An Air Force-funded study at the University of Maryland School of Medicine is raising questions about the safety of transporting troops by air if they sustain head injuries on the battlefield.
The military medical community has seen significant advances in the past 14 years in caring for and evacuating injured service members from combat zones to advanced medical facilities in Europe and the U.S., sometimes in as little as three days.
But a new study suggests that patients with traumatic brain injury who are exposed to the in-flight atmospheric conditions of a C-17A Globemaster III or other aeromedical evacuation aircraft may suffer additional injury to their brains.
For the study, researchers gave rats mild percussive head injuries and exposed them to conditions simulating the cabin pressure of a military aircraft, roughly equal to being at 9,000 feet altitude.
They found that exposure to the hypobaric — or lower air pressure than sea level — conditions within seven days after injury "significantly worsened" the animals' cognitive deficits and caused additional inflammation of their brains.
According to Dr. Alan Faden, director of the Shock, Trauma and Anesthesiology Research Center at the University of Maryland School of Medicine, the rats demonstrated symptoms of a worsening head injury.
"Behaviorally, they were worse, their cognitive function was worse, their depression-like features were worse," Faden said. "The chronic inflammation of the brain was significantly increased, and it was associated with quantitative cell loss in critical brain regions, including the hippocampus, where you make memories."
Nearly 330,000 U.S. service members sustained head injuries in Iraq or Afghanistan, 82 percent of which were considered mild traumatic brain injuries, also called concussions.
The number of head injuries suffered by troops since 2001 — due to blasts, blunt trauma and penetrating wounds — has been so significant that TBI often is referred to as the "signature wound" of the wars.
With their study, Faden and his colleagues sought to prove or disprove a "general impression in the medical community" that troops with head injuries evacuated from theater by air — to Landstuhl, Germany, or elsewhere, or who made two flights, such as going to Landstuhl and eventually the U.S. — after injury did not fare as well as those treated at in-theater medical facilities.
The researchers exposed the injured rats to decreased air pressure for the duration of a flight, usually six to eight hours. They exposed them six hours after injury, one day later and three days afterward.
They also tested the effects of two trips — similar to an evacuation to Landstuhl followed by a trip to the U.S. — by subjecting the rats to two simulated high-altitude flights.
And their prognosis was not good, according to the study, published online Dec. 1 in the Journal of Neurotrama.
"By every measure they got worse. … And if you put an animal in the two-air-evacuation situation, they are worse than one," Faden said.
The group also examined the effects of administrating 100 percent oxygen at high altitude to learn more about the effects of doing so during an air evacuation — a practice sometimes done to humans to ensure that their brains are getting sufficient O2.
But according to Faden, the pure oxygen aggravated the injuries.
While the results of animal studies often are dismissed as preliminary findings with little relevance to human health, Faden and his colleagues believe their conclusions are significant enough to draw the attention of the military medical community.
The researchers recommended that the Air Force weigh the necessity of air evacuations shortly after injury because "standard practices may be subjecting soldiers to unnecessary risk," according to Faden, and urged the service to consider increasing the cabin pressure of air ambulances.
Commercial airliners are pressurized to the equivalent of 6,000 feet — an altitude generally thought to be more accommodating to the human body.
Faden said the study also shows doctors should question the practice of administering 100 percent oxygen to head-injured patients.
"The common practice of giving pure oxygen to patients may be a real problem and make things worse," he said.
In an accompanying editorial, Dr. Patrick Kochanek, director of the Safar Center for Resuscitation Research at the University of Pittsburgh School of Medicine, questioned some of the study's methodology and said more research is needed before making conclusions about the administration of pure oxygen.
Still, Kochanek called the study "highly novel and eye-opening" and said it could have applications for civilian trauma care, including the treatment of sports professionals such as football and ice hockey players who may fly shortly after sustaining a head injury in the arena.
"This has now been identified as an important area of additional investigation," Kochanek wrote.
Faden expressed hope that the Air Force, which provided a $2.5 million grant for the research, would examine the findings and consider making some changes to aeromedical evacuation procedures.
"Unlike many animal studies, which just give you food for thought, we are hoping this would be rapidly translated into practice," Faden said.
Patricia Kime is a senior writer covering military and veterans health care, medicine and personnel issues.