A newly approved clot-preventing drug improves the chances of survival for patients who have suffered a mild heart attack or the severe chest pain that often precedes a heart attack, two studies found.

The drug, Aggrastat, prevents blood platelets from clumping together to form clots that can cause heart attacks. It was approved by the Food and Drug Administration last week for patients who suffer mild heart attacks or severe chest pain, known as unstable angina.In one study, led by researcher Harvey White at Green Lane Hospital in Auckland, New Zealand, 3,232 patients hospitalized with unstable angina were randomly assigned to get either the blood thinner heparin or Aggrastat. All patients got aspirin, which also works as a blood thinner.

One month later, 2.3 percent of the Aggrastat patients had died, compared with 3.6 percent of the heparin patients - a 39 percent reduction in deaths. About the same number in both groups suffered heart attacks.

The other study, led by Dr. Pierre Theroux of the Montreal Heart Institute in Canada, looked at 1,915 more severely ill patients, some of whom had suffered mild heart attacks. Some got heparin, some got Aggrastat and some got both; all got aspirin.

The study was stopped prematurely for the group getting Aggrastat alone because the death rate a week after treatment was about four times higher than it was among the groups getting heparin or the combination.

Among the 1,570 other patients in the study, however, 7 percent taking heparin had heart attacks within a week, while only 3.9 percent of patients taking both drugs had heart attacks - a 44 percent reduction.