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

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

Aldosterone promotes atrial fibrillation.

AIMS: Hyperaldosteronism is associated with an increased prevalence of atrial fibrillation (AF). However, it is unclear whether this is the consequence of altered haemodynamics or a direct aldosterone effect. It was the aim of the study to demonstrate load-independent effects of aldosterone on atrial structure and electrophysiology. METHODS: Osmotic mini-pumps delivering 1.5 µg/h aldosterone were implanted subcutaneously in rats (Aldo). Rats without aldosterone treatment served as controls. After 8 weeks, surface electrocardiogram, the inducibility of AF, and atrial pressures were recorded in vivo. In isolated working hearts, left ventricular function was measured, and conduction in the right atrium (RA) and the left atrium (LA) was mapped epicardially. The atrial effective refractory period (AERP) was determined. Atrial tissue was analysed histologically. RESULTS: Neither systolic nor diastolic ventricular function nor atrial pressures were altered in Aldo rats. All Aldo (11/11) showed inducible atrial arrhythmias vs. two of nine controls (P = 0.03). In Aldo, the P-wave duration and the total RA activation time were longer. Prolongation of local conduction times occurred more often in Aldo, whereas the AERP did not differ between both groups. In Aldo, atrial fibroblasts and interstitial collagen were increased, active matrix metalloproteinase 13 was reduced, and atrial myocytes were hypertrophied. The connexin 43 content was unaltered. CONCLUSIONS: Aldosterone causes a substrate for atrial arrhythmias characterized by atrial fibrosis, myocyte hypertrophy, and conduction disturbances. The described model imputes atrial proarrhythmia directly to aldosterone, since ventricular haemodynamics appeared unaltered in this model. This mechanism may have therapeutical impact for primary and secondary prevention of AF.

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

BACKGROUND: Previous studies suggest that a short temporal excitable gap exists between the fibrillation waves during atrial fibrillation (AF). The aim of this study was to investigate the role of that gap in the development of sustained AF in goats. METHODS AND RESULTS: Eight female goats were instrumented with left atrium (LA) electrodes, and sustained AF (>24 h) was induced by intermittent rapid atrial pacing for 9.3+/-4.6 days. In the process of sustained AF development, the atrial effective refractory period (AERP), refractory period during AF (RP(AF)), mean AF cycle length (AFCL), temporal excitable gap during AF (EG(AF) = AFCL - RP(AF)) and degree of fractionation of fibrillation electrograms at LA were studied. When the induced AF lasted for 3-10 min, AFCL, RP(AF) and EG(AF) were 98.3+/-11.0 ms, 90.5+/-13.2 ms and 7.8+/-2.4 ms, respectively. During sustained AF, the values were 84.9+/-5.2 ms, 63.0+/-4.8 ms and 21.9+/-3.5 ms, respectively (P<0.05). Percentage of single potentials was 94.2+/-3.9% and 75.6+/-5.5%, respectively (P<0.05). CONCLUSIONS: In this model progressive shortening of atrial refractoriness and widening of the temporal excitable gap induced by electrical remodeling created an electrophysiologic substrate for the perpetuation of AF.

Distinct contractile and molecular differences between two goat models of atrial dysfunction: AV block-induced atrial dilatation and atrial fibrillation.

Atrial dilatation is an independent risk factor for thromboembolism in patients with and without atrial fibrillation (AF). In many patients, atrial dilatation goes along with depressed contractile function of the dilated atria. While some mechanisms causing atrial contractile dysfunction in fibrillating atria have been addressed previously, the cellular and molecular mechanisms of atrial contractile remodeling in dilated atria are unknown. This study characterized in vivo atrial contractile function in a goat model of atrial dilatation and compared it to a goat model of AF. Differences in the underlying mechanisms were elucidated by studying contractile function, electrophysiology and sarcoplasmic reticulum (SR) Ca2+ load in atrial muscle bundles and by analyzing expression and phosphorylation levels of key Ca2+-handling proteins, myofilaments and the expression and activity of their upstream regulators. In 7 chronically instrumented, awake goats atrial contractile dysfunction was monitored during 3 weeks of progressive atrial dilatation after AV-node ablation (AV block goats (AVB)). In open chest experiments atrial work index (AWI) and refractoriness were measured (10 goats with AVB, 5 goats with ten days of AF induced by repetitive atrial burst pacing (AF), 10 controls). Isometric force of contraction (FC), transmembrane action potentials (APs) and rapid cooling contractures (RCC, a measure of SR Ca2+ load) were studied in right atrial muscle bundles. Total and phosphorylated Ca2+-handling and myofilament protein levels were quantified by Western blot. In AVB goats, atrial size increased by 18% (from 26.6+/-4.4 to 31.6+/-5.5 mm, n=7 p<0.01) while atrial fractional shortening (AFS) decreased (from 18.4+/-1.7 to 12.8+/-4.0% at 400 ms, n=7, p<0.01). In open chest experiments, AWI was reduced in AVB and in AF goats compared to controls (at 400 ms: 8.4+/-0.9, n=7, and 3.2+/-1.8, n=5, vs 18.9+/-5.3 mmxmmHg, n=7, respectively, p<0.05 vs control). FC of isolated right atrial muscle bundles was reduced in AVB (n=8) and in AF (n=5) goats compared to controls (n=9) (at 2 Hz: 2.3+/-0.5 and 0.7+/-0.2 vs 5.5+/-1.0 mN/mm2, respectively, p<0.05). APs were shorter in AF, but unchanged in AVB goats. RCCs were reduced in AVB and AF versus control (AVB, 3.4+/-0.5 and AF, 4.1+/-1.4 vs 12.2+/-3.2 mN/mm2, p<0.05). Protein levels of protein kinase A (PKA) phosphorylated phospholamban (PLB) were reduced in AVB (n=8) and AF (n=8) vs control (n=7) by 37.9+/-12.4% and 29.7+/-10.1%, respectively (p<0.01), whereas calmodulin-dependent protein kinase II (CaMKII) phosphorylated ryanodine channels (RyR2) were increased by 166+/-55% in AVB (n=8) and by 146+/-56% in AF (n=8) goats (p<0.01). PKA-phosphorylated myosin-binding protein-C and troponin-I were reduced exclusively in AVB goat atria (by 75+/-10% and 55+/-15%, respectively, n=8, p<0.05). Atrial dilatation developing during slow ventricular rhythm after complete AV block as well as AF-induced remodeling are associated with atrial contractile dysfunction. Both AVB and AF goat atria show decreased SR Ca2+ load, likely caused by PLB dephosphorylation and RYR2 hyperphosphorylation. While shorter APs further compromise contractility in AF goat atria, reduced myofilament phosphorylation may impair contractility in AVB goat atria. Thus, atrial hypocontractility appears to have distinct molecular contributors in different types of atrial remodeling.

Synergistic action of atrial dilation and sodium channel blockade on conduction in rabbit atria.

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

Inhibitors of the Na+/H+ exchanger cannot prevent atrial electrical remodeling in the goat.

INTRODUCTION: It has been suggested that blockade of the Na+/H+ exchanger (NHE1) can prevent atrial fibrillation (AF)-induced electrical remodeling and the development of AF. METHODS AND RESULTS: AF was maintained by burst pacing in 10 chronically instrumented conscious goats. Intravenous and oral dosages of two NHE1 blockers (EMD87580 and EMD125021) resulted in plasma levels several magnitudes higher than required for effective NHE1 blockade. Shortening of atrial refractoriness immediately after 5 minutes of AF was not prevented by NHE1 blockade. In remodeled atria, increasing dosages of EMD87580 and EMD125021 did not reverse shortening of the atrial refractory period or reduce the duration of AF episodes. The cycle length during persistent AF also was not affected. Oral pretreatment with EMD87580 (8 mg/kg bid) starting 3 days before AF could not prevent electrical remodeling. After 24 and 48 hours of remodeling, the duration of AF paroxysms was 47 +/- 32 seconds and 135 +/- 63 seconds compared to 56 +/- 17 seconds and 136 +/- 52 seconds in placebo-treated animals (P > 0.8), respectively. CONCLUSION: In the goat model of AF, the Na+/H+ exchanger inhibitors EMD87580 and EMD125021 did not prevent or revert AF-induced electrical remodeling. This indicates that activation of the Na+/H+ exchanger is not involved in the intracellular pathways of electrical remodeling. This does not support the suggestion that blockers of the Na+/H+ exchanger may be beneficial for prevention and treatment of AF.

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

OBJECTIVES: The purpose of this study was to characterize the morphology of fibrillation electrograms in patients in order to provide insight into the underlying electropathologic substrate of atrial fibrillation (AF). BACKGROUND: Electrograms recorded during AF show a high degree of spatiotemporal variation. METHODS: AF was induced by rapid atrial pacing in 25 patients undergoing cardiac surgery. A unipolar mapping array of 244 electrodes was positioned on the free wall of the right atrium to record multiple epicardial fibrillation electrograms. Local anisotropy in conduction and epicardial wavefront curvature during AF were determined by fitting the best quadratic surface on the activation times of rectangular areas of 3 x 3 electrodes. RESULTS: During AF, unipolar epicardial electrograms revealed a clear predominance of S waves. The average RS difference during type I and II AF was -0.15 +/- 0.08 and -0.22 +/- 0.08. During type III AF, the predominance of S waves was less prominent (-0.07 +/- 0.05; P < .005). In all types of AF, the degree of anisotropy in conduction was remarkably low (anisotropy ratio: 1.24 +/- 0.09), and no clear directional effect on the relative amplitude of R and S waves was found. There was a weak relationship between local curvature of wavefronts and RS difference (r = 0.23; P < .01). Computer simulations showed that the negative RS difference could result from transmural activation in an epicardial to endocardial direction. CONCLUSIONS: The clear predominance of S waves in epicardial fibrillation electrograms is not due to anisotropy and can only be partly explained by a high curvature of fibrillation waves. Predominant epicardial to endocardial activation seems to be important in producing rS electrograms on the epicardium. This finding provides indirect evidence that the thin epicardial layer of atrial myocardium plays an important role in propagation of fibrillation waves.

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

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

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

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