Nearly half of all heart attacks may not have classic symptoms, such as chest pain, shortness of breath and cold sweats but are likely to be silent, reveals new research.

A silent heart attack occurs when blood flow to the heart muscle is severely reduced or cut off completely.

"The outcome of a silent heart attack is as bad as a heart attack that is recognised while it is happening," said Elsayed Z. Soliman, director of the epidemiological cardiology research centre at Wake Forest Baptist Medical Centre, in the US.

The findings showed that silent heart attacks are made up 45 percent of all heart attacks.

Also, these are more commonly found in men but are more likely to cause death in women.

Further, the risk of dying from heart disease increases by three times in case of a silent heart attack.

The chance of dying from all other causes rises by 34 percent.

Silent heart attacks are usually detected when patients undergo an electrocardiogram (ECG) -- a process to check heart's electrical activity.

"Doctors need to help patients who have had a silent heart attack quit smoking, reduce their weight, control cholesterol and blood pressure and get more exercise," the researchers noted in the paper published in the journal Circulation.

For the study, the team analysed the records of 9,498 middle-age adults.

For over an average of nine years after the start of the study, 317 participants had silent heart attacks while 386 had heart attacks with clinical symptoms.