Filed at 7:37 p.m. ET

A hormone called vasopressin is clearly better at saving the lives of patients whose hearts have stopped than the drug doctors have been using for the past 100 years, according to a study that could transform the treatment of sudden cardiac arrest.

For a century, cardiac arrest victims have been given epinephrine, a synthetic adrenaline that constricts blood vessels and boosts blood pressure. It is often administered when shocking the heart with a defibrillator fails to revive the patient.

Using vasopressin instead improved the chances of reaching a hospital alive by about 40 percent, and tripled the chances of going home from the hospital, in patients with the most deadly type of cardiac arrest, asystole, where all heart activity has stopped. Still, only 5 percent who got vasopressin made it home.

The large European study was reported in Thursday's New England Journal of Medicine.

The finding should soon change international guidelines for treating people in cardiac arrest outside a hospital, predicted lead researcher Dr. Volker Wenzel, associate professor of anesthesiology and critical care medicine at Leopold-Franzens University in Innsbruck, Austria.

In an accompanying editorial, Dr. Kevin M. McIntyre of Harvard Medical School and Brigham and Women's Hospital in Boston called the findings ``an important breakthrough.'' He noted that vasopressin does not appear to have the risks of epinephrine.

``These advances should be translated into a new standard of care immediately,'' he said.

Vasopressin, a synthetic hormone that narrows blood vessels, has been around for decades. A few years ago, the International Liaison Committee on Resuscitation revised its guidelines to add vasopressin as an alternative first drug. That was partly because epinephrine sometimes increases irregular rhythms and decreases oxygen supply to the brain after resuscitation.

Each year, more than 600,000 people die of sudden cardiac arrest in North America and Europe, usually because of a heart attack or a heart rhythm disturbance.

When medical personnel arrive, nearly 70 percent of victims have ventricular fibrillation, in which the heart's main pumping chambers flutter wildly and pump little blood. Less than 20 percent of these patients survive to go home.

About 30 percent of victims, usually patients who have been down longer, have asystole, and nearly all die.

The study involved 1,186 cardiac arrest patients treated in 33 communities in Austria, Germany and Switzerland from 1999 to 2002.

After ambulance crews tried defibrillation, half the patients not resuscitated then got one or more injections of vasopressin and the other half got epinephrine. Patients in asystole did much better with vasopressin, but among other victims, outcomes were similar for the two drugs.

Wenzel, co-chairman of the advanced cardiac life support committee of the European Resuscitation Council, said he expects that group and its counterparts in several nations to issue a joint position paper in several months recommending vasopressin as the first drug for patients in asystole.

Wenzel will try to persuade his colleagues to go one step further and recommend injecting both vasopressin and epinephrine, because that showed the most benefit and saved eight crucial minutes.

``If truly confirmed by another study it would be a major advancement in the treatment of asystole,'' said Dr. Muhamed Saric, director of echocardiography and cardiac stress testing at University of Medicine and Dentistry of New Jersey in Newark.

One possible drawback, Saric noted, was that vasopressin patients might have been more likely to end up in a coma or vegetative state, because their brains were deprived of oxygen for too long before they were resuscitated.