Endurance Training and A-fib: Relationship Differs by Sex

The previously seen relationship between endurance training and risk of atrial fibrillation appears to differ between men and women, at least among highly trained cross-country skiers, a study out of Sweden shows.

Women participating in the Vasaloppet—an annual long-distance cross-country ski race held in the Scandinavian country—were less likely to develop A-fib compared with nonskiers (HR 0.63; 95% CI 0.55-0.73), regardless of the number of races completed and level of performance, researchers led by Niclas Svedberg, MD (Falun Hospital, Sweden), report.

As seen in a prior analysis by some of the same investigators, however, male skiers were more likely to develop the arrhythmia compared with their nonskiing counterparts after adjustment for diseases related to A-fib (HR 1.11; 95% CI 1.05-1.16). Men who had completed the greatest number of races and had performed the best had the highest risk.

“There are only a few previous studies on female athletes and risk of atrial fibrillation,” Svedberg told TCTMD in an email. “This large cohort study shows that women probably do not respond in the same way as men to training, regarding of risk of atrial fibrillation.”

Another unique aspect of the study was the look at outcomes—specifically stroke and mortality—in these master athletes. Skiers of both sexes had a lower risk of stroke compared with nonskiers (combined HR 0.64; 95% CI 0.60-0.67). And although skiers with A-fib were more likely to have a stroke compared with both skiers and nonskiers who had a normal heart rhythm, among those with a diagnosis of A-fib, skiers had lower risks of stroke (HR 0.73; 95% CI 0.50-0.91) and mortality (HR 0.57; 95% CI 0.49-0.65).

“To our knowledge, no studies have examined both risk of atrial fibrillation and stroke in a large cohort of athletes, Svedberg said. “We find it reassuring that although the group of best-performing skiers had the highest risk of atrial fibrillation compared to other skiers, as a group they had the same low risk of stroke as other skiers when compared to the general population.”

The findings were published online August 26, 2019, ahead of print in Circulation.

For the study, Svedberg et al examined data on 208,654 athletes (mean age 37; 40% women) participating in the Vasaloppet between 1989 and 2011, who were matched to 527,448 nonskiers (mean age 42 years; 33% women) from the general population.

During a median follow-up of 8.8 years, A-fib was diagnosed in 1.2% of skiers and 1.7% of nonskiers. The diagnosis came 1.8 year earlier in skiers, who had a lower CHA₂DS₂-VASc score. Despite that, skiers were more likely to use oral anticoagulation (58.0% vs 52.3%), which could at least partly explain their lower stroke risk (0.3% vs 1.4%), the authors say.

Asked why women do not have a higher risk of A-fib associated with endurance training, Svedberg said he could only speculate. “One reason could be that the morphologic changes in the heart to exercise differs among sexes. Earlier studies of athletes have shown less fibrosis and absolute increase in atrial size in female athletes,” he said. “Further, it is possible that sex hormones in some way play a part, maybe through differences in autonomic response to exercise.”

Regarding the improved outcomes after an A-fib diagnosis in skiers versus nonskiers, Svedberg pointed out that “high levels of physical activity have loads of beneficial effects on general health. For example, high physical activity is associated with lower levels of inflammation and lower risks of hypertension, diabetes, and cardiovascular disease. It is likely that these positive effects remain in people with atrial fibrillation after diagnosis and explain the lower rate of stroke and mortality observed.”

Weighing the Risks

Jonathan Kim, MD (Emory University, Atlanta, GA), said the look at outcomes in this group of master athletes is important because “within the context of those who are engaging in high levels of physical activity, we’re seeing the benefits of exercise, even in those who develop cardiac morbidity.”

He acknowledged that there’s an increased risk of A-fib in men performing at the highest level, which has been seen in other studies, but said that shouldn’t overshadow the generally positive associations seen with exercise in this and other studies.

Kim said there are remaining questions around the mechanisms linking endurance training and A-fib and around the best management strategies for master athletes with the arrhythmia.

To that last point, Kim said physicians should use the same risk criteria for stroke to determine who should be taking anticoagulation as are used in the general population. “Male master skiers who had A-fib were still having more strokes compared to anybody who didn’t have A-fib,” he said. “That’s an important finding. It just shows that we still need to manage them appropriately.”

Svedberg agreed: “The study indicates that athletes with atrial fibrillation should be treated with anticoagulation in the same way as nonathletes, as we could see that skiers with atrial fibrillation, although they had a lower risk of stroke than nonskiers with atrial fibrillation, have a higher risk of stroke compared to nonskiers without atrial fibrillation.”

Importantly, both Kim and Svedberg said, the findings should not dissuade people from participating in an activity like long-distance cross-country skiing if they’re so inclined.

Kim said he discusses the risks of A-fib and a host of other factors with athletes he treats to help make a decision about participation in endurance training “through shared decision-making and going through patient preferences, patient values, what their underlying risk factors are. We talk about it. It’s important for them to be aware of these data. There’s no doubt about that. But there are other important variables as to why individuals engage in extreme amounts of ultra-endurance exercise. So to push people away is not my practice.”

Svedberg also highlighted that discussion between athletes and their physicians. “We do want to underline that for patients with an underlying cardiac disease an individualized assessment of training amount and intensity must be done.”

Overall, though, “from a public health perspective, this study does not motivate a change in the current guidelines for physical activity,” he and his co-authors say.

Of note, the Vasaloppet is the oldest cross-country ski race in the world, first held in 1922. Between 1924 and 1980, race organizers banned women from competing on the grounds that the event was too taxing for women’s health.