Deciding whether heart surgery improves the quality of life -- and assigning a price tag to that improvement -- has become a prime concern of doctors and health planners.

Operations to bypass blockages in blood vessels supplying the heart now are done on 100,000 Americans each year, at an annual cost of almost $2 billion. Measuring the benefits of those operations might well be impossible, some experts concluded yesterday at the close of a conference on bypass surgery sponsored by the National Center for Health Care Technology.

"If anything, there is a consensus that there is no consensus about how you define and measure quality of life, and how you go about . . . using it as an outcome," said Mary C. Gutmann, associate director of behavior medicine at Mount Sinai Medical Center in Milwaukee.

For certain patients with heart disease, coronary bypass surgery has been proven to lengthen life. These patients have severe blockage of all three arteries that supply the heart muscle, or blockage of the so-called left main artery where the three branches originate. For them, there is little argument that the operation is worth what it costs: $15,000 to $20,000 in tests, doctors' fees, and hospital care.

But many of those who have the operation have less severe blockages of blood vessels. They undergo surgery to gain relief from chest pain caused by insufficient blood supply to the heart, which interferes with their work and their lives.

Nationally, the risk of dying from the operation is less than 2 percent, according to Dr. Nicholas T. Kouchoukos, professor of surgery at the University Alabama. About 80 to 90 percent of patients gain some relief from chest pain, and in 60 to 70 percent pain is completely gone, according to Dr. Elliot Rapaport, chief of cardiology at San Francisco General Hospital, chairman of the conference.

But the cost of that relief represents 1 percent of total national health care costs. Since 80 percent of the cost for most patients is borne by health insurance, the major impact of the expenditure falls not on individual patients, but on society at large.

"It's interesting to ask people who had the procedure if they are happy" that they had it, said Harold A. Cohen, executive director of the Maryland Health Services Cost Review Commission. "It also might be interesting to ask them if they would have been happy to have the procedure if they had had to pay for it themselves."

Cohen said the rise in health costs occasioned by bypass surgery results in higher health insurance premiums for employers and higher costs to federal Medicare and Medicaid programs. In turn, these lead to reductions of other kinds of health services or tighter eligibility restrictions for these programs, aimed at containing costs.

Comparing the price of surgery to that of alternative treatments is hampered by the fact that the comparative costs vary from patient to patient, with mild chest pain could be treated with medications at 5 percent of the cost of surgery. But for patients with severe, unstable chest pain, studied at the University of Alabama, the cost of medicine and hospitalization was half that of surgery. And the cost of care for "crossover" patients, who were started on medical therapy but continued to have pain and eventually needed operations, was twice as much as doing surgery at the outset.

Assessing the benefits of surgery on an individual basis also is fraught with pitfalls. Some patients experience relief of pain even though tests may not show proportional improvement in the function of their hearts. Rapaport described a study done in 1960 that showed that "merely incising and suturing the skin over the chest wall in a group of patients who thought they were undergoing a therapeutic cardiac surgical procedure significantly reduced their angina . . . thereby improving their quality of life."

Some of this "placebo effect" may be explained by the tendency of patients to "benefit from the stressful and anxious circumstances that surgery provides," said Carl Cohen, a professor of philosophy at Davidson College in North Carolina. He suggested that rather than regarding the placebo effect as an impediment, doctors should look for alternatives to surgery that might capitalize upon it to relieve symptoms.

The number of annual bypass operations has been increasing since 1968, when doctors at the Cleveland Clinic showed that it could be done effectively with low mortality.Kouchoukos said there is no sign that the numbers -- or the costs -- are about to come down.

On the contrary, he said, the criteria for surgery may expand to include patients who have severe blockages without chest pain and it may eventually become part of the early treatment of heart attack.

"The demand may well go up," he said. "I don't know whether we're at the peak or not."