The purpose of this study is to perform a prospective, randomised study investigating the safety and efficacy of cryoballoon catheter ablation compared with radiofrequency ablation, or both together in the treatment of paroxysmal AF.

The hypotheses for this study are (1) that cryothermal energy is as effective and safe as using radiofrequency energy in the treatment of paroxysmal AF and is associated with a better long term outcome, and (2) that use of both cryothermy and RF in combination is as effective and safe as using either radiofrequency energy or cryothermy alone and is associated with a better long term outcome.

Pulmonary vein isolation is an important treatment for patients with atrial fibrillation (AF), particularly those in whom antiarrhythmic drugs are ineffective or cannot be tolerated. One method involves the use of radiofrequency energy and 3-D mapping system to produce a series of lesions (small burn areas) within the heart. Another method involves passing a balloon (called a cryoballoon) into the heart and freezing the parts of the heart muscle that the veins drain into. Both methods appear to be effective from known data. However, it is not known if use of either method alone or both together is the most effective. We aim to perform a prospective, randomized clinical trial comparing these three strategies.

Substudy 1: Use of cardiac MRI to evaluate ablation lesions. Some patients will also be asked to undergo an MRI scan of the heart before the ablation procedure, and again at three months and one year following the procedure. This will allow us to examine the potential role for MRI in imaging scar tissue formed by the ablation, and help us understand the time course of scar formation and changes to that part of the heart following the ablation.

Substudy 2: Platelet reactivity and activation in AF, and the impact of curative ablation. Blood and urine samples will be taken pre- and 3 months post ablation to see if platelet reactivity and activation are affected by AF compared to established normal ranges, and whether curative ablation impacts on this.

Inclusion Criteria:

• Patients with documented paroxysmal AF on at least 2 occasions and accepted for catheter ablation.

Exclusion Criteria

• Significant valvular disease
• Previous left atrial ablation