Even people without CPR training can save the life of an adult whose heart stops, and the American Heart Association has issued an advisory urging everyone, trained or not, to act immediately in such an emergency.

The procedure is simple: if you see an adult collapse after having a heart attack, immediately call 911 and then push hard and fast in the middle of the chest continuously, until emergency medical personnel arrive or an external defibrillator can be used.

Dr. Michael R. Sayre, the lead author of the recommendations, said in a telephone interview that the ideal would be 100 pushes a minute with enough force to make the chest go down two inches, but, he added, “there is no need to use a metronome and a ruler.” Dr. Sayre is an associate professor of emergency medicine at Ohio State University.

CPR, or cardiopulmonary resuscitation, can more than double the survival rate in cardiac arrest, but only about a third of people who collapse get CPR from bystanders.

The advice does not apply to cardiac arrest in children, or that due to drowning or drug overdose, which usually require rescue breathing. Nor does it apply when the bystander does not see the collapse, since it might not have been cardiac arrest.

But the exceptions, Dr. Sayre said, are only about a quarter of all cardiac arrests. “We want people to act no matter what,” he said, “and one of the reasons it’s important to take a CPR class is because of this other 25 percent.

“But doing chest compressions alone in these situations is better than doing nothing,” he said. “Even for children, the procedure would be the same.”

Chest compression keeps blood flowing, but it does not reoxygenate the blood, as mouth-to-mouth resuscitation does.

The recommendations, being published in the April issue of Circulation, are based on new studies of CPR techniques. They update advisories published in 1997 and 2005.

Numerous studies in animals, and five others in humans, have convinced experts that hands-only CPR from an untrained bystander is usually as effective as CPR with mouth-to-mouth resuscitation performed by a person trained in the technique.

There may be several reasons. Rescuers performing traditional CPR take longer to start than those who use hands only, maybe because it takes more time to prepare, intellectually and emotionally, for the more complex and intimate procedure. Studies have also shown that bystanders performing conventional CPR interrupted chest compressions for longer than the recommended 10 seconds while doing mouth-to-mouth resuscitation, resulting in fewer compressions.

Surveys suggest there may be reluctance to perform mouth-to-mouth resuscitation because of fear of infection, but this is probably a minor barrier. More commonly, CPR-trained bystanders cite panic and fear of causing further harm as reasons for inaction. Such fears are unwarranted.

“If you do nothing, the person will die,” Dr. Sayre said. “And you can’t make them worse than dead.”