People have become concerned about the risks of deep vein thrombosis (DVT) during long air flights, following reports in the press of occasional deaths due to fatal pulmonary embolisms. This advice page aims to:

  1. People are concerned about the risks of deep vein thrombosis (DVT)
  2. during long air flights after reports of occasional deaths from fatal
  3. pulmonary embolisms in the press. This advice page aims to:

What are deep vein thrombosis (DVT) and pulmonary embolism (PE)?
Deep vein thrombosis (DVT) means thrombosis (blood clotting) in the deep veins of the leg. It can cause swelling and pain in the leg, but it often occurs without any symptoms. DVT can completely settle because the thrombosis is resolved by natural processes. When a DVT expands the deep veins, two things can happen:

  1. The thrombosis can separate from the vein and carry through the main veins and heart to lodge in the lungs. This is called a pulmonary embolism. Small pulmonary embolisms can cause chest pain and sometimes coughing up blood. Multiple or larger pulmonary embolisms can lead to shortness of breath. A large pulmonary embolism, which blocks the main blood vessels in the lungs, can be fatal.
  2. The thrombosis can lead to chronic blockages in the deep veins or damage to the valves, leading to long-term swelling and sometimes skin problems on the ankle.

Why does deep vein thrombosis occur?

Comparison: a normal leg versus one with DVT
All veins in the leg have valves that are designed to direct blood flow up to the heart. The deep veins lie between the muscles, and muscle activity (moving your legs, walking, or leg exercises) helps pump blood into these veins. When the legs are inactive, and especially when sitting or standing, the blood in the deep veins stagnates. Blood stagnation can eventually lead to thrombosis. Sitting with your legs bent (like in an airplane or carriage seat) can also restrict blood flow through the veins in the calf.

The longer the stagnation period, the more likely a thrombosis is. If the blood is abnormally “thick” or “sticky”, the risk of thrombosis is higher: this can be caused by dehydration and some diseases.

Who is at risk of deep vein thrombosis?
We have no direct evidence of individuals at particular risk for DVT from long travel, but based on evidence of surgical interventions, the following factors increase risk:

I previously had DVT or pulmonary embolism
Recently had major surgery
The contraceptive pill or hormone replacement therapy (HRT)
Malignant disease (cancer)
Obesity (overweight)
Severe heart disease
Some blood diseases
Varicose veins

The risk of DVT is likely to be highest in people with more than one of these risk factors.

How big is the risk?
For people without any of the risk factors listed above, the risk of DVT (even on a long-haul flight) is tiny – one in hundreds at most. For people with risk factors who do not take precautions against thrombosis, the risk of DVT, which can be detected on special scans, is as high as one in twenty on long-haul flights (but many of these thromboses are minor and do not cause problems).

How do I reduce the risk?
There is now evidence that wearing compression stockings below the knee reduces the likelihood of DVT in those with special risk factors (see above). Because so few people without risk factors from long travel ever develop DVT or pulmonary embolism, there is no clear evidence of other measures to reduce the risk. However, based on the known causes of DVT and the successful prevention methods in hospitals, the following precautionary measures make sense, especially on long-haul flights and other trips lasting several hours:

Move your legs: Don’t sit with your legs bent for hours.
From time to time, straighten your legs and move your feet up and down at the ankles.
Get up to straighten your legs every now and then. Stretching and moving your legs will keep blood from stagnating in the deep veins of the calf and is the easiest and most effective thing you can do.
Go up and down the aisle.
Don’t dehydrate: Drink plenty of fluids – water is ideal.
Avoid excessive alcohol, which can lead to dehydration.
Compression stockings
Wear compression stockings: Graduated compression stockings reduce the risk of DVT. They also help prevent the ankle swelling that many people experience on long trips. Below the knee stockings are the most comfortable type and seem to be just as effective as full length stockings. Medically graded compression stockings come in three classes: Class 1 or 2 stockings are suitable for most people (Class 3 is overly strong for this purpose). Compression stockings can be prescribed by a doctor if there is a medical need. The stockings can be bought at pharmacies, surgical equipment specialists, and now some other stores such as airports. These stockings come in a variety of sizes and your legs will need to be measured to get the right fit. People who have problems with the arteries in their legs should seek medical advice before using compression stockings.

Aspirin: Taking an aspirin tablet (either a 75 mg junior aspirin tablet or a standard 300 mg aspirin tablet) a few hours before a long trip can provide a little extra protection against DVT.
Anticoagulants: Special anticoagulants (e.g., heparin injections or warfarin by mouth) may be advisable for a small number of people with diseases at a particularly high risk of DVT. This type of treatment is always given on the express advice of a doctor.