For the past three years most heart attack victims arriving at George Washington University Hospital have immediately received shots of an experimental drug called TPA. If given early enough, the drug dissolves the deadly clots that cause nearly every heart attack.

Until yesterday George Washington was one of only 13 centers allowed to use tissue plasminogen activator. But within weeks, TPA will be available in nearly every hospital in the country. Few new drugs have been introduced with higher expectations.

Studies have shown that TPA dissolves clots twice as fast as the next best drug. According to officials of Genentech Inc., its South San Fransisco manufacturer, the drug will be available within two weeks, and medical experts expect it to usher in a new era in treating heart disease, the nation's No. 1 killer.

"If the world has a wonder drug this is it," said Dr. Alan Wasserman, associate professor of medicine at George Washington. "When it is put into wider use it will dramatically reduce death from heart attacks in America."

TPA is a genetically engineered replica of a protein created by the body in minute amounts. Yesterday, after years of experiments and months of controversy, the Food and Drug Administration licensed it for immediate use in hospitals. It may become available later for use by emergency paramedics.

Despite public-health efforts to eliminate smoking, reduce obesity and lower blood pressure, and despite advances in drug treatment to lower cholesterol, there are still 1.25 million heart attacks each year in the United States.

Almost 500,000 are fatal, according to the American Heart Association.

But wide use of TPA may help change those statistics. If administered soon enough after heart attacks -- the sooner the better but after six hours it serves little purpose -- the drug melts blood clots and prevents the permanent damage that follows most heart attacks.

In most cases a heart attack occurs after a clot develops in one of the arteries leading to the heart, blocking oxygen-rich blood to that part of the heart muscle. The bigger the blockage and the longer it continues, the more the heart muscle is likely to die.

"Now what we need to drive into public awareness is that they rush to a hospital as soon they have symptoms of a heart attack," said Dr. Burton Sobel, a cardiologist at Washington University in St. Louis and a consultant to Genentech. "The faster people get to the hospital the better their chances will be of survival and improvement."

Sobel said that each year thousands of people ignore the early symptoms of heart attacks or do not know what they are.

The principal signs of heart attacks are chest pain or pressure in the chest which is often accompanied by nausea, unusual sweating and shortness of breath. The pain often spreads to one or both arms, neck or jaw.

"When that happens to you, do not wait at home, do not call your doctor and do not take antacid," Wasserman said. "Get to an emergency room right away. Those people who are obese or smoke or have high cholesterol should be particularly aware that they are at higher risk for heart attacks."

"Time is the single most important factor in treating heart attacks," Sobel said. "Each patient should be given the drug as soon as possible. It is foolhardy to wait an hour when that hour is the most crucial for survival."

Emergency room doctors administering TPA, which Genentech will market under the brand name Activase, will first give patients an electrocardiogram to make certain that their diagnoses are accurate.

Now, treatment for heart attacks includes relieving the severe pain of victims, adding oxygen to the blood flow to aid the damaged heart tissue and administering drugs to help control the rhythms of the heart.

TPA will not eliminate the need for those actions but will help obliterate the atttack's central cause.

The drug will not be cheap. Genentech has not announced a price, but officials said yesterday that it would range from $1,000 to $3,000 or more for the one dose that would normally be given a heart attack victim.

"That is expensive, I agree," Sobel said. "But the cost of that one dose has to be compared to what it would cost if the patient spent three years of congestive heart failure with nine hospital admissions."

The negative side effects are few. Because the drug is designed to break up clots, studies show it can in rare cases cause unintended bleeding.

It could be harmful, for example, after surgery when wounds are kept closed by clotting. The drug could also cause trouble for those with severe high blood pressure.