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Association of echocardiographic atrial size and atrial fibrosis in a sequential model of congestive heart failure and atrial fibrillation
Cardiovascular Pathology 17 (2008) 318 – 324
Original Article
Association of echocardiographic atrial size and atrial fibrosis in a sequential model of congestive heart failure and atrial fibrillation Christian Knackstedt a,1 , Felix Gramley a,1 , Thomas Schimpf a , Karl Mischke a , Markus Zarse a , Jurgita Plisiene a , Michael Schmid c , Johann Lorenzen b , Dirk Frechen a , Philipp Neef a , Peter Hanrath a , Malte Kelm a , Patrick Schauerte a,⁎ a
Department of Cardiology, RWTH Aachen University, Aachen, Germany b Institut of Pathology, RWTH Aachen University, Aachen, Germany c Department of Thoracic and Cardiovascular Surgery, RWTH Aachen University, Aachen, Germany Received 2 April 2007; received in revised form 20 June 2007; accepted 17 December 2007
Abstract Background: Cardioversion (CV) success of atrial fibrillation (AF) inversely correlates to the size of the left atrium (LA). Atrial fibrillation and its most important risk factor, congestive heart failure (CHF), both induce atrial structural enlargement and fibrosis. To investigate the effect of AF and CHF on atrial dilatation and fibrosis, and to estimate whether echocardiographically determined atrial size may be used as a marker for atrial fibrosis. Methods: In six dogs, pacemakers were implanted followed by HIS bundle ablation. After 4 weeks of rapid ventricular stimulation (185 bpm) for CHF induction, additional rapid atrial stimulation (500 bpm) was maintained for 7 weeks to induce AF. Serial determinations of echocardiographic atrial size were performed. Seven dogs with sinus rhythm served as histological controls. Postmortem tissue was obtained to determine the degree and composition of atrial fibrosis. Results: While the ejection fraction of the AF/CHF dogs decreased significantly from 57±5% to 19±7% (Pb.01), an increased degree of atrial fibrosis was found (right atrium [RA], 4.9±2.0% to 19.9±5.4%; LA, 4.4±1.6% to 22.2±3.2%; Pb.01), accompanied by a significant increase of atrial volumes (LA: 21±4 to 44±4 mm3; Pb.01; RA: 10±3 to 18±6 mm3; Pb.05) and LA diameters (34±4 to 43±2 mm, Pb.05). Atrial fibrosis and size significantly correlated. Conclusions: Atrial fibrillation/CHF leads to a significant atrial fibrosis and dilation. The increased echocardiographic size correlates to the degree of atrial fibrosis and may be used as clinical marker for atrial fibrosis. The fibrosis accompanying atrial dilatation may also explain why LA size, as determined by echocardiography, is a strong predictor of CV success. © 2008 Elsevier Inc. All rights reserved. Keywords: Atrial fibrillation; Fibrosis; Echocardiography; Atrial size; Congestive heart failure
1. Introduction Atrial fibrillation (AF) is the most common arrhythmia in man [1]. There is a tendency for AF to become chronic with long-lasting duration [2]. Consequently, many patients with intermittent AF will develop chronic AF [3]. Furthermore, The study was supported by a grant from the DAAD (German Academic Exchange Society) to Dr. Jurgita Plisiene. ⁎ Corresponding author. Department of Cardiology, RWTH Aachen University, Pauwelstrasse 30, 52072 Aachen, Germany. Tel.: +49 241 8089669; fax: +49 214 8082414. E-mail addresses: [email protected] (P. Schauerte). 1 C. Knackstedt and F. Gramley have contributed equally to the study. 1054-8807/08/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.carpath.2007.12.003
conversion and maintenance of sinus rhythm are more difficult the longer the arrhythmia persists [3]. One of the strongest predictors of cardioversion (CV) success and maintenance of sinus rhythm is the degree of left atrial (LA) dilatation [4]. Left atrial dilatation is only incompletely reversible after CV possibly due to structural changes [5]. However, a structural substrate such as atrial fibrosis was not found in experimental AF [6]. Congestive heart failure (CHF) also induces atrial dilatation. In contrast to mere AF, CHF was found to lead to interstitial fibrosis and conduction abnormalities facilitating AF [7]. Furthermore, these changes are only incompletely reversible after cessation of CHF [8].
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In many patients, CHF and AF develop sequentially, and therefore, both conditions are present simultaneously. Additionally, in patients with AF and CHF, there are many factors that affect atrial dilatation and fibrosis, like age, arterial hypertension, valve or coronary artery disease, and also drug treatment with angiotensin inhibitors or aldosterone antagonists. Therefore, an animal model of sequential induction of CHF and AF was investigated (1) to evaluate the mere effect of AF and CHF on atrial dilatation and fibrosis imitating the clinical course, and (2) to estimate whether relatively simple clinical markers like echocardiographically determined atrial size may be associated with atrial fibrosis.
Bar, CA) set at 50 W and 65 °C. After surgery, cefuroxime axetil (0.125–0.25g) was given twice daily for 10 days.
2. Methods
2.4. Congestive heart failure and AF induction
2.1. Experimental setting
After HIS bundle ablation, the pacemaker was programmed to a VVI mode at 100 bpm. After a 6-week recovery, CHF and AF were induced sequentially to mimic the clinical course of this entity: during Week 7 to 17, the ventricles were stimulated at 185 bpm to induce CHF. In addition, during Week 11 to 17, simultaneous high-rate atrial pacing at 500 bpm at twice the diastolic pacing threshold was initiated to induce and maintain AF. Medical treatment was adjusted according to clinical signs and symptoms of AF/CHF and consisted of digoxin and furosemide, but excluded angiotensin-converting enzyme inhibitors (ACE-I), angiotensin-II receptor antagonists, and β-blockers.
Thirteen Labrador dogs (age, 27±5 months) were investigated. Six dogs underwent pacemaker implantation for sequential induction of CHF and AF. Seven healthy dogs (age, 27±4 months) served as a histological control group without pacemaker implantation. All investigations were undertaken with permission of the competent authorities (Regierungspräsident Cologne, AZ 23.203.2 AC37, 5/01). Animal care and euthanasia were performed according to the guidelines of the American Society of Physiology and in accordance with the Guide for the Care and Use of Laboratory Animals (NIH Publication no. 85-23, revised 1996). Anesthesia was induced with 400 mg im of azaperone and maintained by sodium pentobarbital (initial bolus of 16 mg/kg, continuous infusion of 5–20 mg/kg/h). After endotracheal intubation, the dogs were ventilated with N2O/O2 (Dräger Sulla 808V, Dräger, Germany). Surface electrocardiogram and capillary oxygen saturation were continuously monitored, and arterial blood gas analysis was performed hourly. 2.2. Surgical procedure Under sterile conditions, both jugular veins were dissected. A bipolar pacing lead (Elox SR 53; Biotronik, Germany) was introduced via the right jugular vein and positioned into the right atrium (RA) under fluoroscopic guidance. The lead was connected to a pacemaker (Logos, Biotronik), which was implanted in a subcutaneous pocket at the right neck. A second bipolar pacing lead (Kainox RV, Biotronik) was introduced into the right ventricle (RV) inserted via the left jugular vein and connected to an implantable cardioverter defibrillator (ICD, microPhylax06/XM, Biotronik) in a subcutaneous pocket at the left neck. The ICD was programmed to a ventricular pacing mode at 100 bpm. The HIS bundle was then ablated using a 4-mm tip ablation catheter (Biosense Webster, Diamond Bar, CA) and a radio frequency generator (Stockert/Biosense Webster, Diamond
2.3. Hemodynamic evaluation A hemodynamic evaluation was done at baseline and prior to euthanasia. All measurements were performed after ablation during ventricular pacing at 100 bpm, while AF was maintained in the atria by rapid pacing. After percutaneous puncture, a 5-F pigtail catheter (Cordis, Miami Lakes, FL) was introduced into the left ventricle (LV) to record LV pressure. A Swan–Ganz catheter (Becton, Dickinson, UT) was introduced to record pulmonary capillary occlusion pressure (PCOP) and RA pressure, and for cardiac output (CO) calculation.
2.5. Echocardiographic evaluation Transthoracic echocardiographic studies (Philips-Sonos5500/phased-array-S4; Agilent Technologies, Andover, MA) were performed 6 weeks after surgery before CHF induction, after 11 weeks before atrial stimulation was turned on to induce AF, and prior to euthanasia. All studies were performed by two experienced echocardiographers. To achieve comparable conditions, we carried out all measurements after ablation during ventricular pacing at 100 bpm. Control dogs were assessed prior to euthanasia. The dogs were placed in left and right lateral recumbency and gently restrained; anesthesia was not needed. The transducer was positioned in the left parasternal incidence at the level of the maximal manual perception of the precordial stroke, which corresponds approximately to the apex of the heart. All standard views (parasternal long axis/short axis, 2-, 3-, and 4-chamber view) were acquired. The anteroposterior diameter of the LA was determined in the parasternal long axis (D1), whereas the mediolateral (D2) and inferosuperior (D3) diameters were measured in the 4-chamber view. Left atrial volume was calculated by the formula LA volume=4/3 * π* D1/2 * D2/2 * D3/2 [7]. Right atrium diameters were measured in the 4-chamber view (D1: anteroposterior dimension was assumed equal to D2, mediolateral: D2, inferosuperior: D3), and RA volume was calculated [9].
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Fig. 1. Sirius Red (A+B) and Feulgen (C+D) stainings (2 μm). Representative canine LA specimen of healthy controls (A+C) and CHF/AF dogs (B+D) (magnification ×40). Correlation of LA fibrosis (E–H) with echocardiographic diameters and volumes. D1=parasternal long axis, D2=mediolateral in 4-chamber view, D3=inferosuperior in 4-chamber view. Shaded symbols represent controls; unshaded symbols represent the CHF/AF group.
The left ventricular end-diastolic diameter (LVEDD), left ventricular end-systolic diameter (LVESD), and thickness of the ventricular septum were measured in the parasternal long axis [10]. Left ventricular ejection fraction (EF) was calculated according to Simpson's method [11]. Valve function was assessed by color Doppler. 2.6. Histological evaluation After euthanasia by pentobarbital bolus injection (300 mg/ kg), thoracotomy was immediately performed. Then, RA/LA appendages as well as samples from the free wall of RA and
LA were excised, fixed in 10% buffered formalin, and embedded in paraffin. Deparaffinized tissue sections were stained with Sirius Red (staining for 1 h followed by a brief differentiation step in hydrochloric acid) or Feulgen stain (initial incubation in formalin for 50 min followed by two incubations for each 1 h in hydrochloric acid and in Schiff reagent, the latter under dark conditions) (Figs. 1 and 2). Characteristic areas were photographed using a Zeiss Axioplan 2 imaging (Oberkochen, Germany) microscope with a 3200-K lamp and a Color View II digital camera (Soft Imaging System, Germany). The photos were then analyzed using OpenLab version 2.2.5 software (Improvision, GB). To
Fig. 2. Sirius Red (A+B) and Feulgen (C+D) stainings (2 μm). Representative canine RA specimen of healthy controls (A+C) and CHF/AF dogs (B+D) (magnification ×40). Correlation of RA fibrosis (E–G) with echocardiographic diameters and volumes. D2=mediolateral in 4-chamber view, D3=inferosuperior in 4-chamber view. Shaded symbols represent controls; unshaded symbols represent the CHF/AF group.
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Table 1 Echocardiographic and hemodynamic parameters CHF/AF (n=6) Chamber parameters
Control (n=7)
Baseline After RV-stim. After RV/RA-stim. P (baseline vs. P (Week 11 vs. P (baseline vs. P (control vs. baseline (Week 6) (Week 7–11) (Week 11–17) Week 11) Week 17) Week 17) AF/CHF group)
LA D1 (mm) D2 (mm) D3 (mm) LA volume (mm3) RA D2 (mm) D3 (mm) RA volume (mm3) LV LVESD (mm) LVEDD (mm) Septum (mm) EF (%)
34±4 31±3 37±3 21±4 25±2 28±4 10±3 31±5 45±4 11±3 57±5
Hemodynamic measurements Baseline CO (L/min) 3.0±0.8 SV (ml) 32±8.8 RA a-wave (mmHg) 10.3±0.8 RA v-wave (mmHg) 5.7±1.5 PCOP (mmHg) 8.2±1.8
37±4 40±2 41±2 32±4 31±6 32±3 16±8 43±5 50±4 8±2 27±10
43±2 43±2 46±4 44±4 32±4 37±4 18±6 47±2 53±1 8±1 19±7
Final 2.1±0.4 21±3.5 11.7±2.4 5.7±1.9 11.7±6.2
P b.01 b.05 n.s. n.s. n.s.
n.s. b.01 b.05 b.05 n.s. n.s. n.s. n.s. b.05 n.s. b.01
b.05 n.s. b.05 b.05 n.s. n.s. n.s. n.s. n.s. n.s. n.s.
b.01 b.01 b.01 b.01 b.05 b.01 b.05 b.01 b.01 b.05 b.01
33±2 33±4 33±3 18±5 27±1 29±2 11±1 35±4 49±5 10±1 50±9
n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.
D1=anteroposterior in parasternal long axis, D2=mediolateral in 4-chamber view, D3=inferosuperior in 4-chamber view, RV/RA-stim.=RV and RA stimulation, RV-stim.=RV stimulation, n.s.=not significant, SV=stroke volume.
evaluate the amount of mature types I and III collagen, we expressed Sirius Red-stained sections as percentages of the total area. To judge DNA content, nucleus size, and shape, we analyzed Feulgen-stained sections. Size, intensity, and shape factor of the nuclei of each section were calculated using OpenLab in a standardized fashion. 2.7. Statistical analysis Statistical analysis was performed with SPSS version 10.07S. Quantitative data are expressed as mean±1 S.D. General changes of parameters over time were evaluated by analysis of variance. For individual comparisons, a paired Student's t test was applied. Correlation was calculated using the Spearman rank correlation test. For comparison of the two groups, Mann–Whitney U test was used. P values b.05 were considered significant.
to 44±4 mm3 [Pb.01]; RA, 10±3 to 18±6 mm3 [Pb.05]). The increase in volume was not significantly different between RA and LA volumes. Furthermore, LVEDD and LVESD significantly increased while LV EF decreased from 57±5% to 19±7% (Pb.01). At baseline, mild mitral regurgitation was present in one [2] dog of the CHF/AF (control) group, while tricuspid regurgitation was found in one dog of each group. After 17 weeks, mild mitral regurgitation was present in four and moderate in two dogs. Similarly, mild tricuspid valve regurgitation was present in five and moderate in one dog. Establishment of CHF/AF at 17 weeks was accompanied by a significant CO decrease from 3.0±0.8 to 2.1±0.4 L/min (Pb.05) and a decrease of the stroke volume from 32±8.8 to 21±3.5 ml (Pb.05). Atrial pressures (right atrial pressure and pulmonary capillary occlusion pressure as surrogate for LA pressure) increased nonsignificantly. 3.2. Histological changes
3. Results 3.1. Echocardiographic and hemodynamic changes Table 1 provides the echocardiographic and hemodynamic measurements at the three evaluation time points after 6, 11, and 17 weeks. Prior to pacing, neither mean heart rates during echocardiographic assessment (AF/CHF, 100 vs. 92±11.5 bpm in the control group) nor echocardiographic measurements were found to be significantly different between the CHF/AF and control group. After CHF/AF establishment at Week 17, a significant increase of LA/RA diameters and volumes (Table 1) occurred (LA, 21±4
In healthy controls, atrial fibrosis was only scarce in the appendages (LA, 5.6±2.6%; RA, 5.5±4.8%) as well as the free wall (LA, 4.4±1.6%; RA, 4.9±2.0%) and involved only minor patches between cardiomyocytes with regular interstitial matrix (Figs. 1A and 2A). By contrast, atrial myocardium of the CHF/AF group revealed significant fibrosis in the appendages (LA, 24.4±4.1%; RA, 22.6±4.0%) for the free walls (LA, 22.4±3.2%; RA, 19.9±5.4%) covering large areas with fibrotic bundles separating groups of cardiomyocytes (Figs. 1B and 2B). The degree of fibrosis was significantly different from the control group for both atria (appendages: LA, Pb.01; RA, Pb.05; free wall: LA,
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Pb.01; RA, Pb.01). There was no significant difference between the degree of fibrosis in LA and RA regarding atrial appendage and free wall. Cardiomyocytes in the CHF/AF group, as opposed to those of the control samples, displayed well-recognized criteria of hypertrophy, like increased nucleus size and DNA content (Figs. 1C, D and 2C, D and Table 2). Furthermore, the nuclear shape factor in both atria (free wall and appendages) was lower in CHF/AF dogs vs. controls (Pb.01, Table 2). There was no significant statistical difference regarding degree of fibrosis, DNA content, nucleus size, and shape between the two atrial regions, which were analyzed.
The degree of LA fibrosis in the atrial appendages as well as in the free wall significantly correlated to all LA diameters and to LA volume. Basically, the same was applied for the RA. For detailed analysis, see Table 3 and Figs. 1 and 2. Retrospectively, LA volume larger than 20 mm3 had a sensitivity of 100%, detecting LA fibrosis with a specificity of 86%. Similarly, for the RA, a right atrial volume larger than 15 mm3 had a sensitivity of 100% with a specificity of 57%. 4. Discussion The relationship between AF and atrial dilation was first described 50 years ago, observing that an increase in LA Table 2 Histological measurements
Atrial appendage LA RA DNA content (relative units) LA RA Nuclear size (relative units) LA RA Shape factor (0–1) LA RA Atrial free wall LA RA DNA content (relative units) LA RA Nuclear size (relative units) LA RA Shape factor (0–1) LA RA
Control (n=7)
Fibrosis (%)
Atrial appendage LA
RA
Atrial free wall LA
3.3. Correlating atrial fibrosis and atrial dilatation
CHF/AF (n=6)
Table 3 Correlation between fibrosis (%) and size of the atria (n=13)
P
24.4±4.1 22.6±4.0
5.6±2.6 5.5±4.8
b.01 b.05
42.92±4.82 43.68±7.04
37.94±5.47 31.61±3.61
n.s. b.01
22.55±2.3 23.15±3.3
20.60±1.94 17.04±1.59
n.s. b.01
0.31±0.01 0.29±0.04
0.34±0.02 0.36±0.02
b.01 b.01
22.2±3.2 19.9±5.4
4.4±1.6 4.9±2.0
b.01 b.01
42.63±3.64 41.96±5.45
39.54±3.12 33.77±4.92
n.s. b.05
23.71±2.39 22.53±2.44
21.83±2.08 18.01±2.27
n.s. b.01
0.31±0.01 0.29±0.02
0.34±0.02 0.34±0.02
b.01 b.01
n.s.=not significant. DNA content and nuclear size are measured in relative units. The shape factor, a measurement for shape irregularity, was graded between 0 and 1 with 1 representing a perfect circle.
RA
r
P
D1 D2 D3 LA volume D2 D3 RA volume
.89 .76 .78 .76 .59 .79 .57
b.01 b.01 b.01 b.01 b.05 b.01 b.05
D1 D2 D3 LA volume D2 D3 RA volume
.85 .79 .92 .86 .32 .93 .69
b.01 b.01 b.01 b.01 n.s. b.01 b.05
D1=parasternal long axis, D2=mediolateral, 4-chamber view, D3=inferosuperior, 4-chamber view.
diameter led to a higher incidence of AF [12]. Likewise, LA diameter was the only predictor (apart from age) for the occurrence of AF in patients with mitral valve disease [13]. Meanwhile, prospective studies could show that LA size increases with persisting AF [9]. There is still some debate as to whether atrial dilatation is a cause or a consequence of AF. There is, however, increasing evidence that AF and atrial size seem to be mutually dependent [14]. Clinically, LA size is routinely assessed by transthoracic echocardiography. Left atrial diameter larger than 50 mm in the parasternal long axis usually indicates a high recurrence of AF after CV [4]. The present study provides a rationale for this well-known clinical observation as it demonstrates that RA and LA diameters and volumes significantly correlate with the degree of atrial fibrosis. In fact, the loss of atrial myocytes and development of fibrosis in this model created numerous islands of scar tissue, which may provide the morphological substrate for reentrant wavelets. Similar atrial fibrosis as described in this model has been observed in a CHF model without AF and in patients with AF [15–17]. Most importantly, both atrial dilatation and atrial fibrosis are only incompletely reversible after restoration of sinus rhythm [5,8], which is in contrast to short-term electrical remodeling [18]. Although recent studies have questioned the value for repeated CV in order to maintain sinus rhythm in some AF patient [19,20], it is still not yet clear whether this holds true for patients with CHF and AF [21]. This is because atrial transport function and heart rate modulation during sinus rhythm may significantly contribute to the functional capacity of patients with CHF [22]. The present model of sequential induction of CHF and AF provides evidence that, in such circumstances, a simple and widely available noninvasive imaging technique such as transthoracic
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echocardiography of the atria may provide important information on the degree of atrial extracellular remodeling. Of note, the rapid development of atrial dilatation and fibrosis on top of preexisting CHF within weeks raises the question whether the principal decision to cardiovert a patient should be taken early after the onset of AF. The known effects of CHF on atrial fibrosis in sinus rhythm [23] and of AF on top of preexisting CHF as demonstrated in the present study may also be seen as a reasonable argument for an early and more intensive antifibrotic therapy, for example, with ACE-I or angiotensin receptor blockers.
5. Limitations This is only a small study of six animals with AF/CHF and seven animals in the histological control group using a model of sequential development of CHF and AF. A larger number of animals and various subgroups with a defined single influencing parameter like CHF, atrial dilatation, or lone AF would have been needed to fully evaluate the complex interaction among AF, CHF, and atrial fibrosis. However, the aim of this study was to imitate the clinical course of patients with CHF subsequently developing AF, thus, making it rather an observational study Atrial fibrillation and LA dilatation may be two independent predictors of CV success. Potential factors influencing LA diameter are volume and pressure increase as well as structural changes. In this model, all three confounding parameters increased significantly. Because we were not able to control these factors separately, the study does not allow differentiating the independent contribution of these parameters. More specifically, as a sequential model was used, the relative contribution of AF and CHF to the development of atrial fibrosis cannot be estimated. Thus, a single parameter cause–effect relationship cannot be evaluated with the present study. For such a purpose, a multiple subgroup design (e.g., lone AF, lone CHF, lone atrial dilatation) would have been necessary. A sequential model was used to mimic the often found clinical setting of CHF patients developing AF. It was not chosen to evaluate the sequential and relative contribution of CHF and AF on atrial fibrosis. Also, an independency of atrial dilatation and atrial fibrosis on CV cannot be differentiated. In other words, we intended to focus on the combined influence of CHF and AF on the morphological texture of the atria. By connecting these changes to current clinical imaging parameters, we wanted to strengthen the role of these clinical tools. Nevertheless, the study provides the clinician with the recognition that relatively simple clinical imaging parameters of the atria may imply substantial histological changes of the atria. This awareness, in turn, may support the physician's decision to initiate antifibrotic therapies like ACE-I (e.g., as adjunct therapy after CV) or to add antiarrhythmic drugs for prevention of recurrence of AF after CV in very enlarged atria.
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To our knowledge, there is only scarce information about atrial fibrosis without dilatation in CHF or AF. One could imagine that atrial fibrosis in early stages of AF might develop without obvious LA dilatation. However, in our own recent publication, we found increased atrial diameters/ volumes even in short-lasting AF b6 months, which was accompanied by substantial atrial interstitial remodeling [24]. Rapid short-term induction of CHF/AF may not exactly mimic the clinical course of slowly progressing CHF. However, atrial fibrosis in patients with long-lasting AF very much resembles the short-term changes observed in the present study [24]. The slight increase of the degree of mitral regurgitation in the AF/CHF group may have influenced the extent of fibrosis as compared with the control group. It is known that mitral regurgitation leads to atrial dilatation and fibrosis, resulting in an increased vulnerability to AF [25]. However, mitral regurgitation intrinsically occurs in the course of progressive LV dilatation and cannot be excluded in a CHF model. Rather, it mirrors the natural course of the clinical disease. The method used for assessing the atrial volume might have underestimated the real volume [26]. Nevertheless, this should not have affected the relative increase of atrial volumes in each animal. 6. Conclusion During the sequential development of CHF and AF, significant biatrial fibrosis and enlargement occur already after several weeks. Increased atrial diameters and volumes as determined by transthoracic echocardiography correlate to the degree of atrial fibrosis. This may enable the clinician to estimate atrial fibrosis by a simple and routinely performed echocardiographic measurement. Acknowledgment We are very thankful to Drs. Reincke, Wenzel, and Schweika and G. Stäger, Biotronik, Berlin, Germany, for providing pacing devices and leads. References [1] Feinberg WM, Blackshear JL, Laupacis A, Kronmal R, Hart RG. Prevalence, age distribution, and gender of patients with atrial fibrillation. Analysis and implications. Arch Intern Med 1995;155:469–73. [2] Crijns HJ, Van Gelder IC, Van Gilst WH, Hillege H, Gosselink, Lie KI. Serial antiarrhythmic drug treatment to maintain sinus rhythm after electrical cardioversion for chronic atrial fibrillation or atrial flutter. Am J Cardiol 1991;68:335–41. [3] Cuddy TE. Chronic and paroxysmal atrial fibrillation: course, prognosis, and stroke risk. J Thromb Thrombolysis 1999;7:7–11. [4] Henry WL, Morganroth J, Pearlman, Clark CE, Redwood DR, Itscoitz SB, Epstein SE. Relation between echocardiographically determined left atrial size and atrial fibrillation. Circulation 1976;53:273–9. [5] Shi Y, Li D, Tardif JC, Nattel S. Enalapril effects on atrial remodeling and atrial fibrillation in experimental congestive heart failure. Cardiovasc Res 2002;54:456–61.
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tachypacing-induced congestive heart failure. Circulation 2001;104: 2608–14. Cha TJ, Ehrlich JR, Zhang L, Shi YF, Tardif JC, Leung TK, Nattel S. Dissociation between ionic remodeling and ability to sustain atrial fibrillation during recovery from experimental congestive heart failure. Circulation 2004;109:412–8. Wyse DG, Waldo AL, DiMarco JP, Domanski MJ, Rosenberg Y, Schron EB, Kellen JC, Greene HL, Mickel MC, Dalquist JE, Corley SD. A comparison of rate control and rhythm control in patients with atrial fibrillation. N Engl J Med 2002;347:1825–33. Van Gelder IC, Hagens VE, Bosker HA, Kingma JH, Kamp O, Kingma T, Said SA, Darmanata JI, Timmermans AJ, Tijssen JG, Crijns HJ. A comparison of rate control and rhythm control in patients with recurrent persistent atrial fibrillation. N Engl J Med 2002;347: 1834–40. Pedersen OD, Bagger H, Keller N, Marchant B, Kober L, TorpPedersen C. Efficacy of dofetilide in the treatment of atrial fibrillationflutter in patients with reduced left ventricular function: a Danish Investigations of Arrhythmia and Mortality on Dofetilide (DIAMOND) substudy. Circulation 2001;104:292–6. Pozzoli M, Cioffi G, Traversi E, Pinna GD, Cobelli F, Tavazzi L. Predictors of primary atrial fibrillation and concomitant clinical and hemodynamic changes in patients with chronic heart failure: a prospective study in 344 patients with baseline sinus rhythm. J Am Coll Cardiol 1998;32:197–204. Hanna N, Cardin S, Leung TK, Nattel S. Differences in atrial versus ventricular remodeling in dogs with ventricular tachypacing-induced congestive heart failure. Cardiovasc Res 2004;63:236–44. Gramley F, Lorenzen J, Plisiene J, Rakauskas M, Benetis R, Schmid M, Autschbach R, Knackstedt C, Thomas S, Karl M, Gressner A, Hanrath P, Kelm M, Schauerte P. Decreased plasminogen activator inhibitor and tissue metalloproteinase inhibitor expression may promote increased metalloproteinase activity with increasing duration of human atrial fibrillation. J Cardiovasc Electrophysiol 2007;18: 1076–82. Verheule S, Wilson E, Everett T, Shanbhag S, Golden C, Olgin J. Alterations in atrial electrophysiology and tissue structure in a canine model of chronic atrial dilatation due to mitral regurgitation. Circulation 2003;107:2615–22. Khankirawatana B, Khankirawatana S, Porter T. How should left atrial size be reported? Comparative assessment with use of multiple echocardiographic methods. Am Heart J 2004;147:369–74.
Original Article
Association of echocardiographic atrial size and atrial fibrosis in a sequential model of congestive heart failure and atrial fibrillation Christian Knackstedt a,1 , Felix Gramley a,1 , Thomas Schimpf a , Karl Mischke a , Markus Zarse a , Jurgita Plisiene a , Michael Schmid c , Johann Lorenzen b , Dirk Frechen a , Philipp Neef a , Peter Hanrath a , Malte Kelm a , Patrick Schauerte a,⁎ a
Department of Cardiology, RWTH Aachen University, Aachen, Germany b Institut of Pathology, RWTH Aachen University, Aachen, Germany c Department of Thoracic and Cardiovascular Surgery, RWTH Aachen University, Aachen, Germany Received 2 April 2007; received in revised form 20 June 2007; accepted 17 December 2007
Abstract Background: Cardioversion (CV) success of atrial fibrillation (AF) inversely correlates to the size of the left atrium (LA). Atrial fibrillation and its most important risk factor, congestive heart failure (CHF), both induce atrial structural enlargement and fibrosis. To investigate the effect of AF and CHF on atrial dilatation and fibrosis, and to estimate whether echocardiographically determined atrial size may be used as a marker for atrial fibrosis. Methods: In six dogs, pacemakers were implanted followed by HIS bundle ablation. After 4 weeks of rapid ventricular stimulation (185 bpm) for CHF induction, additional rapid atrial stimulation (500 bpm) was maintained for 7 weeks to induce AF. Serial determinations of echocardiographic atrial size were performed. Seven dogs with sinus rhythm served as histological controls. Postmortem tissue was obtained to determine the degree and composition of atrial fibrosis. Results: While the ejection fraction of the AF/CHF dogs decreased significantly from 57±5% to 19±7% (Pb.01), an increased degree of atrial fibrosis was found (right atrium [RA], 4.9±2.0% to 19.9±5.4%; LA, 4.4±1.6% to 22.2±3.2%; Pb.01), accompanied by a significant increase of atrial volumes (LA: 21±4 to 44±4 mm3; Pb.01; RA: 10±3 to 18±6 mm3; Pb.05) and LA diameters (34±4 to 43±2 mm, Pb.05). Atrial fibrosis and size significantly correlated. Conclusions: Atrial fibrillation/CHF leads to a significant atrial fibrosis and dilation. The increased echocardiographic size correlates to the degree of atrial fibrosis and may be used as clinical marker for atrial fibrosis. The fibrosis accompanying atrial dilatation may also explain why LA size, as determined by echocardiography, is a strong predictor of CV success. © 2008 Elsevier Inc. All rights reserved. Keywords: Atrial fibrillation; Fibrosis; Echocardiography; Atrial size; Congestive heart failure
1. Introduction Atrial fibrillation (AF) is the most common arrhythmia in man [1]. There is a tendency for AF to become chronic with long-lasting duration [2]. Consequently, many patients with intermittent AF will develop chronic AF [3]. Furthermore, The study was supported by a grant from the DAAD (German Academic Exchange Society) to Dr. Jurgita Plisiene. ⁎ Corresponding author. Department of Cardiology, RWTH Aachen University, Pauwelstrasse 30, 52072 Aachen, Germany. Tel.: +49 241 8089669; fax: +49 214 8082414. E-mail addresses: [email protected] (P. Schauerte). 1 C. Knackstedt and F. Gramley have contributed equally to the study. 1054-8807/08/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.carpath.2007.12.003
conversion and maintenance of sinus rhythm are more difficult the longer the arrhythmia persists [3]. One of the strongest predictors of cardioversion (CV) success and maintenance of sinus rhythm is the degree of left atrial (LA) dilatation [4]. Left atrial dilatation is only incompletely reversible after CV possibly due to structural changes [5]. However, a structural substrate such as atrial fibrosis was not found in experimental AF [6]. Congestive heart failure (CHF) also induces atrial dilatation. In contrast to mere AF, CHF was found to lead to interstitial fibrosis and conduction abnormalities facilitating AF [7]. Furthermore, these changes are only incompletely reversible after cessation of CHF [8].
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In many patients, CHF and AF develop sequentially, and therefore, both conditions are present simultaneously. Additionally, in patients with AF and CHF, there are many factors that affect atrial dilatation and fibrosis, like age, arterial hypertension, valve or coronary artery disease, and also drug treatment with angiotensin inhibitors or aldosterone antagonists. Therefore, an animal model of sequential induction of CHF and AF was investigated (1) to evaluate the mere effect of AF and CHF on atrial dilatation and fibrosis imitating the clinical course, and (2) to estimate whether relatively simple clinical markers like echocardiographically determined atrial size may be associated with atrial fibrosis.
Bar, CA) set at 50 W and 65 °C. After surgery, cefuroxime axetil (0.125–0.25g) was given twice daily for 10 days.
2. Methods
2.4. Congestive heart failure and AF induction
2.1. Experimental setting
After HIS bundle ablation, the pacemaker was programmed to a VVI mode at 100 bpm. After a 6-week recovery, CHF and AF were induced sequentially to mimic the clinical course of this entity: during Week 7 to 17, the ventricles were stimulated at 185 bpm to induce CHF. In addition, during Week 11 to 17, simultaneous high-rate atrial pacing at 500 bpm at twice the diastolic pacing threshold was initiated to induce and maintain AF. Medical treatment was adjusted according to clinical signs and symptoms of AF/CHF and consisted of digoxin and furosemide, but excluded angiotensin-converting enzyme inhibitors (ACE-I), angiotensin-II receptor antagonists, and β-blockers.
Thirteen Labrador dogs (age, 27±5 months) were investigated. Six dogs underwent pacemaker implantation for sequential induction of CHF and AF. Seven healthy dogs (age, 27±4 months) served as a histological control group without pacemaker implantation. All investigations were undertaken with permission of the competent authorities (Regierungspräsident Cologne, AZ 23.203.2 AC37, 5/01). Animal care and euthanasia were performed according to the guidelines of the American Society of Physiology and in accordance with the Guide for the Care and Use of Laboratory Animals (NIH Publication no. 85-23, revised 1996). Anesthesia was induced with 400 mg im of azaperone and maintained by sodium pentobarbital (initial bolus of 16 mg/kg, continuous infusion of 5–20 mg/kg/h). After endotracheal intubation, the dogs were ventilated with N2O/O2 (Dräger Sulla 808V, Dräger, Germany). Surface electrocardiogram and capillary oxygen saturation were continuously monitored, and arterial blood gas analysis was performed hourly. 2.2. Surgical procedure Under sterile conditions, both jugular veins were dissected. A bipolar pacing lead (Elox SR 53; Biotronik, Germany) was introduced via the right jugular vein and positioned into the right atrium (RA) under fluoroscopic guidance. The lead was connected to a pacemaker (Logos, Biotronik), which was implanted in a subcutaneous pocket at the right neck. A second bipolar pacing lead (Kainox RV, Biotronik) was introduced into the right ventricle (RV) inserted via the left jugular vein and connected to an implantable cardioverter defibrillator (ICD, microPhylax06/XM, Biotronik) in a subcutaneous pocket at the left neck. The ICD was programmed to a ventricular pacing mode at 100 bpm. The HIS bundle was then ablated using a 4-mm tip ablation catheter (Biosense Webster, Diamond Bar, CA) and a radio frequency generator (Stockert/Biosense Webster, Diamond
2.3. Hemodynamic evaluation A hemodynamic evaluation was done at baseline and prior to euthanasia. All measurements were performed after ablation during ventricular pacing at 100 bpm, while AF was maintained in the atria by rapid pacing. After percutaneous puncture, a 5-F pigtail catheter (Cordis, Miami Lakes, FL) was introduced into the left ventricle (LV) to record LV pressure. A Swan–Ganz catheter (Becton, Dickinson, UT) was introduced to record pulmonary capillary occlusion pressure (PCOP) and RA pressure, and for cardiac output (CO) calculation.
2.5. Echocardiographic evaluation Transthoracic echocardiographic studies (Philips-Sonos5500/phased-array-S4; Agilent Technologies, Andover, MA) were performed 6 weeks after surgery before CHF induction, after 11 weeks before atrial stimulation was turned on to induce AF, and prior to euthanasia. All studies were performed by two experienced echocardiographers. To achieve comparable conditions, we carried out all measurements after ablation during ventricular pacing at 100 bpm. Control dogs were assessed prior to euthanasia. The dogs were placed in left and right lateral recumbency and gently restrained; anesthesia was not needed. The transducer was positioned in the left parasternal incidence at the level of the maximal manual perception of the precordial stroke, which corresponds approximately to the apex of the heart. All standard views (parasternal long axis/short axis, 2-, 3-, and 4-chamber view) were acquired. The anteroposterior diameter of the LA was determined in the parasternal long axis (D1), whereas the mediolateral (D2) and inferosuperior (D3) diameters were measured in the 4-chamber view. Left atrial volume was calculated by the formula LA volume=4/3 * π* D1/2 * D2/2 * D3/2 [7]. Right atrium diameters were measured in the 4-chamber view (D1: anteroposterior dimension was assumed equal to D2, mediolateral: D2, inferosuperior: D3), and RA volume was calculated [9].
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Fig. 1. Sirius Red (A+B) and Feulgen (C+D) stainings (2 μm). Representative canine LA specimen of healthy controls (A+C) and CHF/AF dogs (B+D) (magnification ×40). Correlation of LA fibrosis (E–H) with echocardiographic diameters and volumes. D1=parasternal long axis, D2=mediolateral in 4-chamber view, D3=inferosuperior in 4-chamber view. Shaded symbols represent controls; unshaded symbols represent the CHF/AF group.
The left ventricular end-diastolic diameter (LVEDD), left ventricular end-systolic diameter (LVESD), and thickness of the ventricular septum were measured in the parasternal long axis [10]. Left ventricular ejection fraction (EF) was calculated according to Simpson's method [11]. Valve function was assessed by color Doppler. 2.6. Histological evaluation After euthanasia by pentobarbital bolus injection (300 mg/ kg), thoracotomy was immediately performed. Then, RA/LA appendages as well as samples from the free wall of RA and
LA were excised, fixed in 10% buffered formalin, and embedded in paraffin. Deparaffinized tissue sections were stained with Sirius Red (staining for 1 h followed by a brief differentiation step in hydrochloric acid) or Feulgen stain (initial incubation in formalin for 50 min followed by two incubations for each 1 h in hydrochloric acid and in Schiff reagent, the latter under dark conditions) (Figs. 1 and 2). Characteristic areas were photographed using a Zeiss Axioplan 2 imaging (Oberkochen, Germany) microscope with a 3200-K lamp and a Color View II digital camera (Soft Imaging System, Germany). The photos were then analyzed using OpenLab version 2.2.5 software (Improvision, GB). To
Fig. 2. Sirius Red (A+B) and Feulgen (C+D) stainings (2 μm). Representative canine RA specimen of healthy controls (A+C) and CHF/AF dogs (B+D) (magnification ×40). Correlation of RA fibrosis (E–G) with echocardiographic diameters and volumes. D2=mediolateral in 4-chamber view, D3=inferosuperior in 4-chamber view. Shaded symbols represent controls; unshaded symbols represent the CHF/AF group.
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Table 1 Echocardiographic and hemodynamic parameters CHF/AF (n=6) Chamber parameters
Control (n=7)
Baseline After RV-stim. After RV/RA-stim. P (baseline vs. P (Week 11 vs. P (baseline vs. P (control vs. baseline (Week 6) (Week 7–11) (Week 11–17) Week 11) Week 17) Week 17) AF/CHF group)
LA D1 (mm) D2 (mm) D3 (mm) LA volume (mm3) RA D2 (mm) D3 (mm) RA volume (mm3) LV LVESD (mm) LVEDD (mm) Septum (mm) EF (%)
34±4 31±3 37±3 21±4 25±2 28±4 10±3 31±5 45±4 11±3 57±5
Hemodynamic measurements Baseline CO (L/min) 3.0±0.8 SV (ml) 32±8.8 RA a-wave (mmHg) 10.3±0.8 RA v-wave (mmHg) 5.7±1.5 PCOP (mmHg) 8.2±1.8
37±4 40±2 41±2 32±4 31±6 32±3 16±8 43±5 50±4 8±2 27±10
43±2 43±2 46±4 44±4 32±4 37±4 18±6 47±2 53±1 8±1 19±7
Final 2.1±0.4 21±3.5 11.7±2.4 5.7±1.9 11.7±6.2
P b.01 b.05 n.s. n.s. n.s.
n.s. b.01 b.05 b.05 n.s. n.s. n.s. n.s. b.05 n.s. b.01
b.05 n.s. b.05 b.05 n.s. n.s. n.s. n.s. n.s. n.s. n.s.
b.01 b.01 b.01 b.01 b.05 b.01 b.05 b.01 b.01 b.05 b.01
33±2 33±4 33±3 18±5 27±1 29±2 11±1 35±4 49±5 10±1 50±9
n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.
D1=anteroposterior in parasternal long axis, D2=mediolateral in 4-chamber view, D3=inferosuperior in 4-chamber view, RV/RA-stim.=RV and RA stimulation, RV-stim.=RV stimulation, n.s.=not significant, SV=stroke volume.
evaluate the amount of mature types I and III collagen, we expressed Sirius Red-stained sections as percentages of the total area. To judge DNA content, nucleus size, and shape, we analyzed Feulgen-stained sections. Size, intensity, and shape factor of the nuclei of each section were calculated using OpenLab in a standardized fashion. 2.7. Statistical analysis Statistical analysis was performed with SPSS version 10.07S. Quantitative data are expressed as mean±1 S.D. General changes of parameters over time were evaluated by analysis of variance. For individual comparisons, a paired Student's t test was applied. Correlation was calculated using the Spearman rank correlation test. For comparison of the two groups, Mann–Whitney U test was used. P values b.05 were considered significant.
to 44±4 mm3 [Pb.01]; RA, 10±3 to 18±6 mm3 [Pb.05]). The increase in volume was not significantly different between RA and LA volumes. Furthermore, LVEDD and LVESD significantly increased while LV EF decreased from 57±5% to 19±7% (Pb.01). At baseline, mild mitral regurgitation was present in one [2] dog of the CHF/AF (control) group, while tricuspid regurgitation was found in one dog of each group. After 17 weeks, mild mitral regurgitation was present in four and moderate in two dogs. Similarly, mild tricuspid valve regurgitation was present in five and moderate in one dog. Establishment of CHF/AF at 17 weeks was accompanied by a significant CO decrease from 3.0±0.8 to 2.1±0.4 L/min (Pb.05) and a decrease of the stroke volume from 32±8.8 to 21±3.5 ml (Pb.05). Atrial pressures (right atrial pressure and pulmonary capillary occlusion pressure as surrogate for LA pressure) increased nonsignificantly. 3.2. Histological changes
3. Results 3.1. Echocardiographic and hemodynamic changes Table 1 provides the echocardiographic and hemodynamic measurements at the three evaluation time points after 6, 11, and 17 weeks. Prior to pacing, neither mean heart rates during echocardiographic assessment (AF/CHF, 100 vs. 92±11.5 bpm in the control group) nor echocardiographic measurements were found to be significantly different between the CHF/AF and control group. After CHF/AF establishment at Week 17, a significant increase of LA/RA diameters and volumes (Table 1) occurred (LA, 21±4
In healthy controls, atrial fibrosis was only scarce in the appendages (LA, 5.6±2.6%; RA, 5.5±4.8%) as well as the free wall (LA, 4.4±1.6%; RA, 4.9±2.0%) and involved only minor patches between cardiomyocytes with regular interstitial matrix (Figs. 1A and 2A). By contrast, atrial myocardium of the CHF/AF group revealed significant fibrosis in the appendages (LA, 24.4±4.1%; RA, 22.6±4.0%) for the free walls (LA, 22.4±3.2%; RA, 19.9±5.4%) covering large areas with fibrotic bundles separating groups of cardiomyocytes (Figs. 1B and 2B). The degree of fibrosis was significantly different from the control group for both atria (appendages: LA, Pb.01; RA, Pb.05; free wall: LA,
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Pb.01; RA, Pb.01). There was no significant difference between the degree of fibrosis in LA and RA regarding atrial appendage and free wall. Cardiomyocytes in the CHF/AF group, as opposed to those of the control samples, displayed well-recognized criteria of hypertrophy, like increased nucleus size and DNA content (Figs. 1C, D and 2C, D and Table 2). Furthermore, the nuclear shape factor in both atria (free wall and appendages) was lower in CHF/AF dogs vs. controls (Pb.01, Table 2). There was no significant statistical difference regarding degree of fibrosis, DNA content, nucleus size, and shape between the two atrial regions, which were analyzed.
The degree of LA fibrosis in the atrial appendages as well as in the free wall significantly correlated to all LA diameters and to LA volume. Basically, the same was applied for the RA. For detailed analysis, see Table 3 and Figs. 1 and 2. Retrospectively, LA volume larger than 20 mm3 had a sensitivity of 100%, detecting LA fibrosis with a specificity of 86%. Similarly, for the RA, a right atrial volume larger than 15 mm3 had a sensitivity of 100% with a specificity of 57%. 4. Discussion The relationship between AF and atrial dilation was first described 50 years ago, observing that an increase in LA Table 2 Histological measurements
Atrial appendage LA RA DNA content (relative units) LA RA Nuclear size (relative units) LA RA Shape factor (0–1) LA RA Atrial free wall LA RA DNA content (relative units) LA RA Nuclear size (relative units) LA RA Shape factor (0–1) LA RA
Control (n=7)
Fibrosis (%)
Atrial appendage LA
RA
Atrial free wall LA
3.3. Correlating atrial fibrosis and atrial dilatation
CHF/AF (n=6)
Table 3 Correlation between fibrosis (%) and size of the atria (n=13)
P
24.4±4.1 22.6±4.0
5.6±2.6 5.5±4.8
b.01 b.05
42.92±4.82 43.68±7.04
37.94±5.47 31.61±3.61
n.s. b.01
22.55±2.3 23.15±3.3
20.60±1.94 17.04±1.59
n.s. b.01
0.31±0.01 0.29±0.04
0.34±0.02 0.36±0.02
b.01 b.01
22.2±3.2 19.9±5.4
4.4±1.6 4.9±2.0
b.01 b.01
42.63±3.64 41.96±5.45
39.54±3.12 33.77±4.92
n.s. b.05
23.71±2.39 22.53±2.44
21.83±2.08 18.01±2.27
n.s. b.01
0.31±0.01 0.29±0.02
0.34±0.02 0.34±0.02
b.01 b.01
n.s.=not significant. DNA content and nuclear size are measured in relative units. The shape factor, a measurement for shape irregularity, was graded between 0 and 1 with 1 representing a perfect circle.
RA
r
P
D1 D2 D3 LA volume D2 D3 RA volume
.89 .76 .78 .76 .59 .79 .57
b.01 b.01 b.01 b.01 b.05 b.01 b.05
D1 D2 D3 LA volume D2 D3 RA volume
.85 .79 .92 .86 .32 .93 .69
b.01 b.01 b.01 b.01 n.s. b.01 b.05
D1=parasternal long axis, D2=mediolateral, 4-chamber view, D3=inferosuperior, 4-chamber view.
diameter led to a higher incidence of AF [12]. Likewise, LA diameter was the only predictor (apart from age) for the occurrence of AF in patients with mitral valve disease [13]. Meanwhile, prospective studies could show that LA size increases with persisting AF [9]. There is still some debate as to whether atrial dilatation is a cause or a consequence of AF. There is, however, increasing evidence that AF and atrial size seem to be mutually dependent [14]. Clinically, LA size is routinely assessed by transthoracic echocardiography. Left atrial diameter larger than 50 mm in the parasternal long axis usually indicates a high recurrence of AF after CV [4]. The present study provides a rationale for this well-known clinical observation as it demonstrates that RA and LA diameters and volumes significantly correlate with the degree of atrial fibrosis. In fact, the loss of atrial myocytes and development of fibrosis in this model created numerous islands of scar tissue, which may provide the morphological substrate for reentrant wavelets. Similar atrial fibrosis as described in this model has been observed in a CHF model without AF and in patients with AF [15–17]. Most importantly, both atrial dilatation and atrial fibrosis are only incompletely reversible after restoration of sinus rhythm [5,8], which is in contrast to short-term electrical remodeling [18]. Although recent studies have questioned the value for repeated CV in order to maintain sinus rhythm in some AF patient [19,20], it is still not yet clear whether this holds true for patients with CHF and AF [21]. This is because atrial transport function and heart rate modulation during sinus rhythm may significantly contribute to the functional capacity of patients with CHF [22]. The present model of sequential induction of CHF and AF provides evidence that, in such circumstances, a simple and widely available noninvasive imaging technique such as transthoracic
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echocardiography of the atria may provide important information on the degree of atrial extracellular remodeling. Of note, the rapid development of atrial dilatation and fibrosis on top of preexisting CHF within weeks raises the question whether the principal decision to cardiovert a patient should be taken early after the onset of AF. The known effects of CHF on atrial fibrosis in sinus rhythm [23] and of AF on top of preexisting CHF as demonstrated in the present study may also be seen as a reasonable argument for an early and more intensive antifibrotic therapy, for example, with ACE-I or angiotensin receptor blockers.
5. Limitations This is only a small study of six animals with AF/CHF and seven animals in the histological control group using a model of sequential development of CHF and AF. A larger number of animals and various subgroups with a defined single influencing parameter like CHF, atrial dilatation, or lone AF would have been needed to fully evaluate the complex interaction among AF, CHF, and atrial fibrosis. However, the aim of this study was to imitate the clinical course of patients with CHF subsequently developing AF, thus, making it rather an observational study Atrial fibrillation and LA dilatation may be two independent predictors of CV success. Potential factors influencing LA diameter are volume and pressure increase as well as structural changes. In this model, all three confounding parameters increased significantly. Because we were not able to control these factors separately, the study does not allow differentiating the independent contribution of these parameters. More specifically, as a sequential model was used, the relative contribution of AF and CHF to the development of atrial fibrosis cannot be estimated. Thus, a single parameter cause–effect relationship cannot be evaluated with the present study. For such a purpose, a multiple subgroup design (e.g., lone AF, lone CHF, lone atrial dilatation) would have been necessary. A sequential model was used to mimic the often found clinical setting of CHF patients developing AF. It was not chosen to evaluate the sequential and relative contribution of CHF and AF on atrial fibrosis. Also, an independency of atrial dilatation and atrial fibrosis on CV cannot be differentiated. In other words, we intended to focus on the combined influence of CHF and AF on the morphological texture of the atria. By connecting these changes to current clinical imaging parameters, we wanted to strengthen the role of these clinical tools. Nevertheless, the study provides the clinician with the recognition that relatively simple clinical imaging parameters of the atria may imply substantial histological changes of the atria. This awareness, in turn, may support the physician's decision to initiate antifibrotic therapies like ACE-I (e.g., as adjunct therapy after CV) or to add antiarrhythmic drugs for prevention of recurrence of AF after CV in very enlarged atria.
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To our knowledge, there is only scarce information about atrial fibrosis without dilatation in CHF or AF. One could imagine that atrial fibrosis in early stages of AF might develop without obvious LA dilatation. However, in our own recent publication, we found increased atrial diameters/ volumes even in short-lasting AF b6 months, which was accompanied by substantial atrial interstitial remodeling [24]. Rapid short-term induction of CHF/AF may not exactly mimic the clinical course of slowly progressing CHF. However, atrial fibrosis in patients with long-lasting AF very much resembles the short-term changes observed in the present study [24]. The slight increase of the degree of mitral regurgitation in the AF/CHF group may have influenced the extent of fibrosis as compared with the control group. It is known that mitral regurgitation leads to atrial dilatation and fibrosis, resulting in an increased vulnerability to AF [25]. However, mitral regurgitation intrinsically occurs in the course of progressive LV dilatation and cannot be excluded in a CHF model. Rather, it mirrors the natural course of the clinical disease. The method used for assessing the atrial volume might have underestimated the real volume [26]. Nevertheless, this should not have affected the relative increase of atrial volumes in each animal. 6. Conclusion During the sequential development of CHF and AF, significant biatrial fibrosis and enlargement occur already after several weeks. Increased atrial diameters and volumes as determined by transthoracic echocardiography correlate to the degree of atrial fibrosis. This may enable the clinician to estimate atrial fibrosis by a simple and routinely performed echocardiographic measurement. Acknowledgment We are very thankful to Drs. Reincke, Wenzel, and Schweika and G. Stäger, Biotronik, Berlin, Germany, for providing pacing devices and leads. References [1] Feinberg WM, Blackshear JL, Laupacis A, Kronmal R, Hart RG. Prevalence, age distribution, and gender of patients with atrial fibrillation. Analysis and implications. Arch Intern Med 1995;155:469–73. [2] Crijns HJ, Van Gelder IC, Van Gilst WH, Hillege H, Gosselink, Lie KI. Serial antiarrhythmic drug treatment to maintain sinus rhythm after electrical cardioversion for chronic atrial fibrillation or atrial flutter. Am J Cardiol 1991;68:335–41. [3] Cuddy TE. Chronic and paroxysmal atrial fibrillation: course, prognosis, and stroke risk. J Thromb Thrombolysis 1999;7:7–11. [4] Henry WL, Morganroth J, Pearlman, Clark CE, Redwood DR, Itscoitz SB, Epstein SE. Relation between echocardiographically determined left atrial size and atrial fibrillation. Circulation 1976;53:273–9. [5] Shi Y, Li D, Tardif JC, Nattel S. Enalapril effects on atrial remodeling and atrial fibrillation in experimental congestive heart failure. Cardiovasc Res 2002;54:456–61.
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