Bernard Bertrand, 74, lies in one of three catheterization laboratories of the Washington Hospital Center, awake but heavily sedated. As he fights the angina pain that has plagued him for months, two doctors carefully thread a specially modified balloon catheter through the femoral artery in his leg and up into his heart.

An hour later, the end of the catheter is squarely positioned inside a severely clogged vessel in Bertrand's heart. As doctors inflate the balloon, a thin stainless-steel tube expands around the balloon, compressing the blockage against the vessel walls, which allows blood to flow freely through the vessel again. Once the catheter is removed, the tube, called a stent, remains behind.

Bertrand, a retired Maryland hospital inspector from Laurel, who was operated on April 19, is one of about 600 heart patients nationwide to receive a stent, an experimental device that some physicians are predicting may be the first significant addition to the balloon angioplasty technique since the procedure came into widespread use in 1980.

Balloon angioplasty works by compressing fatty deposits in the coronary arteries that restrict blood flow to the heart. The technique has saved thousands of people from open-heart surgery, and 227,000 patients underwent the procedure in 1988.

The current interest in stents stems largely from the drawbacks of balloon angioplasty. But while angioplasty is viewed as an important alternative to bypass surgery, it doesn't work for everyone. For reasons still not completely understood, about 30 percent of vessels treated by angioplasty eventually become blocked again by excessive growth of scar tissue, a process called restenosis. And in 3 to 5 percent of cases, the vessel being dilated collapses suddenly, and the patient must undergo emergency bypass surgery.

Stents are "a major step forward" in preventing the re-blockage, said William Winters Jr., clinical professor of medicine at Methodist Hospital in Houston and president of the American College of Cardiology.

After six months, data on 150 of the 350 patients who received stents show that their arteries are 33 percent less likely to become blocked than are the arteries of those treated with balloon angioplasty alone.

Like Bertrand, these patients received a stent developed by Julio C. Palmaz, a radiologist at the University of Texas Health Science Center at San Antonio. The Palmaz stent, 15 millimeters long, 1.5 millimeters in diameter and two-thousandths of an inch thick, is covered with tiny rectangular slots. When it expands, the slots open up into diamond shapes, forming a stiff scaffold that mashes against the inside of the artery.

"It's like taking a piece of chicken wire and pressing it into Silly Putty," said Richard A. Schatz, director of research and education at the Arizona Heart Institute and a co-developer, with Palmaz, of a segmented stent that can navigate sharply twisted arteries.

The late Charles Dotter, a radiologist, first proposed placing a permanent prosthesis inside the heart in the late 1960s. He called it a "splint." Andreas Gruentzig, a Swiss cardiologist, was the first to perform a balloon angioplasty procedure in Zurich in 1977. Another Swiss cardiologist, Ulrich Sigwart, implanted the first stent in a human coronary artery in March 1986.

Originally a term used in dentistry, "stent" has come to mean any medical device that provides support for a tubular structure. It is named after Charles Stent, a 19th century London dentist.

Three varieties of stents have been developed, and two are being used on an experimental basis in humans. Each has a corporate sponsor. The Palmaz stent -- a rigid device -- is licensed to Johnson & Johnson Interventional Systems, a division of the parent health care company.

A flexible, serpentine wire-coil stent designed by cardiologists Cesare Gianturco and Gary Roubin and manufactured by Cook Inc. in Bloomington, Ind., has been used on an experimental basis in about 275 cardiac patients. Follow-up data on this device have not been released.

The third stent, developed by Schneider Inc. in Minneapolis but not yet in human trials, is a self-expanding, braided wire mesh, which opens up inside the artery without the use of a balloon catheter.

Patients who have failed to improve after one or more angioplasties form the bulk of those treated with stents.

Martin B. Leon, a cardiologist and former National Institutes of Health researcher who heads the experimental angioplasty program at the Washington Cardiology Center, said that if stents are approved for wider use by the Food and Drug Administration, they may be appropriate for as many as one third of angioplasty patients.

But stents are not without risks. About 4 percent of the 350 patients who received the Palmaz device suffered heart attacks, while 6 percent required surgery or transfusions to counteract excessive bleeding.

Because metal tends to attract the blood's clotting elements, patients who receive stents must take potentially dangerous anti-coagulation medicines for weeks or months. "It's very difficult to manage patients on those drugs," noted Michael Crawford, chief of cardiology at the University of New Mexico School of Medicine and chairman of the American Heart Association's council on clinical cardiology.

To avoid the risks of internal bleeding associated with giving anticoagulants, several research teams are now experimenting with ways of coating stents with synthetic polymers or genetically engineered cells that will release clot-preventing substances directly at the site of the stent.

At least one stent manufacturer is exploring the idea of a biodegradable stent which, like dissolving sutures, would gradually be reabsorbed into the body over a six-month period.

In addition, even though stents appear to reduce the incidence of blockages, about 20 percent do close up again, not from the fatty deposits characteristic of atherosclerosis, but from excessive growth of fibrous tissue.

One solution might be to coat the stent with a polymer impregnated with anti-cancer agents, which interfere with cells' ability to divide.

It's also possible that stent patients who suffer this excessive growth of tissue may benefit from having the stent re-expanded by balloon angioplasty. About 15 patients who have undergone this procedure appear to be doing well, but follow-up studies have not yet been performed to see if the improvement is permanent, according to Donald Baim, director of invasive cardiology at Beth Israel Hospital in Boston.

Ironically, this same fibrous growth is the basis of the normal healing process that covers the inside of the stent with a layer of cells within the first few weeks of implantation. Once the covering of cells is in place, blood no longer comes in contact with the stent and the risk of clotting is greatly reduced.

Another drawback to the current assortment of stents is that their thin walls and alloy construction make them nearly invisible to X-ray and therefore hard to see during placement inside the body. New stent materials like tantalum, an elemental metal that X-ray can't penetrate, may solve this problem.

Although most research on stents has focused on the coronary arteries, other types of blood vessels have been treated, notably the iliac arteries of the legs and the arteries that join the aorta to the kidneys.

For all their potential, no one is suggesting the devices will have the same revolutionary impact on heart disease that angioplasty did a decade ago. "This is a refinement, an improvement, a further step along the continuum of making that revolution, one that is more perfect and will allow us to treat more patients with fewer risks," said cardiologist Leon.

Greg Pearson is a freelance writer in Washington.