The Role Of Renin Angiotensin System In Atrial Fibrillation

Girish M. Nair. MBBS, MSc., FRCPC, Pablo B. Nery. MD, Calum J. Redpath. MB, ChB, PhD., MRCP, David H. Birnie. MB, ChB, MD, MRCP

Arrhythmia Service, Division of Cardiology University of Ottawa Heart Institute 40 Ruskin Ave, Ottawa Canada – K1Y 4W7.

Abstract

Atrial fibrillation (AF) is the most prevalent arrhythmia and its incidence is on the rise. AF causes significant morbidity and mortality leading to rising AF-related health care costs. There is experimental and clinical evidence from animal and human studies that suggests a role for the renin angiotensin system (RAS) in the etiopathogenesis of AF. This review appraises the current understanding of RAS antagonism, using angiotensin converting enzyme inhibitors (ACE-I), angiotensin receptor blockers (ARB) and aldosterone antagonists (AA), for prevention of AF. RAS antagonism has proven to be effective for primary and secondary prevention of AF in subjects with heart failure and left ventricular (LV) dysfunction.However, most of the evidence for the protective effect of RAS antagonism is from clinical trials that had AF as a secondary outcome or from unspecified post-hoc analyses. The evidence for prevention in subjects without heart failure and with normal LV function is not as clear. RAS antagonism, in the absence of concomitant antiarrhythmic therapy, was not shown to reduce post cardioversion AF recurrences. RAS antagonism in subjects undergoing catheter ablation has also been ineffective in preventing AF recurrences.

Key Words : Atrial Fibrillation, Renin Angiotensin System, Angiotensin Converting Enzyme- inhibitors, Angiotensin Receptor Blockers, Aldosterone Antagonists, Angiotensin Converting-Enzyme Gene Polymorphism, Clinical Trials, Primary and Secondary Prevention, Review.

Corresponding Address : Girish M. Nair. MBBS, MSc., FRCPC H-1285 B University of Ottawa Heart Institute 40 Ruskin Ave, Ottawa – K1Y 4W7

Introduction

Atrial Fibrillation (AF) is the most commonly encountered cardiac arrhythmia and affects 1% of the North American population. The prevalence of AF increases to 8-10% in people older than 80 years.1-4 AF independently increases the risk of heart failure, stroke, dementia and mortality. There is also a steep increase in AF related morbidity and hospital admissions with increasing age.5-11 The rising burden of AF and related health care costs are responsible for placing a heavy economic burden on health care systems around the world.12-16 Anti-arrhythmic medications and non-pharmacological interventions, such as catheter ablation, aimed at secondary prevention of AF have thus far been unsuccessful in curing.17,18 There is a pressing need for primary and secondary prevention strategies to reduce AF related morbidity, mortality and health care costs.2,15,19-21 This review appraises the role of the Renin-Angiotensin system (RAS) in the etiopathogenesis of AF and the evidence for therapeutic RAS blockade in primary and secondary prevention of AF.

The Renin Angiotensin System And Human Atrial Fibrillation

RAS is an important neuro-endocrine/paracrine system involved in the regulation of multiple cardiovascular, pulmonary and renal processes in humans.22 Systemic hypertension and heart failure are the most important risk factors associated with the development of AF.3,6,23,24 The activation of RAS plays an integral part in the neuro-humoral processes leading to changes seen in systemic hypertension and heart failure. There is some evidence to suggest that RAS is associated with the development of AF in subjects with systemic hypertension and heart failure.22,25,26 In addition multiple RAS gene polymorphisms have been linked to the development of AF in subjects with known conditions that directly or indirectly result in increased left atrial pressure, such as systemic hypertension or heart failure.27-32 Analysis of human atrial myocytes in subjects undergoing cardiac surgery has demonstrated increased tissue levels of angiotensin converting enzyme (ACE) and angiotensin II (AT-II) receptors in subjects with AF compared to those in sinus rhythm.33 Reduced density of AT-II -type 1 receptors, responsible for atrial fibrosis subjects with AF, was also noted and this was thought to be secondary to down regulation in response to high tissue ACE levels.34 The activation of RAS with consequent electrical and ultrastructural changes, called “atrial remodeling”, is thought to play a role in the development of AF in humans.

Postulated Mechanisms Linking the Renin Angiotensin System and Atrial Fibrillation (See Table 1)

Activation of RAS in hypertension and heart failure results in Angiotensin II mediated elevation in left atrial (LA) pressure, secondary to rise in left ventricular end diastolic pressure (LVEDPP).35-37 Atrial dilatation is associated with stretch related alteration in ion-channels that is believed to be responsible for electrophysiological changes such as shortened refractory periods (electrical remodeling).38,41 Prolonged activation of RAS results in high myocardial tissue levels of ACE and density of AT-II receptors triggering inflammation and fibrosis. These effects are mediated by fibroblast-derived cytokines such as transforming growth factor-  (TGF-B) and AT II receptor activated phosphorylation cascade causing release of mitogen-activated protein kinases (MAPK). Extensive atrial collagen deposition results from uncontrolled extracellular matrix metabolism and angiotensin II mediated modulation of matrix –metalloproteinases (structural remodeling). AF is considered to be one of the clinical manifestations of atrial remodeling.22,25,42,43 Animal models of rapid atrial pacing induced AF have shown high atrial tissue levels of ACE, chymase and angiotensinogen. Increased production of tissue level AT II mediated by paracrine activation of ACE, chymase and angiotensinogen is also thought to be responsible for atrial remodeling leading to AF. The cascade of events leading up to AF has been summarized in (Figure 1).

Figure 1. Figure summarizing the role of RAS in the etiopathogenesis of AF


RAS: Renin angiotensin system; LVEDP: Left Ventricular End Diastolic Pressure; TGF: Transforming growth factor; MAPK: Mitogen-activated protein kinase; ACE-I: Angiotensin converting enzyme; ACE: Angiotensin converting enzyme; ARB: Angiotensin receptor blocker.

Interruption of key steps in the RAS cascade (RAS antagonism) using angiotensin converting enzyme inhibitors (ACE-I), angiotensin-II receptor blockers (ARB) and aldosterone antagonists (AA) has been shown to reverse some of the electrical and ultrastructural changes in patients with AF.44,46 The important basic science data has been summarized in (Table 1).

Table 1. Summary of studies demonstrating the role of the renin-angiotensin system (RAS) in atrial remodeling and reversal of atrial remodeling after pharmacological antagonism of RAS
MechanismAuthorExperimental ModelMain Findings
Electrical Remodeling Wijffels MC, et al.96Goat; Artificial AF maintenance using pacemakerAERP shortening; increase in rate, inducibility and stability of AF
Electrical and Structural Remodeling Sakabe M, et al.97Canine; Pacing induced AF; Placebo vs. EnalaprilEnalapril prevented AF and tachycardia-mediated cardiomyopathy by suppressing interstitial fibrosis, connexin 43 over-expression and conduction delay
Electrical Remodeling Laszlo R, et al.98Rabbit; Rapid pacing induced atrial remodeling; Enalapril PretreatmentIncreases iCa,L current density, no effect on Ito current density. Beneficial in preventing early remodeling in AF model
Electrical Remodeling Doronin SV, et al.99Canine and Human Cell Lines AT-II type 1 receptor complex associates with Kv4.3 alpha subunit. AT-II stimulation raises the activation voltage threshold to more positive values
Electrical Remodeling Nakashima H, et al.100Canine; Rapid atrial pacing; control, candesartan, captopril and AT- IIAERP unchanged with candesartan and captopril pretreatment. AT-II linked to electrical remodeling
Electrical and Structural RemodelingKumagai K, et al.101Canine; Rapid atrial pacing; Candesartan pretreatment compared to controlAERP unaltered; Lesser interstitial fibrosis and shorter intra-atrial conduction time in candesartan treated animals
Structural Remodeling Milliez P, et al.102Rat; Post MI- Heart Failure model; spironolactone, lisinopril or atenololAtrial fibrosis reduced by spironolactone
Electrical Remodeling Tillmann HC, et al.103Human; Action potential duration before and after aldosterone infusionAldosterone increases monophasic action potential duration
Electrical Remodeling Cheng CC, et al.104Rabbit pulmonary vein cardiomyocytes, whole cell patch clamp; heat stressHeat stress attenuated the electrophysiological effects of AT-II
Electrical and Structural RemodelingReil JC, et al.26Rat; Aldosterone infusionIncrease in p-wave duration, total right atrial activation time, increase in atrial fibroblasts and interstitial collagen, atrial myocyte hypertrophy
Structural Remodeling Crabos M, et al.105Rat adult cardiac fibroblast cultureAT-II, via AT-II- Type 1 receptors, mediates cardiac fibroblast growth and increases collagen synthesis in cardiac tissue
Structural Remodeling Lee AA, et al.106Rat adult cardiac fibroblast cultureAT-II effects on myocardium mediated by production and release of TGF-beta 1 by cardiac fibroblasts
Structural Remodeling Kallergis EM, et al. Human; serum markers of collagen type I turnoverMarkers of collagen synthesis and breakdown were increased in subjects with AF compared to those in sinus rhythm. Subjects with higher burden of AF had more intense evidence of increased collagen synthesis and breakdown.
Structural Remodeling Goette A, et al.33Human; Atrial tissue samples from open heart surgery subjects ACE and extracellular signal related kinase (Erk1/Erk2) increased in subjects with AF and may be responsible for atrial fibrosis
Structural Remodeling Goette A, et al.34Human; Atrial tissue samples from open heart surgery subjects AT-II Type 1 and 2 (AT1 and AT2) receptors were analysed. AT1 receptor density was reduced and AT2 receptor density was increased in subjects with AF. This was associated with increased interstitial fibrosis.
Structural Remodeling Dahl JS, et al.107Human; Subjects with aortic valve replacement treated with Candesartan; Placebo controlled prospective studyCandesartan treatment resulted in greater LV mass index reduction, improvement in LV systolic function and greater reduction in LA volume
Electrical and Structural Remodeling; ArrhythmogenesisXiao HD, et al.108Mouse; ACE 8/8 with 4.3 fold increased levels of cardiac angiotensin II levelsAtrial fibrosis, morphological changes resulting in atrial fibrillation and conduction block

AERP: Atrial effective refractory period; AF: Atrial fibrillation; iCaL: long acting L-type calcium ion channel; AT-II: Angiotensin II; MI: Myocardial infarction; TGF: Transforming growth factor.

The Renin Angiotensin System Gene Polymorphisms And Atrial Fibrillation

The evidence linking RAS to atrial remodeling in AF and the inherent variation among individuals with respect to the extent and consequences of RAS activation had led investigators to suspect a role for genetic polymorphisms in the ACE gene. The human ACE gene is situated in chromosome 17q23.3 and demonstrates a polymorphism consisting of insertion (1) or deletion (D) in the intron.16 Consequently three genotypes are encountered in human populations- homozygous D/D and I/I and heterozygous I/D. ACE I/D polymorphism accounts for half of the variance noticed in ACE levels in humans, with the D/D alleles manifesting highest levels of the enzyme.47 The I/D heterozygous polymorphism has been associated with cardiovascular diseases including left ventricular hypertrophy, essential hypertension, dilated cardiomyopathy and myocardial infarction.48 ACE I/D polymorphism has been shown to increase the risk for development of AF in case-control studies.27,49 ACE D/D polymorphism has been shown to be associated with poor response to anti-arrhythmic medications in subjects with AF.28 ACE I/D polymorphisms have also been identified in certain cases of nonfamilial AF.27A recent meta-analysis of case-control studies failed to demonstrate a significant association between the ACE I/D polymorphism and AF risk.However, there was a significant association noted between the I/D polymorphism and AF risk in subjects with hypertension.31 Another prospective study evaluating 238 consecutive subjects with paroxysmal or persistent AF undergoing catheter ablation found that the ACE D/D homozygous gene variant to be associated with an increased risk of post ablation AF recurrence.50

The aldosterone synthase (CYP11B2) T-344C gene polymorphism and resultant raised aldosterone levels have been independently linked to an increased risk of AF in subjects with symptomatic heart failure (left ventricular ejection fraction <40%).29 A more recent case-control study in 620 Chinese subjects showed that the aldosterone synthase (CYP11B2) T-344C gene polymorphism (the CC homozygous allele) was associated with echocardiographic markers of atrial remodeling in hypertensive subjects. However, the distribution of the different alleles of this gene (TT/TC/CC) did not differ among hypertensive and normotensive subjects.30

Renin Angiotens In System Antagonism And Primary Prevention Of Atrial Fibrillation (Table 2)

Table 2. Summary of clinical studies evaluating RAS antagonism for primary prevention of AF
Patient PopulationAuthorsSubjects EnrolledStudy Interventions Study DesignPrincipal Findings
Post – MI; Heart FailurePedersen OD, et al.641,577Trandolapril vs. PlaceboRCT; AF primary outcome of trialReduced AF incidence
Subjects with multiple CV risk factors; Intolerant to ACE-I Yusuf S, et al.745,926Telmisartan vs. PlaceboRCT; AF primary outcome of trialNo effect on AF incidence
Post Cardiac Surgery Ozaydin M, et al.76128ACE-I with Candesartan vs. ACE-I vs. controlsRCT; Small sample size and not adequately poweredReduced AF incidence
Post - MIPizzetti F, et al.7217,944Lisinopril vs. PlaceboPost-hoc analysis of RCTReduced AF incidence
HypertensionHansson L, et al.666,614Enalapril or Lisinopril vs. older AHT medicationsPost-hoc analysis of RCTNo effect on AF incidence
HypertensionHansson L, et al.6710,985Captopril vs. BB or diuretics Post-hoc analysis of RCTNo effect on AF incidence
HypertensionWachtell K, et al.698,851Losartan vs. BBPost-hoc analysis of RCTReduced AF incidence
HypertensionSchmieder RE, et al.7115,245Valsartan vs. AmlodipinePost-hoc analysis of RCTReduced AF incidence
Heart FailureVermes E, et al.51374Enalapril vs. PlaceboPost-hoc analysis of RCTReduced AF incidence
Heart FailureMaggioni AP, et al.524,395Valsartan vs. PlaceboPost-hoc analysis of RCTReduced AF incidence
Heart FailureDucharme A, et al.651,794EplerenonePost-hoc analysis of RCTReduced AF incidence
Heart FailureSwedberg K, et al.546,379Candesartan vs. PlaceboPost-hoc analysis of RCTReduced AF incidence
Subjects with multiple CV risk factorsSalehian O, et al.738,335Ramipril vs. PlaceboPost-hoc analysis of RCTNo effect on AF incidence
Post Cardiac Surgery Mathew JP, et al.754,657ACE-IProspective, observational studyReduced AF incidence
HypertensionL’Allier PL, et al.6810,926ACE-I vs. CCBRetrospective, longitudinal, cohort analysisReduced AF incidence
HypertensionSchaer BA, et al.7023,303ACE-I or ARB vs. BB vs. CCBNested Case-controlReduced AF incidence
Post Cardiac Surgery White CM, et al.77338ACE-I or ARB vs. controlsObservational studyNo effect on AF incidence
Post Cardiac Surgery Rader F, et al.786744ACE-I or ARB vs. controlsObservational study; propensity matchedNo effect on AF incidence

MI: Myocardial infarction; RCT: Randomized controlled trial; AF: Atrial fibrillation; AHT: Anti hypertensive treatment; BB: Beta blockers; CCB: Calcium channel blockers; ACE-I: Angiotensin converting enzyme; ARB: Angiotensin receptor blocker.

Heart Failure Trials

Retrospective, sub-group analyses from multiple large trials evaluating the role of RAS antagonism in subjects with heart failure and LV systolic dysfunction have found a lower incidence of new-onset AF.51-54 Systematic reviews of these studies have demonstrated a 21-50% risk-reduction for new-onset AF in heart failure subjects receiving RAS antagonists.55-64 However, RAS antagonism in subjects with heart failure and preserved LV systolic function has not shown benefit in preventing new onset AF.65

Systemic Hypertension Trials

Systematic review of hypertension trials found a 25% reduction in new-onset AF. This was principally due to a 33% reduction noted in one trial evaluating losartan for AF prevention.66-71

Post Myocardial Infarction Trials

Two trials have evaluated the incidence of new-onset AF in subjects treated with RAS antagonists following myocardial infarction (MI). Subjects with impaired LV function following MI had lower incidence of AF after treatment with trandolapril. The GISSI-3 trial reported a lower incidence of new-onset AF in post MI subjects treated with lisinopril. However, about a third of subjects showed AF on their admission EKG bringing into question whether this study truly evaluated RAS antagonism for primary prevention of AF.64,72

Subjects With Multiple Cardiovascular Risk Factors

RAS antagonism (ramipril and telmisartan) for prevention of major adverse cardiac events in patients with multiple cardiovascular risk factors has not shown a reduction in the incidence of AF.73,74

Post-Cardiac Surgery Trials

A prospective, multicenter analysis of subjects who had undergone coronary artery bypass graft (CABG) surgery found that postoperative use of ACE-I was associated with reduction in new-onset AF.75 A randomized trial in subjects undergoing cardiac surgery demonstrated that the use of ACE-I or the combination of ACE-I and candesartan reduced postoperative AF. However, this trial enrolled a relatively small number of subjects (N= 60) and was not adequately powered to answer the primary hypothesis that RAS antagonism was capable of reducing the incidence of AF post cardiac surgery.76 Subgroup analyses from two large retrospective observational studies in patients undergoing cardiac surgery failed to demonstrate a protective effect for RAS antagonism.77,78

Renin Angiotensin System Antagonism And Secondary Prevention Of Atrial Fibrillation (Table 3)

Table 3. Summary of clinical studies evaluating RAS antagonism for secondary prevention of AF
Patient PopulationAuthorsSubjects EnrolledStudy Interventions Study DesignPrincipal Findings
Paroxysmal AFGissi-AF Disertori M, et al.791,442Valsartan vs. PlaceboRCT; AF primary outcome of trialNo effect on AF recurrence
Lone Paroxysmal AFYin Y, et al.111177Losartan with Amiodarone vs. Perindopril with Amiodarone vs. AmiodaroneRCT; AF primary outcome of trialACE-I and ARB with Amiodarone reduced AF recurrence
Paroxysmal AF; Hypertension Fogari R, et al.112222Losartan with Amiodarone vs. Amlodipine with AmiodaroneRCT; AF primary outcome of trialARB with Amiodarone reduced AF incidence
Paroxysmal AF; Hypertension Fogari R, et al.113369Valsartan vs. Ramipril vs. AmlodipineRCT; AF primary outcome of trialACE-I and ARB reduced AF recurrence
Paroxysmal AF; Hypertension Yamashita T, et al.80318Candesartan vs. AmlodipineRCT; AF primary outcome of trialNo effect on AF recurrence
Paroxysmal AF; Hypertension and metabolic syndrome Fogari R, et al.81391Telmisartan vs. Ramipril vs. AmlodipineRCT; AF primary outcome of trialTelmisartan more effective than Ramipril in reducing AF recurrence
Paroxysmal AFGoette A, et al.82430Olmesartan vs. PlaceboRCT; AF primary outcome of trialNo effect on AF recurrence
Post-Surgical AblationWang JG, et al.8683Irbesartan vs. ControlRCT; AF primary outcome of trialReduced AF recurrence
Post-CardioversionTveit A, et al.94171Candesartan vs. PlaceboRCT; AF primary outcome of trialNo effect on AF recurrence
Post-CardioversionMadrid AH, et al.88154Irbesartan with Amiodarone vs. AmiodaroneRCT; AF primary outcome of trialARB with Amiodarone reduced AF recurrence
Post-Cardioversion(Lone AF)Madrid AH, et al.9090Irbesartan (150 or 300 mg) with Amiodarone vs. AmiodaroneRCT; AF primary outcome of trialARB with Amiodarone reduced AF recurrence in a dose dependent fashion
Post-CardioversionUeng KC, et al.89145Enalapril with Amiodarone vs. AmiodaroneRCT; AF primary outcome of trialACE-I with Amiodarone reduced AF recurrence
Post-CardioversionBelluzzi F, et al.9362Ramipril vs. PlaceboRCT; AF primary outcome of trialReduced AF recurrence
Chronic AF with Congestive Heart FailureVan den Berg MP, et al.8730Lisinopril vs. PlaceboRCT; AF not primary outcome, exercise capacity primary outcome Non-significant decrease in AF recurrence
Paroxysmal AFMurray KT, et al.1091,153ACE-I/ARB vs. controlPost-hoc RCTNo effect on AF recurrence
Paroxysmal AFPalardy M, et al.110403ACE-I/ARB with Amiodarone vs ACE-I/ARB with Sotalol/Propafenone vs. Sotalol/Propafenone vs. Amiodarone Post-hoc RCTNo effect on AF recurrence
Paroxysmal AFKomatsu T, et al.11458Enalapril with Amiodarone vs. Amiodarone Retrospective analysisACE-I with Amiodarone reduced AF recurrence
Post-Catheter AblationRichter B, et al.115234ACE-I or ARB vs. Statins vs. combination Statin with ACE-I or ARB Prospective non-RCTNo effect on AF recurrence
Post-Catheter AblationAl Chekakie MO, et al.83177ACE-I or ARB vs. StatinsRetrospective analysisNo effect on AF recurrence
Post-Catheter AblationZheng B, et al.84139ACE-I or ARB vs. ControlRetrospective analysisNo effect on AF recurrence
Post-Catheter AblationZheng B, et al.85372ACE-I or ARB with Statins vs. ControlRetrospective analysisNo effect on AF recurrence

MI: Myocardial infarction; RCT: Randomized controlled trial; AF: Atrial fibrillation; AHT: Anti hypertensive treatment; BB: Beta blockers; CCB: Calcium channel blockers; ACE-I: Angiotensin converting enzyme; ARB: Angiotensin receptor blocker.

Prevention Of Paroxysmal And Recurrent Persistent AF

The GISSI-AF trial did not demonstrate an additional benefit for adding ARB (Valsartan) to ACE-I therapy for prevention of AF recurrence.79 Three recent trials evaluating the role of RAS antagonism (J-RHYTHMII, Fogari et al. and ANTIPAF) showed conflicting results, with two of the trials (J-RHYTHM II and ANTIPAF) failing to show any benefit of RAS antagonism for secondary prevention of AF.80-82 RAS antagonism does not seem to be effective for secondary prevention of AF in subjects without structural heart disease or LV dysfunction.

Prevention Of Recurrent AF After Catheter Or Surgical AF Ablation

RAS antagonism following catheter ablation has not proven to be effective in reducing AF recurrence.83-85 In contrast a recent study in subjects undergoing minimally invasive surgical ablation for AF showed that Irbesartan reduced the incidence of AF recurrence.86

Prevention Of AF After Cardioversion

Multiple small prospective, randomized trials have demonstrated the benefit of RAS antagonism for preventing post cardioversion recurrence, in subjects with persistent AF. The benefit of RAS antagonism in this setting is complementary to concomitant antiarrhythmic medications, usually with amiodarone.87-93 A well-designed prospective, randomized- controlled trial (CAPRAF) failed to demonstrate the efficacy of candesartan for prevention of post cardioversion AF recurrence. In contrast to the previously mentioned trials patients in the CAPRAF trial did not receive concomitant antiarrhythmic medications before and after cardioversion.94,95

Conclusions

There is experimental and clinical evidence from animal and human studies that suggests a role for RAS in the etiopathogenesis of AF. Genetic polymorphisms of the ACE and aldosterone synthase genes have been linked to the development of AF in subjects with pre-existent risk factors for AF such as systemic hypertension and heart failure. RAS antagonism has shown to reduce the incidence of AF in subjects with heart failure and left ventricular (LV) dysfunction. However, most of the evidence for the protective effect of RAS antagonism is from clinical trials that had AF as a secondary outcome or from unspecified post-hoc analyses. The evidence for prevention in subjects without heart failure and normal LV function is not as clear. RAS antagonism, in the absence of concomitant antiarrhythmic therapy, was not shown to reduce post cardioversion AF recurrences. RAS antagonism in subjects undergoing catheter ablation has also been ineffective in preventing AF recurrences. There is need for ongoing research to identify novel targets for intervention and develop effective therapeutic agents to combat the rising burden of AF.

Disclosures

None.

References

  • [ PubMed ]
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  • [ PubMed ]
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  • [ PubMed ]
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  • [ PubMed ]
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  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
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  • [ PubMed ]
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  • [ PubMed ]
  • [ PubMed ]
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  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
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  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]
  • [ PubMed ]