For laypeople, that usually means doing "hands-only" CPR -- just chest compressions, and no mouth-to-mouth breathing -- until paramedics arrive. Studies have shown that hands-only CPR is just as effective as the traditional way when it comes to helping adult cardiac arrest victims. (The recommendations for children are different.)

Now the new study, reported in the journal Resuscitation, suggests that cardiac arrest victims stand a better chance when more than one bystander comes to their aid. Japanese researchers found that among more than 5,000 adults who went into cardiac arrest outside of a hospital, the odds of surviving were up to two-fold higher when more than one person tried to help.

Six percent of victims were alive one year later when three or more "rescuers" were there, versus 3 percent when only one person came to their aid. When two people responded, the survival rate was 4 percent. The researchers do not know if all of those good Samaritans performed, or even knew, CPR.

Some may have just tried to help in some way, note the researchers, led by Dr. Hideo Inaba of Kanazawa University Graduate School of Medicine.

Still, the findings do show that the more bystanders who jump into action, the better, said Dr. Michael Sayre, an associate professor of emergency medicine at Ohio State University in Columbus and a spokesman for the AHA.

"This study confirms the importance of bystanders responding to cardiac arrest, and the importance of early CPR," Sayre told Reuters Health.

When it came to cardiac arrests suffered at home (where most do occur), Inaba's team found no survival advantage to having multiple rescuers. It's not clear why that was, the researchers say.

Sayre agreed that the reasons are uncertain. But he said that many factors could be at work. For one, people who are "out and about" when cardiac arrest strikes may be relatively healthier.

Cardiac arrests in the elderly and frail are very likely to happen at home.

Cardiac arrest occurs when electrical impulses in the heart suddenly became chaotic, and the heart cannot pump blood and oxygen to the rest of the body. Cardiac arrest is fatal within minutes without treatment.

CPR cannot "restart" the heart, but it can keep the flow of blood and oxygen moving until medical help arrives.

An electrical shock from a device called a defibrillator can reverse cardiac arrest, but time is crucial. It's estimated that for every minute defibrillation is delayed, the odds of survival drop by 10 percent.

So along with performing CPR, bystanders need to immediately call for emergency help.

According to the AHA, more than 380,000 Americans go into cardiac arrest outside of a hospital each year. But most Americans have either not learned CPR at all, or their training has lapsed.

CPR classes are available, but the hands-only approach is easy enough to learn without formal training.

"You really don't have to go to a class," Sayre said.

He noted that the AHA website has video instructions on how to do hands-only CPR here: bit.ly/LhVoQl.

The basic instruction is to give strong, steady chest compressions at a rate of 100 per minute. Experts have pointed out that humming the Bee Gees' 1970s disco song "Stayin' Alive" will help you find the 100-beat-per-minute rhythm.

"Learning CPR is something people often feel that they can put off," Sayre said. "But you never know when you'll be called on to act."

SOURCE: bit.ly/LjRENu Resuscitation, online June 15, 2012.

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