The investigators will prospectively evaluate the extent of antral lesion created by the 31 mm balloon size of a novel cryoballoon catheter (POLARx FIT). In 20 patients with paroxysmal atrial fibrillation, the investigators will perform pre- and post-ablation high-definition 3D maps to quantify the extent of lesion created by the 31 mm POLARx FIT cryoballoon.

Rationale: The next generation POLARx cryoablation balloon (CB) catheter will have the option of to deliver cryotherapy with the current balloon size of 28 mm or a new, larger 31 mm size by changing the inner balloon pressure (POLARx FIT, Boston Scientific). Currently, there is no data on the extension of left atrial (LA) lesion formation with the 31 mm balloon size of the POLARx FIT cryoablation balloon catheter. The hypothesis is that a larger CB size results in a wide antral circumferential lesion.

Objective: The primary objective is to evaluate the antral lesion size using ultrahigh-density (UHDx) mapping pre- and post-cryoablation with the 31 mm balloon size CB. Secondary objectives are the proportion of pulmonary veins (PVs) where a 31 mm CB could be positioned to achieve a grade 4 occlusion and the single shot success rate with the 31 mm balloon size CB.

Study design: Single-center, single-arm, prospective study with pre- and post-PVI UHDx mapping.

Study population: Twenty adult patients with paroxysmal AF who are scheduled to undergo pulmonary vein isolation (PVI) with a CB.

Intervention: Patients will undergo pre- and post-ablation UHDx mapping (Orion catheter and Rhythmia 3D-mapping system, Boston Scientific) during the index procedure.

Main study parameters/endpoints: The primary endpoint is the extent of the antral lesion size. Secondary endpoints are the proportion of PVs with grade 4 occlusion with the 31 mm balloon size CB, the single shot success rate of the 31 mm balloon size CB, difference in rate of achieving grade 4 occlusion in comparison to the standard (28 mm) size.

Nature and extent of the burden and risks associated with participation, benefit, and group relatedness: A UHDx mapping will be acquired before and after complete PVI. The procedures will be performed under deep sedation, which is standard practice for CB procedures in our institution. The risk of additional mapping is limited. LA mapping is a standard diagnostic method for patients undergoing PVI with radiofrequency ("standard-of-care"). In the current study, UHDx mapping will now be used in patients undergoing PVI with CB. The most important complication of additional intracardiac mapping is cardiac tamponade, but this risk is low (<0.5%).

Inclusion Criteria:

1. History of symptomatic paroxysmal AF
2. Subjects who are indicated for a PVI according to the 2020 ESC guidelines for the diagnosis and management of AF
3. Subjects who are willing and capable of providing informed consent
4. Subjects who are willing and capable of participating in all testing associated with this clinical investigation
5. Subjects whose age is 18 years or above

Exclusion Criteria:

1. Any known contraindication to an AF ablation or anticoagulation

2. History of previous left atrial ablation or surgical treatment of AF, atrial flutter, or atrial tachycardia

3. AF secondary to electrolyte imbalance, thyroid disease, or any other reversible or non-cardiac cause

4. Significant structural heart disease as evidenced by:

   1. Left ventricular ejection fraction <45% based on most recent transthoracic echocardiogram (TTE) performed <6 months prior to enrollment
   2. LA diameter >55 mm based on most recent TTE performed <6 months prior to enrollment
   3. Previous cardiac surgery
   4. Previous cardiac valvular surgical or percutaneous procedure
   5. Interatrial baffle, closure device, patch, or occluder
   6. Unstable angina or ongoing myocardial ischemia
   7. Moderate or severe valvular heart disease on most recent TTE performed <6 months prior to enrolment
   8. Congenital heart disease
   9. Left atrial thrombus

5. History of blood clotting or bleeding disease

6. Stroke or transient ischemic attack <3 months prior to enrollment

7. Active systemic infection

8. Common ostium PV >24 mm defined by CT-scan

9. Pregnant, lactating, or women of childbearing potential who are, or plan to become, pregnant during the time of the study

10. Subjects who are currently enrolled in another investigational study or registry that would directly interfere with the current study

3 months