The US Food and Drug Administration (USFDA) has approved the first fully implantable artificial heart - the AbioCor Implantable Replacement Heart - made by the Danvers-based company Abiomed Inc.

The whirring 2-pound pump made of plastic and titanium, is the result of three decades of research and three years of human trial.

One of the most complex and expensive devices in medical history, the $250,000 device may not help keep patients alive indefinitely, but rather extend the lives of dying people too sick to receive a transplant.

The device would help patients whose hearts are so weak that they can no longer keep blood flowing to vital organs.

The artificial heart will be allowed to be sold for "humanitarian use", an exemption for products that help some patients with no alternatives.

FDA has set a sale limit of around 4,000 units for Abiomed's artificial heart. The company must also closely monitor patients who receive the devices.

The first experimental AbioCor heart was tested in a retired phone-company worker with a life expectancy of less than a month. He lived for five months.

So far, 14 patients have received experimental AbioCor hearts, with mixed results. The first was Robert Tools. Another patient lived 17 months and was healthy enough to check out of the hospital.

Others, however, did not survive the initial operation, which requires doctors to remove the heart's pumping chambers and stitch the device into place.

Several patients suffered strokes after blood clots formed around certain parts of the artificial heart.

FDA is still unsure whether the device truly improved the quality of life of its recipients. In fact, a panel of FDA science advisers a year ago had narrowly voted to deny the company's request to begin selling it.

The current approval follows further discussion and analysis of the trial results. FDA contented that the sick patients should have the right to decide whether to try the artificial heart.