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Thrombosis is the formation of blood clots within vessels. Blood does not normally clot within vessels and the development of such clots may lead to problems. Thrombosis can potentially occur in any blood vessel and may have fatal consequences if it obstructs the flow of blood to a vital organ. In the context of air travel, we are concerned with thrombosis occurring in the deep veins of the lower legs.

The deep veins of the lower legs are situated in the muscles of the calf. Compression of the veins by muscular contraction, cause a pumping effect (the muscle pump) aiding the return of blood to the heart. If the muscle pump is not working (e.g. due to immobilization), the blood flow in the veins may decrease (known as venous stasis) to the point where small clots may form. Most of these are too small to cause problems. However, they may sometimes enlarge significantly or link up to produce a large obstructive thrombus.

Immobility in a seated position is an obvious predisposition to DVT as the veins of the legs can get compressed and cause stasis. DVT is known to occur in passengers on long-distance road or rail journeys as well as those by air. The common factor in all of these is immobility rather than the environment. Pressurized cabins and high altitude of aircraft have no impact on the risk of developing a DVT.

The term "Economy Class Syndrome" was first used in 1977 in a paper entitled "Pulmonary thromboembolism after travel". It is, in fact, an inaccurate term as there is no firm link between air travel in any class per se and the development of DVT. Long periods of immobility in trains, buses and automobiles carry similar risks. The House of Lords report recommends the use of the more appropriate term "traveler’s thrombosis".

The normal rate of occurrence in the general population in the UK is 1 per 1,000 to 1 per 10,000 people every year, depending on age. According to the Wright Study 1 in 6,000 passengers develop DVT after periods of immobility of 4 hours or more.

There are certain predisposing factors that are well known to contribute to the risk of having a DVT. These are:

  • Increasing age over 40 years old
  • Pregnancy *
  • Smoking
  • Obesity
  • Previously or currently suffering from malignant cancer *
  • Blood disorders which lead to an increased clotting tendency *
  • Heart disease or blood vessel disease *
  • Personal or family history of DVT or pulmonary embolism*
  • Recent surgery or major injury, especially below the waist level *
  • Oestrogen hormone therapy, including oral contraception *
  • Previous / recent immobilization *
  • Depletion of body fluids causing increased blood viscosity
  • Varicose veins

Obviously, if more than one of the above applies, the risk of having a DVT is further increased. If this is the case, or you have any one of the conditions marked '*', we suggest you seek the advice of your doctor before flying.

As the major cause is immobility, you can do quite a lot to reduce this, and hence minimize your risk of getting DVT. Some suggestions are:

  • Walk around the cabin, if safe, once every 2-3 hours to break up long periods of immobility. Moving around the cabin during long flights will help to reduce the period of immobility, although this may not always be possible. Any potential health benefits must be balanced against the risk of injury if the aircraft were to experience sudden turbulence.
  • Move your legs and feet for 3 or 4 minutes per hour while in your seat to get the muscle pump working and avoid stasis.
  • Do some stretching exercises when waiting to use the washroom.
  • Avoid wearing tight clothing around the upper thighs and body.
  • Don’t place hand luggage where it restricts the movement of legs and feet.
  • At risk individuals should consult their doctor about preventive measures which may include consuming low dosage of soluble aspirin, wearing anti-embolism stockings or even taking anti-coagulant therapy for those at highest risks.
  • Avoid commencing an air journey in a dehydrated state (e.g. after drinking a lot of alcohol, having a hangover or being exposed to very hot weather for long periods).
  • Drinking coffee, tea and alcohol only in moderation as these are diuretics and may cause dehydration.

Our inflight magazine, Discovery, and inflight safety video also includes tips and exercises to do inflight to help prevent DVT.

The most common features of a DVT are:

  • Pain or tenderness of the calf muscles.
  • Swelling of the leg, especially if it is only on one side. Most people have a small degree of swelling of both feet after a long journey, but this is often due to the accumulation of water as a result of gravity and will soon recover.
  • Increased skin temperature and/or redness of the leg.
  • Dilation of the veins right below the skin of the leg.

If you experience any of these after your flight; we suggest you consult a doctor to exclude DVT.
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