The study involves endocardial cryoablation of the left and/or right atrium of the heart using the CoolCryo system as an adjunct to mitral valve surgery, with the aim of eliminating atrial fibrillation (AF), which is a common sequelae of mitral valve defects and is associated with an increased risk of ischaemic stroke, heart failure and death.

An open-label, multi-centre, non-randomised study involving 16 patients, male and female, to evaluate the safety and efficacy of the CoolCryo system.

The procedure with the medical device will be performed on female and males aged ≥18 years, qualified for mitral heart valve surgery with a documented comorbidity of paroxysmal or persistent atrial fibrillation (AF) since at least 3 months prior to surgery excluding acute conditions.

The study involves endocardial cryoablation of the left and/or right atrium of the heart using the CoolCryo system as an adjunct to mitral valve surgery, with the aim of eliminating atrial fibrillation (AF), which is a common sequelae of mitral valve defects and is associated with an increased risk of ischaemic stroke, heart failure and death.

Data to assess the safety and efficacy of the CoolCryo system are collected at the time of the procedure and during the 6-month follow-up of the patient. In the clinical trial of the CoolCryo system, patient management at the Centre is conducted in accordance with the guidelines and standards of Polish and European medical societies, as well as taking into account the best practices developed at the Centre.

Due to the nature of the study population, 14 patients were taken as the minimum group size for which the CoolCryo device will be used. This size appears sufficient to analyse the safety and efficacy of the medical device used and was chosen for practical reasons and not on the basis of a formal sample size estimate. Given the 6-month follow-up period, the proportion of patients who will be lost to follow-up during the study was assumed to be 10 %. Therefore, 16 patients will be treated.

Inclusion Criteria:

1. Written consent from the patient to participate in the study.
2. age ≥18 years.
3. patient qualified for mitral valve surgery (mitral valve plication or mitral valve replacement "with" or "without" left atrial appendage closure).
4. a documented diagnosis of co-occurrence of paroxysmal or persistent atrial fibrillation (AF) since at least 3 months prior to surgery with exclusion of acute conditions.

Exclusion Criteria:

1. Lack of written consent from the patient to participate in the study.
2. Failure to meet the inclusion criteria.
3. Contraindications to the cryoablation procedure.
4. Patient breastfeeding, pregnant or planning pregnancy within 6 months of study treatment.
5. co-morbidities and/or conditions that may cause atrial fibrillation (uncompensated hyperthyroidism or hypothyroidism, acute infection, pheochromocytoma, significant dyselectrolitemia).
6. significant calcification of the left atrium.
7. Condition after previous cardiac surgery.
8. significant left atrial enlargement (size greater than 10 cm on echocardiography in one of the projections performed). projection).
9. the patient is currently a participant in another clinical trial.
10. The patient is on active biological therapy.
11. emergency surgery.
12. Life expectancy <12 months.
13. Any circumstances that, in the opinion of the Investigator, may prevent the patient from participating in the study, limit the ability to perform the procedures outlined in the study protocol or place an unreasonable risk on the patient.

Sixteen patients, male and female, aged ≥18 years, qualified for mitral heart valve surgery with documented (in at least one ECG recording) co-occurrence of paroxysmal or persistent atrial fibrillation (AF) will undergo the procedure with the medical device.

There is also a prediction of 10 per cent of patients (screen failure) who do not meet the inclusion and/or exclusion criteria after signing informed consent.

Additionally, there may be perioperative situations where the cryoablation procedure is not performed due to potential acute aortic dissection or undiagnosed conditions necessitating a change in surgical tactics (e.g. pericarditis). It is estimated that the above situation may affect 1-2 study participants who will be given drop-out status.