Study population:
Cross-country (XC) skiing ranks among the sports with the highest cardiovascular demands and profound cardiac remodeling.20 The Norwegian Birkebeiner XC ski race is one of the world’s most demanding XC ski races, with a course length of 54 km, covering approximately 1000 altitude meters. Previous studies have demonstrated how participation in the Birkebeiner XC ski race is associated with athletic cardiac remodeling and an increased risk of AF.4, 13, 21 To cover the whole range of physical activity in the population, the Birkebeiner Atrial Fibrillation study (2012) comprised two independent cohorts; one cohort consisting of persons who had completed the Birkebeiner XC ski race, and another cohort of individuals that had participated in a population-based health survey (The Oslo Health study).4 With the aim to investigate athletic cardiac remodeling in relation to AF, we invited 520 former participants in the Birkebeiner Atrial Fibrillation study to participate in a cross-sectional clinical study performed between January 2019 and October 2020. All veteran skiers with AF living within a 2-hour perimeter from the study site were invited. Participants with supraventricular tachycardia other than AF, prior open-heart surgery, heart failure (EF <35%), or clinically significant valvular disease were excluded. Veteran skiers without AF, age-matched to the included skiers with AF, were invited until reaching the same number of included skiers with and without a history of AF. Age-matched participants from the non-athletic cohort with and without a history of AF were consecutively invited until reaching the same number of included individuals in all four groups. Because the original study cohort lacked age-matched non-athletic controls, the group of non-athletes with AF was supplemented by male patients visiting the AF outpatient clinic at Diakonhjemmet Hospital between May and September 2020. All patients were screened, and men of the proper age were consecutively invited to participate. Participants with AF during the examination, paced rhythm, or left bundle branch block were excluded from the current analysis (Figure 1). The methods of the study have been described in detail earlier.10
All participants answered a questionnaire and underwent physical examination by the main investigator (ES). Data from the questionnaire (age, sex, history of heart failure, hypertension, stroke, coronary heart disease, and diabetes mellitus) were used to estimate the CHA2DS2-VASc score as a marker of cardiovascular risk.22 Regular endurance exercise was self-reported in the questionnaire on an eight-level ordinal scale. To evaluate the dose-response relationship between years of endurance exercise and LA MD, we condensed this scale into four categories in the same matter used previously.13
A 12-lead electrocardiogram (ECG) was obtained and exported in digital format for offline processing using standard Glasgow algorithm with automatic identification of fiducial points, intervals and amplitudes of the components of the PQRST-complex.23 We affirmed the history of AF by review of medical journals, adjudicating AF according to guidelines.24 In 13 participants (5 athletes and 8 non-athletes), it was challenging to differentiate paroxysmal AF from persistent AF, these were included as having paroxysmal AF.
The study complies with the Declaration of Helsinki and was approved by the Regional Committee for Medical and Health Research Ethics (ref.nr: 2016/565). All participants gave written consent.