A number of studies have shown a relationship between long time endurance exercise and increased risk of atrial arrhythmias. and there is a growing consensus that this is a causal relationship. As a long time marathoner I don't welcome the news but I cannot no longer ignore it and have to worry about it.
One of Bradford Hill's classic criteria for assessing if a relationship between variables is causal is the dose response principle also referred to as biological gradient. This simply means that a larger dose or exposure should lead to a greater incidence of the effects.Myrstar and co authors have published a study demonstrating a dose response relationship between years of endurance type activity and risk of atrial fibrillation and atrial flutter. In this article Myrstad and co-authors reported an odds ratio of development of atrial fibrillation of 1.16 per ten years of endurance exercise. (confidence interval 1.06--1.29).
In another study from Norway the same lead author reported in regard to a cohort study of 2626 long time cross-country skiers and 2326 people from the general population. He found a prevalence of 12.3% of self reported atrial fibrillation (AF) in the skiers versus 5% in the non-skiers. Of those, 64% continued to engage in regular endurance exercise after the onset of AF.Interestingly some 1/3 of AF patients did not use oral anticoagulants even though they had a CHA2DS-VASc scored of greater than or equal to 2,a score that current conventional medical wisdom believes anti coagulation is needed to decrease stroke risk.
To get some approximation of context consider the ATRIA study (Go,A, et al, JAMA 2001:285:2370) which reported a prevalence of 0.1% in those younger than 60 years,3.8 % age 60 and older, and 9 % age 90 and older More extensive data can be found here in the discussion of a pooled analysis of five randomized clinical trials in which the relative risk of a fib increased 1.4 by decade ( C.I. 1.1 to 1.8). Some older runners would like to take those factoids to suggest that getting old is even more risky than running.
An enlarged left atrium has thought to be the likely link between atrial fibrillation and hypertension.Some long term endurance athletes have been demonstrated to have larger left atria and while it can be considered a "physiological" adaption to increased exercise and periods of increased cardiac output the increased surface per se may predispose to atrial rhythm mischief.Is the enlarged athletes' left atrium less pathological than that of the long term hypertensive patient?
Most of this is a "dog bites man" or duhh story but I continue to be impressed with how easily one can effortlessly find medical articles even in Journals that are not that widely read. The second article mentioned above was first published in the official journal of the German Cardiac Society, one that I do not typically read. The ease with which you can follow current journal articles on a number of topics of your choosing on the free app QxMD is amazing particularly for someone who grew up wrestling the unwieldy Index Medicus tomes and wandering the medical library book stacks to find the volume of interest missing.You can easily learn more in an hour with that app and your tablet than you could in all day at the medical library.
addendum:10/1/15 some editorial flourishes made.
addendum: 12/2/2016 Another article presented some data on dose response relationship.Anderson and co workers from Sweden compered the risk of atrial fib and brady rhythm problems in a large cohort of cross-country skiers. Those who took part in five or more events were compared with those who only completed on one 90km race and found a hazard ratio of 1.3 (1.08--1.58) for those who raced more. Though a increased risk of atrial fibrillation gets most of the attention , this study showed a greater risk of bradyarrhythmias with a HR.of 1.85 , though not statistically significant with a CI of 0.97--3.54.The quantification of the "dose" is crude as well but data are suggestive.
(Anderson, K et al., Risk of arrhythmias in 52 775 long-distance cross country Skiers: a cohort study.
European Heart Journal 2013 34: 3264 )