Exercise is beneficial to heart health, however, there appears to be a 'U' shaped relationship where too much exercise may increase the risk of an irregular heart rhythm, called atrial fibrillation. Endurance athletes may have up to a 2.5-fold higher risk of developing atrial fibrillation than non-athletic controls.

The mechanisms behind this increased risk of atrial fibrillation are not the well understood. It is thought to be a mixture of enlarged heart chambers, low resting heart rate, genetic predisposition and possibly scarring in the heart. In this study, the investigators will investigate the electrical activity changes in the heart, using a high-quality electrocardiogram (ECG) and relate this to changes in the heart size measured by ultrasound and MRI. Cardiopulmonary exercise testing will determine fitness (V̇O2 max) and assess the heart's electrical activity during exercise.

This will be a case-control study where athletes with and without atrial fibrillation will be recruited. The investigators hope the results of this study can improve our understanding of atrial fibrillation in athletes by associating atrial fibrillation with structural and electrical differences which may aid the prediction of future atrial fibrillation development and help guide more athlete-specific treatment pathways.

Inclusion Criteria:

• ≥18 years of age at the time of enrolment, male and female.

• History of atrial fibrillation confirmed on ECG - either paroxysmal or persistent.

• Competitive athlete. Defined as:

  1. Competed in endurance sports with a total cumulative moderate to high intensity of > 1500 hours.
  2. Have participated in at least one competitive event in the last 10 years.

Exclusion Criteria:

• Permanent atrial fibrillation.

• History of pre-existing cardiovascular disease :

  1. Atherosclerotic disease: previous myocardial infarction, symptomatic coronary artery disease or Peripheral peripheral arterial disease
  2. Left ventricular systolic dysfunction (EF < 45%)
  3. Heart muscle disease: cardiomyopathies, Infiltrative diseases of the heart
  4. Complex Congenital heart disease
  5. Moderate or severe valvular disease
  6. Uncontrolled hypertension (>180/100mmHg)

Endurance athletes with and without atrial fibrillation from across the United Kingdom. Recruitment through social media, sports clubs, and word of mouth.

Individual participant identifiable data will not be shared. Imaging scans anonymised may be shared with future potential collaborators.