Federal regulators approved use of an artificial heart yesterday, marking the first time a mechanical device has been made widely available to replace a failing human organ.

In contrast with early hopes of permanently replacing an ailing heart with a man-made pump, the new device substitutes for the bottom half of the heart and can be used only to sustain critically ill patients in the hospital until they can get a heart transplant.

But the CardioWest Total Artificial Heart offers hope to the dozens of Americans who would otherwise die each year without it, and its approval by the Food and Drug Administration was hailed as an important, long-sought moment in the troubled quest to replace the human heart with a machine.

"It's a milestone," said Robert Jarvik, whose Jarvik-7 heart electrified the world when it was implanted in Barney Clark in 1982. "It's a shame it took so many years. But this is very gratifying."

The approval of the device, an updated version of the Jarvik-7, could help invigorate the field, which has seen a flurry of activity in recent years with the development of a new, totally implantable artificial heart and a variety of pumps that can sustain patients dying of heart disease -- the nation's No. 1 killer, Jarvik and others said.

"It's remarkable, and I think very worthwhile and very encouraging for the whole field," said heart surgeon Michael DeBakey of the Baylor College of Medicine in Houston. "This shows that we're moving along and progress is being made."

But critics questioned the value of the $100,000 device, saying it will only add expense to the nation's already bloated health care bill without increasing the number of heart patients who survive.

"From an individual's perspective, it's a wonderful thing -- it can help an individual survive," said medical ethicist George J. Annas of Boston University. "But from the medical system's perspective, it's a useless device. The number of lives you save with heart transplants is limited by the number of hearts that are available. And this does nothing about that."

About 4,000 Americans need heart transplants every year, but only about 2,200 donor hearts are available. Small implantable pumps can keep some patients alive while they await transplants by supplementing their weakened organs. But some patients -- the FDA estimates about 100; SynCardia Systems Inc., which developed the new device, estimates several hundred -- need something more because both sides of their hearts are failing.

"This is a parachute device that can save the lives of patients who have no alternatives," said Marvin J. Slepian, a cardiologist and chief executive officer of Tucson-based SynCardia.

To implant the device, surgeons cut out the diseased bottom part of the heart, which contains the pumping chambers, and sew the six-ounce polyurethane CardioWest in its place. Two flexible tubes protrude through the abdomen to attach the pump to an air compressor that powers the unit and to a large computerized console that monitors and controls it. Patients can walk and even exercise because the console is on wheels.

The device has been implanted into about 250 people worldwide on an experimental basis, including 81 patients who received the heart at five centers in the United States as part of a study the company submitted to the FDA. In the study, 79 percent of the recipients survived long enough to undergo a transplant, with patients being sustained for an average of 79 days.

The heart has been updated since the original Jarvik-7 with new valves and other features designed to minimize the risk of infection and blood clots that plagued the iconic predecessor. But about 72 percent of patients still suffer infections and about 25 percent experience neurological problems, such as a stroke.

Despite the device's limitations, an expert panel in March recommended the heart be approved for patients who have no other options and are likely to die within 30 days unless they get a transplant or the device to keep them alive until a heart becomes available.

"We will be able to save the lives of many critically ill patients," Slepian said.

Although the Food and Drug Administration has approved artificial hips, knees, skin, cochlear implants for the deaf, heart valves and small pumps to assist parts of ailing hearts, the new device is the first to supplant most of the functions of the heart or any other major organ.

"This is the first implantable organ replacement technology that's ever been approved," Slepian said. "That's a milestone in American medicine."

The company is testing smaller control consoles in the hope that eventually some patients may be able to leave the hospital and perhaps even remain on the device indefinitely, Slepian said.

The quest to develop an artificial heart began with heady ambitions of permanently replacing damaged organs. But the work foundered when Clark and other early recipients, tethered to bulky external consoles, suffered serious complications, including devastating strokes from blood clots that formed in the devices.

Efforts shifted largely to smaller devices that could sustain patients while they waited for donor organs or assist weakened hearts in the hope they would begin to recover on their own.

Work also has continued on developing a fully implantable artificial heart that would avoid the shortcomings of the early attempts. Abiomed Inc. of Danvers, Mass., has tested its AbioCor device in 14 patients and is seeking approval for patients on the brink of death who are ineligible for a transplant.