More Than Immobility Triggers In-Flight Clots

Low cabin pressure might also be to blame, but more study is needed, experts say

FRIDAY, March 10, 2006 (HealthDay News) -- It's not just sitting still for hours that puts air travelers at higher risk for blood clots.

The low-pressure, low-oxygen environment of the plane cabin may also play a role, new research suggests.

"Something in the environment of an airplane, for instance the low pressure, affects the clotting system of some people in a way that predicts a higher risk of thrombosis, and particularly those with risk factors," said Dr. Frits R. Rosendaal, senior author of a new study and a professor of clinical epidemiology of haemostasis and thrombosis at Leiden University Medical Centre in the Netherlands.

But because the risk of thrombosis after air travel appears to be only mildly increased, "indiscriminate medicine prophylaxis" -- excessive attempts to lower the risk -- are not warranted, Rosendaal added.

Although the study, which appears in this week's issue of The Lancet, was a small one, Dr. Steven Deitelzweig, chairman of hospital medicine at the Ochsner Clinic Foundation in New Orleans, called the finding "provocative," one that could point the way to effective prevention strategies.

Previous research has found a two-fold to four-fold increased risk of blood clots after plane travel (a phenomenon sometimes known as "economy class syndrome"). However, the mechanisms behind the phenomenon remain unclear. Data also suggest that longer flights carry a greater risk than shorter ones.

"Whilst recent research has shown that air travel is related to an increased risk of thrombosis, there were questions about the cause: just sitting still for a long time (known to increase the risk in various circumstances, as shown in London during WWII when people sought shelter in the underground) or also factors specific to air travel," Rosendaal explained. "For the latter, people have suggested that the low pressure in the cabin (equal to a mountain altitude of 2,500 meters) could increase the tendency to clot of the blood," he added.

The medical name for these clots is "deep-vein thrombosis" (DVT) whenever a clot forms in the leg, and a pulmonary embolism (PE) when a clot breaks free and travels to arteries feeding the lungs. Both are also known as venous thromboembolism.

This study involved a small group of people, 71 men and women. Concentrations of blood-clotting markers in the blood were measured before, during and immediately after an eight-hour flight as well as before, during and after eight hours of sitting in a movie theater, and during eight hours of a regular, mobile day.

Forty percent of the participants started out with an increased risk of clots either because they had a mutation in the factor V Leiden clotting gene, or because they were taking birth control pills.

"About 10 percent of people showed clear clotting activation, or a change in blood indicative of higher risk of clotting, during air travel, and only very few during cinema or daily life circumstances," Rosendaal said. "These were mainly the women with factor V Leiden or oral contraceptive use," he added.

In other words, something about air travel above and beyond the forced immobilization contributed to an increased risk of blood clots.

The findings do not point to any clear change in current practice, however.

"It doesn't change the management approach," Deitelzweig said. "People at high risk still get low molecular heparin [an anticlotting drug]."

He added, "Everybody needs to be well-hydrated, and compression stockings may play a role" in preventing clots..

Rosendaal stressed that "indiscriminate use of aspirin, heparin or other drugs to prevent thrombosis should be discouraged, since the risks are likely to outweigh the benefits. However, while the risks are low on average, they are higher in some people, i.e., those with risk factors and the elderly. Those [people] should consider safe preventive measures, such as refraining from sleeping pills or excess alcohol during air travel, and regular exercise."

Rosendaal's team has proposed doing further research to find effective and safe preventive strategies.

A larger study, Deitelzweig pointed out, might indeed point to different ways to manage the problem.

More information

For more on "economy class syndrome," check out the American Heart Association.

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