Long-Haul Flights May Promote High-Altitude Symptoms

SEATTLE, July 5 -- Commercial airline passengers flying long distances at typical cabin pressures may experience discomfort that falls just short of mountain sickness, researchers reported.

In a study of volunteers in a hypobaric chamber simulating altitudes from 650 to 8,000 feet, ascent from ground level to 8,000 feet was associated with about a 4% drop in oxygen saturation, found J. Michael Muhm, M.D., M.P.H., and colleagues, of the Boeing Company here, and Oklahoma State University in Tulsa.

The induced hypoxemia was sufficiently severe to add discomfort to airline passengers already enduring three to nine hours in cramped confines, the authors reported in the July 5 issue of the New England Journal of Medicine.

"On the basis of our findings we conclude that maintaining a cabin altitude of 6,000 feet or lower (equivalent to a barometric pressure of 609 mm Hg or higher) on long-duration commercial flights will reduce the occurrence of discomfort among passengers," they wrote.

Commercial airline cabins are typically pressurized to 565 mm Hg or lower, equivalent to a terrestrial altitude of about 8,000 feet, or roughly the altitude of Bogota, Colombia.

Mountain sickness may occur among travelers who are not acclimated to altitudes above 6,500 feet. Also known as altitude sickness, the condition is a self-limited syndrome characterized by headache, nausea, vomiting, anorexia, lassitude, and sleep disturbance. The occurrence rises with climbing heights and is related to rapidity of ascent, they wrote.

The symptoms of acute mountain sickness are believed to be caused primarily by hypoxia in a hypobaric environment, such as high in the Himalayas, with the severity of symptoms increasing as arterial oxygen saturation drops, the authors noted.

To see whether similar effects occurred among airline passengers at typical cabin pressures, they conducted a prospective, single-blind, controlled hypobaric-chamber study in 502 volunteers.

In sessions of 12 simulated fliers at a time, the volunteers were placed in the chamber, and spent most of their time during the simulated 20-hour flight (roughly the time it takes to fly from New York to Sydney, Australia) in economy-class style airline seats. They were encouraged to walk or stand when not taking part in specific tests.

During hours one through nine of every test session, five randomly selected volunteers from the ages of 21 to 60 were asked to exercise by walking on a horizontal treadmill at three miles per hour for each hour for 10 minutes at a stretch.

Tests were conducted under atmospheric pressures equivalent to 650, 4,000, 6,000, 7,000 and 8,000 feet above sea level. The participants reported discomfort with responses to the Environmental Symptoms Questionnaire IV.

The investigators found that as the altitude went up, mean oxygen saturation went down, to a maximum decrease of 4.4% (95% confidence interval, 3.9% to 4.9%) at 8,000 feet compared with sea level.

In all, 7.4% of the volunteers suffered from acute mountain sickness, with symptoms such as malaise, muscular discomfort, fatigue, and ear, nose and throat discomfort. There were no differences in the frequency of mountain sickness among any of the altitudes studied. Exercise reduced the prevalence of muscle discomfort, but did not affect any other symptoms.

"The frequency of reported discomfort increased with increasing altitude and decreasing oxygen saturation and was greater at 7,000 to 8,000 feet than at all the lower altitudes combined," they wrote.

Post hoc analysis showed that the five Environmental Symptoms Questionnaire IV symptoms that most contributed to discomfort were backache, headache, light-headedness, shortness of breath, and impaired coordination.

Volunteers older than 60 were less likely to report discomfort than younger participants, and men were less likely than women to have discomfort.

The authors suggested that the reported discomfort "may represent subclinical acute mountain sickness." As they found evidence that the level of hypoxemia manifested at 7,000 to 8,000 feet played an important role in the development of discomfort, they concluded that cabin altitude of 6,000 feet or lower will reduce passengers' discomfort.

No serious adverse events occurred during testing, but four occurred within one month after testing, one of which, pneumonia, may have been study related.

There were 15 additional adverse events during the study, none serious. Nine may have been study related, including painful ear pressure, a panic attack, and low oxygen in an older women at the 8,000 feet level.

The authors noted that their study may have underestimated the degree of decline in oxygen saturation, which was greater in two other studies looking at the effects of altitude in commercial airplanes. Additionally because they used a modified version of the Environmental Symptoms Questionnaire IV, their results are not comparable with those of other studies using standard versions of the questionnaire.