Regular use of drugs known as "beta-blockers" by patients who have suffered heart attacks could save at least 6,500 lives each year in the United States alone, a large-scale federal study has found.

Officials at the National Heart, Lung and Blood Institute announced new findings yesterday indicating that long-term administration of the drug propranolol, starting shortly after a heart attack has occurred, reduced deaths by 26 percent over a two-year period.

Calling the results "statistically and medically important," Dr. William T. Friedewald, an associate director of the institute, said that this represented an "exciting breakthrough" in the treatment of heart attack survivors.

The drug appeared to be so effective that the government researchers decided to halt their nationwide experiment almost a year before its scheduled completion date.

The government study involved more than 3,800 patients at 31 medical centers across the country. It was a "double-blind" controlled study in which the participants were divided into two groups, with one receiving propranolol and the other a "placebo" or inactive drug. Neither the patients nor their doctors knew which drug they were receiving.

The drugs were begun from 5 to 21 days after a patient was hospitalized for a heart attack, or "myocardial infarction," with the patients followed for an average of 24 months.

The heart institute officials reported yesterday that, regardless of age, sex, race, type of heart attack or its severity, there appeared to be a benefit from administration of the drug, with the greatest effect in the first 12 months.

The institute's acting director, Dr. Peter Frommer, said that deaths during a two-year period were "reduced from 9 1/2 percent to 7 percent, a 26 percent reduction in mortality."

Since 350,000 Americans are discharged from hospitals each year following heart attacks and are at added risk of sudden death from subsequent heart attacks, the institute estimates that a minimum of 6,500 lives might be saved annually.

Friedewald said that about three-fourths of the heart attack survivors might be likely candidates for the therapy. It would not be suitable for those with conditions such as bronchial asthma, for example, because of the drug's possible side effects.

In the study, those receiving the drug experienced a greater degree of tiredness, faintness, depression and gastrointestinal problems than those who did not.

The federal heart officials emphasized that the study provided no information on the possible benefit of propranolol if it is not started soon after the heart attack occurs. Nor did it conclude at what point treatment should be discontinued.

Institute officials also acknowledged that the long-term benefits, in terms of prolonged life span, were not known. Patients "are still likely to die of a second heart attack, but we don't know when," said Frommer. Friedewald noted that the drug's protective effect appears to even out after about 18 months.

Propranolol is already on the market under the trade name Inderal because it is federally approved for the treatment of high blood pressure and angina.

Although the U.S. study is the largest and longest of its kind, foreign investigations have also found two similiar drugs, timolol and metoprolol, to be effective.

They are known as "beta-blockers" because they block parts of the "sympathetic nervous system," said Frommer, which stimulate the heart and may increase its susceptibility to "rhythm disturbances."