.

Electrical remodeling due to atrial fibrillation in chronically instrumented conscious goats: roles of neurohumoral changes, ischemia, atrial stretch, and high rate of electrical activation

BACKGROUND

Recently, we developed a goat model of chronic atrial fibrillation (AF). Due to AF, the atrial effective refractory period (AERP) shortened and its physiological rate adaptation inversed, whereas the rate and stability of AF increased. The goal of the present study was to evaluate the role of (1) the autonomic nervous system, (2) ischemia, (3) stretch, (4) atrial natriuretic factor (ANF), and (5) rapid atrial pacing in this process of electrical remodeling.

METHODS AND RESULTS

Twenty-five goats were chronically instrumented with multiple epicardial atrial electrodes. Infusion of atropine (1.0 mg/kg; n=6) or propranolol (0.6 mg/kg; n=6) did not abolish the AF-induced shortening of AERP or interval (AFI). Blockade of K+(ATP) channels by glibenclamide (10 micromol/kg; n=6) slightly increased the AFI from 95+/-4 to 101+/-5 ms, but AFI remained considerably shorter than during acute AF (145 ms). Glibenclamide had no significant effect on AERP after electrical cardioversion of AF (69+/-14 versus 75+/-15 ms). Volume loading by 0.5 to 1.0 L of Hemaccel (n=12) did not shorten AERP. The median plasma level of ANF increased from 42 to 99 pg/mL after 1 to 4 weeks of AF (n=6), but ANF infusion (0.1 to 3.1 microg/min, n=4) did not shorten AERP. Rapid atrial pacing (24 to 48 hours; n=10) progressively shortened AERP from 134+/-10 to 105+/-6 ms and inversed its physiological rate adaptation.

CONCLUSIONS

Electrical remodeling by AF is not mediated by changes in autonomic tone, ischemia, stretch, or ANF. The high rate of electrical activation itself provides the stimulus for the AF-induced changes in AERP.

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.

Electrophysiologic characteristics and radiofrequency catheter ablation in atrioventricular node reentrant tachycardia with second-degree atrioventricular block

INTRODUCTION

Detailed electrophysiologic study of AV nodal reentrant tachycardia (AVNRT) with 2:1 AV block has been limited.

METHODS AND RESULTS

Six hundred nine consecutive patients with AVNRT underwent electrophysiologic study and radiofrequency catheter ablation of the slow pathway. Twenty-six patients with 2:1 AV block during AVNRT were designated as group I, and those without this particular finding were designated as group II. The major findings of the present study were: (1) group I patients had better anterograde and retrograde AV nodal function, shorter tachycardia cycle length (during tachycardia with 1:1 conduction) (307 +/- 30 vs 360 +/- 58 msec, P < 0.001), and higher incidence of transient bundle branch block during tachycardia (18/26 vs 43/609, P < 0.001) than group II patients; (2) 21 (80.8%) group I patients had alternans of AA intervals during AVNRT with 2:1 AV block. Longer AH intervals (264 +/- 26 vs 253 +/- 27 msec, P = 0.031) were associated with the blocked beats. However, similar HA intervals (51 +/- 12 vs 50 +/- 12 msec, P = 0.363) and similar HV intervals (53 +/- 11 vs 52 +/- 12, P = 0.834) were found in the blocked and conducted beats; (3) ventricular extrastimulation before or during the His-bundle refractory period bundle could convert 2:1 AV block to 1:1 AV conduction.

CONCLUSIONS

Fast reentrant circuit, rather than underlying impaired conduction of the distal AV node or infranodal area, might account for second-degree AV block during AVNRT. Slow pathway ablation is safe and effective in patients who have AVNRT with 2:1 AV block.

Heart failure and atrial fibrillation: current concepts and controversies

Heart failure and atrial fibrillation are very common, particularly in the elderly. Owing to common risk factors both disorders are often present in the same patient. In addition, there is increasing evidence of a complex, reciprocal relation between heart failure and atrial fibrillation. Thus heart failure may cause atrial fibrillation, with electromechanical feedback and neurohumoral activation playing an important mediating role. In addition, atrial fibrillation may promote heart failure; in particular, when there is an uncontrolled ventricular rate, tachycardiomyopathy may develop and thereby heart failure. Eventually, a vicious circle between heart failure and atrial fibrillation may form, in which neurohumoral activation and subtle derangement of rate control are involved. Treatment should aim at unloading of the heart, adequate control of ventricular rate, and correction of neurohumoral activation. Angiotensin converting enzyme inhibitors may help to achieve these goals. Treatment should also include an attempt to restore sinus rhythm through electrical cardioversion, though appropriate timing of cardioversion is difficult. His bundle ablation may be used to achieve adequate rate control in drug refractory cases.

Effects of lisinopril in patients with heart failure and chronic atrial fibrillation

Although atrial fibrillation is common in patients with heart failure, patients with atrial fibrillation are often excluded from congestive heart failure trials or are not analyzed separately. Consequently, while the effect of angiotensin-converting enzyme inhibitors in patients with sinus rhythm is well established, the effect on patients with atrial fibrillation is unknown. The authors hypothesized that these agents might be particularly effective in this patient category, given their antiadrenergic properties and the importance of adequate rate control. Therefore, the effects of lisinopril 10 mg once daily were evaluated in 30 patients with congestive heart failure and chronic atrial fibrillation (mean age, 68 +/- 6 years) in a double-blind, randomized, placebo-controlled trial. All patients were in New York Heart Association class II or III and were stable on conventional therapy (digoxin, diuretics, nitrates). After 6 weeks, mean peak oxygen consumption increased from 14.7 +/- 3.4 to 15.9 +/- 2.9 mL/min/kg in the lisinopril group (P = .034). Plasma norepinephrine levels during exercise and at peak exercise tended to be lower when the patients were taking lisinopril (10.8 +/- 4.2 to 8.9 +/- 4.4 nmol/L and 16.3 +/- 9.2 to 14.3 +/- 7.7 nmol/L, P < .1). Heart rate during exercise and ambulatory monitoring was not significantly affected. Left ventricular fractional shortening tended to increase after lisinopril (23 +/- 7 to 27 +/- 9%, P = .073). Left atrial volume was unchanged, as were plasma atrial natriuretic peptide levels. After subsequent electrical cardioversion, treatment was continued for 6 more weeks, allowing assessment of the effect of lisinopril on maintenance of sinus rhythm; maintenance of sinus rhythm was 71% in the lisinopril group and 36% in the placebo group (P = NS). This study shows that treatment with an angiotensin- converting enzyme inhibitor improves peak oxygen consumption in patients with congestive heart failure and chronic atrial fibrillation. Attenuation of adrenergic drive during exercise may play a role in mediating this effect.

Atrial pressure and experimental atrial fibrillation

A possible profibrillatory effect on the atria of an elevated atrial pressure and the site of atrial stimulation was examined. In 15 anesthetized dogs, right or left atrial or biatrial pacing was applied at a high rate (300-600/min) for 5 seconds at double threshold intensity under a wide range of atrial pressures achieved by venous or arterial transfusion or bleeding. Induction of atrial fibrillation in 236 of 1,971 pacing runs was associated with a significantly higher (P < 0.001) atrial pressure (21.6 +/- 12.2 mmHg, mean +/- SD) than maintenance of sinus rhythm (16.8 +/- 11.1 mmHg in 1,735 of 1,971 pacing runs). Stimulation of the right atrium resulted in atrial fibrillation more frequently than left atrial or biatrial stimulation, with biatrial stimulation less frequent than right or left atrial stimulation. The induction of atrial fibrillation was related to the atrial pressure and to the site of stimulation but not to the pacing rate or the prepacing heart rate. The prepacing heart rate, associated with failure to induce sustained atrial fibrillation, was higher than that associated with atrial fibrillation in 12 of 15 experiments (significantly in 6) and not significantly lower in 3 of 15. Atrial fibrillation lasting 1 minute or more was more frequently associated with simultaneous stimulation of both atria than of either atrium alone. Thus, an elevated atrial pressure may facilitate the induction of atrial fibrillation. The site of stimulation also plays an important role for both the induction and maintenance of atrial fibrillation in this model.

Relation between cycle length, volume, and pressure in type I atrial flutter

Assuming that type I atrial flutter is a macroreentrant circuit, its cycle length should vary with the atrial dimensions. In order to test this hypothesis, flutter cycle length was measured while inducing atrial volume and pressure changes by postural and pharmacological means in seven patients undergoing a therapeutic programmed stimulation for type I atrial flutter conversion. Right atrial volume was estimated from B-mode echocardiography data. Basal values were compared with those obtained during inspiration, expiration, Valsalva maneuver, negative tilt (head down), and positive tilt (head up) with 0.8-1.6 mg p.o. nitroglycerin. The right atrial size increased slightly from 17.8 to 18.3 cm2 (P = 0.04) during the pressure load induced by negative tilt (+3 mmHg), with a corresponding lengthening of the flutter cycle length from 228 to 233 msec (P = 0.02). Similarly, pressure unloading of -2 mmHg by positive tilting and nitrates was accompanied by a decrease in right atrial size to 16.6 cm2 (P = 0.04), with a corresponding decrease in cycle length from 228 to 219 msec (P = 0.03). Respiratory maneuver yielded similar results with an inspiratory cycle lengthening, expiratory shortening, and further shortening during Valsalva maneuver. These experiments demonstrate a direct relation between cycle length and atrial volume in human type I atrial flutter. They underline the importance of the right heart preload and atrial size for the electrophysiological characteristics of type I atrial flutter. Beside its fundamental interest, this finding is important for the understanding of the mechanism of maintenance and therapeutic responses of this common arrhythmia.

Ventricular beats induce variations in cycle length of rapid (type II) atrial flutter in humans. Evidence of leading circle reentry

BACKGROUND

Slight variation in cycle lengths of common and rapid atrial flutter in humans is an established phenomenon, but its mechanisms have not been completely clarified. In a previous study, we demonstrated that in common atrial flutter the variations in atrial cycle length were due to atrial stretch affecting the revolution time of a reentrant circuit. In the present study, we investigate the nature of atrial cycle length variations in the rapid type of human atrial flutter.

METHODS AND RESULTS

Atrial interval variations of 17 episodes of rapid atrial flutter in 14 patients were investigated by measuring the sequence of atrial intervals from intraesophageal or intra-atrial leads and the onset of QRS complexes from a surface lead (V1). To study whether interval variation in flutter cycle was related to ventricular activity, a phase plot was constructed in which the flutter cycle length was plotted against the time after the previous QRS complex. This showed that the interval fluctuations were strictly coupled to the moment of ventricular activation. After the onset of the QRS complex, the rapid atrial flutter interval gradually decreased by an average of 4.1% (P < .001) and reached a minimum value after 300 to 600 milliseconds. Thereafter, the intervals increased again until the next ventricular beat occurred. In 10 patients developing both common and rapid atrial flutter, two different phase relations were found. Whereas during common atrial flutter the atrial interval increased after the QRS complex, it decreased during rapid atrial flutter. In three patients, intra-atrial pressure was recorded together with the electrical activity during both common and rapid atrial flutter episodes. This showed that variations in atrial flutter cycle length were associated with the rise of atrial pressure during ventricular contraction.

CONCLUSIONS

These findings indicate a role of contraction-excitation feedback caused by atrial stretch after a ventricular activation. The shortening of the atrial interval after the onset of the QRS complex as found in patients during rapid atrial flutter can be explained by stretch-induced shortening of atrial refractoriness and consequent shortening of the revolution time of a functionally determined intra-atrial circuit.

Electrophysiological effects of flecainide acetate on stretched guinea pig left atrial muscle fibers

The electrophysiological effects of flecainide acetate (3 x 10(-6) M) on stretched atrial tissue were investigated using guinea-pig left atrial muscle fibers. Before stretching, the resting membrane potential was not affected by flecainide at 1 Hz, although the overshoot potential (Eov) and the action potential duration at 50% repolarization (APD50) were slightly but significantly decreased by 2 +/- 1 mV and 2 +/- 1 msec, respectively. The effective refractory period (ERP) was increased by 3 +/- 1 msec. The reduction of Vmax was 20.6 +/- 1.2%. The half-maximum potential (Vh) of the relationship between Vmax and the resting potential was shifted to become more negative by flecainide (from -60.6 +/- 2.1 mV to -63.2 +/- 1.7 mV). After 90-120 min of washout with drug-free Tyrode's solution, the tissue was mechanically stretched to 150% of its slack length. Stretching significantly decreased the Vmax by 16.9 +/- 3.1%, along with a slight but significant increase in ERP (3 +/- 1 msec) and shifted Vh to become more negative (from -60.6 +/- 2.1 to -63.1 +/- 1.8 mV). In the presence of flecainide, Vmax further decreased by 20.2 +/- 2.6%, and Vh shifted from -63.1 +/- 1.8 to -65.0 +/- 1.5 mV. Comparison with the control unstretched fibers showed that flecainide significantly decreased Vmax by 34.0 +/- 2.7%, reduced the resting membrane potential by 3 +/- 1 mV, decreased Eov by 4 +/- 1 mV, and shifted Vh from -60.6 +/- 2.1 to -65.0 +/- 1.5 mV, while the APD50 and ERP did not change. In conclusion, the reduction of Vmax in the presence of flecainide was much greater in the stretched atrial muscle fibers than in the unstretched fibers, because the Vmax-resting potential relationship was shifted towards more negative potentials by both flecainide and stretching. These results suggest that flecainide exerts a stronger antiarrhythmic action on stretched atrial muscle fibers than on normal fibers.