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PARIS (AFP) - The chances of surviving a heart attack outside a hospital double if a bystander performs chest-compressions but omits the mouth-to-mouth resuscitation widely regarded as part of standard rescue procedure, according to a study released Friday.

Nearly everyone has witnessed the scene dozens of times on television, and perhaps a time or two in real life: someone, mostly likely a man getting on in years, collapses to the pavement clutching his chest.

A take-charge passerby drops to his side, pinches the victim's nose and begins mouth-to-mouth resuscitation, alternating this treatment by pushing repeatedly and vigorously on his chest.

But there is something wrong with this textbook picture of CPR -- shorthand for cardio-pulmonary resuscitation -- according to the study, published in the British journal The Lancet: it does more harm than good.

Not only is there "no evidence for any benefit from the addition of mouth-to-mouth ventilation," writes Ken Nagao, a doctor at the Nihon University hospital in Tokyo who led the study of more than 4000 heart arrest cases in the Kanto area of Japan.

The chances of surviving with a "favorable neurological outcome" are twice as high when would-be rescuers skip the mouth-to-mouth and focus exclusively on trying to revive the heart by rhythmic chest-compressions.

"This finding ... should lead to a prompt interim revision of the guidelines for out-of-hospital cardiac arrest," wrote Gordon Ewy, director of the University of Arizona's Sarver Heart Center, in a commentary.

The purpose of pushing air into a heart attack victim's lungs is to oxygenate the blood, while the massaging the chest aim to restart the heart or re-establish a regular heart-beat.

But this first-ever, large-scale comparison of survival rates of cardiac arrest patients puts the lie to the standard CPR technique, which has been taught to millions of people around the world, wrote Ewy.

"We have found that the survival rate is higher even when the blood has less oxygen content but is moved through the body by continuous chest compressions," he noted.

If results of the Japanese study are used to revise the standard guidelines for helping cardiac arrest victims before medical professionals arrive at the scene, it could, in fact, have another positive effect: more people might be willing to try.

Of the 4,068 adults examined who had heart attacks witnessed by strangers, 439 received cardiac-only resuscitation and 712 were given conventional CPR.

But 2,917 -- more than 70 percent -- were left to fend for themselves.

"Studies have shown that because current CPR guidelines call for mouth-to-mouth ventilations, the majority of people would not perform CPR on a stranger, partly out of fear of contracting diseases," said Ewy.

While the study provides what Ewy called "unequivocal evidence" that chest-compression-only resuscitation improves survival rates, the authors of the study caution that the same does not apply to respiratory failure brought on by near-drowning, drug overdose or choking.

In these cases, they say, an alternation of two breaths and 30 chest compressions is still the appropriate method.