Argentina fans smile before the Group C World Cup 2006 soccer match between the Netherlands and Argentina in Frankfurt June 21, 2006. [Reuters]

The World Cup can bring both joy and despair, and can even end marriages, but now doctors are studying whether the thrill of it all can be literally heart stopping.

Previous research during international soccer tournaments has found an increase in the general incidence of heart attacks, particularly on days when tense matches have fans on the edge of their seats.

In the new FIFA-approved study researchers will receive blood samples from heart attack victims from all over Germany who were watching football at the time of the attack.

They plan to search the samples for traces of stress hormones which can clot the blood.

Doctors will also receive samples from anyone who collapses in a stadium during a World Cup match to check whether their blood has higher levels of hormones than fans watching at home.

"Patients are asked precisely what they were doing at the time of the attack whether they were following football on the radio or television, or even watching the pundits after the game," David Leistner of Munich's Ludwig Maximilians University, told Reuters.

"So far, on the days when Germany has played we have received a lot more blood samples," he added.

The study's first results are due in October.

A study in 1998 found the number of heart attacks increased by 25 per cent on the day and in the two days after England lost to Argentina in a World Cup penalty shoot-out.

Researchers in Switzerland also found heart attacks in the country increased by 60 per cent during the 2002 World Cup in Japan and South Korea even though the Swiss team was not competing.

The findings prompted calls for emergency heart attack equipment to be installed in stadiums during Euro 2004.

"If it really is the case that higher stress levels increase the chance of a heart attack then attending soccer games may have to carry a health warning," said Leistner.

Doctors have advised fans who may already be at greater risk of heart attack due to obesity, high-cholesterol or diabetes, to refrain from drinking excessively during the World Cup.