CHICAGO (Reuters) - Paramedics in Los Angeles who spent at least 20 minutes giving advanced life support to cardiac arrest victims at the scene were far more successful at restarting hearts compared with older guidelines, US researchers said on Monday.

Firefighters and paramedics in Los Angeles who used the new guidelines succeeded in restoring heart beats 29 percent of the time, a 70 percent improvement over the 17 percent success rates seen in a prior study in Los Angeles.

"We obviously want to improve upon these numbers, but this is a big step forward," said Dr. Marc Eckstein of the University of Southern California and the Los Angeles Fire Department, who presented his findings at an American College of Emergency Physicians meeting in Chicago.

Eckstein said the new protocol calls for emergency workers to deliver 2 minutes of cardiopulmonary resuscitation, or CPR, before giving the victim a shock with a defibrillator. They call for at least 20 minutes of resuscitation on the scene, when chances of restoring a pulse are greatest, before rushing a person to the hospital.

Cardiac arrest occurs when the heart stops circulating blood. Survival rates for cardiac arrest that occurs outside of a hospital are very poor.

Most often, people with cardiac arrest have a type of heart rhythm known as ventricular fibrillation, in which the heart quivers but does not pump blood.

If no shock is delivered in the first four minutes of this deadly rhythm, the heart stops altogether and it becomes much harder to get it restarted. During this phase, old-fashioned chest compressions can help push blood back into the heart, making it more likely to restart.

Eckstein and colleagues looked to see how patients in Los Angeles with cardiac arrest witnessed by a bystander fared in 2007 — after the new guidelines were adopted — compared with a similar group in 2000.

In addition to requiring longer life support on the scene, they also call for continuous, uninterrupted chest compressions and delivering no more than 10 breaths per minute to avoid hyperventilation, which studies have shown can reduce the chances of survival.

"It's very encouraging that a change of protocol could show such a dramatic improvement," Eckstein said in a statement.

He said the next step will be to collect hospital data to see how well people fared who were treated using this new method when they were discharged.