Implantable defibrillators, which protect against deadly heart rhythms, may offer little benefit for older people who have other serious health problems at the time of heart failure, U.S. researchers said on Monday.

They said implantable heart defibrillators — which detect abnormal heart rhythms and deliver a life-saving shock to restore a regular beat — provide their greatest benefits for people under age 65 who have few other health complications.

"What we found was older patients with co-morbidities, such as chronic kidney disease, cancer and dementia — those patients are less likely to benefit," said Dr. Soko Setoguchi of Harvard Medical School and Brigham and Women's Hospital in Boston, whose study appears in the Canadian Medical Association Journal.

"Patients with multiple hospitalizations are also unlikely to benefit," Setoguchi said in a telephone interview.

Implantable defibrillators, which cost between $20,000 to $30,000 to implant, are recommended for people with heart failure to prevent sudden cardiac death, which occurs when the heart stops circulating blood.

An estimated 5.3 million Americans suffer from heart failure, a chronic disease in which the heart gradually loses its ability to pump blood efficiently. The American Heart Association estimates the disease cost the United States $34.8 billion last year in direct and indirect treatment costs.

Many clinical trials have found the defibrillators to be life-saving and cost-effective devices, but such studies tended to exclude people with other health complications, Setoguchi said.

Her team's findings were based on records from more than 14,000 people in Canada with heart failure. The average age of the study group was 77 and many also had heart disease, diabetes, chronic lung disease and kidney disease.

Everyone in the group had been hospitalized at least once for heart failure.

Setoguchi and her colleagues found that having a defibrillator prolonged lives by just over six months in half of the patients, and that this benefit shrank with each subsequent hospital admission.

She said people under 65 and older patients without kidney disease, cancer or dementia would be most likely to benefit from implantable defibrillators and that doctors could use the findings, which need to be confirmed, to advise patients.