• Age ≥18 years

• Signed written informed consent

• Clinical evidence of symptomatic heart failure (NYHA Class II-III)

• Paroxysmal or persistent atrial fibrillation (less than 24 months after first diagnosis, documented at least on one 12-lead ECG)

• Left ventricular ejection fraction (LVEF) 40-49%

• OR

• LVEF ≥ 50% with at least one of the following HFpEF echocardiography findings (any local measurement made during the screening epoch):

• A. LA enlargement defined by at least 1 of the following: LA width (diameter) ≥3.8 cm or LA length ≥5.0 cm or LA area ≥20 cm2 or LA volume ≥55 ml or LA volume index ≥29 ml/m2

• B. Left ventricular hypertrophy (septal thickness or posterior wall thickness ≥1.1 cm or relative wall thickness \>0.42)

• Patients with at least 1 of the following:

⁃ A. HF hospitalization (defined as HF listed as the major reason for hospitalization) within 6 months prior to screening visit and NT-proBNP \>200 pg/ml for patients in sinus rhythm (SR) or \>600 pg/ml for patients in AF at the time of blood sampling

⁃ B. NT-proBNP \>300 pg/ml for patients in SR or \>900 pg/ml for patients in AF on screening ECG