April 3, 2006
A study reported in the March 11, 2006 issue of Lancetsays that the syndrome of venous thrombosis (blood clots in the legs) that some passengers experience after long airplane flights are due to more than just being immobile for a long time.
They studied 71 healthy volunteers during 8-hour airplane flights, then during an 8-hour "movie marathon" conducted at sea level, then during 8 hours of daily living. They measured blood products during each of these test periods that determine whether the blood has a tendency to clot.
What they found was a higher tendency to clot during air travel than during either immobility at sea level or during daily living. Individuals who had either Factor V Leiden genetic mutations (a fairly common but but usually mild clotting disorder) or who were taking oral contraceptives had the greatest tendency to clot. Overall, 17% of these normal volunteers showed a clotting propensity during air flight. (Only 3% showed the same tendency during the sedentary movie test, and only 1% during daily living.)
The investigators speculate that there must be something specific about air travel - most likely either the reduced oxygen level or the reduced atmospheric pressure that occurs during high-altitude flight - that increases clotting propensity. Individuals on oral contraceptives or who have a factor V mutation should take special precautions.
Avoiding alcohol or sedatives on long flights, using carefully-fitted elastic compression stockings, and exercise are the current recommendations for reducing the risk. Of these, exercising the calf muscles on long air flights appears to be especially helpful.
