Medical papers have been published since the 1950s about a possible link between deep vein thrombosis (DVT) and air travel. However, it was the publicity surrounding 2 unexpected deaths from pulmonary embolism — a blood clot travelling from the legs to the lungs — after long-haul air travel in 2000 that sparked renewed interest in the issue and much public debate.

Each year, DVT occurs in about 1-2 of every 1,000 people in the general population and in up to one-third of people who have had major surgery. Scientific study to quantify the risk of DVT posed by air travel — although it is suspected to be small in most people — is ongoing.

In the meantime, if you’re planning to travel by air, it’s a good idea to be aware of DVT and its symptoms, and to follow the currently accepted advice aimed at helping to prevent DVT.

What is DVT?

DVT is a condition in which a blood clot or ‘thrombus’ forms in the deep veins of the legs.

The return of blood from these deep veins to the heart is made more difficult by the force of gravity and the relatively long distance that blood needs to travel back to the heart, compared with return from other parts of the body.

The veins in the legs therefore use the squeezing, pump-like action of the leg muscles — as occurs with normal walking — and a system of non-return (one-way) valves in the walls of these veins to help move blood back towards the heart.

If the blood flow from the legs to the heart is further hampered, for example, by a person not moving around for a long time, then blood can pool in the leg veins, sometimes leading to a clot forming inside the leg veins — so-called deep vein thrombosis.

What are the symptoms of DVT?

DVT can result in no symptoms, or it can cause swelling and pain in the affected leg, for example, pain in the calf when the foot is flexed upwards.

Although DVT is a serious condition, it is the relatively rare complications of DVT that can be life-threatening. One such complication involves a piece of the blood clot in the leg vein breaking away and travelling through the circulation to lodge in a small blood vessel elsewhere in the body, blocking blood supply in this area. This fragment of the original clot is called an embolus.

An embolus that lodges in a blood vessel in the lungs results in the life-threatening condition called pulmonary embolism (PE). Symptoms of PE include shortness of breath, chest pain and, in the extreme, collapse and death due to respiratory failure and lack of blood flowing from the lungs back to the heart.

How is DVT diagnosed?

Diagnosis usually involves a special type of ultrasound scan of the leg called a duplex ultrasound. If this test does not show a DVT but the doctor still suspects a DVT based on the symptoms, then further tests such as venography may be carried out. Venography uses an X-ray image to track the distribution of a special dye injected into the deep veins of the leg. If DVT is being considered as a diagnosis, but is not thought to be likely, a simple blood test called a D-dimer may be ordered instead of an ultrasound.

How is DVT treated?

If symptoms are confined to the leg, the main aim of treatment is to prevent complications such as an embolus. This usually involves giving 2 medications to thin the blood. The first is given by injections under the skin (which can be done at home), or sometimes an intravenous infusion (this necessitates admission to hospital). The first medication is necessary because the second medication, usually warfarin tablets, takes several days to become effective. Once the warfarin is working well, the injections or infusion can be stopped.

In addition, you will be advised to elevate the leg and wear a compression stocking. The blood thinning tablets are usually continued for several months. Throughout treatment, blood samples are taken to monitor the blood’s clotting ability to make sure that it is ‘thin’ enough but not so ‘thin’ that bleeding becomes a high risk.

Pulmonary embolism requires urgent medical treatment that centres on measures to support heart and lung function, pain relieving medication and, again, blood thinning medication.

Who gets DVT?

Several factors, unrelated to travel, have been recognised as increasing the risk of a person getting DVT (see table). Your likelihood of getting DVT is thought to increase with the number of risk factors for DVT that you have.

(The following 2 tables are based on Chapter 6 of the 5th Report of the Select Committee on Science and Technology, UK Parliament, entitled ‘Air Travel and Health’, published in November 2000 and have been updated from a 2005 review by Australian experts.)

Factors considered to increase the risk of DVT
(‘risk factors’ for DVT)
Increasing age above 40 years (especially being elderly)
Being pregnant or recently having had a baby
Having had a DVT or pulmonary embolism previously or having a family member who has had a DVT
Having impaired blood clotting, especially any disorder that increases the tendency of your blood to clot
Having major medical disorders, e.g. chronic cardiorespiratory disease, inflammatory bowel disease, nephrotic syndrome or myeloproliferative disorders
Having cancer either now or in the past
Having a recent major injury or recent surgery, especially involving the abdomen or legs
Taking oestrogen hormone therapy or the contraceptive pill (especially if started within the last 2 weeks)
Being immobile for one or more days (for example as a result of being ill in hospital or following major surgery, wearing a plaster cast or having a paralysed leg)
Losing body fluids, for example through dehydration, which can make your blood more viscous (‘sticky’)
Being obese (body mass index over 30)*
Having varicose veins*
*Experts differ in their views as to whether these factors increase the risk of DVT.

It is also suspected that some characteristics of long-distance travel by car or train, and of air travel, may be risk factors for DVT because, for example, they can hinder the return flow of blood from the legs and/or make the blood more likely to clot.

Travel-related factors that may increase the risk of DVT
Road, rail and air
Increasing age above childhood
Travelling for a long time
Frequently undertaking long-distance travel
Being immobile (and this can happen just as easily in business class as in economy class)
Being constrained by seating, especially having insufficient leg-room
Sitting and sleeping in positions during travel that discourage return blood flow from the legs
Wearing tight underwear and/or clothes that restrict your movement
For obese people: having their immobility and seating discomfort made worse by their extra weight
For tall or short people: having their immobility and constrained seating and posture made worse by their height
For smokers: the psychological effects and the effects on their bodies of abiding by a non-smoking policy
Air (These are aircraft cabin factors that are considered to increase the risk of DVT.)
Lowered air pressure resulting in distension of the abdomen which slows down return blood flow from the legs
Lowered oxygen levels and/or pressure resulting in an increased tendency of blood to clot
Low humidity affecting the body’s fluid balance**
High consumption of alcohol and caffeine resulting in dehydration**
Restricted mobility due to aircraft safety procedures and cabin-crew service
Increasingly long non-stop flight sectors
** These factors are not unique to the aircraft cabin.

Air travel and the risk of DVT

An ongoing investigation by the World Health Organization (WHO) — WHO Research Into Global Hazards of Travel (WRIGHT) Project on Air Travel and Venous Thromboembolism — has found that the risk of DVT approximately doubles after a long-haul flight (more than 4 hours). This increased risk also applies to other forms of travel (such as car, bus or train) where people are exposed to prolonged seated immobility. The risk increases with the duration of travel and with multiple flights within a short period.

The WHO research also revealed that the risk of DVT increased significantly when other risk factors were present. Factors that contribute to the increased risk of travel-related DVT include obesity, extremes of height, use of oral contraceptives and pre-existing blood clotting abnormalities. The absolute risk of DVT in healthy passengers per long-haul flight (more than 4 hours) was found to be about 1 in 6000.

If you are planning to travel by air, and you know or suspect that you have any predisposing risk factors for DVT, you should discuss your risk of DVT with your doctor and find out what precautions you should take, and whether or not it is advisable for you to travel.

General precautions to take when travelling by air

Current advice suggests that, where possible, air travellers should adhere to the following precautions to help prevent DVT.

  • Drink plenty of water during the flight.
  • Avoid alcohol and caffeinated drinks before and during the flight.
  • Wear clothing that does not restrict your movement and avoid tight underwear.
  • Don’t take sleeping tablets when flying as the effect of these will further limit your mobility.
  • If possible, don’t let your thighs press on the edge of your seat — slide your legs and bottom forward so that the angle between your legs and abdomen is more open and your bottom is nearer the front of the seat. (This may not be possible if you are tall or have long legs.)
  • Use footrests where available or rest your feet on luggage to get your feet up.
  • Do in-flight exercises every half-hour throughout the flight that include flexing and stretching your legs and feet.
  • Occasionally, and only if it is safe to do so, take a walk around the aircraft cabin.

People with pre-existing risk factors for DVT

If you have pre-existing risk factors for DVT you may need to take additional precautions such as wearing prescribed support stockings or taking prescribed blood thinning medication, as discussed with your doctor.

In-flight exercises

Here is a sequence of in-flight exercises that you can do every half-hour during a flight as a precaution against getting DVT. The leg exercises aim to encourage blood flow from your legs to your heart and the upper body exercises are aimed at improving your circulation overall.

foot circles 1. Foot circles. Lift your feet off the floor. Moving both feet together, draw an imaginary circle with each big toe so that each foot rotates about the ankle joint. Continue several times in one direction, then repeat in the other direction.
heel lift/ toe lift 2. Heel lift/Toe lift. Start with your feet flat on the floor. Then lift your heels as high as is comfortable while leaving your toes and the balls of your feet on the floor. Lower your heels and repeat several times. Now leave your heels on the floor and gently lift your toes and the front of your feet off the floor, thus flexing your ankle as far as is comfortable. Lower and repeat several times.
knee raises 3. Knee raises. Sitting upright with your feet flat on the floor, lift one leg up while keeping your knee bent, hold for 2-3 seconds, then lower. Do the same with the other leg. Repeat the sequence at least 20 times for each leg.
knee curls 4. Knee curls. Sitting upright, gently lean forward while at the same time raising one knee. Grasp the knee with both arms and gently pull the leg towards your chest as you then lean back. Hold for 15 seconds, then release and gently lower the leg. Do the same for the other leg. Repeat the sequence 10 times.
neck stretches 5. Neck stretches. Start with your head in an upright position. Gently drop your right ear towards your right shoulder as far as is comfortable. Then gently roll your head forwards until you are looking down at your lap. Finally roll your head gently up towards your left shoulder, then lift your head to the upright position. Alternate the direction and repeat several times.
shoulder rolls 6. Shoulder rolls. Sitting comfortably, roll your shoulders gently backwards, continuing in a circular motion, several times. Repeat the circles, rolling your shoulders forwards several times.

After the flight

If you have travelled by air you should be alert for the symptoms of DVT for up to one month after flying, and should seek medical help if any symptoms such as a swollen or painful leg, especially the calf, and/or breathing difficulties occur.