NATICK, Mass. (August 10, 2016) -- What goes up must come down. There has never been a more perfect expression to describe what altitude missions are like for Soldiers.
Soldiers deployed in mountainous terrains like Afghanistan often must make multiple trips from low altitude (less than 4,000 feet) to high altitude (greater than 11,500 feet) and back again with days to weeks separating the ascents. As Soldiers continue to make rapid ascents to altitude without acclimatizing, it is undeniable they are going to get sick. Over the years, Dr. Stephen Muza and Dr. Beth Beidleman of the U.S. Army Research Institute of Environmental Medicine have received numerous inquiries from operational forces requesting improved guidance for sustaining altitude acclimatization after descent.
Rapid ascent to altitude causes illness, properly known as Acute Mountain Sickness, during the first 48 hours at altitude because of the environment's lower oxygen pressure levels, which then causes less oxygen to get to critical organs, like the brain, and working muscles. Altitude acclimatization attained over days of exposure to high altitude is the most effective way to prevent AMS. However, how quickly acclimatization is lost upon descent and if acclimatization can be sustained at low altitude is not well known.
USARIEM researchers recently concluded a 12-day study at the Institute's Maher Memorial Altitude Laboratory, located on the summit of Pikes Peak in Colorado, to test a method to sustain altitude acclimatization. Researchers studied how AMS symptoms decreased and exercise performance improved as test volunteers acclimatized to the hypoxic environment.
"When the test volunteers first get to the top of Pikes Peak at 14,115 feet, they could potentially suffer from AMS, which would degrade their physical and cognitive performance," said Beidleman, a research physiologist and the principal investigator of the altitude study. "By the end of those 12 days, their bodies are well adjusted to the hypoxic environment. Although we did a 1992 altitude study to see how long it takes to lose acclimatization, we do not know whether or not we can sustain (acclimatization) as Soldiers return to sea level by intermittently exposing them to hypoxia (low oxygen) in 'hypoxia rooms.'"
Next, Dr. Mark Buller, a computational physiologist and Beidleman used physiological status monitors to develop individualized models that could help predict which Soldiers are likely to get AMS and which Soldiers will remain symptom-free. When test volunteers stepped foot on Pikes Peak, they immediately donned chest harnesses that tracked minute-by-minute heart rates and arterial oxygen saturations for the first 24 hours of their exposure to 14,115 feet to help detect development of AMS during the altitude exposure.
Then three times throughout the 12-day study, volunteers participated in time trials, where researchers assessed volunteers' physical performance. The volunteers ran for 3.1 miles at a 1-percent grade on a treadmill while researchers monitored their breathing, oxygen consumption and heart rates. This physical performance test can be especially grueling during the first 48 hours, when AMS is at its worst.
"We know when Soldiers go to altitude, their performance is impacted," Beidleman said. "Whatever task you do at sea level, it takes you longer to do at altitude. We are looking at how physical performance improves over time at altitude as your body adjusts to the hypoxia."
Volunteers also settled at computers to take daily cognitive tests, using the Automated Neuropsychological Assessment Metrics battery, a computer-based tool designed to detect reaction time, speed and accuracy of attention, short-term and long-term memory and thinking abilities, and the Synthetic Work Program test, which evaluates multitasking ability.
"Cognitive performance is very important for the military because Soldiers have to be able to identify a friend or foe," Beidleman said. "That involves quick reaction times and short- and long-term memory."
Next, blood and ventilation teams tested the volunteers' blood and resting breathing. According to Beidleman, during acclimatization, a person begins to breathe more, which helps transport more oxygen to the working cells, and also loses plasma volume, which concentrates the red blood cells and also improves transport of oxygen to the working muscles. Once arterial oxygen saturation levels start to creep up and they start producing more red blood cells, overall health and performance improves at altitude.
Research continued even while volunteers were asleep. The low oxygen and AMS can also affect Soldiers' sleep quality and quantity by causing periodic or irregular breathing and even instances of sleep apnea.
"The volunteers have a lot of awakenings before acclimatizing," Beidleman said. "You do not breathe for about 15 to 25 seconds, and, all of a sudden, you gasp for air. We use sleep watches to measure sleep quality and quantity before, during and after acclimatization."
On Day 13, the researchers and volunteers flew back to USARIEM for a 12-day treatment phase. During the treatment phase, volunteers spent either three hours a day in USARIEM's normobaric hypoxia room, a simulated altitude room that lowers the amount of oxygen but maintains the same pressure at sea level, or a sham normobaric hypoxia room that keeps oxygen levels the same as at sea level. Volunteers were blinded to what treatment they were receiving. Finally the volunteers spent 24 hours in USARIEM's hypobaric chamber, which can simulate Pikes Peak's oxygen and pressure levels at 14,115 feet, to see if the normobaric treatment sustained their acclimatization.
Surprisingly, not everyone is susceptible to AMS. Some Soldiers might display severe symptoms while at altitude, such as headache, vomiting, fatigue, lassitude or trouble sleeping that is so severe they need to be put on supplemental oxygen. Others are seemingly immune. Beidleman wants to learn why.
Dr. Nisha Charkoudian, a research physiologist, is looking at muscle sympathetic nervous activity as a potential predictor of who will and will not get AMS or have greater than normal decrements in physical or cognitive performance.
"What makes this individual different from that individual?" Beidleman asked. "We are looking at blood and genetic biomarkers that may be able to predict who is more likely to get or not get sick."
According to Beidleman, USARIEM is moving toward individualized medicine and eventually wants to create "something like a urine pregnancy test to see whether or not Soldiers are more likely to get AMS." This would not prevent Soldiers from going to altitude, but it would make the leaders aware of what to expect and know who is susceptible and who may need intervention early in altitude exposure.
"It is a scientific advancement to move toward these individualized models and develop a method to help our troops when they have to go on repeated missions to altitude," Beidleman said. "If we could prevent AMS upon return trips to altitude by sustaining acclimatization, Soldiers might have to experience AMS during the first exposure to altitude, but we could sustain that acclimatization so they would not have to go through those miserable 48 hours again. That is a huge benefit for the Soldier and the Army."