Despite being extremely fit, top cross-country long-distance skiers are at increased risk of developing arrhythmia, or abnormal heart rhythm. This was the conclusion of a Swedish-led study published online this week in the European Heart Journal.

Lead author Kasper Andersen, a cardiologist at Uppsala University Hospital, and colleagues, examined health data on nearly 53,000 elite male skiers who took part in the Swedish 90 km Vasaloppet, one of the world’s most challenging ski races.

They found the risk of developing either atrial fibrillation, where the heart beats irregularly or abnormally fast, or bradyarrhythmia, where the heart beats too slowly, to be highest among those contestants with the highest and most intense “exposure”, that is who completed the most races and had the shortest finishing times.

Andersen says in a statement that their findings show “a dose-response relationship: the more races skiers complete and the faster they go, the greater their risk of subsequently developing arrhythmia“.

For example, skiers who completed five or more races over ten years had a 30% higher risk of developing any arrythmia compared with those who completed only one race in the same period.

And similarly, skiers who had the fastest finishing time of a race also had a 30% higher risk of developing any arrhythmia in subsequent years compared to slower contestants.

For their study, Andersen and colleagues identified 52,755 cross-country skiers who completed Vasaloppet between 1989-1998, and followed them through to 2005. Over this period 1.74% (919 skiers) experienced some form of arrhythmia.

They ranked them according to duration and intensity of physical exercise by using finishing times and number of races completed.

They split them into four groups according to percentages of the winning times for that year: thus the fastest finishing time was 100% and the slowest times were over 240%.

Similarly, they split them into four groups according to number of races they completed, the groups being one race, two races, three to four races and five or more races.

They found, after taking out any effects from other influencers such as age, education and occupation, that not only was there a 30% increased risk of any arrhytmia with the highest number of races completed and the fastest finishing time, but the risk went up 10% from one group to the next.

Thus, from the least exposed group (one race completed) to the next (two races), there was a 10% increase in risk, and so on, until the most exposed group.

There was also a similar pattern going up the groups for the finishing times, but this was not statistically significant, and the numerical increase in risk between the slowest finishing times and the next group was 6%.

The most frequent type of arrhythmia was atrial fibrillation: this affected 681 skiers.

After adjusting for the potential influencers, they found a 29% higher risk of atrial fibrillation among the highest exposure group (five or more races) compared to the lowest (one race).

And for the group with the fastest finishing times, there was a 20% higher risk for atrial fibrillation compared to the slowest, but the figure was not statistically significant.

Bradyarrhythmia affected 119 skiers.

After adjusting for the potential influencers, the risk more than doubled (raised by 110%) in the highest exposure group (five or more races) compared to the lowest (one race).

While faster finishing times tended to go with higher risk (85% higher between the slowest and fastest), the result was not statistically significant.

Andersen says the study is important because although others have researched the effects of endurance exercise on other types of cardiovascular events such as ischemic heart disease and stroke, very few have looked at effect on heart rhythm, and those that have, have tended to study much smaller numbers of less physically active participants, or compared people who do no exercise with very active people.

Their study, says Andersen, looks at the “higher end” of the physical activity scale, and shows how that level of exercise affects risk of arrhythmia.

“The skiers in our study are as a group healthier than the general population. We have previously shown that besides higher leisure time physical activity, the participants in Vasaloppet smoke less, have lower fat and higher fibre consumption and better physical and mental health than the general population,” says Andersen.

They have to be fit and healthy even to take part in the race, he explains.

Andersen is keen to point out that “this study does not show that the exercise causes arrhythmias, only that it is associated with an increased risk”.

He says many studies have shown that exercise protects against heart disease and other conditions.

“Our findings should not deter people from exercising, especially as we did not find any increased incidence of arrhythmias leading to sudden death,” he urges.

He says the results show that compared to the general population, Vasaloppet participants had a lower rate of deaths during the follow up, and that rate of death went down as number of races completed went up.

“This has also been shown in a Dutch long-distance skating event. Therefore, we believe that it is generally safe to prepare for and participate in the Vasaloppet races,” he concludes.

In 2012, a series of articles in The Lancet explored recent developments in the biology, treatment, and diagnosis of arrhythmias.

Written by Catharine Paddock PhD