Like many other top athletes, Harald Sjoelshagen, a 66-year-old Norwegian, has an enlarged heart. When doctors noticed it in the early 1960s, he was already training hard, skiing or running almost every day. He didn’t need to worry, they told him, because he showed no sign of underlying disease.

So for the next 45 years, he strapped on his Fischer cross-country skis almost every March to compete in Norway’s Birkebeiner ski marathon, slightly longer than the 31 miles facing contestants on Sunday in this year’s Winter Olympics.

Then in 2004, researchers studying heart health in veteran Birkebeiner skiers told Sjoelshagen he had a heart-rhythm disturbance called atrial fibrillation, which can cause fatigue and strokes.

“I feel it immediately when I have it,” Sjoelshagen said. “You don’t have any power at all. An old woman can pass you.”

Sjoelshagen is just one example of what might amount to a larger health problem for aging sportsmen and sportswomen.

For the past few decades, case reports of endurance athletes with atrial fibrillation have trickled out of physicians’ offices around the globe. Of those studied so far, the Birkebeiner data show that skiers have the highest risk.

“One man’s psychological high from cross-country skiing could be another man’s cardiac venom,” sports cardiologist Dr. Sanjay Sharma, who was not involved in the study, told Reuters Health.

Although he calls himself an “exercise enthusiast,” Sharma said extreme endurance training had become increasingly common and was likely to take a toll on the heart.

“There comes a limit in some individuals where grueling exercise just becomes too much,” said Sharma, of St. George’s Healthcare NHS Trust in London.

A considerable number of the 78 skiers in the Birkebeiner study, published this February in the European Journal of Cardiovascular Prevention & Rehabilitation, seem to have hit that limit: 13 of them had atrial fibrillation.

This is about what would have been expected from other studies showing that as many as 15 percent of Norwegian men older than 75 may suffer from atrial fibrillation. But the skiers, who had been followed by scientists since 1976, developed the condition at an average age of 58 - and the condition is much rarer in younger people.

Even more unexpectedly, 10 of the skiers - 13 percent - had none of the risk factors that usually go hand in hand with atrial fibrillation, such as high blood pressure, heart disease or heavy drinking. That makes it the highest rate of so-called “lone” atrial fibrillation ever reported in endurance athletes.

“We were surprised by the high incidence,” said Dr. Jostein Grimsmo of The Feiring Heart Clinic in Norway, who led the research. “We knew that some cross-country skiers had developed atrial fibrillation, but we didn’t know how many.”

EXERCISE TAKES ITS TOLL

At 5 foot 9 and 165 pounds, Harald Sjoelshagen is fit and healthy apart from his heart condition. He works out regularly. And on weekends, he often dons skis to traverse the 30 snowy miles to his cottage outside Oslo. This year will be the fortieth time he participates in the Birkebeiner race.

But some days his heart gives him trouble, forcing him to slow down - way down.

“When you’re going up the stairs you feel like you don’t have any power at all,” he said. “I feel in good shape but I’m also very concerned.”

During atrial fibrillation, the small chambers of the heart - known as the atria - go into overdrive. They beat so fast they can’t get enough blood into the bigger chambers - the ventricles - which provide blood to the whole body.

This causes fatigue. In rare cases, the blood left sitting in the atria may begin to clot. Such clots may then travel to the brain, where they can block the smaller arteries and cause strokes.

The link between endurance training and atrial fibrillation is still murky. Doctors have long known that intense, prolonged exercise causes the heart to grow, eject more blood and beat slower during rest - a phenomenon aptly called athlete’s heart.

Traditionally, athlete’s heart has been considered harmless, said cardiologist J. Jason West of the University of Texas in Houston. But the Birkebeiner study hints otherwise.

“What these folks are suggesting is that maybe athlete’s heart is not that benign,” said West, who was not involved in the study. “Maybe it does have implications in the long term.”

As the heart stretches to adapt to hard exercise, it forms small scars, which in principle could explain why the cells begin to beat out of sync. In the Birkebeiner study, the researchers found that both a larger heart and a slower heart beat were linked to atrial fibrillation, bolstering the theory.

HEALTHY WITH A BAD RHYTHM

The study was relatively small, and needs to be confirmed. If the findings pass muster, it could mean that a considerable proportion of old endurance athletes should expect to develop atrial fibrillation, said West.

Still, West said less than one percent of people with atrial fibrillation go on to have a stroke. In fact, athletes generally tend to live longer than the average citizen. Sjoelshagen “is probably the healthiest 66-year-old in his region,” he chuckled.

But Sjoelshagen is vexed by his condition. He doesn’t know what triggers the episodes, and doctors are reluctant to give firm advice.

“We don’t have strong enough evidence yet to recommend a specific age,” Grimsmo, who examined Sjoelshagen, told Reuters Health. He hasn’t advised the skier to cut back on exercise, although he said that “perhaps you shouldn’t compete very hard when you reach 55.”

Sharma of St. George’s is not in doubt, though. “Anyone who experiences atrial fibrillation should cut back on the exercise,” he said.

For older athletes in general, he added, “I think what we should be doing is checking their hearts every two years to see if their hearts are getting worse.”

For the next few days, before he hits the snow himself, Sjoelshagen will be glued to his television, awaiting the men’s 31-mile ski marathon on Sunday. And some hearts will definitely be glowing after the race, as the slogan for the 2010 Games goes.

“I will definitely watch that,” he said. “I’m very keen on sports, you know.”

SOURCE: The European Journal of Cardiovascular Prevention and Rehabilitation, February, 2010.