Gertrude Hale was desperate. A vigorous, 60-year-old woman with diabetes and high blood pressure, she had begun to have chest pain with the slightest exertion. She could no longer even make her bed.

Doctors at George Washington University Hospital told Hale her pain was caused by a blockage in one of the arteries in her heart. To unblock it would require open-heart surgery -- extremely risky for Hale because of her medical problems.

To save her from the operation Dr. Allan. Ross, the hospital's chief of cardiology, offered to try a new treatment that sounds as unlikely as a magic wand: a balloon. He proposed to feed a slender catheter (tube) into the blocked artery, and dilate the blockage by blowing up a tiny balloon at the catheter's tip.

Coronary angioplasty, as the balloon-dilation technique is called, is a trail-blazing treatment for heart disease. For 5 to 10 percent of the more than 100,000 Americans who annually undergo open-heart surgery to restore the flow of oxygen-rich blood through blocked vessels, it can provide a seemingly miraculous alternative.

Using what looks like a miniature birthday-party balloon, it can open vessels without surgery, often abolish the heart pain caused by lack of oxygen, and save each successful patient from at least two weeks" hospitalization and at least $15,000 in medical bills.

In these patients, whose cornary vessels have a single, relatively recent blockage that the balloon can reach, angioplasty can substitute for surgery, postpone it, and even be repeated as needed.

No one is sure how it works. But the technique has transformed doctors' mental image of the plaques that block arteries -- which, instead of being rocklike deposits, seem to behave more lke spreadable pats of butter.

And although the ballon's long-term effect on the blood vessel is still uncertain, there is evidence that balloon-dilated vessels not only stay open, but even get better with time.

In this area, coronary angioplasty is performed only at George Washington University Hospital and at the National Institutes of Health. In April the U.S. Food and Drug Administration approved marketing of the special catheter used for the technique. Ross and other, while elated by the treatment's success in expert hands, fear its promise may touch off a great number of angioplasties performed indiscriminately by cardiologists not trained in its use and limitations.

Early one morning last month, Hale -- a massive woman with gray hair braided in short cornrows -- lay patiently on the "cath lab's operating table as cardiologists connected a forest of tubes to the catheter protruding from the left side of her groin.

Down the hall, a team of surgeons waited to take her to the operating room if the balloon failed. While the radio played, "If this isn't love, then the whole world is crazy", Ross' assistants maneuvered a large, guiding catheter toward the opening of Hale's blocked right coronary arter. They checked their progress on a television screen that showed a fluroscopic image of her beating heart. Once the large catheter was in place, the skinnier balloon catheter would be threaded through it like a train through a tunnel.

Meanwhile Ross -- whom interns nicknamed "the cowboy" because of his love for the trigger-activated balloon -- freed the delicate balloon-tipped catheter of air bubbles and explained its design.The slender tube contained two lmens or tunnels, one leading to the balloon and one through which dye could be injected and blood pressure measured. Both are ways for the cardiologist to locate the blockage.

The balloon differs from an ordinary one, because it is made from a form of polyvinylchloride that expands only to a certain size, so that it will neither blow out the side of the artery nor worm its way past the obstruction. A pressure gun inflates and deflates it quickly since leaving it inflated in the artery could bring on a heart attack.

The balloon catheter was perfected for use in arteries of the legs an kidneys, then redesigned by a Swiss doctor to serve the coronary vessels. It was tried on dogs, then on human cadavers. "But nothing was the same as the living, beating human heart," Ross said. This and the brain are the last frontier, for obvious reasons."

When the guiding catheter reached the entrance of Hale's right coronary attery, Ross and his colleagues hit a snag. Despite doses of nitrglycerin to dilate the vessel, Hale developed chest pain every time the guiding catheter entered the opening.

"The catheter is virtually as big as her coronary [opening] and every time you get it in position it totally obstructs flow -- which is not good," Ross muttered.

He tried another strategy. He maneuvered the guiding catheter as near as possible to the opening, then -- as he described it later -- rammed the smaller balloon catheter through. The guiding catheter backed off, but the balloon catheter was in the artery.

"Mrs. Hale, your luck is OK so far today," Ross said. "You must have picked the right number."

"I get a little hyper after this, you have to excuse me," Ross said afterward, puffing a cigar in the corridor. He said the procedure gave him a thrill usually only enjoyed by surgeons: seeing a blocked vessel and being able to do something about it.

Yet, like others experimenting with the balloon, Ross tempers enthusiasm with caution. The first living human coronary artery was dilated by a balloon catheter only 2 1/2 years ago, and many experts are still incredulous that the treatment works.

Balloonists from 30 medical centers met last year at the National Institutes of Health and agreed to stay within strict guidelines governing which patients should have the treatment. They also agreed to enter their results in an international patient register.

What they hope to avoid is the confusion that currently plagues the use of open-heart surgery to bypass blocked arteries. Research has so far proven that the surgery increases the lifespan of only the most severely ill patients -- those with blockages in all three vessels or in the main left coronary artery. But surgeons in community hospitals all over the country operate on patients with much more minor disease, hoping to reduce their chest pain and increase their ability to exercise.

As a result, there are no accepted criteria dictating who should have by-pass surgery, and few means of resolving the conflict. "Everybody equipped to do one, does one," Ross said. In the United States, "a study of efficacy about coronary surgery is virtually impossible now, because everybody gets operated on."

Dr. Andreas Gruntzig, who developed the balloon catheter for coronary arteries, concluded from his research that it should be used only on patients with severe blockage of only one vessel, in whom the blockage had recently brought on disabling chest pain. The patient had to be well enough to withstand surgery, since the risks of the balloon -- severe damage to the artery, or worsening of the blockage because of spasm of the vessel -- might make an emergency operation necessary.

Gruntzig began using his technique in 1977. By the following year, he reported success in 32 of his first 50 patients. "I found it incredible," Ross recalled.

Ross' and his colleagues' amazement stemmed partly from the prevalent view that coronary blockages were rigid, brittle deposits that might break apart and cause further damage to the heart if tampered with.

"We've all had our eyes opened," Ross said. "If you can dilate so many of them [arteries], they're not like rocks. . . . They're compressible, resorbable."

Dr. Robert I. Levy, director of the National Heart, Lung and Blood Institute, shared Ross' astonishment.He said he still shudders to look at microscopic pictures of what the balloon does to the vessel wall.

"It looks like an H-bomb explosion occurred," he said. "The whole endothelium [inner layer of the wall] is denuded."

No one really knows how the balloon does its job. There are several theories. One, the butter-pat theory, holds that the plague of deposited materials is simply compressed and spread thinner, so that more blood gets by.

Another states that the balloon cracks the inner layer of the wall, which normally overlies the deposits. This puts blood in direct contact with the plaque, so the parts of it dissolve and are flushed away.Also, like janitors, specialized blood cells may gain access to the plaque and clean it up. Ross thinks both the butter-part and the crack theories are partly right.

A third theory is that the balloon merely stretches the vessel, leaving a permanent bulge. Ross disputes this because after a balloon dilation, vessels appear even wider open than they did right after the procedure.

"That can only be ascribed to the body's own systems doing the right job again," he said.

From the viewpoint of Ross' patients, the balloon certainly seems to help the heart to do a better job. He has had failure -- on a 31-year-old lawyer who artery took such a sharp turn that the balloon could not reach the blockage.

But Ross said the patients whose arteries were successfully dilated all showed significantly greater ability to exercise on treadmill tests done several months later. Several could run as long as a normal person without getting chest pain.

John M. Carroll, a 60-year-old retired Pentagon staff member who had a blockage dilated July 17, passed his treadmill test last week with no chest pain. Carroll, whose artery had been so clogged that he had suffered chest discomfort even after a spaghetti dinner, said he had been testing himself ever since the procedure by exercising and eating different foods.

By the weekend following the dilation, he said, "I was digging ditches, riding horseback and driving a tractor. No more angina. I have not had any angina, so far as I know, since the procedure."

"I would recommend it," Carroll said. "There are prople walking around, understandably fearful of open-heart surgery, and having angina. This . . . would offer alternative to them."

Ross said the reason patients improve so dramatically is that when a coronary vessel is severely blocked, widening the opening even by 20 to 30 percent greatly increases blood flow.

Angioplasty seems to carry about the same risk as by pass surgery. For both, the mortality rate is about 1 percent. Of the 620 angioplasty patients in the register, five have died.

Two died of damage probably caused by the catheter, and one during surgery after the dilation failed to work. Two other died several months later of uncertain causes.

The major risk appears to be heart damage from lack of oxygen, either because the balloon damages the artery or because the vessel wall goes into spasm from irritation.

The treatment is so new that no one knows what its long-term success will be. Ross said patients whose leg arteries were dilated 10 years ago still have good circulation, leading him to hope for lasting results.

He said research indicates about 10 percent of balloon-dilated coronary arteries will narrow again, but that unlike bypass surgery, a patient can have a second balloon dilation without increased risk.

"I have no reason to think that I wouldn't just do it again," he said.

The question now is whether the balloon can also benefit other heart disease victims -- those with narrowing of more than one of the heart's three major arteries. Levy of the National Heart, Lung and Blood Institute cautioned that until further research is done, the balloon should be used on the few paitents whose pain is caused by single blockages -- about 5,000 of the 100,000 who annually have bypass surgery.

But Ros said there are times when he uses the balloon on patients with more than one blockage. "You wind up being backed into it" by the patient, his family or his doctors, he said.

Other cardiologists has been rushing to order ballons since last April, when the Food and Drug Administration approved marketing of the catheter. An FDR spokesman said this means the technique is no longer officially experimental, as long as patients who receive it fit the specified criteria. NIH officials said it means there is far less control over which doctors try the technique and how much training they receive.

That prospect unnerves Ross, who over an 18-month period learned to dilate leg arteries and assisted at several coronary angioplaties before he did one himself.

Cardiologists at Georgetown University Hospital, Howard University Hospital and Fairfax Hospital said they considered the technique experimental and had no plans to begin doing coronary angioplasties.

Ross predicted that rather than replacing bypass surgery, the ballon catheter will be used more and more during surgery to improve results by opening more vessels.

He believes the balloon's greatest contribution -- changing doctors' understanding of the plaques that block vessels -- will eventually make it obsolete, because medicines will be found to dissove blockages so neither a balloon nor a scalpel will be needed.

"The time will come that we all sit around and laugh at what we did today," Ross said.