High resolution mapping of the pulmonary vein and the vein of Marshall during induced atrial fibrillation and atrial tachycardia in a canine model of pacing-induced congestive heart failure

Structural and electro-anatomical characterization of the equine pulmonary veins: implications for atrial fibrillation

INTRODUCTION/OBJECTIVES

Spontaneous pulmonary vein (PV) activity triggers atrial fibrillation (AF) in humans. Although AF frequently occurs in horses, the origin remains unknown. This study investigated the structural and electro-anatomical properties of equine PVs to determine the potential presence of an arrhythmogenic substrate.

ANIMALS, MATERIALS AND METHODS

Endocardial three-dimensional electro-anatomical mapping (EnSite Precision) using high-density (HD) catheters was performed in 13 sedated horses in sinus rhythm. Left atrium (LA) access was obtained retrogradely through the carotid artery. Post-mortem, tissue was harvested from the LA, right atrium (RA), and PVs for histological characterization and quantification of ion channel expression using immunohistochemical analysis.

RESULTS

Geometry, activation maps, and voltage maps of the PVs were created and a median of four ostia were identified. Areas of reduced conduction were found at the veno-atrial junction. The mean myocardial sleeve length varied from 28 ± 13 to 49 ± 22 mm. The PV voltage was 1.2 ± 1.4 mV and lower than the LA (3.4 ± 0.9 mV, P < 0.001). The fibrosis percentage was higher in PV myocardium (26.1 ± 6.6 %) than LA (14.5 ± 5.0 %, P = 0.003). L-type calcium channel (CaV1.2) expression was higher in PVs than LA (P = 0.001). T-type calcium channels (CaV3.3), connexin-43, ryanodine receptor-2, and small conductance calcium-activated potassium channel-3 was expressed in PVs.

CONCLUSIONS

The veno-atrial junction had lower voltages, increased structural heterogeneity and areas of slower conduction. Myocardial sleeves had variable lengths, and a different ion channel expression compared to the atria. Heterogeneous properties of the PVs interacting with the adjacent LA likely provide the milieu for re-entry and AF initiation.

Is extensive atrial fibrosis in the setting of heart failure associated with a reduced atrial fibrillation burden?

Atrial fibrillation (AF) affects 10-50% of patients with chronic heart failure (HF) and is associated with poor long-term prognosis. AF is commonly associated with atrial structural remodeling (ASR), principally characterized by atrial dilatation and fibrosis. However, the occurrence of AF in the full spectrum of ASR encountered in patients with HF is poorly defined. Experimental studies have presented evidence that extensive ASR can be accompanied with a reduced burden of AF, secondary to a prominent depression of atrial excitability. This reduction in AF burden is associated with severe atrial fibrosis rather than with dilatation. Clinical studies of patients with HF point to the possibility that advanced ASR is associated with a less frequent AF occurrence than moderate ASR. Our goal in this review is to introduce the hypothesis that AF is less likely to occur in severe versus moderate atrial ASR in the setting of HF and that it is severe atrial fibrosis-associated depression of atrial excitability that reduces AF burden.

Focal activation patterns: breaking new grounds in the pathophysiology of atrial fibrillation

INTRODUCTION

High-resolution atrial mapping studies have provided novel insights in the pathophysiology of atrial fibrillation (AF) in the last few years. Increasing attention is being drawn to the so-called focal activation patterns (FAPs); however, there is no consensus on criteria to identify and characterize these patterns. Areas covered: In this expert review, an overview of definitions and criteria used to examine FAPs obtained from atrial mapping studies is provided and studies reporting on the underlying mechanisms are discussed. Expert commentary: High-resolution cardiac mapping has revealed the importance of FAPs in the pathophysiology of AF. There is increasing evidence supporting the concept of endo-epicardial (E-E) asynchrony enabling transmural conduction of electrical waves resulting in FAPs. Uniform reports of FAPs in future studies are needed to provide more knowledge on its clinical importance.

Atrial Electrophysiological Remodeling and Fibrillation in Heart Failure

Heart failure (HF) causes complex, chronic changes in atrial structure and function, which can cause substantial electrophysiological remodeling and predispose the individual to atrial fibrillation (AF). Pharmacological treatments for preventing AF in patients with HF are limited. Improved understanding of the atrial electrical and ionic/molecular mechanisms that promote AF in these patients could lead to the identification of novel therapeutic targets. Animal models of HF have identified numerous changes in atrial ion currents, intracellular calcium handling, action potential waveform and conduction, as well as expression and signaling of associated proteins. These studies have shown that the pattern of electrophysiological remodeling likely depends on the duration of HF, the underlying cardiac pathology, and the species studied. In atrial myocytes and tissues obtained from patients with HF or left ventricular systolic dysfunction, the data on changes in ion currents and action potentials are largely equivocal, probably owing mainly to difficulties in controlling for the confounding influences of multiple variables, such as patient’s age, sex, disease history, and drug treatments, as well as the technical challenges in obtaining such data. In this review, we provide a summary and comparison of the main animal and human electrophysiological studies to date, with the aim of highlighting the consistencies in some of the remodeling patterns, as well as identifying areas of contention and gaps in the knowledge, which warrant further investigation.

Atrial fibrillation: to map or not to map?

Isolation of the pulmonary veins may be an effective treatment modality for eliminating atrial fibrillation (AF) episodes but unfortunately not for all patients. When ablative therapy fails, it is assumed that AF has progressed from a trigger-driven to a substrate-mediated arrhythmia. The effect of radiofrequency ablation on persistent AF can be attributed to various mechanisms, including elimination of the trigger, modification of the arrhythmogenic substrate, interruption of crucial pathways of conduction, atrial debulking, or atrial denervation. This review discusses the possible effects of pulmonary vein isolation on the fibrillatory process and the necessity of cardiac mapping in order to comprehend the mechanisms of AF in the individual patient and to select the optimal treatment modality.

A temporal window of vulnerability for development of atrial fibrillation with advancing heart failure

AIMS

Heart failure (HF) is associated with development of AF and life-threatening ventricular tachycardia and fibrillation (VT/VF). Vulnerability to development of AF and VT/VF at different stages of HF and the underlying pathophysiological mechanisms are poorly defined. The present study was designed to determine the time-course of development of electrical and structural remodelling of the atria and ventricles, and their contribution to induction of AF and VT/VF in a canine model of HF.

METHODS AND RESULTS

Dogs were ventricular tachypaced (VTP) for 2-3 weeks or 5-6 weeks ('early' and 'late' HF, respectively). Electrophysiological studies were performed in isolated atrial and ventricular preparations and correlated with cardiac dimensions and haemodynamic parameters recorded in vivo. Vulnerability to programmed electrical stimulation-induced AF was greater in early vs. late stages of HF (78% vs. 38%). In contrast, VT/VF was inducible in late but not in early stages of HF (38% vs. 0%). The temporal distinction in atrial and ventricular arrhythmia susceptibility was associated with a much more rapid development of electrical and structural remodelling in atria. Vulnerability to AF developed following moderate electro-structural remodelling and waned with further progression to severe remodelling, which averted rapid atrial activation.

CONCLUSIONS

A temporal window of vulnerability for AF appears relatively early during development of VTP-induced HF in dogs, whereas VT/VF vulnerability is observed at more advanced stages of HF. These findings, if confirmed in humans, may have clinical implications with regard to prognosis and approach to therapy of patients with HF.

Experimental evaluation of the cardiac rhythm originating in myocardial sleeves of pulmonary veins using a monophasic action potential

Spontaneous depolarization similar to that from the sinus node was documented from the myocardial sleeves of pulmonary veins (PV) after isolation procedures. It was then hypothesized that sinus node-like tissue is present in the PVs of humans. Based on a number of features, the myocardium of myocardial sleeves (MS) is highly arrhythmogenic. Membrane potentials originating from MS are invariably recordable at the PVs ostia in patients with atrial fibrillation (AF) and delayed conduction around the PVs ostia may play a role in re-entry process responsible for the initiation and maintenance of AF. Diagnostic and therapeutic evidence of premature atrial beats induced in MS of PVs and resulting in launch of AF was detected by 3D electroanatomic method of monophasic action potential (MAP). MAP recording plays an important role in a direct view of human myocardial electrophysiology under both physiological and pathological conditions. Its crucial importance lies in the fact that it enables the study of the action potential of myocardial cell in vivo and, therefore, the study of the dynamic relation of this potential with all the organism variables. The knowledge of pathological MAPs from PV myocardial sleeves can help us to confirm a diagnosis when finding the similar action potential morphology. MAP can be also used to evaluate the therapeutic efficiency of vagal nerves suppression, radiofrequency ablation or other treatment procedures in PVs myocardial sleeves as well as for post-treatment following up.

Atrial fibrosis: a risk stratifier for atrial fibrillation

Atrial fibrillation (AF), especially persistent and long-standing persistent AF, may result in electro-anatomical changes in the left atrium, resulting in remodeling and deposition of fibrous tissue. There are emerging data that atrial substrate modification may increase the risk of thromboembolic complications, including stroke. Several studies have reported that atrial fibrosis is due to complex interactions among several cellular and neurohumoral mediators. Late gadolinium enhancement MRI has been reported to allow quantitative assessment of myocardial fibrosis in patients at risk of developing a stroke. Current stroke risk stratification criteria for AF do not utilize atrial fibrosis as an independent risk factor despite its association with AF and stroke. Further research is required in developing adequate risk stratification tools for predicting the stroke risk and catheter ablation outcomes in AF.

Autonomic cardiac innervation: development and adult plasticity

Autonomic cardiac neurons have a common origin in the neural crest but undergo distinct developmental differentiation as they mature toward their adult phenotype. Progenitor cells respond to repulsive cues during migration, followed by differentiation cues from paracrine sources that promote neurochemistry and differentiation. When autonomic axons start to innervate cardiac tissue, neurotrophic factors from vascular tissue are essential for maintenance of neurons before they reach their targets, upon which target-derived trophic factors take over final maturation, synaptic strength and postnatal survival. Although target-derived neurotrophins have a central role to play in development, alternative sources of neurotrophins may also modulate innervation. Both developing and adult sympathetic neurons express proNGF, and adult parasympathetic cardiac ganglion neurons also synthesize and release NGF. The physiological function of these “non-classical” cardiac sources of neurotrophins remains to be determined, especially in relation to autocrine/paracrine sustenance during development.

 

Cardiac autonomic nerves are closely spatially associated in cardiac plexuses, ganglia and pacemaker regions and so are sensitive to release of neurotransmitter, neuropeptides and trophic factors from adjacent nerves. As such, in many cardiac pathologies, it is an imbalance within the two arms of the autonomic system that is critical for disease progression. Although this crosstalk between sympathetic and parasympathetic nerves has been well established for adult nerves, it is unclear whether a degree of paracrine regulation occurs across the autonomic limbs during development. Aberrant nerve remodeling is a common occurrence in many adult cardiovascular pathologies, and the mechanisms regulating outgrowth or denervation are disparate. However, autonomic neurons display considerable plasticity in this regard with neurotrophins and inflammatory cytokines having a central regulatory function, including in possible neurotransmitter changes. Certainly, neurotrophins and cytokines regulate transcriptional factors in adult autonomic neurons that have vital differentiation roles in development. Particularly for parasympathetic cardiac ganglion neurons, additional examinations of developmental regulatory mechanisms will potentially aid in understanding attenuated parasympathetic function in a number of conditions, including heart failure.

Heart failure enhances arrhythmogenesis in pulmonary veins

1. Heart failure (HF) predisposes to atrial fibrillation (AF) as a result of substrate remodelling. The present study aimed to investigate the impact of HF on the electrical remodelling of the pulmonary veins (PV) and left atrium (LA). 2. The electrical activity was recorded in LA and PV from control rabbits and rabbits with rapid ventricular pacing-induced HF, using a multi-electrode array system and conventional microelectrodes. 3. Compared with the control-PV (n = 21), the HF-PV (n = 13) had a higher incidence and frequency of rapid pacing-induced spontaneous activity (85 vs 29%, P = 0.005; 3.5 ± 0.2 vs 1.7 ± 0.1 Hz, P < 0.001) and high-frequency irregular electrical activity (92 vs 38%, P = 0.01; 23 ± 1 vs 19 ± 1 Hz, P = 0.003), greater depolarized resting membrane potential (-59 ± 1 vs -70 ± 2 mV, P < 0.001), higher incidence of early afterdepolarizations (EAD; 69 vs 6%, P = 0.001) and delayed afterdepolarizations (DAD; 92 vs 25%, P = 0.001), and slower conduction velocity (38 ± 2 vs 63 ± 2 cm/s, P < 0.05). In comparison to the HF-LA, the HF-PV had a higher incidence of spontaneous activity and high-frequency irregular electrical activity (85 vs 39%, P = 0.04; 92 vs 46%, P = 0.03), and higher incidence of EAD and DAD, and those differences were not found between the control-LA and control-PV. The control-PV with high-frequency irregular electrical activity had a higher incidence of DAD and spontaneous activity as compared with those without it. 4. HF contributed to an increased automaticity, triggered activity and conduction disturbance in the PV. The PV possessed more arrhythmogenic properties, which might play an important role in the genesis of AF in HF.

20-Hydroxyeicosatetraenoic acid induces apoptosis in neonatal rat cardiomyocytes through mitochondrial-dependent pathways

OBJECTIVE

20-Hydroxyeicosatetraenoic acid (20-HETE), a [omega]-hydroxylation product of arachidonic acid catalyzed by cytochrome P450 4A, may play a role in the cardiovascular system. It is well known that cytochrome P450 [omega]-hydroxylase inhibitors markedly reduced the cardiac ischemia reperfusion injury. However, the direct effect of 20-HETE on cardiomyocytes is still poorly investigated. Here, we studied the effect of 20-HETE on cardiomyocyte apoptosis and the apoptosis-associated signaling pathways.

METHODS AND RESULTS

The cardiomyocyte apoptosis was measured by fluorescein isothiocyanate conjugated annexin V/propidium iodide double staining cytometry, indicating that the percentage of early apoptotic cells increased from 15.6% +/- 2.6% to 25.5% +/- 2.5% in control and 20-HETE-treated cells, respectively. The mitochondrial membrane potential ([DELTA][PSI]m) was measured by detecting the ratio of JC-1 green/red emission intensity. A significant decrease in the ratio was observed after treatment with 20-HETE for 24 hours in comparison with control group, suggesting the disruptive effect of 20-HETE on mitochondrial [DELTA][PSI]m. In addition, 20-HETE stimulated caspase-3 activity and Bax mRNA expression in cardiomyocytes. In contrast, the Bcl-2 mRNA levels were significantly decreased by 20-HETE treatment.

CONCLUSION

These results demonstrate that 20-HETE induces cardiomyocyte apoptosis by activation of several intrinsic apoptotic pathways. The 20-HETE-induced apoptosis could contribute to the cytochrome P450 [omega]-hydroxylase-dependent cardiac injure during cardiac ischemia-reperfusion.

Atrial Coronary Arteries: Anatomy And Atrial Perfusion Territories

Coronary anatomy has traditionally focused on ventricular circulation. This is largely due to the extent to which coronary artery disease contributes to ischemic heart disease through ventricular myocardial damage. Atrial fibrillation and other tachyarrhythmias that involve the atria, however, remain a major cause of morbidity and mortality. In order to increase mechanistic research and therapeutic interventional procedures for diseases of the atria, an optimal knowledge of atrial anatomy is necessary. While substantial clarity exists regarding the distribution of nerve terminals and the organization of muscle bundles, the anatomy of coronary atrial circulation remains understudied. Historically, the high anatomical variability of atrial coronary branches led to unstandardized nomenclature in the literature. In this review, we delineate the anatomic courses of key atrial coronary branches and their perfusion territories, clarify their nomenclature, and propose unifying anatomical concepts of atrial circulation that we believe to be critical to the success of modern electrophysiologic and surgical procedures.

High-density epicardial mapping of the pulmonary vein-left atrial junction in humans: insights into mechanisms of pulmonary vein arrhythmogenesis

BACKGROUND

The pulmonary veins (PVs) and the PV-LA (left atrium) junction are established sources of triggers initiating atrial fibrillation. In addition, they have been implicated in the maintenance of arrhythmia.

OBJECTIVE

To undertake high-density electrophysiological characterization of the right superior PV-LA junction in humans.

METHODS

Mapping was performed in 18 patients without a history of atrial fibrillation undergoing cardiac surgery. A high-density epicardial plaque was positioned at the anterior right superior pulmonary vein covering 3 regions: LA, PV-LA junction, and the PV. Isochronal maps were created during (1) sinus rhythm (SR); (2) LA pacing (LA-Pace); (3) PV pacing (PV-Pace); (4) LA programmed electrical stimulation (LA-PES); and (5) PV programmed electrical stimulation (PV-PES). Regional differences in conduction slowing/conduction block (CS/CB) and the prevalence of fractionated signals (FS) and double potentials (DPs) were assessed.

RESULTS

A region of isochronal crowding representing CS/CB developed at the PV-LA junction in 84% of the maps. Three distinct activation patterns were seen. Pattern 1: Uniform SR activation without CS/CB. LA-Pace and PES caused 1 to 2 lines of isochronal crowding (CS/CB) at the PV-LA junction. Pattern 2: CS/CB occurred at the PV-LA junction in SR. LA/PV-Pace and LA/PV-PES caused an increase in CS/CB at the PV-LA junction with widely split DPs and FS. Pattern 3: A single incomplete line of CS at the PV-LA junction in SR. With LA/PV pacing and LA/PV-PES, multiple lines (≥3) of CS/CB developed at the PV-LA junction with evidence of circuitous activation and a marked increase in DPs and FS.

CONCLUSION

High-density epicardial mapping of the right superior pulmonary vein demonstrates marked functional conduction delay and circuitous activation patterns at the PV-LA junction, creating the substrate for reentry.

Electroanatomic properties of the pulmonary veins: slowed conduction, low voltage and altered refractoriness in AF patients

INTRODUCTION

Rapid PV activity is critical in initiating and maintaining AF. The underlying substrate responsible for this remains uncertain. We sought to identify if patients with paroxysmal (PAF) and persistent atrial fibrillation (PeAF) have an abnormal substrate within the pulmonary veins (PVs).

METHODS AND RESULTS

Thirty-nine patients with AF (21 PAF, 18 PeAF) were compared with 15 age-matched controls with left-sided accessory pathways (AVRT). High-density 3D electroanatomic maps of the PVs were created. PV voltage, conduction, PV muscle sleeve length, effective refractory periods (ERPs) of the PVs, posterior left atrium (PLA), left atrial appendage (LAA) and distal coronary sinus (CSd), and signal complexity were assessed. Compared with controls, the PVs of AF patients had (1) lower mean-voltage and a higher % low-voltage; (2) shorter PV muscle sleeves; (3) slower conduction; (4) shorter ERP; and (5) more prevalent complex signals. Compared with the PAF group, the PeAF group had (1) higher % low voltage; (2) slower conduction; and (3) more complex signals. In PAF patients, the PLA and LAA ERPs were longer than controls and the PV ERP was shorter than controls; in PeAF patients PLA and LAA ERPs were reduced, but to a lesser extent than in the PVs. AF induction occurred during PV ERP testing in both AF groups, but not controls.

CONCLUSIONS

PAF and PeAF patients demonstrate electrical and electroanatomic remodeling of the PVs compared to control patients without prior AF. Some of these changes were more marked in PeAF. 

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

Animal models for atrial fibrillation: clinical insights and scientific opportunities

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. A variety of animal models have been used to study the pathophysiology of AF, including molecular basis, ion-current determinants, anatomical features, and macroscopic mechanisms. In addition, animal models play a key role in the development of new therapeutic approaches, whether drug-based, molecular therapeutics, or device-related. This article discusses the various types of animal models that have been used for AF research, reviews the principle mechanisms governing atrial arrhythmias in each model, and provides some guidelines for model selection for various purposes.

Autonomic nervous system activity measured directly and QT interval variability in normal and pacing-induced tachycardia heart failure dogs

OBJECTIVES

This study sought to find out more about the relationship between sympathetic and vagal nerve activity and the cardiac repolarization in a canine model of pacing-induced tachycardia congestive heart failure (CHF).

BACKGROUND

The QT variability index (QTVI), a noninvasive marker of temporal cardiac repolarization dispersion, is among the risk factors for sudden death during CHF. Among factors influencing this variable are the myocardial damage and the autonomic nervous system activity typical of dilated cardiomyopathy.

METHODS

We assessed autonomic nervous system activity recorded from an implanted data transmitter that monitored integrated left stellate-ganglion nervous activity, integrated vagus nerve activity, and electrocardiogram. We collected 36 segments recorded at baseline and 36 after induced CHF. We then arbitrarily identified recording segments as containing low or high sympathetic activity values, and we compared corrected QT intervals and the QTVI under a given sympathetic activity condition at baseline and after inducing CHF.

RESULTS

In the high sympathetic activity subgroup, both QT variables increased from baseline to CHF (corrected QT intervals, p < 0.01; QTVI, p < 0.05) whereas in the low sympathetic activity subgroup they remained unchanged. The baseline QTVI correlated inversely with integrated vagus nerve activity (r(2) = 0.16; beta = -0.47; p < 0.05) whereas, during CHF, the QTVI correlated directly with integrated left stellate-ganglion nervous activity (r(2) = 0.32; beta = 0.27, p < 0.01).

CONCLUSIONS

During CHF, sympathetic activation is associated with an increase in the QT interval and QTVI. Because these changes vary over time, they could result from myocardial structural damage and sympathetic activation combined. Conversely, under normal conditions, no relationship exists between sympathetic activation and the QT variables.

The role of pulmonary veins in atrial fibrillation: a complex yet simple story

Atrial fibrillation (AF) is the most common cardiac arrhythmia, with increased incidence among the elderly population. The concept that ectopic activity in pulmonary veins (PVs) could be responsible for triggering AF has been put forward, and the inter-relationship between PVs and left atrium has been the subject of many anatomical and physiological investigations. Variable configuration of action potentials among various PV cardiomyocytes has been reported. PV myocytes were shown to have a higher resting membrane potential and a lower action potential amplitude and duration than the left atrium. Much evidence has accumulated to indicate that spontaneous depolarization and/or re-entry from PVs could be the mode by which AF is initiated and/or sustained. Attempts have been made to link AF in certain pathophysiological states, notably, congestive heart failure, valvular disease and hyperthyroidism to PVs. There has been evidence to suggest that an increase in PV diameter may be the trigger for initiating AF. However, there is limited clinical knowledge available on the nature of the antiarrhythmic drugs that act upon PVs to alleviate AF. Most drugs currently employed are the standard agents generally utilized for the treatment of AF. Radiofrequency ablation (RFA) of the PVs and its isolation from the left atrium has become a major curative measure of AF. It is also possible that pharmacotherapy may be more effective or provide extra benefit to patients after a RFA procedure. The trend of the clinical evidence seems to suggest that a hybrid treatment may be beneficial in some population of patients.

New concepts in atrial fibrillation: neural mechanisms and calcium dynamics

Atrial fibrillation (AF) is a complex arrhythmia with multiple possible mechanisms. It requires a trigger for initiation and a favorable substrate for maintenance. Pulmonary vein myocardial sleeves have the potential to generate spontaneous activity, and this arrhythmogenic activity is surfaced by modulation of intracellular calcium dynamics. Direct autonomic nerve recordings in canine models show that simultaneous sympathovagal discharges are the most common triggers of paroxysmal atrial tachycardia and paroxysmal AF. Autonomic modulation as a potential therapeutic strategy has been targeted clinically and experimentally, but its effectiveness as an adjunctive therapeutic modality to catheter ablation of AF has been inconsistent. Further studies are warranted before application can be widely implied for therapies of clinical AF.

Power spectral analysis of heart rate variability and autonomic nervous system activity measured directly in healthy dogs and dogs with tachycardia-induced heart failure

BACKGROUND

Heart rate variability (HRV), calculated in the frequency or time domain, decreases in congestive heart failure (CHF). In HRV power spectral analysis, the low-frequency (LF) component diminishes in patients with CHF and the decrease is related to an increased risk of sudden death.

OBJECTIVE

Our aim was to clarify the nature of HRV power spectral analysis in normal and CHF dogs.

METHODS

Using an implanted radiotransmitter, we directly studied integrated left stellate ganglion nerve activity (iSGNA), integrated vagal nerve activity (iVNA), and electrocardiographic tracings before and after pacing-induced CHF in 6 ambulatory dogs. In a short-term power spectral analysis of HRV, we measured power spectral density during high, medium, and low sympathetic and vagal nerve activity. We analyzed 38 data segments characterized by the same autonomic nerve activity patterns at baseline and after pacing-induced CHF.

RESULTS

As compared with baseline, the spectral variables during CHF showed decreased total power (P = .002), LF power (P < .0001), and the LF/high frequency (HF) ratio (P = .005) and increased iVNA and iSGNA (P < .0001 for both). Only at baseline, iSGNA correlated positively with LF power (P < .05, r = 0.314). Under the same condition iVNA correlated positively with the HF component expressed as normalized units (P < .05, r = 0.394) and negatively with the LF component expressed both as absolute power (P < .05, r = -0.464) and normalized units (P < .05, r = -0.425).

CONCLUSION

The spectral variables (LF power and the LF/HF ratio) and direct variables measuring sympathetic nerve activity (iSGNA) correlate at baseline but not during CHF. At baseline, the vagal activity (iVNA) is associated with an increase in HFnu and a decrease in LFnu. These data indicate that the reduction in LF power and LF/HF ratio observed in heart failure dogs are likely to reflect a diminished sinus node responsiveness to autonomic modulation or an abnormal baroreflex function rather than an increased sympathetic activity.

Calcium-Handling Abnormalities Underlying Atrial Arrhythmogenesis and Contractile Dysfunction in Dogs With Congestive Heart Failure

Background— Congestive heart failure (CHF) is a common cause of atrial fibrillation. Focal sources of unknown mechanism have been described in CHF-related atrial fibrillation. The authors hypothesized that abnormal calcium (Ca 2+ ) handling contributes to the CHF-related atrial arrhythmogenic substrate.

Methods and Results— CHF was induced in dogs by ventricular tachypacing (240 bpm �2 weeks). Cellular Ca 2+ -handling properties and expression/phosphorylation status of key Ca 2+ handling and myofilament proteins were assessed in control and CHF atria. CHF decreased cell shortening but increased left atrial diastolic intracellular Ca 2+ concentration ([Ca 2+ ] i ), [Ca 2+ ] i transient amplitude, and sarcoplasmic reticulum (SR) Ca 2+ load (caffeine-induced [Ca 2+ ] i release). SR Ca 2+ overload was associated with spontaneous Ca 2+ transient events and triggered ectopic activity, which was suppressed by the inhibition of SR Ca 2+ release (ryanodine) or Na + /Ca 2+ exchange. Mechanisms underlying abnormal SR Ca 2+ handling were then studied. CHF increased atrial action potential duration and action potential voltage clamp showed that CHF-like action potentials enhance Ca 2+ i loading. CHF increased calmodulin-dependent protein kinase II phosphorylation of phospholamban by 120%, potentially enhancing SR Ca 2+ uptake by reducing phospholamban inhibition of SR Ca 2+ ATPase, but it did not affect phosphorylation of SR Ca 2+ -release channels (RyR2). Total RyR2 and calsequestrin (main SR Ca 2+ -binding protein) expression were significantly reduced, by 65% and 15%, potentially contributing to SR dysfunction. CHF decreased expression of total and protein kinase A–phosphorylated myosin-binding protein C (a key contractile filament regulator) by 27% and 74%, potentially accounting for decreased contractility despite increased Ca 2+ transients. Complex phosphorylation changes were explained by enhanced calmodulin-dependent protein kinase IIδ expression and function and type-1 protein-phosphatase activity but downregulated regulatory protein kinase A subunits.

Conclusions— CHF causes profound changes in Ca 2+ -handling and -regulatory proteins that produce atrial fibrillation–promoting atrial cardiomyocyte Ca 2+ -handling abnormalities, arrhythmogenic triggered activity, and contractile dysfunction.

Spatial distribution of fibrosis governs fibrillation wave dynamics in the posterior left atrium during heart failure

Heart failure (HF) commonly results in atrial fibrillation (AF) and fibrosis, but how the distribution of fibrosis impacts AF dynamics has not been studied. HF was induced in sheep by ventricular tachypacing (220 bpm, 6 to 7 weeks). Optical mapping (Di-4-ANEPPS, 300 frames/sec) of the posterior left atrial (PLA) endocardium was performed during sustained AF (burst pacing) in Langendorff-perfused HF (n=7, 4 micromol/L acetylcholine; n=3, no acetylcholine) and control (n=6) hearts. PLA breakthroughs were the most frequent activation pattern in both groups (72.0+/-4.6 and 90.2+/-2.7%, HF and control, respectively). However, unlike control, HF breakthroughs preferentially occurred at the PLAs periphery near the pulmonary vein ostia, and their beat-to-beat variability was greater than control (1.93+/-0.14 versus 1.47+/-0.07 changes/[beats/sec], respectively, P<0.05). On histological analysis (picrosirius red), the area of diffuse fibrosis was larger in HF (23.4+/-0.4%) than control (14.1+/-0.6%; P<0.001, n=4). Also the number and size of fibrous patches were significantly larger and their location was more peripheral in HF than control. Computer simulations using 2-dimensional human atrial models with structural and ionic remodeling as in HF demonstrated that changes in AF activation frequency and dynamics were controlled by the interaction of electrical waves with clusters of fibrotic patches of various sizes and individual pulmonary vein ostia. During AF in failing hearts, heterogeneous spatial distribution of fibrosis at the PLA governs AF dynamics and fractionation.

Left Stellate Ganglion and Vagal Nerve Activity and Cardiac Arrhythmias in Ambulatory Dogs With Pacing-Induced Congestive Heart Failure

OBJECTIVES

The purpose of this study was to determine the patterns of autonomic nerve activity in congestive heart failure (CHF).

BACKGROUND

The relationship between autonomic nerve activity and cardiac arrhythmias in CHF is unclear.

METHODS

We implanted radiotransmitters in 6 dogs for continuous (24/7) simultaneous monitoring of left stellate ganglion nerve activity (SGNA), vagal nerve activity (VNA), and electrocardiography before and after pacing-induced CHF.

RESULTS

Congestive heart failure increased both SGNA and VNA. The SGNA but not VNA manifested a circadian variation pattern. There was extensive sinus node fibrosis. We analyzed 2,263 episodes of prolonged (>3 s) sinus pauses (PSP) and 1,420 long (>10 s) episodes of paroxysmal atrial tachycardia (PAT). Most (95.3%) PSP episodes occurred at night, and 56% were preceded by a short burst of SGNA that induced transient sinus tachycardia. Long PAT episodes were typically (83%) induced by simultaneous SGNA and VNA discharge, followed by VNA withdrawal. Premature ventricular contractions and ventricular tachycardia were preceded by elevated SGNA.

CONCLUSIONS

The reduction of sympathovagal balance at night in ambulatory dogs was due to reduced sympathetic discharge rather than a net increase of vagal discharge. The tachybrady syndrome in CHF might be triggered by an intermittent short burst of SGNA that resulted in tachycardia and sinus node suppression. Simultaneous sympathovagal discharge is a cause of long PAT episodes. These data indicate that there is an association between the specific patterns of autonomic nerve discharges and cardiac arrhythmia during CHF.

Atrial fibrosis and the mechanisms of atrial fibrillation

Atrial fibrillation (AF) is commonly associated with congestive heart failure (CHF), and CHF has been shown to be associated with atrial structural remodeling resulting in fibrosis. Atrial interstitial fibrosis has been seen in patients with CHF and in animal models of pacing-induced heart failure. With atrial fibrosis, conduction abnormalities result in increased AF vulnerability. The mechanism of AF associated with CHF is under debate, as both focal and reentrant mechanisms have been observed in animal models of CHF. However, recent studies using frequency-domain analysis have shown that the AF within this model is characterized by discrete, stable, high-frequency areas. The precise signaling processes involved in the development of atrial fibrosis are unknown. Angiotensin appears to play a role, as inhibition of angiotensin-converting enzyme (or angiotensin-receptor blocker) blunts atrial fibrosis in animal models of heart failure and decreases the incidence of AF in patients with heart failure. Transforming growth factor-beta (TGF-beta) also appears to play an important role. Mouse models that overexpress TGF-beta1 have profound atrial fibrosis and AF (with normal ventricles). Heart failure in canine models also produces increases in atrial TGF-beta1 expression, and inhibition of this expression prevents atrial fibrosis and the development of a substrate for AF. Atrial fibrosis appears to play a role in the development of a vulnerable substrate for AF, especially in the setting of CHF.

Triggered activity and atrial fibrillation

In 1999, Haissaguerre et al published a landmark article showing that atrial fibrillation can be initiated by electrical activity in the pulmonary veins. Not only does it appear that electrical activity in the veins initiates fibrillation, but it also may be responsible for perpetuating fibrillation. Subsequently, similar evidence has suggested that other thoracic veins (vena cavae, coronary sinus, ligament of Marshall) initiate and perpetuate atrial fibrillation. How does electrical impulse initiation occur in the veins? The results of numerous in vivo and in vitro studies on this subject have not conclusively defined a mechanism. Impulse initiation by automaticity and triggered activity as well as impulse initiation resulting from reentry have been suggested. In this article, we focus only on those data suggesting the possibility that triggered activity initiates and/or perpetuates atrial fibrillation.

The Mechanisms of Atrial Fibrillation

In this article we have reviewed the mechanisms of atrial fibrillation (AF) with special emphasis on the thoracic veins. Based on a number of features, the thoracic veins are highly arrhythmogenic. The pulmonary vein (PV)-left atrial (LA) junction has discontinuous myocardial fibers separated by fibrotic tissues. The PV muscle sleeve is highly anisotropic. The vein of Marshall (VOM) in humans has multiple small muscle bundles separated by fibrosis and fat. Insulated muscle fibers can promote reentrant excitation, automaticity, and triggered activity. The PV muscle sleeves contain periodic acid-Schiff (PAS)-positive large pale cells that are morphologically reminiscent of Purkinje cells. These special cells could be the sources of focal discharge. Antiarrhythmic drugs have significant effects on PV muscle sleeves both at baseline and during AF. Both class I and III drugs have effects on wavefront traveling from PV to LA and from LA to PV. Separating the thoracic veins and the LA with ablation techniques also prevents PV-LA interaction. By reducing PV-LA interaction, pharmacological therapy and PV isolation reduce the activation rate in PV, intracellular calcium accumulation, and triggered activity. Therefore, thoracic vein isolation is an important technique in AF control. We conclude that thoracic veins are important in the generation and maintenance of AF.

Structural atrial remodeling alters the substrate and spatiotemporal organization of atrial fibrillation: a comparison in canine models of structural and electrical atrial remodeling

Several animal models of atrial fibrillation (AF) have been developed that demonstrate either atrial structural remodeling or atrial electrical remodeling, but the characteristics and spatiotemporal organization of the AF between the models have not been compared. Thirty-nine dogs were divided into five groups: rapid atrial pacing (RAP), chronic mitral regurgitation (MR), congestive heart failure (CHF), methylcholine (Meth), and control. Right and left atria (RA and LA, respectively) were simultaneously mapped during episodes of AF in each animal using high-density (240 electrodes) epicardial arrays. Multiple 30-s AF epochs were recorded in each dog. Fast Fourier transform was calculated every 1 s over a sliding 2-s window, and dominant frequency (DF) was determined. Stable, discrete, high-frequency areas were seen in none of the RAP or control dogs, four of nine MR dogs, four of six CHF dogs, and seven of nine Meth dogs in either the RA or LA or both. Average DFs in the Meth model were significantly greater than in all other models in both LA and RA except LA DFs in the RAP model. The RAP model was the only one with a consistent LA-to-RA DF gradient (9.5 ± 0.2 vs. 8.3 ± 0.3 Hz, P < 0.00005). The Meth model had a higher spatial and temporal variance of DFs and lower measured organization levels compared with the other AF models, and it was the only model to show a linear relationship between the highest DF and dispersion ( R2 = 0.86). These data indicate that structural remodeling of atria (models known to have predominantly altered conduction) leads to an AF characterized by a stable high-frequency area, whereas electrical remodeling of atria (models known to have predominantly shortened refractoriness without significant conduction abnormalities) leads to an AF characterized by multiple high-frequency areas and multiple wavelets.

Atrial fibrillation may be a vascular disease: the role of the pulmonary vein

Recent years have seen an enormous amount of experimental and clinical research into role of the pulmonary veins (PVs) in atrial fibrillation (AF). The PVs contain cardiomyocytes with easily inducible arrhythmogenic activity due to the enhanced automaticity, induction of triggered activity, and genesis of microreentrant circuits. The enhanced automaticity, induced triggered activity, either alone or in combination with the reentrant mechanisms, may play a role in the initiation of PVs AF. Detailed mapping studies suggest that reentry within the PVs is most likely responsible for their arrhythmogenicity. There is no doubt that the PVs represent the most important source of arrhythmogenic activity in patients with paroxysmal AF. In AF patients with risk factors for development of AF, the presence of the pathological situation is important in enhancing the PV arrhythmogenic activity. Coronary sinus or superior vena cava may also be a source of rapid repetitive electrical activity during AF. Thus, AF should be considered a kind of vascular disease. Moreover, in patients with paroxysmal AF originating from the PVs, a wide spectrum of atrial arrhythmias may coexist, including paroxysms of atrial premature, tachycardia, flutter and fibrillation. This kind of arrhythmias should be named as PV atrial arrhythmias. These new views will help understand the mechanism, diagnosis and treatment method for AF.

Vein of Marshall Activity During Sustained Atrial Fibrillation

BACKGROUND

The human vein of Marshall (VOM) activation patterns during sustained (persistent or permanent) atrial fibrillation (AF) have not been studied in detail.

METHODS

VOM was cannulated via coronary sinus in six patients (67.3 +/- 7.5 years old) having either persistent (N = 4) or permanent (N = 2) AF presenting for radiofrequency catheter ablation.

RESULTS

At sinus rhythm in patients with persistent AF, there were frequent ectopic beats from the VOM as well as from left and right pulmonary veins (PVs). The ectopic activity originating from the VOM was highly fragmented. The P wave morphology associated with VOM ectopy was isoelectric in leads I and aVL, positive in leads II, III, aVF, and V2-V5, which is similar to the P wave morphology associated with left PV ectopic beats. During AF the activation cycle length at VOM was 140 +/- 31 msec, which was significantly shorter than that in other atrial sites (P < 0.05). Similarly, the dominant frequency at VOM (9.71 Hz +/- 1.52 Hz) was significantly higher than that at other atrial sites (P < 0.0001). In one patient, VOM ablation was associated with AF termination during radiofrequency energy application.

CONCLUSIONS

Ectopic activity with complex local electrogram originating from VOM is commonly seen in patients with sustained AF. The P wave morphology associated with the ectopic beats from VOM and from the left PVs was similar. The rate of activation at VOM is significantly faster than other atrial and PV sites during AF. These findings show that rapid activation is present in the VOM during sustained AF in human patients.

Marshall vein as arrhythmogenic source in patients with atrial fibrillation: correlation between its anatomy and electrophysiological findings

BACKGROUND

Atrial fibrillation (AF) may originate from catecholamine-sensitive vein of Marshall (VOM) or its ligament in addition to pulmonary veins (PVs). The anatomy of VOM and its relation to arrhythmogenic foci in the left atrium are unknown. We studied the anatomy of VOM and its relation to foci in patients with AF.

METHODS

The study population consisted of 100 patients with AF (mean age, 62 years; chronic AF, n = 15). AF sources were determined at baseline and after isoproterenol administration without sedation. VOM was identified by balloon-occluded coronary sinus (CS) angiography. We determined its anatomy in relation to left PVs.

RESULTS

VOM was visualized in 73 patients (73%). Ninety-seven patients had 269 arrhythmogenic foci (PV, n = 77; non-PV, n = 48). Non-PV foci included left atrial posterior wall (24, 9%), left lateral area (12, 4.5%), roof (6, 2.2%), superior vena cava (28, 10.4%), crista terminalis (8, 3.0%), CS (10, 3.7%), and others (10, 3.7%). The incidence of PV foci in the left superior PV (LSPV) was significantly higher in patients with well-developed VOM than in those without (66% vs 42%, P < 0.05). Twenty-eight patients had 30 non-PV foci around the LSPV ostium. We successfully ablated the non-PV foci at the distal end of VOM in 11 patients. The ends of the VOM branches were good markers to search for non-PV foci. Seven of 11 (64%) patients with successful ablation of non-PV foci were free from arrhythmia, whereas only 6 of 17 (35%) were free from arrhythmia in those with residual non-PV foci.

CONCLUSIONS

To determine VOM anatomy is important to identify non-PV foci around the ends of VOM.

Impact of Transforming Growth Factor-β1 on Atrioventricular Node Conduction Modification by Injected Autologous Fibroblasts in the Canine Heart

Background— Atrioventricular (AV) nodal ablation for management of atrial fibrillation (AF) is irreversible and requires permanent pacemaker implantation. We hypothesized that as an alternative, implantation of autologous fibroblasts in the perinodal region would focally modify AV nodal conduction and that this modulation would be enhanced by pretreatment with transforming growth factor-β1 (TGF-β1), a stimulant of fibroblasts.

Methods and Results— Skin biopsies were taken from 12 mongrel dogs, and derived fibroblasts were dissociated and grown in culture for 2 weeks. Multiple injections (0.25 mL) were made through an 8F NOGA catheter along the fast/slow AV nodal pathways as guided by an electroanatomic mapping system. Seven dogs received fibroblasts alone (1×10 6 cells/mL), 7 dogs received TGF-β1 (5 μg), 4 dogs received fibroblasts and TGF-β1 (1×10 6 cells/mL+5 μg), and 4 dogs received saline only. AV node function was assessed at baseline and after 4 weeks. Saline (80 mL) with assigned therapy (0.25 mL per injection) was injected into the peri-AV nodal region in each dog. At baseline, the AH interval (66±3 ms) and the average RR interval (331±17 ms) in pacing-induced AF were similar in each cohort. The increase in AH interval in normal sinus rhythm was longer after fibroblast (23±4 versus 5±5 ms; P =0.05) and fibroblast plus TGF-β1 (50±5 versus 5±5 ms; P <0.001) injections than with saline alone, with similar findings during high right atrium and distal coronary sinus pacing. The AH interval was not significantly increased after TGF-β1 injections. The AH interval was significantly longer after fibroblast plus TGF-β1 injections than with either therapy (TGF-β1 or fibroblasts) alone. The RR interval during AF was increased in dogs that received fibroblasts alone (110±36 versus −41±34 ms) and to a greater extent with the addition of TGF-β1 (294±108 versus −41±34 ms). No AV block was seen in any cohort at 4 weeks. Labeled fibroblasts that expressed vimentin were identified in all dogs that received cell injections at 4 weeks.

Conclusions— AV nodal modification can be achieved with injected fibroblasts without the creation of AV block. The effect on AV node conduction is substantially enhanced by pretreatment of fibroblasts with TGF-β1. These data have therapeutic potential for the management of rapid ventricular rate during AF without pacemaker implantation.

Mapping of atrial activation during sustained atrial fibrillation in dogs with rapid ventricular pacing induced heart failure: evidence for a role of driver regions

INTRODUCTION

Dogs with rapid ventricular pacing (RVP)-induced congestive heart failure (CHF) have inducible atrial tachycardia, flutter, and fibrillation (AF). We tested the hypothesis that rapid atrial activation in multiple regions and at different rates is responsible for sustained AF in this CHF model.

METHODS AND RESULTS

We studied 12 episodes of sustained (>10 minutes) AF induced in 12 dogs with CHF produced by 3-6 weeks of RVP at 230 beats/minute. High-density mapping of AF was performed using 382 unipolar atrial electrograms recorded simultaneously from epicardial electrodes on the right (RA) and left atria (LA) and Bachmann's bundle. AF mechanisms were based on Fast Fourier Transform (FFT) analysis and activation sequence mapping. A driver was defined as a rapid stable activation region with a single dominant frequency peak in FFT analysis. During AF, three FFT and activation patterns were seen: (1) a single LA driver (7.8 +/- 1.1 Hz) near the pulmonary veins (PVs) with irregular activation in the rest of the atria (n = 4); (2) simultaneous, multisite, biatrial drivers at differing frequencies (LA vs RA dominant frequency gradient: 1.3 +/- 0.8 Hz) near the PVs (8.4 +/- 0.3 Hz) and high RA (8.5 +/- 1.5 Hz) (n = 7); and (3) biatrial irregular activation with multiple and/or broadband frequency peaks without a dominant frequency. (LA: 7.1-11.4 Hz; RA: 5.9-7.7 Hz) (n = 1). Atrial drivers had either a focal activation pattern or were due to a macroreentrant circuit around the PVs.

CONCLUSIONS

In this CHF model, FFT analysis and activation sequence mapping demonstrate that sustained AF is characterized by single and multiple, stable LA and RA drivers with predominant sources in the PVs and high RA causing fibrillatory conduction.

Electrophysiology of Pulmonary Veins

Pulmonary veins were found to be important foci for the genesis and maintenance of atrial fibrillation. Morphological studies have demonstrated the presence of complex anatomic structures and different types of cardiomyocytes in pulmonary veins. Numerous studies have suggested that the combination of reentrant and nonreentrant mechanisms (automaticity and triggered activity) are the underlying arrhythmogenic mechanisms of atrial fibrillation initiation from the pulmonary veins. Electropharmacological studies further indicated that pulmonary veins contained distinct arrhythmogenic activity. Several experimental models have been used to study the pulmonary vein electrical activity and demonstrate the precipitating factors for enhancing the pulmonary vein arrhythmogenic activity. The aim of this review article is to provide a critical overview of the current understanding of the basic and clinical electrophysiology of pulmonary veins and to underscore the importance of future research in this field.

Increased Extracellular Collagen Matrix in Myocardial Sleeves of Pulmonary Veins: An Additional Mechanism Facilitating Repetitive Rapid Activities in Chronic Pacing-Induced Sustained Atrial Fibrillation

Increased ECM in canine PVs.

INTRODUCTION

Cell uncoupling due to fibrosis or increased extracellular collagen matrix (ECM) affects the formation of ectopic focal activity. Whether or not the increase of ECM also exists in the pulmonary veins (PVs) with rapid atrial pacing is unknown. We sought to test the hypothesis that in chronic atrial pacing dogs with sustained atrial fibrillation (AF), the amount of ECM was increased in both atria and the PVs.

METHODS AND RESULTS

We induced sustained AF in dogs by rapid atrial pacing. Computerized mapping techniques were used to map both atria and the PVs. We also used histological assessment to quantify the amount of ECM. After 118+/-24 days of rapid atrial pacing, sustained AF was induced in 7 dogs. Repetitive rapid activities (RRAs) either continuously or intermittently arose from the PVs during sustained AF. Histological study shows that there was no fibrosis in both atrial free walls and the PVs. However, the amount of ECM was increased in these regions. The mean ECM surface area fraction at each region in the dogs with sustained AF was all significantly higher compared to the corresponding region in normal dogs. Similarly, the heterogeneity of the ECM surface area fraction at each region in the dogs with sustained AF was also all significantly higher compared to normal dogs.

CONCLUSIONS

In chronic atrial pacing-induced sustained AF, structural remodeling (i.e., inhomogeneous increase of ECM) also involves the PVs. Reduced coupling of the myocytes in the PV due to histological changes may provide an additional mechanism facilitating RRAs.

Intracellular Calcium Dynamics and Anisotropic Reentry in Isolated Canine Pulmonary Veins and Left Atrium

Background— Rapid activations due to either focal discharge or reentry are often present during atrial fibrillation (AF) in the pulmonary veins (PVs). The mechanisms of these rapid activations are unclear.

Methods and Results— We studied 7 isolated, Langendorff-perfused canine left atrial (LA) and PV preparations and used 2 cameras to map membrane potential alone (Vm, n=3) or Vm and intracellular calcium simultaneously (Ca i , n=4). Rapid atrial pacing induced 26 episodes of focal discharge from the proximal PVs in 5 dogs. The cycle lengths were 223±52 ms during ryanodine infusion (n=13) and 133±59 ms during ryanodine plus isoproterenol infusion (n=13). The rise of Ca i preceded Vm activation at the sites of focal discharge in 6 episodes of 2 preparations, compatible with voltage-independent spontaneous Ca i release. Phase singularities during pacing-induced reentry clustered specifically at the PV-LA junction. Periodic acid-Schiff (PAS) stain identified large cells with pale cytoplasm along the endocardium of PV muscle sleeves. There were abrupt changes in myocardial fiber orientation and increased interstitial fibrosis in the PV and at the PV-LA junction.

Conclusions— PV muscle sleeves may develop voltage-independent Ca i release, resulting in focal discharge. Focal discharge may also be facilitated by the presence of PAS-positive cells that are compatible with node-like cells. During reentry, phase singularities clustered preferentially at sites of increased anisotropy such as the PV-LA junction. These findings suggest that focal discharge caused by spontaneous calcium release and anisotropic reentry both contribute to rapid activations in the PVs during AF.

Atrial fibrillation

Atrial fibrillation (AF) is the most common sustained dysrhythmia in adults. It is ironic, then, that although mechanisms and effective treatments for most other supraventricular tachyarrhythmias have been discovered, AF remains incompletely understood and poorly treated. Nonetheless, our understanding of the pathophysiology of AF has improved in the last half-century, including some groundbreaking observations made in the last 10 years. Indeed, for some patients, the potential for cure now appears to be available. Because no unifying mechanism of AF has been proven, the aim of this review is to describe some of the common and important concepts behind current mechanistic theories of AF and how they contribute to our clinical understanding of AF.

Pulmonary Veins:. Anatomy, Electrophysiology, Tachycardia, and Fibrillation

Recent years have seen an enormous amount of experimental and clinical research into role of the pulmonary veins (PVs) in atrial fibrillation (AF). Advanced imaging techniques have confirmed the findings of earlier postmortem studies and added further dimension to our knowledge of PV anatomy. Such work is vital for an effective approach to successful ablation of AF. Detailed mapping studies suggest that reentry within the PVs is most likely responsible for their arrhythmogenicity, although focal or triggered activity cannot be excluded. Further work also implicates the posterior left atrium in the genesis of AF. Investigation into the interplay between the PVs and left atrium has led to a reevaluation of the mechanisms underlying AF and suggests that the PVs may play a role in both the initiation and maintenance of this arrhythmia. In order for electrophysiologists to further develop the technical approach to ablation of AF and improve the clinical outcomes, these crucial issues must be resolved.