World Cup soccer causes joy and despair and even ends 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 had fans on the edge of their seats.

In the new FIFA-approved study researchers will receive blood samples from heart attack victims all over Germany watching football at the time of the attack, allowing them to look 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 and whose blood may show higher levels of hormones then those 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.

First results are due in October.

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

Researchers in Switzerland also found heart attacks in the country increased by 60 percent during the 2002 World Cup in Japan and South Korea -- even though the Swiss team was not even 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 can increase the chance of a heart attack then attending soccer games may have to carry a health warning," said Leistner.

Doctors advise those soccer fans who may be at greater risk of heart attack anyway due to obesity, high-cholesterol or diabetes, to refrain from drinking excessively during the World Cup tournament.