There's been a dramatic increase in the number of Australians having heart attacks without any obvious risk factors.

The heart attack that killed Dean Mercer has shocked many who believe the 47-year-old Ironman champion was a healthy lifestyle pin-up but the number of Australians having heart attacks without any known risk factors appears to be on the rise.

High blood pressure, high cholesterol, diabetes, or cigarette smoking, have long been used as predictors of developing heart disease.

However, a new study published in the European Journal of Preventive Cardiology, found three-out-of-10 patients treated for a heart attack at Sydney's Royal North Shore Hospital between 2006 to 2014, had none of these risk factors.

Heart Research Australia's Professor Gemma Figtree says the findings highlight the need to identify new triggers for heart disease.

"It may be that identification and treatment of standard risk factors like high blood pressure has been successful enough that patients without these treatable conditions are making up a greater proportion of patients having heart attacks," Prof Figtree said.

Of the 695 patients in the study who were treated for heart attacks, 132 had no known risk factors.

Mercer died on Monday after having a heart attack while driving home from a training session on the Gold Coast.

The proportion of heart attack patients who had no risk factors in 2006 was 11 per cent and during the eight year study that more than doubled to an "astounding" 27 per cent, Prof Figtree said.

The cause for this increase in proportion was not clear, Prof Figtree said.

"The next step is to identify new ways of diagnosing heart disease to enable early identification and treatment of these patients to better protect them," she said.

One possible unique trigger could be a drop in air temperature.

A 16-year study involving more than 280,000 heart attack patients in Sweden, presented at the ESC Congress in Barcelona this week, suggests air temperature is an external trigger for heart attack.

"There is a seasonal variation in the occurrence of heart attack, with incidence declining in summer and peaking in winter," said author Dr Moman Mohammad from the Department of Cardiology at Lund University.

It is unclear, however, whether this link is due to colder temperatures or behavioural changes, noted Dr Mohammad.
Cardiologist and Sydney University Associate Professor Tom Buckley says it's plausible and adds to the broad literature on environmental factors that contribute to heart attacks.

"It makes sense from a physiological perspective knowing that blood vessels don't dilate as well in the cold, heart rate increases, the extra load that is on the body," Prof Buckley told AAP.

"What's interesting with this study, more so than previous studies, is a small change in temperature having a medium or long term effect. It wasn't immediate at the time of the temperature change," he noted.

Australian research, published earlier this year, also linked upper respiratory infection to an increased risk of heart attack.

The paper, co-authored by Prof Buckley, found the risk of having a heart attack is 17 times higher in the seven days after a serious respiratory infection.