A medical emergency helicopter can swoop down to pick up a trauma victim and often deliver him or her to a hospital much faster than a road-bound ambulance can. So it might seem obvious that these speedy aircraft, dispatched from major medical centers with top-of-the-line personnel and equipment, are more likely to save a life than an ambulance.

But research has not clearly supported that conclusion. A new study released today brings some clarity: helicopters save a slightly higher percentage of lives than ambulances when the severity of the injury is taken into account. The findings were published online in JAMA, The Journal of the American Medical Association.

The study examined 223,457 patients older than 15 who needed to be treated at level I or II trauma centers (the two highest levels) and who were transported either by helicopter or by a ground vehicle, such as an ambulance. Overall, a higher percentage of patients who had a medevac flight died. However, those patients tended to be in worse condition (with a higher so-called Injury Severity Score). After adjusting for this difference, the researchers found that patients taken by helicopter to level I trauma centers were associated with a 15 percent increased chance of survival. The gain for trips to level II trauma centers was 16 percent.

Traumatic injury is the leading cause of death in the U.S. for people under the age of 45, killing 169,000 individuals annually. And in terms of years of potential life lost, trauma ranks the highest—more than all cancers combined or heart disease. Nevertheless, research into trauma treatment has lagged far behind cancer and cardiovascular disease. Finding what is most likely to save trauma victims is of major public health importance.

Researchers still are unsure what aspect of helicopter lifts plays the biggest role in saving lives. "Is it the speed?" Adil Haider, of the Department of Surgery at Johns Hopkins University and co-author of the new study, asked at a briefing about the new findings. Is it that "the crew that makes it to the helicopter is the best of the best?" Or is it that patients arriving at a medical center via helicopter receive quicker and higher-quality medical attention once they land? "We need to figure out what makes the difference," Haider said.

In the real world, cost is also an issue. Helicopter transport is "one of the most expensive interventions in contemporary healthcare," the researchers wrote. The flights cost some medical centers as much as $4.5 million per year—a sum that often gets passed along to insurance companies and consumers. First responders are usually the ones to decide whether to call in an airlift or not, so triage and on-site decision making needs to be improved to ensure that helicopters get called to the right victims, Haider noted.

A medical helicopter flight is not without risks, either. Between 2004 and 2010, 53 emergency medical helicopters crashed, killing 77 crew and patients. Private medevac helicopter companies have been criticized for risking additional lives by flying in dangerous conditions.

Despite the latest results, Haider cautions that, "helicopters are not the panacea." Often an ambulance is a perfectly logical choice. If a victim is injured 10 miles from a hospital, "with sirens blaring" an ambulance might be able to reach him or her in the same amount of time.