A Boston medical team is using a clot-dissolving drug to protect the heart from crippling damage after an attack.

In what they call "an exciting new approach," doctors at Beth Israel Hospital and Harvard Medical School are using a powerful enzyme to reopen the clogged arteries that caused the attack. This reestablishes the blood flow before a large area of heart muscle dies.

The imaginative new treatment must be more widely tested, however. It apparently can work only when the enzyme is given within a few hours of the attack.

The method could bring a large reduction in the nation's leading cause of death. Its use could spread as rapidly as the coronary bypass surgery and new drugs that have been revolutionizing heart care.

In today's New England Journal of Medicine, Dr. John Markis and colleagues report favorable results in seven of nine men aged 47 to 67 who arrived within three hours of their first chest pains.

The cause of most heart attacks, he said, is a blood clot that blocks one of the coronary arteries that surround the heart and feed it fresh blood and oxygen.

The researchers threaded a catheter through a blood vessel to the heart. Then they injected streptokinase. Within 20 minutes, the blocked arteries usually opened and, in seven of the nine patients, blood flow was restored to heart muscle that ordinarily would soon have died.

One of the patients has died.

The method is not without possible harm. Disturbances in heart rhythm often occur when clots dissolve. But problems seem minor, Markis said, compared with extensive heart-muscle death.

Now he will see whether patients whose clots are dissolved live longer.