For climbers, Mt. Everest is a monumental challenge. For scientists, it's a natural laboratory. At nearly 5 1/2 miles high, the summit is about as extreme as it gets. Studying people who live or climb that high offers insights into how the body works – and how it doesn’t in cases of illnesses that deprive the body of oxygen, including congestive heart failure and sleep apnea.
“A lot of people like me who have an interest in human physiology and have been athletes in the past are curious to know how far someone can go, how high someone can go and what are the limits to plasticity in the human body,” said Bruce Johnson, a physiologist at the Mayo Clinic in Rochester, Minn. “Here’s a mountain that’s almost 30,000 feet tall where oxygen is a third of normal. What does it take for humans to go up there? How does the body adapt or maladapt? These are the questions that drive us.”
The history of scientific investigation on Everest is nearly as long as the history of climbing on the mountain. Many of the first American climbers who made it to the summit were physicians and researchers, Johnson said, intrigued by what many view as the greatest physical challenge on Earth.
Extreme elevations are tough on the human body, not because there is less air but because there is less air pressure to drive oxygen into the blood. Up to about 10,000 feet, the body does a pretty good job of protecting itself by breathing a little more and increasing heart rate and blood pressure. Red-blood cell count goes up, as does hemoglobin -- the protein in blood that binds to oxygen. And extra urine output concentrates hemoglobin to allow each unit of blood to carry more oxygen.
Acclimatization is a slow process that becomes more difficult the higher you go. Not taking enough time to ascend causes just about everyone to feel sick above 15,000 feet, Johnson said. Going to 18,000 feet without acclimatizing causes a loss of consciousness after about half an hour. As for getting dropped directly onto the summit of Everest, most lowlanders would only last a minute or two before passing out.
A climber on Everest resembles a heart failure patient in a variety of ways, including a revved up nervous system response to low oxygen levels as well as constricted blood vessels in the lungs and brain. Both groups can feel light-headed, nauseous and exhausted. And both groups experience sleep apnea that causes periodic waking in the night, gasping for breath.
“The physiology of a healthy person trying to adapt to high-altitude looks a lot like a disease state of someone who is trying to tolerate a condition like heart failure, where blood flow to tissues is reduced,” Johnson said.
Recognizing these parallels has allowed scientists to pursue treatments and drugs for people under both kinds of distress. Diamox, which is often used to prevent altitude sickness, was originally designed to treat high blood pressure. And in recent work, Johnson’s team has been trying to figure out how to combat the build-up of fluid in the lungs of people with heart failure, a problem that also plagues high-altitude climbers.
Studying climbers on Everest also allows researchers to better understand the effects of hypoxia, a lack of oxygen supply to tissues, said Cynthia Beall, an anthropologist at Case Western Reserve University in Cleveland. “One of the beauties of high-altitude studies of healthy people is we can look at the range of things that hypoxia does,” Beall said. “We can separate the disease from the hypoxia.”
Something strange happens to many people who travel to extremely high altitudes: they have trouble thinking straight, probably as a result of low-oxygen levels in the brain. Impaired judgment begins at about 10,000 feet, according to research by the U.S. Army.
During a 2012 expedition to Everest, Johnson and colleagues used a video game to test cognitive function of climbers between about 17,000 and 18,000 feet.
When climbers were well acclimatized, results showed, they did pretty well. But if they were more aggressive about getting to altitude quickly, they had trouble completing tasks and making decisions.
People live in mountainous regions around the world, but there are no permanent settlements above about 17,000 feet and most of the highest villages lie below 15,000 feet, at which point, birth rates plummet and it becomes difficult to maintain good health.
Genetic studies are starting to explain how some populations manage to thrive at altitudes that make many people fill loopy and ill. In 2010, Beall’s group identified a gene called EPAS1 that’s mutated in Tibetan highlanders, allowing them to breathe in more air per minute than other people. They also produce higher than normal levels of nitric oxide, which dilates the blood vessels, all while maintaining low hemoglobin levels.
Other groups manage to live at extreme heights without the same adaptations that Tibetans have developed. Andean highlanders, for example, simply increase hemoglobin concentrations like almost everyone else, despite the added stress of pumping thicker blood. Ethiopian highlanders seem to have their own ways of coping with extreme elevations, including genetic variations that allow them to tolerate hypoxia, according to a sequencing study published this year.
“By comparing Tibetan, Andean and Ethiopian highlanders, what we see is that there are whole populations exposed to the same stress of altitude but with different patterns of responses, and they’re all successful,” Beall said. “That gives us a sense of how human biology is much more flexible than we thought. We are enormously adaptable.”