In the proposed study the investigators aim to clarify the relative contribution of these different mechanisms to the progression of atrial fibrillation (AF). Also the contribution of the individual genetic background will be investigated. Furthermore, the investigators aim to identify clinical parameters and biomarkers informing on the main mechanisms of AF progression in atrial tissue.

For this purpose, in all included patients atrial biopsies will be taken during cardiac surgery.

An estimated 380 patients will be included

Four patient categories will be included enabling to study patients with different stages of AF progression;

1. Patients without history of atrial fibrillation, without new onset atrial fibrillation detected by continuous rhythm monitoring after surgery (control group),
2. Patients without history of atrial fibrillation, with new onset atrial fibrillation detected by continuous rhythm monitoring,
3. Patients with self-terminating atrial fibrillation at inclusion, and
4. Patients with non-self-terminating atrial fibrillation at inclusion. At baseline in-depth phenotyping and genotyping will be performed. Continuous rhythm monitoring will also be performed in all patients. The combination of extensive phenotyping, genotyping and atrial fibrillation burden follow-up offers the unique opportunity to study the atrial tissue alterations and atrial gene expression changes in different stages of atrial fibrillation progression and to correlate these data to the phenotype of the patients.

Inclusion Criteria:

• Age > 18 years;
• Undergoing first elective open chest cardiac surgery or surgical ablation for atrial fibrillation;
• Able and willing to sign informed consent for the registry;
• Able and willing to undergo implantation of implantable loop recorder (unless the patients has a pacemaker or implantable cardioverter-defibrillator (ICD) with atrial leads)

Exclusion Criteria:

• • Deemed unsuitable or not willing to undergo implantation of implantable loop recorder or attend follow-up visits.

  • Pregnancy.
  • Life expectancy of less than 2.5 years.
  • History of prior cardiac surgery or ablation for atrial fibrillation.

380 patiënts undergoing elective open chest heart surgery.