Americans will live longer in the 1980s because of "huge and dramatic" progress against heart disease and strokes, Dr. Robert Levy, the government's chief heart doctor, forecast yesterday.

To the sheer surprise of most physicians, who thought heart attacks and strokes were an inevitable part of aging, this nation made great inroads against both diseases in the 1970s.

The heart disease death rate, which measures the number of persons per 100,000 who die each year, fell by 25 percent and the stroke death rate by 40 percent.

If these death rates had not changed or gone up--and the heart death rate had been going up--"a million more Americans would have died in the '70s, but instead over a million lives were saved," Dr. Robert Levy, head of the National Heart, Lung and Blood Institute, reported.

With continued progress, he said, "there is no reason to think the decline over the next decade will not be as great," with at least 1 million more Americans spared to live longer.

Levy spoke at the National Institutes of Health in a working group discussion on arteriosclerosis, or hardening or clogging of the arteries. Artery disease is the basic cause of heart attacks and strokes.

To an extent, Levy said, progress will take more effort and money, and an "optimum" program would require adding $200 million a year in "real dollars" to the heart institute's current $550 million budget.

That will not happen immediately, he said, noting that the pace of change is so great that "I'm sure we'll progress." Unknown, he said, is what will happen in the late 1980s and early 1990s when current initiatives lose their impact.

There is no scientific way to pinpoint why heart and stroke deaths have declined. Probable reasons, said Levy and Dr. Alfred Fishman of the University of Pennsylvania, arteriosclerosis working group chairman, are:

* A major effort against high blood pressure, a main precursor of strokes and heart attacks, with millions of mildly hypertensive persons placed on drugs or diets to lower their blood pressure.

* A great increase in fundamental knowledge of artery disease, which is helping to lead to "tremendous improvements" in care, including "dramatically effective" new drugs and operations.

* New ways of diagnosing heart and artery disease earlier and better.

* A sharp change in Americans' life styles, with less smoking, consumption of less fatty and less cholesterol-laden food and a national exercise habit. Exercise may or may not lengthen life, but exercisers often become more aware of their bodies and stop smoking and overeating, Fishman said.

Additional research is needed in many areas, the working group said.

In particular, Fishman said, if young doctors and science graduates are to choose research, federal support of research must become more reliable from year to year, instead of dwindling because of inflation.

"One of our biggest problems," he said, is that "we are now seeing a dearth of clinical investigators" because "young doctors don't want to embark on a career" that depends on unreliable federal funding.