The study's lead author said he doesn't know for sure why a helicopter is better than an ambulance or other ground transportation, but two possibilities are the speed of transport and the medical team on the chopper.

"It's likely some combination. It's likely not going to be one specific element," said Dr. Samuel Galvagno Jr., from the University of Maryland School of Medicine in Baltimore.

For their study, published in the Journal of the American Medical Association, Galvagno and his fellow researchers consulted a national database of severely injured patients treated at 900 specialized trauma centers across the United States between 2007 and 2009.

Trauma centers are hospital units that specialize in severe injuries.

Car crashes were the most common cause of injury among transported patients. The database included information on about 62,000 teens and adults who were taken to a trauma center by helicopter and 162,000 who were taken by ambulance.

At first look, ambulance-transported patients seemed to fare best. About 11 percent of them eventually died from their injury, compared to 13 percent of those transported by helicopter.

However, after accounting for factors that might influence the results -- such as patients' age, type of injury and vital signs -- in order to isolate the effect of transportation, the researchers found that being taken to the hospital by helicopter was linked to a higher chance of survival.

Patients' chances of surviving were between one and two percent better if they were transported to trauma centers by helicopter rather than by ambulance.

The researchers didn't have data on how far away patients were from the hospital when they got injured or what treatments they received from paramedics in the ambulance or helicopter.

Galvagno said there could be other unmeasured differences between patients that influenced the findings.

"It may be that (ground crews) don't have the same tools available to them that helicopter crews do," he told Reuters Health.

"There is a lot of work to be done on this area still. This is by no means a final answer."

The new findings echo the results of some previous studies, including one published last year and led by Dr. Mark Gestring, director of the Kessler Trauma Center at the University of Rochester Medical Center in New York.

Gestring told Reuters Health that until the last few years it's been hard to say if there's a clear benefit to transporting patients by helicopter, but now doctors can say those people tend to do better.

"I think the trick is when do you use helicopters and when do you not use helicopters… and that is very regional," he said.

What might work for Rochester or Baltimore, Gestring said, might not work for the rest of the country.

"It certainly doesn't make sense to take a helicopter if you are within 30 minutes by ground," said Galvagno. But he added, "If it was my family member, I'd want them flown if they didn't have timely access to a trauma center."

There is also a cost associated with using helicopters to transport patients.

While cost estimates vary widely, Galvagno and his colleagues estimated that each helicopter transport costs about $5,000 in Maryland, where flights are typically done by the state police.

That means $325,000 would be spent transporting people to the most specialized trauma centers for each life saved, they calculated.

Galvagno said that amount is not insignificant, but is comparable to other lifesaving procedures such as open-heart surgery.

SOURCE: bit.ly/IzZWKK Journal of the American Medical Association, online April 17, 2012.

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