Current guidelines recommend radiofrequency catheter ablation of the cavotricuspid isthmus as treatment for symptomatic/drug-refractory atrial flutter, in spite of the fact that recurrences of flutter and incidence of post-ablation atrial fibrillation are common.

In this study, the investigators assess the hypothesis that the use of cryoballoon Pulmonary Vein Isolation ('novel' treatment) to achieve the electrical disconnection between the pulmonary veins and the heart will lead to higher rates of freedom from abnormal heart rhythms (atrial flutter, atrial fibrillation, or atrial tachycardia) and more improved quality of life than treatment using heat energy (radiofrequency ablation) directed at the cavotricuspid isthmus ('conventional treatment').

Atrial flutter and atrial fibrillation are believed to share the same initiating triggers in the form of pulmonary vein ectopy. Cavo-tricuspid isthmus-dependent atrial flutter almost always results from short bursts of antecedent atrial fibrillation. Radiofrequency (RF) ablation of the cavo-tricuspid isthmus (CTI) is the current accepted first-line treatment for atrial flutter, although post-ablation atrial fibrillation commonly occurs, even in the absence of pre-existing atrial fibrillation.

Cryoballoon Pulmonary Vein Isolation (PVI) has become an established treatment for atrial fibrillation. In patients with both atrial flutter and fibrillation, PVI alone has been shown to control both types of atrial arrhythmia, with no benefit derived from supplemental RF CTI ablation.

This study aims to demonstrate that standalone cryoballoon PVI for typical atrial flutter without RF CTI ablation will lead to a significant difference in preventing recurrence of atrial arrhythmia compared to radiofrequency ablation of the CTI, and should be offered as first-line therapy.

Inclusion Criteria:

• 1. Age 18-80 years
• 2. Patients referred for catheter ablation for typical atrial flutter. The atrial flutter may be either persistent or paroxysmal, with at least one episode having been documented on 12-lead ECG. In the view of the treating physician, the ECG morphology should be compatible with a CTI-dependent circuit, either counterclockwise or clockwise.

Exclusion Criteria:

• 1. Any evidence of previously documented atrial fibrillation
• 2. Previous cavo-tricuspid isthmus ablation or atrial fibrillation ablation
• 3. Atrial flutter documented solely on Ambulatory monitoring
• 4. Atrial flutter morphology on ECG suggestive of a left atrial flutter
• 5. History of atrial flutter with 1:1 atrioventricular conduction and haemodynamic compromise
• 6. Indwelling atrial-septal defect occluder device, or any anatomical reason that precludes left atrial access
• 7. Left atrial diameter (PLAX M-mode) >5.5 cm
• 8. Severe left ventricular dysfunction (LV ejection fraction < 30% on Echocardiography)
• 9. Recent stroke/transient ischaemic attack within 3 months
• 10. Inability or unwillingness to take oral anticoagulant treatment
• 11. Morbid obesity (Body Mass Index ≥40)
• 12. Extreme frailty (A score of 7,8 or worse on the Clinical Frailty Scale)
• 13. Implanted metal prosthetic valve(s) in mitral position
• 14. Indwelling cardiac resynchronisation therapy device, pacemaker or implantable cardioverter defibrillator
• 15. Advanced Renal dysfunction (eGFR<30 ml/min)
• 16. Pregnancy
• 17. Severe valvular heart disease of any kind as assessed by the investigator
• 18. Previous valve replacement surgery or other prosthetic heart valve

There is no plan to share Individual Participant Data with other researchers.

Supporting information that will be shared are only:

Study Protocol, Statistical Analysis Plan (SAP), and Clinical Study Report (CSR)

Data will be available after about 6 months for about 5 years.

Criteria for which Individual Participant Data and any additional supporting information will be shared, including with whom, for what types of analyses, and by what mechanism:

These will be shared with research co-workers, for results, statistical analysis and conclusions. Communication of anonymized data will be done via secure mails and emails.

Blinded outcome assessment