In patients with symptomatic atrial fibrillation (AF), current clinical guidelines recommend ablation for rhythm control.

While percutaneous ablation has good results in patients with paroxysmal AF, it is not clear which is the best technique in patients with persistent or long-standing persistent AF. Our group performed the first randomized study of thoracoscopic epicardial ablation vs. endocardial catheter ablation (FAST Trial), which showed better results for epicardial ablation but with a higher rate of complications. Hybrid epicardial and endocardial ablation strategies have recently been described sequentially, showing better results in patients with persistent AF, but 40% of patients are still refractory to ablation.

Our study aims to compare the conventional strategy (catheter ablation) with partial electrical isolation of the left atrium endo-epicardially (pulmonary veins, posterior wall and left atrial appendage) in a single procedure in the group of patients most refractory to ablation. Likewise, using magnetic resonance imaging and echocardiography to analyze the atrial remodeling factors that can predict the success and failure of both therapies.

Methodology: A 1:1 randomized clinical trial in patients with persistent and long-standing persistent AF in two groups: conventional ablation vs. partial endo-epicardial isolation. Prior to ablation, cardioversion will be performed and functionality and atrial size will be verified by echocardiography and delayed-enhancement magnetic resonance imaging (MRI) will detect the degree of fibrosis and atrial remodeling. During the ablation procedure, a continuous rhythm recording device will be implanted. Likewise, at three months the MRI will be repeated to detect post-ablation fibrosis. At six months the echocardiography will be performed to assess atrial function and size.

Main Expected Results: The expected results include: 1. an improvement in the rate of patients with no atrial arrhythmias recurrence in patients undergoing endo-epicardial ablation; 2. a lower AF burden in patients undergoing endo-epicardial ablation who have had recurrence; 3. To know the degree of atrial fibrosis associated with the success/failure of each technique; 4. To know the degree of fibrosis that both techniques produce; 5. To know the degree of ventricular function associated with the success/failure of each technique; 6. To know the degree of potentially lost atrial function; 7. To Compare the safety of both techniques

Inclusion Criteria:

1. Patients between the ages of 18 and 75, both inclusive, at the time of the consent date.
2. Patients with persistent atrial fibrillation refractory to at least one antiarrhythmic drug (Class I or III).
3. Signature of written informed consent.

Exclusion Criteria:

1. Persistent atrial fibrillation of more than 4 years.
2. Moderate to severe valvular heart disease.
3. Severe atrial dilatation (left appendage AP diameter > 55 mm or > 50ml/m2).
4. Patients requiring concomitant cardiac surgery or with previous cardiac surgery.
5. Left ventricular ejection fraction <40%.
6. History of pericarditis.
7. Previous stroke.
8. Presence of active infection or sepsis, esophageal ulcer stricture and/or esophageal varices.
9. Patients with renal dysfunction defined as GFR less than or equal to 40 ml/min/m2.
10. Contraindication for chronic anticoagulation.
11. Patients who have undergone prior left atrial catheter ablation for atrial fibrillation or who are receiving treatment for ventricular arrhythmia.
12. Patients with severe chronic obstructive pulmonary disease considered GOLD III.
13. Patients with active neoproliferative disease or other concomitant disease where the medical team considers that ablation places the patient at unacceptable risk.
14. Pregnant, lactating women.
15. Patients who have previously presented allergic reactions to gadolinium.
16. Patients with claustrophobia that makes it impossible to perform nuclear magnetic resonance.