Marathons and triathalons are fast-growing events, more than 10,000 people regularly running, cycling and swimming long distances. But the super-fit athletes who train hard for such races can develop a life-threatening condition called ventricular arrhythmia (VA), in which the heart beats at an irregular rate and rhythm, according to the Belgian study.

The condition increases the chance of sudden arrhythmic death syndrome, which kills 500 healthy Britons a year.

The conditions, often treated as hereditary, are thought to have various underlying causes. The study, published today in the European Heart Journal, suggests that 82 per cent of athletes with VA have dysfunctional right ventricles, indicating that endurance sports contribute to changes in the chambers of the heart that can lead to heart attacks. Previous studies have found that VA in endurance athletes often originates from damage to the right ventricle, which pumps blood to the lungs.

Researchers at the University Hospital Gasthuisberg set out to study whether abnormal functioning of the right ventricle might be present in endurance athletes with VA and studied 22 endurance athletes aged between 18 and 55 who had been referred to them with VA. The men’s hearts were X-rayed and measured, and the results compared with the hearts of 15 male endurance athletes without VA and of a control group of non-athletes. The endurance athletes had participated in endurance sports sessions of at least two hours, three times a week, for at least five years.

Hein Heidbüchel, a professor of cardiology said that all the athletes had larger right ventricles compared with the control group, as would be expected. But the right ventricles of those with VA held 20 per cent more blood at the end of the heart’s cycle than those of athletes without VA. The difference indicated that in those with VA the ventricle was not pumping blood out of the heart as effectively.

“A right ventricular origin of the arrhythmia was manifest in 12 [55 per cent] of the 22 and probable in six [27 per cent] of them,” he said.

An irregular heartbeat may have been a consequence of the effects of strenuous exercise on the heart, he suggested. “An explanation for our findings is that exercise also acted as a promoter of the right ventricle changes, maybe in synergy with other environmental or genetic factors, and hence led to arrhythmias.

Those who do endurance sports should not worry too much, the professor said: ventricular arrhythmias are rare. “But they need to be vigilant and honest with themselves: if they have a family history of heart complaints, or if they have experienced exercise-induced lightheadedness, palpitations or fainting, they should see their doctor for an evaluation.” The British Triathalon Association said that people interested in taking up endurance sports should build up training gradually with an accredited coach and consult a GP about any previous medical conditions.

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