CHICAGO (Reuters) - Patients showing heart attack symptoms who received a mixture of glucose, insulin and potassium from paramedics were half as likely to go into cardiac arrest or die than those who did not receive the dose, a study found.

Researchers trained paramedics to administer the treatment after determining with an electrocardiograph-based instrument that a patient was likely having a heart attack. The results of the study were presented at a meeting of the American College of Cardiology in Chicago.

Although the treatment did not stop the heart attack from occurring, patients who received it were 50 percent less likely to go into full cardiac arrest, in which the heart suddenly stops beating, or to die, than those who received a placebo.

The study of 911 patients showed the treatment also reduced the severity of damage to heart tissue from the heart attack. In patients who received the mixture, 2 percent of heart tissue was destroyed, compared with 10 percent in those who received a placebo.

Researchers said the study was the first to test the effectiveness of administering the treatment at the first signs of a threatening heart attack, rather than waiting for a diagnosis to be confirmed at the hospital, which can take hours.

The study was funded by the National Institutes of Health.

We've done a lot of studies of acute cardiac care in emergency departments and hospitals, but more people die of heart attacks outside the hospital than inside the hospital, said Dr. Harry Selker of Tufts Medical Center, who led the study. We wanted to direct our attention to those patients.

Although 23 percent of suspected heart attacks in the study turned out to be false alarms, the treatment did not appear to have any harmful effect on those patients, the researchers said.

The treatment costs about $50.

We wanted to do something that is effective and can be used anywhere, Selker said.

(Editing by Gerald E. McCormick)