Six months ago, it took all of Rex Wood's energy to climb the stairs to his front door and collapse gasping into an easy chair. He had advanced heart failure and was sinking rapidly toward premature death at 46.

But on April 14, doctors at Annandale's Fairfax Hospital implanted a special man-made blood pump in his abdomen to do most of his diseased heart's work. Today, Wood can ride a bicycle, drive a car and think about returning to his job.

If it hadn't been for his HeartMate, a device 4 inches in diameter and 1 1/2 inches thick, Wood would quite likely be dead. "I was out of it," he acknowledged. "As far as I'm concerned, it's keeping me going."

Wood, an auto parts clerk from Prince George's County's southern tip, is living proof that a victory anticipated by medical researchers for more than three decades may now be within their grasp: the permanent replacement of heart function -- or the heart itself -- by man-made machines.

For years, the idea of exchanging the human heart for an artificial substitute was dismissed as too complex, too dangerous and too macabre. Early efforts ended badly and were fraught with controversy, causing the public to condemn the research.

But researchers have quietly continued to pursue the goal, hoping to exploit technological advances that have occurred over the last decade and a greater understanding of how the heart works.

Wood is waiting for a transplant, but in a path-breaking study authorized by the Food and Drug Administration, HeartMates for the first time are now being implanted permanently in a randomly selected group of patients too old or too sick to qualify for a transplant.

Without help, most of these could expect to die within two years. The study seeks "to test the hypothesis that the device will double the survival rate and that the quality of life will be at least as good, if not better," said Eric A. Rose of New York's Columbia-Presbyterian Medical Center, who is overseeing the project.

Until now, HeartMate and other similar devices have been used only as a "bridge to transplant," and the Columbia-Presbyterian study is being closely watched at more than a half-dozen research facilities across the country, where doctors and engineers are developing a new generation of machines designed specifically as permanent implants.

Most, like the HeartMate, are "left ventricular assist devices" (LVADs) -- auxiliary pumps designed to boost the performance of the left ventricle, the chamber that does 80 percent of the heart's work.

About 4,000 LVADs have been implanted in patients worldwide. HeartMate, like most LVADs, is surgically placed below the heart, where it receives blood through a conduit attached to the bottom of the left ventricle, and discharges it to the body through another conduit attached to the aorta.

HeartMate was designed in the 1970s, first implanted in the 1980s and reconfigured as a portable, electric device in the last few years. It requires a battery pack that drives the motor, and a tube that sucks air into the chamber and vents it as the pump drives blood into the aorta. An air tube and an electric wire enter the body in a single cable through a hole in the abdomen. The external paraphernalia weighs "about four pounds," Wood said, and looks like a bandolier hanging from his shoulders.

The sophistication of the new devices contrasts sharply with the bulky accouterments of the air-driven Jarvik-7 artificial heart that was implanted in patients Barney Clark and William Schroeder in the early 1980s.

Robert Jarvik himself has shifted his focus to a new LVAD called the Jarvik 2000, small enough to fit inside a patient's left ventricle.

And with the help of the National Aeronautics and Space Administration, world-famous heart surgeon Michael E. DeBakey has designed a high-speed turbine-shaped LVAD "about the size of a double-A battery," DeBakey said. It has been implanted in 11 transplant candidates in Europe.

While both of these pioneers have bet on LVADs, two research teams, led by Danvers, Mass.-based Abiomed Inc., and Pennsylvania State University, are preparing for final tests on new, portable, "total replacement devices" -- artificial hearts.

The goal of all the technologies is to save the lives of as many as 150,000 "end-stage" heart patients who need transplants each year but will not get them because age, weight or overall ill health automatically disqualifies them.

Although some of the patients may have localized heart damage, most suffer from "heart failure," in which repeated heart attacks or a prolonged heart condition has caused a deterioration in muscle and valve function. Patients with heart failure have enlarged, inefficient hearts that cannot pump adequate blood to vital organs. An end-stage heart patient can expect to die within six months.

Because of longer life expectancy and medical advances in other areas, the number of heart failure cases is rising. At the same time, added the Texas Heart Institute's O.H. Frazier, co-director of the Abiomed artificial heart research in Houston, "we're having fewer homicides and fewer auto accidents," which is "a good thing," except that it is depleting two prime sources of transplant donors. Last year, doctors performed 2,340 heart transplants in the United States but would have done many more if there had been more organs.

LVADs and the artificial heart are the answer, scientists believe, and even though LVADs have so far been used only as a "bridge to transplant," the results are dramatic.

Wood has an inherited form of cardiomyopathy, in which heart muscle cells die off and are replaced by scar-tissue, progressively impairing heart function. His mother died from it at 62, an uncle in his thirties and a brother at 36. Wood's sister in Arizona is also a transplant candidate.

From his first heart attack in 1993 until his HeartMate implantation, Wood watched his body deteriorate. He lost feeling on his left side. He got the shakes and dizzy spells, and was afraid to drive. His kidney function was impaired, causing his lower legs and chest to swell with fluid.

"I would eat one-half or one-third of what I used to eat," he said. Since he left the hospital May 28, however, things have changed. Wood has gained 13 pounds. He is recovering use of his left hand, and his arms and legs are acquiring muscle definition for the first time in years. He rides a stationary bicycle comfortably for 15 minutes at a time, unthinkable a year ago, and "I was able to watch my daughters play softball," although "I'm not ready to play catch with them yet."

A HeartMate was used by one transplant candidate for 835 days, said Victor Poirier, president of the Woburn, Mass.-based Thermo Cardiosystems Inc., which makes the device. Fifty patients have used another LVAD made by Novacor, Thermo Cardiosystem's biggest competitor, for more than a year and one patient who is entering his fifth year, according to Novacor's Linda Strauss.

Most remarkably, researchers have documented a handful of cases in which a diseased heart, given a rest while the LVAD carries the pumping load, has cured itself to such an extent that doctors have been able to remove the device and forget about the transplant.

"Seeing is believing," said the National Heart, Lung and Blood Institute's Alan Berson, who supervises the federal grants that fund most LVAD and artificial heart research. "These are people who would have been dead. There is no question that the device has saved lives."

The next step is to see whether the devices can keep people alive indefinitely. The study of the HeartMate as a permanent device began in April 1998 and will track 140 heart-failure patients for two years at up to 20 different hospitals nationwide. Half the patients will have implanted HeartMates, and the other half will receive medication. Participating hospitals have enrolled 43 patients to date, and the entire study could take as long as five years.

Success in the study would go a long way toward vindicating the small group of specialists who have struggled for decades to prove that man can make a mechanical substitute for the most sacred and storied part of the human body.

It was DeBakey, still vigorous today at 90, who urged the National Institutes of Health to establish its artificial heart program in 1964. "The idea was that if we can plan on sending someone to the moon, why can't we build an artificial heart," said Berson. "It was sort of naive."

In 1969, Houston heart surgeon Denton A. Cooley implanted the first artificial heart as a bridge to transplant, but the patient died after three days. The controversy surrounding the operation led to a falling-out between Cooley and DeBakey that continues to the present.

Also controversial were the two Jarvik-7 implants in 1982 and 1984. Clark lived for nearly four months, but Schroeder suffered four strokes and ran a fever for most of the 620 days he was alive. His ordeal soured public opinion on the artificial heart for years.

"I wrote an article [in the early 1980s] indicating that I was not going to do any more research on the artificial heart," DeBakey said. "I still feel that way now. Once we have reached the stage where we can install LVADs safely, we don't need to replace the heart. That is a far more complicated problem."

The biggest challenges to scientists working on both artificial hearts and LVADs are infection and clotting, the same difficulties that dogged the Clark and Schroeder implants.

The switch to electric pumps from earlier pneumatic designs has helped control infection, because the upcoming generation of devices will not need external air hoses entering the body.

Abiomed's AbioCor and the new LVADs have also taken the final step of transmitting electricity through the patient's skin, sending the current from the external battery to an internal wire in a process much like that of a common transformer.

The new technologies also pay close attention to the materials they use and their engineering techniques, so that blood cells will not be scarred, trapped or allowed to clot as they move out of the man-made device and into the bloodstream.

The Abiomed AbioCor is a yoyo-shaped, clear plastic disc with a metal housing holding the halves together. It has the diameter of a grapefruit and weighs just under 2 pounds. Each side functions as a heart ventricle, while the metal band contains an electrically driven centrifugal pump that pushes hydraulic fluid against plastic diaphragms facing each chamber. This alternating movement propels blood out of one chamber as it draws it into the other.

The Texas Heart Institute's Frazier said that Abiomed has not had a case of clotting in eight years of implanting AbioCor in calves. Berson said formal bench testing of the Abiomed and Penn State "final devices," in calves and in the laboratory, may begin as early as August.

The two teams must demonstrate that their devices can beat 100,000 times a day without breaking down and that they can support life without complications. Berson said the testing "has to go on for two years" and will serve as "the prelude for clinical studies," but Abiomed research chief Robert Kung and the Texas Heart Institute expect to implant an artificial heart next year.

"To me the first use should be as a rescue device," Frazier said. "But as a clinician, I'm ready. I have to talk to the families; they don't."

A Helping Hand

The HeartMate, implanted in the abdomen and attached between the natural heart and the aorta, takes over the pumping function of the heart's left ventricle. Researchers say the device more than doubles the probability that heart-transplant candidates will survive long enough to receive a donor heart.

A healthy heart

1. "Old" blood -- to be pumped to lungs to receive oxygen -- enters heart through right atrium.

2. "Fresh," oxygen-rich blood from lungs enters via left atrium.

3. Blood flows into lower ventricles.

4. Heart muscle contracts and outlet valves open, pumping blood throughout the body.

SOURCE: Thermo Cardiosystems

CAPTION: Rex Wood, with daughter Lauren, left, can now ride a bike with the aid of an implanted heart pump. He has advanced heart failure and is a candidate for a transplant.