By Brenda L. Mooney
BALTIMORE — U.S. warfighters injured in combat in Iraq and Afghanistan had a 90% or more rate of survival, with a substantial part of that success attributed to medical evacuation teams that swiftly flew wounded servicemembers to locations such as Landstuhl Regional Medical Center in Germany or to Brooke Army Medical Center in Texas.
A new study funded by the Air Force noted, however, that one-fourth of all wounded warriors evacuated to receive more-extensive treatment outside of battle zones had head injuries and questioned whether air evacuation was the best course of action for them.
The University of Maryland researchers said they have uncovered evidence that those air evacuations might have caused more damage to already injured brains by exposing them to reduced barometric pressure, as well as vibration, acceleration and temperature variations.
“This research shows that exposure to reduced barometric pressure, as occurs on military planes used for evacuation, substantially worsens neurological function and increases brain-cell loss after experimental TBI — even when oxygen levels are kept in the normal range,” explained lead researcher Alan Faden, MD. “It suggests that we need to carefully re-evaluate the cost-benefit of air transport in the first days after injury.”
Background information in the article, published recently in the Journal of Neurotrauma, pointed out that more than 330,000 military personnel suffered a traumatic brain injury (TBI) during the last 15 years.
Faden, director of the University of Maryland’s Shock, Trauma and Anesthesiology Research Center (STAR) as well as the National Study Center for Trauma and Emergency Medical Services, and colleagues performed an animal study using a model that simulates key aspects of human brain injury.
For the research, rats were exposed to six hours of lowered air pressure, hypobaria, at levels that simulated conditions during transport. They then were compared to a control group exposed to normal pressure. All of the animals received extra oxygen to restore normal oxygen concentrations in the blood.
In a companion study, animals received oxygen, either as in the first study or at much higher 100% concentration, which is often used during military air evacuations, according to the researchers. >>Long-Term Cognitive Function
Long-Term Cognitive Function
Results indicated that, on its own, low air pressure worsened long-term cognitive function and increased chronic brain inflammation and brain tissue loss. Furthermore, the study found that pure oxygen further worsened outcomes.
Although some researchers have promoted the use of pure oxygen in TBI treatment, others have posited that breathing pure oxygen could worsen outcomes by increasing production of dangerous free radicals in the brain which could flood the injury site.
“Hypobaric exposure up to seven days following injury significantly worsened cognitive deficits, hippocampal neuronal loss and microglial/astrocyte activation in comparison to injured controls not exposed to hypobaria,” the researchers wrote. “Hyperoxia during hypobaric exposure or two exposures to prolonged hypobaric conditions further exacerbated spatial memory deficits. These findings indicate that exposure to prolonged hypobaria up to 7 days after traumatic brain injury, even while maintaining physiological oxygen concentration, worsens long-term cognitive function and neuroinflammation. Multiple exposures or use of 100% oxygen further exacerbates these pathophysiological effects.”
Study authors said their investigation, the first to look at the risks of air evacuation in TBI, suggests that the potential downsides be weighed against the advantages of relatively early air evacuation.
“The present study demonstrates that HB exposure during simulated air transport following TBI exacerbates neuroinflammatory secondary injury mechanisms, leading to increased deficits in learning and memory, as well as hippocampal neuronal cell loss,” Faden and colleagues pointed out. “The present studies suggest possibilities for mitigating injury exacerbations by HB exposure following TBI, including retaining patients at in-theater hospitals for longer periods prior to transport; delaying the second AE; increasing cabin pressurization to reduce barometric effects or specialized enclosures for individual pressurization; and/or changes to the supplemental oxygenation protocols.”
In an accompanying editorial entitled “Hidden perils of the ‘wild blue yonder’ after traumatic brain injury,” Patrick Kochanek, MD, and Hulya Bayir, MD, of the University of Pittsburgh, called the findings “highly novel and eye-opening,” and said that they could have “impactful clinical relevance for the field of traumatic brain injury in both military and civilian applications.”2
The study team is now examining the process by which hypobaria worsens TBI by increasing persistent brain inflammation after injury and is looking for methods that can reduce the risks of air evacuation.
However it is altered for brain injured troops, aeromedical evacuation (AE) is likely to remain a critical tool in battlefield medicine. A “Talking Paper” from Air Mobility Command notes that, during the Vietnam War, returning the wounded to U.S. facilities took an average of 45 days, with the time dropping to 10 days for Operation Desert Storm it took 10 days. Now, AE takes three days, with critical cases returning within 24 hours.
The document also notes that survival rates have improved from 75% to 94%, with 98% of those who make it into the AE system surviving.
1 Skovira JW, Kabadi SV, Wu J, Zhao Z, DuBose J, Rosenthal RE, Fiskum G, Faden, AI. Simulated Aeromedical Evacuation Exacerbates Experimental Brain Injury. J Neurotrauma. 2015 Nov 23. [Epub ahead of print] PubMed PMID: 26593382.
2 Kochanek PM, Bayir H. Hidden perils of the “wild blue yonder” after traumatic brain injury. J Neurotrauma. 2015 Dec 1. [Epub ahead of print] PubMed PMID:26625357