The findings suggest a big need for more knowledge and training, the researchers said.
The study looked at data on more than 14,000 people in 29 U.S. cities. It's one of the largest to show how race, income and other neighbourhood characteristics combine to affect someone's willingness to offer heart-reviving help.
More than 300,000 people suffer a cardiac arrest in their homes or other non-hospital settings every year, and most don't survive. A cardiac arrest is when the heart stops, and it's often caused by a heart attack, but not always. Quick, hard chest compressions can help people survive.
For their study, researchers looked at the makeup of neighbourhoods and also the race of the victims. They found that blacks and Hispanics were 30 per cent less likely to be aided than white people. The odds were the worst if the heart victim was black in a low-income black neighbourhood.
The researchers also found that regardless of a neighbourhood's racial makeup, CPR was less likely to be offered in poor areas. That shows that socio-economic status makes more difference than the neighbourhood's racial makeup, said lead author Dr. Comilla Sasson, of the University of Colorado in Denver.
While few people in poor black neighbourhoods got CPR, those who did faced double the odds of surviving. Overall, only 8 per cent of patients survived until at least hospital discharge, but 12 per cent of those who got bystander CPR did versus just 6 per cent of those who did not.
About 80 per cent of the cardiac arrest victims in the study had collapsed in their own homes. That suggests lack of knowledge about how to do CPR. But also, people tend to panic and freeze when they encounter someone in cardiac arrest, and they need to know that cardiopulmonary resuscitation is easier than many realize, Sasson said.
She said the study results should prompt public outcry — especially since most people who suffer cardiac arrest in non-hospital settings won't survive and those statistics haven't changed in 30 years.
"We can't accept that anymore," she said. "It shouldn't matter where I drop to have someone help me."
The study appears in Thursday's New England Journal of Medicine.
The researchers analyzed data from 2005-2009 from a cardiac arrest registry co-ordinated by the federal Centers for Disease Control and Prevention and Emory University. They also examined U.S. Census data in cities where study patients were stricken — including Atlanta; Boston; Columbus, Ohio; Denver; Houston; Nashville; and San Francisco. Whether similar results would be found in small cities or rural areas isn't known.
Much of the research was done before experts changed CPR advice in a move many think may encourage bystanders to offer help. American Heart Association guidelines issued in 2008 emphasize quick, hard chest compressions rather than mouth-to-mouth resuscitation — removing some of the discomfort factor.
Mary Tappe owes her life to bystanders' willingness to offer help.
In 2004, she collapsed at her office in Iowa. A co-worker called 911; another quickly began CPR and someone else used the office's automated heart defibrillator. An ambulance took Tappe to the hospital, where doctors said her heart had stopped. They never determined why but implanted an internal defibrillator.
Tappe, 51, who now lives in Englewood, Colo., said raising awareness about the importance of CPR is "incredibly important because that's the first step" to helping people survive.
CPR specialist Dr. Dana Edelson, an assistant professor at the University of Chicago Medical Center, said the new research echoes smaller studies showing bystander CPR depends on neighbourhood characteristics, including a Chicago study that found intervention occurred most often in integrated neighbourhoods.
"Nothing that we do has as big an impact on survival as CPR, and it's so cheap," Edelson said, noting that online videos demonstrate how to do CPR.
It involves pushing hard and fast on the victim's chest; research has shown using the beat of the old Bee Gees song "Stayin' Alive" is a good guide.
"It's your ultimate low-budget solution to improving survival," Edelson said.
Dr. David Keseg, an emergency medicine specialist at Ohio State University, has helped teach CPR to eighth-graders in inner-city Columbus, Ohio. That includes giving them free classes and CPR kits.
"We tell them to take them home and show their families and neighbourhoods how to do it," Keseg said.
"It's kind of a drop in the bucket," but it's the kind of targeted approach that is needed to improve the odds of surviving a cardiac arrest, he said.
New England Journal of Medicine: http://www.nejm.org
AP Medical Writer Lindsey Tanner can be reached at http://www.twitter.com/LindseyTanner