Apparently healthy endurance athletes could have scar tissue building up in their heart, increasing their risk of dangerous abnormal heart rhythms, a new study says.

About half of a large group of middle-aged male cyclists and triathletes showed signs of scarring in their heart’s left ventricle, the lower chamber responsible for pumping oxygenated blood out to the body, researchers found.

Further, around 1 in 5 of the longtime competitive athletes were experiencing short bursts of rapid heartbeat that could be linked to heart problems, according to findings published in the journal Circulation: Cardiovascular Imaging.

Overall, the presence of heart scarring increased the  athletes’ risk of irregular ventricular rhythm by nearly fivefold, researchers concluded.

“We did not expect the association between scarring and dangerous rhythms to be this strong,” said senior researcher Dr. Peter Swoboda, an associate professor of cardiology at the University of Leeds in the U.K. 

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“Similar findings have been reported in patients with heart muscle disease — however, we did not expect this in healthy, performance athletes,” he said in a news release.

Irregular heart rhythms emanating from the ventricles are believed to be the main reason for sudden cardiac death among athletes, researchers noted. It’s considered one of the most serious types of heart rhythm disorders.

For the study, researchers recruited 106 male cyclists and triathletes older than 50 who’d trained 10 or more hours a week for 15 years or longer.

“In certain highly trained athletes — particularly as they get older — it is possible to identify scarred heart tissue via magnetic resonance imaging,” Swoboda said. “We wanted to investigate whether dangerous heart rhythms in endurance athletes might be caused by heart scarring.”

The main aim, according to Swoboda: "To help endurance athletes continue to participate safely in sports, particularly as they get older.”

The athletes all underwent MRI scans that produced detailed images of their hearts, and had an ECG implanted under the skin of their chest that recorded every heartbeat for two years.

After an average follow-up of two years, researchers found that:

• More than 47% of the athletes had signs of scarring on their heart’s left ventricle.

• About 19% had experienced short bursts of rapid heartbeats.

• Around 3% had sustained, potentially dangerous fast heart rate originating in the ventricles.

• Heart scarring increased an athlete’s risk of rapid heartbeat by 4.7 times.

Participants who developed a potentially dangerous heart rhythm got an urgent call to assess their symptoms and advise them to seek medical care, researchers said.

Swoboda urges any athlete who experiences chest pain, dizziness or heart palpitations during exercise to seek treatment.

“It’s important that everyone involved in sports knows CPR and how to use an automated external defibrillator,” he added.

However, researchers noted that because this was an observational study, a direct cause-and-effect link between endurance athleticism and heart scarring can’t be proved.

Further study is needed to verify the link, to figure out why the heart scarring occurs, and to establish whether it causes abnormal heart rhythms or is part of a larger health issue, researchers said.