More than half of elite professional cyclists apparently have an enlarged heart chamber, a new study from France indicates.

Although endurance athletes are known to develop large hearts, the finding that such a high percentage of top-class cyclists are affected could have implications for screening elite athletes for heart conditions, researchers say.

Dr. Eric Abergel of the Hopital Europeen Georges Pompidou in Paris and colleagues examined the hearts of 286 cyclists who competed in the Tour de France in 1995 or 1998 or both years. The researchers used echocardiography, which uses ultrasound to create an image of the heart, to examine cyclists’ hearts two days before the race.

As a comparison, 52 non-cyclists also underwent echocardiography.

The dimensions of the left ventricle, the heart’s main pumping chamber, were much greater in cyclists than in non-athletes. A little more than half of cyclists had a substantially enlarged left ventricle, the researchers report in the Journal of the American College of Cardiology.

In about 12 percent of cyclists, the left ventricle was so enlarged that it had an effect on heart function, according to the report.

Typically, the heart wall thickens when a chamber enlarges, but the researchers noticed something unusual when they compared cyclists who competed in 1995 with those who competed three years later. Cyclists in the 1998 race tended to have a larger left ventricle, but the walls of the chamber were thinner than in 1995 cyclists.

Abergel and his colleagues are not sure why the 1995 and 1998 cyclists differed. Although performance did not vary significantly between the years, the variations could reflect excessive training or the use of steroids or other performance-enhancing drugs, the authors suggest.

“This modification could be due not only to intensive physical training but also to pharmacological training - drug abuse,” Abergel told Reuters Health.

It is uncertain whether the changes to the heart can be harmful to cyclists, according to the French researcher.

Whatever the reasons for the enlarged hearts in elite cyclists, the changes could make it more difficult to detect certain heart problems, Abergel and his colleagues suggest.

For example, a genetic disorder called hypertrophic cardiomyopathy that causes the walls of the heart to thicken and stiffen, can cause sudden death.

Although the study does not prove that extreme exercise causes an enlarged heart, “it is hard to ignore the possibility that extreme amounts of exercise may cause harm,” according to Dr. Pamela S. Douglas of the University of Wisconsin in Madison.

However, very few people train at the same level as riders in the Tour de France, she notes in an accompanying editorial.

“It appears that, even with exercise, one of the most powerful strategies for good health and longevity is moderation,” Douglas concludes.

SOURCE: Journal of the American College of Cardiology, July 7, 2004.