Pulse
Pressure Work
Researchers are studying the physiologic effects of altitude.
By Trout Lowen
Anyone who has flown on a commercial airline has no doubt heard these words: “In the unlikely event of a loss of cabin pressure, an oxygen mask will drop down. Place the mask over your nose and face. Put your mask on first.”
Those instructions haven’t changed substantially in decades, and neither have the emergency oxygen systems onboard airplanes. The yellow “Dixie cup” mask and the method of oxygen delivery is much the same as it was when it was introduced on commercial flights during the early 1950s. But that may soon change, thanks in large part to the work of Mayo Clinic physicians.
For the past two years, researchers in the Division of Preventive, Occupational, and Aerospace Medicine have been working with aircraft manufacturers and engineering firms to test new types of emergency oxygen systems for the next generation of commercial aircraft. One system will be operational on Boeing’s new 787 Dreamliner, which is scheduled for delivery later this year.
Mayo Clinic’s history of altitude and aviation-related research dates back to the 1930s, when its scientists and physicians first began studying the effects of high altitude and G forces on the body. That research led to the creation of the first oxygen masks and G-suits worn by pilots, and advances such as the “oxygen tent” and delivery of supplemental oxygen to the patient through a mask. It also contributed to the physiologic principles of what today is known as hyperbaric and altitude medicine. In fact, in an article published in the Journal of the American Medical Association in 1939, Mayo physicians Walter M. Boothby, Charles W. Mayo, and Randolph Lovelace II laid out indications for treatment with 100 percent oxygen.
Today, Mayo researchers conduct much of their work, including testing of new emergency oxygen systems for aircraft, using a 3,800-square-foot facility installed in 2007 that has both hyperbaric (compression) and hypobaric (decompression) chambers.
The facility is primarily used to provide hyperbaric oxygen therapy for patients with a wide range of illnesses and injuries including nonhealing diabetic ulcers and soft-tissue injuries including those resulting from radiation therapy and surgery. But with its hypobaric capabilities, it also serves as a laboratory for physicians and researchers studying the physiologic effects of altitude on air travelers and high-altitude climbers. When the chamber was built, Mayo officials had anticipated working with NASA on research related to a manned mission to Mars. But the economic downturn and cutbacks in space exploration shifted priorities, says Paul Claus, M.D., medical director of the Altitude and Hyperbaric Medicine Program. Still, he says, Mayo plans to be ready when the climate for space exploration changes. “We’re not going anywhere,” he says. “We have a piece of equipment that will last 50 years.”
Safe Travels
In the meantime, researchers are using the facility to do research that will benefit the traveling public. Clayton Cowl, M.D., a pulmonary specialist, chief of the Aviation and Aerospace Medicine section and principal investigator for
Traveling with Oxygen
Physicians should consider prescribing supplemental oxygen for patients who will be flying if they
- Use oxygen at baseline altitude
- Have a baseline PaO2 < 70 mmHg
- Are NYHA Class III or IV
- Have angina Class III or IV
- Have cyanotic congenital heart disease
- Have primary pulmonary hypertension
- Have severe anemia or cardiovascular disease with baseline hypoxemia.
Source: American College of Chest Physicians Expert Panel on Patient Travel (2010)
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aviation and aerospace research, says understanding the effect of hypoxia and preventing it during flight are increasingly important to public safety because of the large number of air travelers.
As airlines seek to cut fuel costs and find less-crowded airspace free from adverse weather conditions, they are flying at higher average altitudes than ever before, which Cowl says can pose health risks to some chronically ill or elderly travelers. Add a mechanical mishap, and the situation can become deadly: In 2006, the cabin of a Boeing 737 operated by Helios Airways failed to pressurize. Everyone, including the pilots on board, succumbed to hypoxia, and the plane flew on automatic pilot until it ran out of fuel and crashed outside of Athens, Greece, killing all passengers and crew members.
Cowl recently used Mayo’s hypobaric chamber to simulate different altitudes and to gauge the rate of oxygen flow needed in a prototype oxygen mask and delivery system to prevent hypoxia (the amount is different for different people). He tested 38 subjects, ranging in age from 18 to 61 years. All were free of cardiac, pulmonary, and other conditions that could have caused problems during the test. Cowl monitored the subjects’ blood oxygen levels and other parameters at different flow rates and different altitudes and did tests while they were breathing normally, hyperventilating, and performing various tasks to assess their mental acuity. He plotted the data to determine the average responses and padded those averages to ensure flow rates would be adequate for all passengers, regardless of their age or health status. “The FAA requires that you identify a ‘traveling population,’ so it can’t just be young, healthy, military fighter pilots that you’re testing,” he says.
Cowl says airlines are trying to minimize the amount of oxygen they carry because they are concerned about extra weight, as that means extra fuel costs. “We have to know, in a new system, how much flow you need at 40,000 feet, at 30,000 feet, at 20,000 feet because the flow rates would presumably decrease as you get down to that goal of 10,000 feet, at which point most people don’t need oxygen,” he says. “If you are flying over the Himalayas, you may not be able to descend to 10,000 feet above sea level very quickly.”
Although the FAA doesn’t require private planes to carry emergency oxygen as it does commercial aircraft, more aircraft manufacturers are interested in doing so, given the risk. This summer, Cowl will work with AVOX Systems Inc. on the development of in-plane oxygen systems for private aircraft.
The Body in Flight
In addition to what it has provided for industry, the testing done by Cowl and his research team has also yielded data that indicate that more people should travel with supplemental oxygen.
Most air travelers don’t realize that at a cruising altitude of 40,000 feet, the plane’s cabin isn’t fully pressurized. In fact, the air pressure is closer to what you might find at the top of a 7,000-foot mountain than to that at sea level, Cowl says. That creates problems, especially when combined with the stress of flying and the exertion of lugging heavy bags through the airport.
The passengers most at risk are not those with chronic or severe disease who are already likely to be traveling with oxygen. It’s those who may not normally use it or even know they need it such as those with moderately severe obstructive lung disease, pulmonary fibrosis, or with subclinical or asymptomatic coronary artery disease. Consequently, primary care physicians need to be aware of the risks and know how to advise their patients, Cowl says. “A lot of doctors don’t think about their elderly and not-so-elderly patients with respiratory illnesses who want to travel, mostly because the conversation about travel doesn’t come up in the medical history.” Cowl and colleagues wrote a guide for patients about traveling with oxygen that was published online in 2010 by the American College of Chest Physicians (ACCP). It is available free on the ACCP website, www.chestnet.org/accp/patient-guides/traveling-oxygen.
Altitude Adjustment
In addition to the work they are doing in Rochester, Mayo researchers have traveled to remote locations to learn how the body adapts to high altitudes. Bruce D. Johnson, Ph.D., recently completed a three-year attempt to identify factors that determine how well an individual can acclimate to high altitudes and to identify predictors for altitude illness.
Johnson’s findings are of interest to the U.S. government, which operates a research station at the South Pole as well as to NASA and the military. Altitude sickness has been a problem for some troops arriving in Afghanistan, he says. In the most severe cases, it can result in pulmonary or cardiac edema, or even death. “They’re interested in who’s at risk, how they can acclimatize people, how you can get them ready for work at high elevations,” he says.
Next year, Johnson will study noncompetitive climbers ascending Mount Kilimanjaro in Tanzania to determine why some people acclimate to altitude better than others. (In addition to work he does in the field, Johnson uses the hypobaric chamber and the Mayo Clinic fitness center for research. But, he says, it is difficult to mimic conditions such as humidity and temperature.)
Johnson says his high-altitude research has broad applications—especially for people with chronic conditions such as lung disease and heart failure. “A lot of what we do in studying altitude physiology in healthy people has carryover to patient populations,” he says. “We try to blend the two.”