SUNDAY, May 18 (HealthDay News) -- Flying on commercial flights may put people who suffer from obstructive sleep apnea at greater risk of cardiac stress, Australian researchers report.

Sleep apnea is a common condition in which breathing stops or becomes very shallow. Obstructive sleep apnea is the most common form of the condition. When it occurs, people can't get enough air through their mouth or nose, and the levels of oxygen in the blood may drop.

"Falls in oxygen in flight are important but may not be the only important change that occurs," said lead researcher Leigh Seccombe, a senior scientist in the Department of Thoracic Medicine at Concord Repatriation General Hospital in Sydney. "They may not be the only factor separating normal subjects from those with obstructive sleep apnea."

People with obstructive sleep apnea clearly have increased metabolic demands during air travel, Seccombe said. "For many who have heart or other vascular disease, this may place them at increased risk of cardiac events such as heart attack or arrhythmia, but the extent of that risk is uncertain," she said.

Seccombe said the researchers performed the study, because obstructive sleep apnea is becoming much more common as obesity increases. The findings were expected to be presented Sunday at the American Thoracic Society's 2008 International Conference, in Toronto.

For the study, Seccombe's team compared oxygen levels and breathing patterns in healthy people and 22 people with severe obstructive sleep apnea during a simulated flight. The simulator replicated the oxygen and pressure levels found in the passenger cabin during commercial airline flights.

The researchers found that people with obstructive sleep apnea had lower levels of oxygen in their blood before and during the simulated flight. They also found that those with obstructive sleep apnea increased their breathing intensity at about the same rate as healthy people during the simulation.

However, while it's normal for the rate of breathing to increase as air pressure falls, people with obstructive sleep apnea experienced higher heart rates and more demand for oxygen, compared with healthy people, the researchers found.

"Half of the patients with obstructive sleep apnea would require supplemental oxygen in-flight if current guidelines for those with lung disease were strictly followed if these results are typical," Seccombe said. "In patients with obstructive sleep apnea or other cardiac or vascular disease, it might be wise to have a lower threshold for giving supplemental oxygen until more results are available."

Dr. Ronald D. Chervin, who's with the Michael S. Aldrich Sleep Disorders Laboratory at the University of Michigan, said he agreed that people with obstructive sleep apnea have a greater risk for heart problems during long airline flights, though there has been little research on the topic.

"I suspect that part of the problem may arise from co-morbid conditions, and especially obesity, which can compromise ventilation and perhaps account for some of the investigators' findings of differences between groups," Chervin said.

More information

To learn more about sleep apnea, visit the U.S. National Library of Medicine.

SOURCES: Leigh Seccombe, M.Sc., senior scientist, Department of Thoracic Medicine, Concord Repatriation General Hospital, Sydney, Australia; Ronald D. Chervin, M.D., Michael S. Aldrich Sleep Disorders Laboratory, University of Michigan, Ann Arbor; May 18, 2008, presentation, American Thoracic Society 2008 International Conference, Toronto

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