Determinants of fibrotic atrial cardiomyopathy in atrial fibrillation. A multicenter observational study of the RETAC (reseau européen de traîtement d’arrhythmies cardiaques)-group

In this multicenter prospective observational study, consecutive patients undergoing first PVI between January 2016 and October 2019 at the participating sites (University Heart Center Freiburg-Bad Krozingen, Bad Krozingen, Germany; Clinique Pasteur, Toulouse, Centre Hospitalier Universitaire Limoges, Limoges, and Centre Hospitalier Universitaire Grenoble, all in France; Hopital de la Tour, Geneva, Switzerland, and Hopital de Sant Pau, Barcelona, Spain) were screened for inclusion into the determination cohort and underwent high-density endocardial mapping as outlined below. An outcome-database of patients who underwent their first PVI at University Heart Center Freiburg-Bad Krozingen and Centre Hospitalier Universitaire Grenoble served as external outcome-validation cohort (Supplemental Fig. 1). Details on ablation and follow-up are described below. Inclusion criteria were symptomatic paroxysmal (< 7 days duration) and persistent (> 7 days and < 12 months duration) atrial fibrillation. Exclusion criteria were prior left atrial ablation, presence of left atrial thrombus or contraindication to anticoagulant therapy. Primary endpoints were the derivation of a scoring system to predict left-atrial low voltage-substrate as a surrogate of FAM, and external validation of this score for prediction of FAM. Secondary endpoint was the external validation of this score to predict arrhythmia-freedom following PVI.

Patients in the determination cohort underwent high density left-atrial voltage mapping during sinus rhythm with a minimum of 1200 mapped points per patient [8]. All patients underwent mapping in sinus rhythm prior to any ablation using an endocardial electro-anatomical contact mapping-system (Carto3, Biosense-Webster, Diamond Bar, CA, US) in combination with a 20-pole circumferential mapping catheter (electrode size: 1 mm; spacing: 2–6–2 mm). Peak-to-peak intracardiac bipolar electrograms were recorded at 15 to 250 Hz and amplified 0.1–0.2 mV/cm to visualize low-voltage electric activity. For highest accuracy, mapping was performed with respiratory gating and under mechanical ventilation. Low interpolation settings (17 in Carto3-system) were used. Mapping points that were > 5 mm away from the atrial geometry were excluded. Areas demonstrating potential low voltage when mapped with the 20-pole lasso catheter were reconfirmed using a contact force-enabled mapping catheter with a contact threshold of > 5 g. LVS was defined using a cutoff value for bipolar peak-to-peak voltage in sinus rhythm of < 0.5 mV and further expressed in cm² of absolute cumulative left-atrial surface area. The pulmonary vein antrae and pulmonary veins that physiologically show voltages < 0.5 mV as well as the mitral valvular area were excluded. For the current study, FAM was defined as LVS exceeding 5 cm².

All patients of the outcome-validation cohort underwent proximal circumferential pulmonary vein isolation without adjunctive left- or right-atrial ablations using either contact-force enabled radiofrequency-ablation in combination with the Carto3-Mapping system (Biosense Webster) or cryo-balloon-ablation (Medtronic Arctic Front Advance). The decision per ablation type was at the discretion of the treating physician. Antiarrhythmic drugs, where applicable, were continued for three months following ablation and then stopped. Patients underwent routine ambulatory cardiological examinations including 12-lead ECG at four weeks following ablation. Regular 24 h-Holter-ECGs were scheduled in 6-months-intervals. Patients experiencing symptoms suggestive of arrhythmia recurrence (e.g. palpitations, dyspnea, fatigue) underwent additional cardiological examinations including ECG and symptom-triggered ECG (event recorder and 24 h-Holter ECG). AF or atrial tachycardia lasting for > 30 s experienced beyond the blanking period of three months were considered as recurrence. The study protocol was approved by the ethic committees of the participating sites.

In total, 440 patients were included in the determination cohort and underwent high-density left atrial endocardial voltage mapping. Patients of the determination cohort were randomized 1:1 to the derivation cohort or the mapping-validation cohort (n = 220 each, Supplemental Fig. 1). These cohorts did not differ with regard to key patient characteristics (Supplemental Table 1). Factors associated with FAM in the derivation cohort in univariate models were age, female sex, arterial hypertension, persistent AF, left atrial (LA) dilatation and creatinine clearance. In a multivariate logistic model, only age, sex and AF-phenotype remained as independent factors associated with FAM (Table 1).

For the derivation cohort, the cumulative left-atrial area demonstrating bipolar voltages < 0.5 mV in relation to the patient’s age is given in Fig. 1. Patients who met the prespecified criterion for diagnosis of FAM (≥ 5 cm² low-voltage areas at < 0.5 mV) had on average 19.2 ± 15.9 cm² of cumulative low-voltage areas, compared to 0.8 ± 1.3 cm² in patients who did not. Extensive low-voltage areas were primarily found in patients aged 60 years or older (8.4 ± 13.6 cm² vs. 1.4 ± 5.4 cm² in patients younger 60 years, p < 0.0001). The criterion for FAM (≥ 5 cm² low-voltage areas at < 0.5 mV) was met in 59 of 146 patients (40.4%) aged 60 years or older, compared to only 4 of 70 (5.4%) in younger patients (p < 0.0001). The percentage of patients in a given age group with FAM increased steadily from 0% (0/38) in patients younger 44 years to 36% (31/87) in patients aged 60–69 years and reached 70% (12/17) in those aged 75 years or older (Fig. 1).

Persistent AF with the longest recorded episode of AF exceeding 7 days was present in 42% (31/74) of younger and 77% (113/146) of patients aged 60 years or older (p < 0.0001). The presence of FAM increased incrementally with age in patients with persistent AF, reaching up to 84% in those aged 75 years or older (Fig. 2). Younger patients (< 60 years) rarely had FAM despite a clinical phenotype of persistent AF (4/31, 12.9%).

This observation was also made regarding the absolute extent of low-voltage areas, which were higher in persistent compared to paroxysmal AF only in patients older than 60 years (10.5 ± 14.8 cm² vs. 1.2 ± 1.7 cm² and 3.0 ± 8.1 cm² vs. 0.3 ± 0.8 cm² for persistent vs. paroxysmal AF in patients older and younger than 60 years, respectively; Supplemental Fig. 2).

Irrespective of age, FAM was rare in patients with paroxysmal AF and occurred in 1.3% (1/76) of cases only. The negative predictive value of a paroxysmal AF-phenotype for the absence of FAM was 100% in younger and 96.9% in patients aged 60 years and older. Irrespective of age, AF-persistency was highly sensitive, yet not specific, for FAM (Supplemental Fig. 2B).

While the majority of study patients were male (160/220, 72.7%), women had FAM more often and more severe than men (26/60 females with FAM [43.3%] vs 37/160 males [23.1%], p < 0.0001; and low-voltage areas < 0.5 mV of 10.9 ± 17.2 cm² in females vs. 4.2 ± 8.7 cm² in males, p < 0.0001, Supplemental Fig. 3). The higher overall prevalence of FAM in females was driven by older women with persistent AF (Figs. 3 and 4; 18.1 ± 19.5 cm² in females with persistent AF aged ≥ 60 years vs. 7.1 ± 10.5 cm² in males with persistent AF aged ≥ 60 years, p < 0.0001). In younger patients, sex did not affect the extent of FAM (Supplemental Fig. 3, p = 0.389).

Based on multivariate logistic models (Table 1), age, sex and AF-phenotype were incorporated in a simple risk stratification scheme called AF-SCORE that ranges from 0 to 4 points (Fig. 5). One point is given when a patient’s age is ≥ 60 years, with additional points in older patients for female sex (1 point) and persistent AF-phenotype (2 points).

Forty-nine percent of patients in the mapping-validation cohort had an AF-SCORE ≤ 2. For “rule-out” of significant LVS, this threshold yielded a sensitivity of 92.0% and a negative-predictive value of 95.3%. A pathological threshold for AF-SCORE of ≥ 3 selected 51.4% of patients in the validation cohort and yielded a specificity of 64.9% and a positive-predictive value of 51.3% to “rule-in” significant LVS. Increasing the pathological threshold for AF-SCORE to 4 selected 15.9% of study patients and yielded a specificity of 93.6% and an increased positive-predictive value of 71.4% for “rule-in”.

Application of AF-SCORE to the mapping-validation cohort is shown in Fig. 6A–C. Receiver-operating-curve-analysis yielded an area-under-the-curve (c-statistic) for presence of FAM of 0.792 (Fig. 6A). An AF-SCORE of ≤ 2 is related to absence of, respective very minor FAM, which increases with an incremental AF-SCORE (Fig. 6B; 1.7 ± 3.9 cm² for an AF-SCORE of ≤ 2, 9.6 ± 13.5 cm² for an AF-SCORE of 3, and 24.8 ± 28.2 cm² for an AF-SCORE of 4, p < 0.0001). Accordingly, the proportion of patients with FAM (> 5 cm² at < 0.5 mV) increased incrementally from AF-SCORE ≤ 2 to AF-SCORE of 4 (Fig. 6C).

AF-SCORE as determined above was next applied on the outcome-validation cohort comprising 518 patients in total who underwent proximal-circumferential pulmonary vein isolation for symptomatic AF. Baseline characteristics of these patients with regard to outcome are given in supplemental table 2 and with regard to AF-SCORE in supplemental table 3. Patients with recurrence were older, predominantly female, had more often persistent AF, were more frequently ablated using radiofrequency than cryo-ablation and had a higher AF-SCORE. During a mean follow-up of 421 ± 208 days, 172 patients experienced AF-recurrence.

Patients with AF-SCORE of 3 or 4 demonstrated significantly more recurrences as opposed to patients with AF-SCORE 2 or lower (unadjusted hazard ratio 1.994 [95% confidence interval 1.44–2.77] for AF-SCORE 3 and HR 2.866 [95% CI 1.89–4.37] for AF-SCORE 4 compared to AF-SCORE ≤ 2; Fig. 6D). These differences persisted when the analysis was adjusted for type of ablation (HR 1.595 [95% CI 1.1–2.34] for AF-SCORE 3 and HR 2.231 [95% CI 1.39–3.59] for AF-SCORE 4 compared to AF-SCORE ≤ 2) or pre-procedural antiarrhythmic medications (HR 2.081 [95% CI 1.46–2.96] for AF-SCORE 3 and HR 3.443 [95% CI 2.22–5.33] for AF-SCORE 4 compared to AF-SCORE ≤ 2. Arrhythmia freedom rates were 74.3% (AF-SCORE ≤ 2), 54.7% (AF-SCORE 3) and 45.5% (AF-SCORE 4).

Fibrotic remodeling of previously healthy atrial myocardium is associated with slow-conduction sites that serve as arrhythmogenic substrate and contribute to the perpetuation of AF even in the absence of pulmonary vein-inputs [3, 5, 9]. Fibrotic regions can be identified either as areas with reduced local voltage by endocardial contact mapping or by delayed Gadolinium-enhanced areas using MRI [10]. No matter the diagnostic modality, advanced fibrotic remodeling was demonstrated to be a main factor determining the long-term success of PVI [5, 6, 8, 11, 12]. In general practice, the use of these tools is, however, limited by prohibitive costs (MRI), invasiveness (contact mapping) and/or required expertise (electrocardiographic markers) [8, 12].

These limitations led to the development of several scoring systems that aim at stratifying patients for the presence of fibrotic atrial remodeling such as the DR-FLASH and APPLE-score [13, 14]. With an area-under-the-curve of 0.711 (APPLE-score) and 0.797 (DR-FLASH), they offer moderate to good discriminative properties for the diagnosis of atrial low-voltage substrate, but require echocardiographic or laboratory parameters such as left atrial diameter, left-ventricular ejection fraction or renal function in addition to routine patients’ clinical characteristics. In contrast, AF-SCORE as described in the current study relies solely on patient characteristics that are part of every AF-patient’s medical history: age, sex, and whether AF is paroxysmal or persistent. This greatly condensed clinical risk stratification model yields diagnostic properties (AUC 0.792) that are on par with the abovementioned scores and others, and AF-SCORE proved equally efficient to predict PVI-outcome.

The current study underlines that age is a major determinant for the development of extra-pulmonary vein arrhythmogenic fibrotic substrate in the left atrium. This finding spreads new light on the interpretation of established clinical trials on AF-ablation, particularly on those that focus on persistent forms of AF: the STAR-AF II-trial compared PVI-only to complex fractionated atrial electrogram (CFAE)-ablation or linear ablation in addition to PVI, and found no benefit of adjunctive left atrial ablations [15]. This trial enrolled younger patients at a mean age of 58–61 years. Similar findings were obtained in the CHASE-AF trial, which compared a stepwise approach including widespread bi-atrial ablations (“full-defragmentation”) to PVI-only in persistent-AF patients [16]. At a mean age of 61 in the “full-defrag”-group, no additional benefit to PVI-only was observed.

Taking the current study’s data into account, a large proportion of patients in STAR-AF II and CHASE-AF likely had no relevant left-atrial arrhythmogenic fibrotic substrate, but rather isolated pulmonary vein-dependent AF – no matter of the clinical phenotype of persistent AF.

As a result, ablation strategies aiming at non-pulmonary vein-sources of AF such as linear ablations and ablation of high-voltage complex-fractionated atrial electrograms were in retrospect unlikely to show an additional benefit in these trials [15].

In patients aged 60 years or older, the current study found extensive non-pulmonary vein fibrotic substrate predominantly in females. However, women of any age are widely underrepresented in key clinical trials on AF-ablation: STAR-AF II and CHASE-AF included approximately 80% of male participants, and 87% of patients randomized to ablation in CASTLE-AF (comparing ablation to conservative medical therapy in patients with atrial fibrillation and heart failure) were males [17]. Even so, women in CASTLE-AF had no discernable benefit of catheter ablation in contrast to men in subgroup analysis.

Female sex seems disadvantageous with regard to fibrotic myocardial remodeling in at least two aspects. First, women, but not men, develop myocardial fibrosis with age also in the absence of cardiovascular risk factors [18]. And second, in specific cardiovascular disease such as valvular cardiomyopathy and despite a more beneficial overall cardiovascular risk profile, women have a higher burden of myocardial fibrosis than men [19].

Guidelines define persistent AF as individual AF-episodes that vary in duration from seven days to less than 12 months. The current study illustrates that this population is not homogeneous, but composed of two very different groups of patients: younger patients, in whom AF is largely PV-dependent, no matter the clinical AF-phenotype or sex, and older patients, particularly women (74% of females ≥ 60 years with persistent AF vs. 55% of males had FAM), who likely have fibrotic atrial substrate resulting in a “non-PV-substrate-dependent” form of persistent AF. Unfortunately, many key clinical trials on AF-ablation in general and ablation of persistent AF in particular were conducted in patient populations consisting of relatively young, male, and presumably PV-dependent AF-patients (Supplemental Fig. 4). Future studies investigating adjunctive left-atrial ablations in addition to PVI should therefore focus on patients with the highest likelihood of non-PV-substrate-dependency: senior patients and women with persistent AF.