People with proportionately higher quantities of harder calcified plaque best predicted risk of heart attacks, while soft plaque did not, representing a potential paradigm shift, researchers said.

The study may be a "game-changer" for determining who is at risk of a heart attack.

"It's a disease marker, not a risk marker. And we think it's possibly a very important predictor," said Brent Muhlestein from the Intermountain Medical Centre Heart Institute in Utah, US.

Atherosclerosis is caused when plaque -- a sticky deposit known to cause ruptures -- builds up in the arteries, narrowing and hardening them. The notion that soft plaque is more likely to rupture and cause heart attacks than hard calcium deposits in coronary arteries may be wrong, according to the study that will be presented at the American College of Cardiology Scientific Sessions in Washington D.C.

"We previously thought the lipid-laden soft plaque was more likely to rupture and cause heart attacks, but based on our new research, it's more the calcified plaque that appears to be associated with adverse cardiovascular events," Muhlestein added. Furthermore, the study potentially could mean a lot of patients may not require statin therapy, even though they have high cholesterol.

"Maybe we can find and identify them. If there's no atherosclerosis, you're not going to have a heart attack. So the coronary calcium score may allow us to much more effectively select who we treat," Muhlestein said. For the study, the team evaluated the composition of coronary artery plaque identified in the subjects through computerised tomography (CT) coronary angiography.

The patients were followed for an average of nearly seven years to see if their plaque composition had predicted whether they'd have a cardiac event.