Denver Magazine / Denver Magazine May 2010 / Changes in Altitude

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Changes in Altitude

Denver researchers lead the charge to cure acute mountain sickness, with economic and military implications that reach far beyond improving your ski vacation.

The Colorado high country is a natural laboratory for the study of short- and long-term health effects of altitude. It is a magnet for outdoor enthusiasts, attracting an estimated 25 million visitors each year, most from lower elevations. Visitors often experience the effects of the change in altitude, and we may feel it ourselves after the exertion of hiking or skiing more than 3,000 feet above our usual mile-high lives. Hypoxia, the medical term for when the body’s supply of oxygen is insufficient for proper mental and physical functioning, can be more than an inconvenience on vacation. U.S. soldiers fighting in Afghanistan’s high mountainous terrain while carrying heavy packs can be stressed by the altitude, for example. It’s important to understand how their judgment may be affected by the stresses of exertion and hypoxia.

The Altitude Research Center (ARC) at the University of Colorado School of Medicine at the Anschutz Medical Campus brings scientists and doctors together to study the complex medical and physiological challenges of hypoxia on human health and performance. The facility boasts state-of-the-art laboratory facilities, sophisticated instrumentation and computing resources, and a 10'-by-28' hypobaric chamber that can simulate altitudes up to and beyond the summit of Mt. Everest.

You may be familiar with hyperbaric chambers. They are relatively common and are used therapeutically to increase air pressure and, therefore, oxygen saturation of the blood. Hypobaric chambers, on the other hand, are rare. They are used experimentally in aerospace studies and in high-altitude simulations. Highly accurate pressure controls gradually reduce the air in the chamber for extremely precise control of the simulated altitude. Temperature and humidity are also rigorously controlled. Subjects exercise on stationary bikes under hypoxic conditions and are monitored for the effects on their body tissues and blood flow in their brains.

Into Thin Air

“There’s no air here!” visitors often joke. In fact, the percentage of oxygen in the air remains constant at 21 percent, but the barometric pressure decreases, so there is less air driven into the lungs and less oxygen crossing into the blood stream. The average healthy person’s oxygen saturation level is close to 100 percent at sea level and about 94 percent in Denver. It can drop below 80 percent in high-altitude environments, and people often start to feel lightheaded. Judgment may be impaired. Simple mental and physical tasks become difficult. Some people become physically ill.

Humans adapt by a process known as acclimatization. The heart beats faster and harder to pump more blood. People breathe deeper and faster. The kidneys release a hormone called erythropoietin to stimulate red blood cell production in the bone marrow. All of which can cause minor, transient discomfort: a rapid pulse, shortness of breath, lightheadedness. If enough oxygen is not available, muscles employ a process called anaerobic metabolism that produces lactic acid, the buildup of which causes the burn skiers often feel.

Most travelers to Colorado adapt within a day or two. Roughly a quarter of visitors, however, will experience acute mountain sickness (AMS): headaches plus insomnia, fatigue, or nausea. This is especially common when flatlanders head immediately for the mountains above 8,000 feet and begin active exercise. These symptoms generally resolve fairly quickly, but a small percentage of people may progress to dangerous, life-threatening conditions such as high-altitude pulmonary edema or cerebral edema, which require immediate medical attention. The risk is higher at elevations over 12,000 feet.

What Can Be Done?

The ARC, one of the few research centers of its kind in the world, helps to establish scientific principles that will allow a mountain climber to climb higher without consequence and an oxygen-starved ICU patient to heal faster. Understanding basic mechanisms of hypoxia may help doctors better understand conditions ranging from headaches to strokes.

According to ARC director Ben Honigman, M.D., the center has two main focuses. First is the study of integrative physiology, how hypoxia affects the whole person as well as at cellular and molecular levels. This includes the search for a prevention and cure for AMS — which could have implications both economic and military. Investigators are also identifying specific genes that can predict who will get sick and studying the responses of individuals who exercise vigorously in hypoxic environments to better understand physiology at altitude and minimize the risk of deadly edema.

The other focus is epidemiology, the effects of hypoxia on lifespan and progression of cancer, heart disease, obesity, lung diseases, and neurological diseases, including Alzheimer’s and multiple sclerosis. Projected studies will investigate multiple sclerosis progression, longevity, cardiovascular disease, and behavior of malaria at altitude. Medical applications for these studies are being realized in conjunction with sophisticated geographic information systems technology that tracks migrations of populations.

Research director Robert Roach, Ph.D., is investigating the brain response to hypoxia and its relation to AMS and the genetics of AMS and human athletic performance. Vaughn Browne, M.D., Ph.D., studies the genetics of low birth-weight babies at altitude, investigating the differences in pregnancy adaptations of women whose ancestors lived in the Bolivian Andes for millennia compared to European women whose ancestors lived at altitude for only a few generations.

ARC scientists are offering their expertise to high-altitude clinical research projects worldwide. Ultimately, experts hope understanding gained through research improves the clinical treatment of problems related to hypoxia.

Barbara T. Zimmerman, a freelance biomedical writer, has a Ph.D. in pathology from the University of Colorado Health Sciences Center and worked as a bench scientist for many years.