Giving oxygen to heart attack victims may cause additional damage to the heart, Ambulance Victoria says.

Giving oxygen to heart attack victims may do more harm than good, a study suggests.

Ambulance Victoria treated 440 patients - half of whom were given oxygen, with the other half breathing normal air.

The study found heart attack victims treated with oxygen endured more heart damage than patients not given oxygen.

For decades it has been routine practice to treat heart attacks with oxygen.

"This study is likely to change the practice of ambulance services in Australia and around the world," Ambulance Victoria paramedic and research co-ordinator Ziad Nehme said on Thursday.

Mr Nehme said there were several reasons why oxygen may cause additional damage to the heart.

High levels of oxygen can narrow the coronary arteries and reduce the heart's blood flow.

It can also accelerate the production of harmful reactive oxygen molecules, which increase heart inflammation and stress.

But, he noted, that giving oxygen is still appropriate for patients with low levels of oxygen in their blood.

"These patients do need supplemental oxygen and were excluded from the trial," he said.

The results of the trial were presented at an American Heart Association conference in Chicago on Wednesday.

AAP.