Heart pumps can buy time for people with failing hearts in need of a transplant but implanting heart-assist devices in the elderly as a substitute for a heart transplant benefits only some — and at a high financial cost, U.S. researchers said on Tuesday.

They said about half of people 65 or older who got the devices between 2000 and 2006 under Medicare, the U.S. health insurance program for the elderly, were alive after one year.

"We need to have predictive tools in terms of assigning those patients who are likely to do well with this and being honest with the patients and families and ourselves for those patients who are extremely unlikely to do well with this," Dr. Adrian Hernandez of Duke University School of Medicine in Durham, North Carolina, said in a telephone interview.

Ventricular assist devices, or VADs, help weak hearts by circulating oxygen-rich blood throughout the body. They are often used to keep patients alive until they can get a heart transplant.

But many older people with end-stage heart failure are poor candidates for a transplant, and for these patients, heart pumps are used as a permanent therapy.

Given their cost, which Hernandez estimates at $60,000 to $70,000 per device, he said it is important to understand which patients are most likely to benefit.

Hernandez and colleagues analyzed the long-term outcomes of two categories of Medicare beneficiaries between 2000 and 2006: 1,476 patients who got the devices as a primary therapy, and 1,467 patients who got one within a month after having open-heart surgery, such as a coronary bypass or valve replacement surgery.

"Basically, 50 percent of those patients who received a VAD as their primary therapy were alive at one year," said Hernandez, whose study appears in the journal of the American Medical Association. But 55.6 percent of that group was readmitted to hospital six months later.

Of the surgery patients, only 30.8 percent were alive after a year, and 48.3 percent of those were readmitted to the hospital within six months.

The team also looked at total costs of the devices.

"On average, it cost $176,000 including hospital care, any other procedures and the device for one year," he said.

Hernandez said the health of patients prior to having a VAD implanted appears to play a significant role in survival.

"If you are half in the grave when you go into a VAD procedure, you will be fairly high risk," Hernandez said.

Currently, only about 500 Medicare patients get the devices per year but that number could swell as the baby boomer population ages.

"We don't have the tools or the information we need to inform patients and their families with accuracy who will do well and who will not," Hernandez said.

"That is where we need to go next."

An estimated 5.3 million Americans have heart failure, a chronic disease in which the heart gradually loses its ability to pump blood efficiently.

The American Heart Association estimates heart failure will cost $34.8 billion this year in the United States for direct and indirect treatment costs.