Evidence-based research into in-flight stressors on critical patients

aeromedical evacuation research US Air Force
Taking all the evidence into account

Amy Gallagher reports on new US Air Force evidence-based research on aeromedical evacuation in-flight stressors, the findings of which may alleviate a ‘second hit’ to patients

US Air Force medevac flight

Clear evidence at last

Although substantial research on the science of flight stressors on aircraft and medical equipment has been widely published, the evidence-based science explaining how and why in-flight physiological stressors affect the human system has remained a mystery, until now.

Understanding the science of how flight stressors can ‘add insult to injury’ to an AE patient is explained in the research study

Understanding the science of how flight stressors can ‘add insult to injury’ to an AE patient is explained in the research study, Assessment of Aeromedical Evacuation Transport Patient Outcomes with and without Cabin Altitude Restriction, August 2017 Final Report as researched and published by the 711th Human Performance Wing USAFSAM, EC Research Division, which produces knowledge and technologies to improve EC for the Joint Force. The three-year evidence-based research (EBR) report showed patient outcomes that were impacted by a ‘second hit’, during aeromedical transport, with the ‘first hit’ incurred on the battlefield.

Practicing EBR to ‘close the gap’

AE is the ‘means of moving patients from one level of care to the next, always bringing the patient to a higher level of care’, explained Averett-Brauer. “We focus on the evacuation and in-flight phases and transitions of care, and the austere environment within the transport aircraft,” she said. “At the same time, we now know AE patients are exposed to a number of in-flight stressors. Unfortunately, we also know AE is not without risks.”

The practice of conducting EBR is designed to open doors to greater understanding in all areas of medicine while closing the ‘theory-practice gap’, according to Averett-Brauer. More specifically, the multi-phased, retrospective matched case control study investigated whether there were differences in the clinical outcomes, in-flight status, and mission costs between CAR and non-CAR patients as prescribed by the TVFS, she added. “Although guidelines for the use of CAR exist, those guidelines are not necessarily supported with EBR,” explained Averett-Brauer. “This study followed the EBR protocols to identify if a patient receives any clinical benefits with a CAR.”

Prescription: CAR

It is current practice that the TVFS makes the flight prescription for a CAR based on the potential negative impacts of flight on the patient’s clinical condition, said Dr Burch. One example is determining whether or not the patient will have adequate haemoglobin saturation throughout the transport. “This can be impacted by the patient’s haemoglobin levels, cardiac output, lung shunt ratios, and the TVFS determination of whether the patient will have adequate tissue oxygen delivery,” explained Dr Burch. “Our research aims to expand and validate the paradigm of this tissue oxygen delivery to account for more variables that may influence patient outcomes and provide decision support for the TVFS and the in flight clinical team. We are contributing to the evidence base needed to make changes to the TVFS Primer, which is a clinical guide used within the patient movement system,” he added. “CAR is but one of several prescriptions available to the TVFS, such as supplemental oxygen, more gradual ascent/descent profiles, and blood transfusions, to name a few.”

Paradigm of tissue oxygen delivery

Traumatic brain injury (TBI) research conducted by the Air Force and US Department of Defense (DoD) has been instrumental in the development of various breakthroughs that are improving the lives of airmen and all other service members who have sustained a TBI, explained Averett-Brauer. “Research and studies examining the interactions between TBI and low-pressure aeromedical conditions, the impact of hypobaric environment, and understanding of how TBI is impacted by the aeromedical environment help guide the way the Air Force treats TBI received on the battlefield,” she added.

The research lines of inquiry included understanding the impact of transport on patients and providers; improving operational clinical care and patient safety; developing medical technology solutions; and simulation, education and training research, said Averett-Brauer. “We use a variety of research approaches including quantitative and qualitative methods; retrospective and prospective studies; interventional studies; and clinical trials,” she added. “We also evaluate medical technologies for use in the EC environment.”

Research approaches include explorations surrounding the stresses of flight, including hypobaria, hypoxia, vibration and the biodynamics of the environment

Research approaches include explorations surrounding the stresses of flight, including hypobaria, hypoxia, vibration and the biodynamics of the environment, noise, temperature changes, and humidity changes; all of these stresses of flight culminate in fatigue.

“Well-established research has demonstrated reductions in tissue health associated with acute high-altitude exposure due to hypoxic hypoxemia,” said Dr Burch. “However, at much lower altitudes, including those common for patient transport (8,000 feet or below), we are seeing some changes in oxygen saturation.”

However, lower partial pressure of oxygen alone cannot fully explain the effects observed in controlled studies, he added. “We are still investigating the mechanisms to ascertain if they are more dependent on hypoxia, hypobaria, or interaction effects of both factors,” said Averett-Brauer. “Once we better understand the stresses of flight on the patients, our research can better focus on identifying mechanisms that may cause secondary insults due to transport, so that we can then focus on determining optimal mitigating strategies to improve patient outcomes. This is only the beginning.”

Potential outcome for the soldier

The goal is to maintain and improve patient status during the transport, using the transport time for therapeutic gains, according to Colonel Nicole Armitage, Chief EC Research Division.

Consolidating electronic and handwritten documentation into the full implementation of the electronic health record also leads to improved patient outcomes

“All this feeds into improved patient outcomes, facilitates return to duty and improved quality of life, or improved transition to civilian life,” she added.

Consolidating electronic and handwritten documentation into the full implementation of the electronic health record (EHR) also leads to improved patient outcomes, said Averett-Brauer. “The vision is that the EC EHR will be part of the entire health record continuum,” she said. “Access to improved patient care data will translate into higher-quality evidence and analyses.” With the continued development of retrospective and prospective research capabilities, the team will be able to provide stronger evidence to inform clinical protocols, according to Averett-Brauer.

Physiological phenomena from additional altitude

The AE flight has a number of physiological stressors that could potentially cause a ‘second hit’ to the patient, said Averett-Brauer. The research concluded that hypoxia and hypobaria are the most prominent features associated with the cabin environment. “As a result, CAR is often prescribed to counter the effects of hypoxia and hypobaria on such patients,” she said.

As explained in the research report, the pressurisation imposes both hypoxia (reduced oxygen availability) and hypobaria (reduced barometric pressure that has a physiologic effect of shifting intravascular fluid into the extravascular space) upon patients. The research further stated that the two physiological phenomena can have an adverse impact on tissue oxygen delivery (DO2), particularly critical to patients’ compromised physiology (eg. massive transfusions, significant trauma).

“The results suggest that the AE flight may not be innocuous,” said Dr Burch. “Indeed, it may pose a ‘second hit’ risk to our patients, resulting in both devitalised and compromised tissue.”

As explained by Dr Burch, the CAR is generally accompanied by a drop in the cruising altitude. “Normally, the cabin is pressurised around 8,000 feet,” he said. “In an already compromised patient, this lower cabin altitude (CAR) can potentially benefit the patient in a number of ways.”

US Air Force medevac flight

Post-flight: the clinical effect of CAR

The salutary clinical effect of CAR might well alleviate the ‘second hit’ based on our understanding of the DO2 paradigm, said Dr Burch, citing his mentor and the study’s author Dr William Butler. In addition to hypoxia and hypobaria, the research indicated other stressors including acceleration forces, vibration, noise, thermal instability and reduced humidity.

“These flight stressors can ‘add insult to injury’, including multiple organ system post-flight complications,” said Averett-Brauer. “Knowing the cabin altitude consequences, we can expect a physiological milieu in which oxygen diffusion into tissues is impaired,” she said. “This can cause a potential drop in oxygen delivery to the tissue or DO2.”

However, the CAR is a prescription that can enhance DO2 by bringing the patient ‘closer to the ground level’, which would lessen the impact of hypoxia and hypobaria, Averett-Brauer said.

Additionally, the findings revealed that those patients who were transported with a CAR had a significantly lower systolic blood pressure and a statistically higher 24-hour fluid intake.

The most significant finding was in post-flight procedures with the CAR group, which experienced fewer major and minor procedures, according to Dr Burch. “The study, however, did not find a significant difference in length of stay, days in the ICU, post-flight transfusions, or discharge status,” he said.

More to be done

Investigations into the uses of CAR and the impact on patient outcome will continue, Averett-Brauer said: “One special means through which the TVFS can mitigate clinical impact is the CAR. The effect is most likely a consequence of improved DO2, making a ‘second hit’ to already compromised tissues less likely. Thus, these results suggest a CAR benefit. This study is just the beginning.”

Dr Burch concluded: “We anticipate closing more of the gaps in knowledge between patients with survivable injuries, and those that ultimately survive and thrive.”

US Air Force medevac flight transfer patient