Heart attack survivors face the highest risk of sudden cardiac death in the 30 days following the attack, according to a historical study.

In Wednesday's issue of the Journal of the American Medical Association, researchers in the U.S. from the Mayo Clinic in Rochester, Minn., reported that sudden deaths declined by more than 40 per cent over the past 25 years.

Sudden cardiac death occurs when the heart's electrical system fails. If CPR and defibrillation do not happen quickly, a person dies.

"There are three key findings here that can be immediately applied to heart attack patients today," said Dr. Veronique Roger, a cardiologist at the Mayo Clinic and the study's lead author.

"One is that the first month post-heart attack is the highest risk period for patients to suffer sudden cardiac death — and acute surveillance is warranted.

"A second is that the risk drops rapidly after the first month, but this does not mean the patient is out of danger. Surveillance is still required after the first month because our third finding shows that even though the risk drops after the first month, the onset of symptoms of heart failure at any time after the heart attack markedly increases the risk of [sudden cardiac death.]"

In people with heart failure, the heart becomes too weak or stiff to pump enough blood to the rest of the body.

The researchers said patients and their families should watch for heart failure symptoms that require immediate attention:

• Shortness of breath.
• Persistent coughing or wheezing.
• Inordinate fatigue.
• Confusion.
• Bloating and swelling in the legs.

In the study, Roger and her colleagues analyzed the records of 2,997 men and women in Olmsted County, Minn., who had heart attacks between 1979 and 2005. The average age of the subjects was 67 and 57 per cent of them were men.

The subjects were monitored through their medical records until they died or their follow-up in February 2008.

During the study period, 1,160 patients died, with 282 of these deaths caused by sudden cardiac death.

The cumulative incidence of sudden cardiac death was 1.2 per cent in the first 30 days, which is four times higher than expected, the researchers said.

The findings emphasize the need for physicians to stay in close contact with their heart attack patients to recognize symptoms, Roger said.

If heart failure is suspected, cardiologists say echocardiogram images increase the likelihood of diagnosis.

Depending on the cause of the heart failure, therapies include surgery, a variety of medications, devices and lifestyle changes such as a better diet that includes less salt, according to the Mayo Clinic and the University of California, San Francisco Medical Center.