There has been much recent media coverage about the risk of developing a DVT (deep vein thrombosis) whilst flying on long haul flights and this has become known as “Economy Class Syndrome”.
Deep vein thrombosis cartoon
Who says that your legs don’t swell on long flights!
Veins carry blood from the tissues to the heart. In the legs, the superficial veins (which sometimes develop into varicose veins) collect blood from the skin, whilst much bigger veins – the deep veins – are situated within the muscles. During exercise the blood flow to the muscles increases and the volume of the flow in these deep veins (squeezed out as the muscles contract) is very large, whereas at rest the flow can be sluggish.
In order to stop us bleeding if we are injured, there is a complex coagulation system which converts liquid flowing blood into jelly like clots which block the damaged blood vessels thereby plugging the leak. This may be life saving but in certain circumstances blood may clot within normal blood vessels causing a “thrombosis”. This can occur if blood flow is reduced for long periods and this is why deep vein thrombosis used to be common after surgery when patients were immobilised in bed for long periods. Fortunately this is now uncommon since we use anti coagulant drugs to reduce clotting and we also get people moving quickly to increase the flow in their veins.
Other factors also increase the tendency for thrombosis. These include some drugs and increased viscosity (thickness) of blood.
What happens in DVT?
Blood clots in the deep veins cause the leg to swell as fluid leaks out into the tissues. This is often very painful. The clots develop first in the calf but may extend into the veins in the thigh or even up into the very large Iliac veins in the pelvis. Sometimes if appropriate treatment is started early, the clots will dissolve but this may still leave the veins permanently damaged leading to prolonged swelling and sometimes leg ulcers many years later.
The most dangerous effect however is Pulmonary Embolus where the clot becomes dislodged from the legs and rushes up through the abdominal veins into the heart. The clot may then lodge in the large pulmonary arteries blocking flow to the lungs. This usually causes severe chest pain with sudden collapse and may be fatal. This catastrophe may occur as the passenger gets up after many hours on the plane and walks to the terminal. Sometimes, however, swelling may start some days after the flight and collapse may follow. These cases are seen away from hospitals close to airports and in the past the connection may not have been realised!
Some things increase the risk for an individual. These include obesity and previous history of DVT. Women taking the contraceptive pill are always at a slightly higher risk of thrombosis, and HRT may marginally increase the risk. Flying should be avoided for some weeks after surgery.
What increases the risk?
Although DVT can occur after any form of travel the evidence that flying is a real risk is mounting and indeed would be expected to take in to account the factors listed above.
Why is there an increased risk?
If we now imagine ourselves on a long flight there are a combination of factors which will increase the risk of a DVT!
Firstly in the sitting position, particularly if there isn’t much room and our knees are bent, flow in the deep veins is restricted. If we sleep for long periods without moving the danger is even greater.
Secondly, we often become dehydrated when flying the cabin air is warm and dry and alcohol encourages excess fluid loss. This blood may become thicker than usual.
There is also the possibility that cabin pressure may have an effect on clotting. At normal cruising altitude (30,000 feet) you couldn’t breathe outside the plane the air pressure is too low. Within the cabin air is pressurised to approximately the same as 7,000 feet allowing us to breathe comfortably. The air at this altitude is, however, much thinner than at ground level; as you would know if you have ever skied at this height.
Can the risk be reduced?
Simple measures like drinking quantities of non alcoholic liquids during the flight (have a bottle of spring water with you). Get up and move around the cabin as often as you can and waggle your feet up and down when in your seat – This is a great way to increase vein flow! Wearing support stockings may help.
Aspirin is very effective in reducing the risk of clots. A junior aspirin daily starting some days before travel might be advisable, particularly if you are at high risk. But remember that this may be dangerous if you have had a stomach ulcer or problems with abnormal bleeding in the past.
Will I ever dare fly again?
The biggest danger in air travel is still overwhelmingly road accidents during the trip to the airport. Tens of millions of people fly long haul every year and very, very few of them get a DVT.
“Forewarned is forearmed”, and a few simple precautions may make all the difference. Enjoy your trip.
What to do if I am worried?
Ask your doctor before the trip if you think you might be at increased risk of DVT, or indeed if you are fit to fly if you have other medical problems. Conditions on board are very different from ground level and people with heart or chest problems may need advice. Diabetics may also need to change the timings of their medication.
If your doctor thinks you may have developed a DVT he could send you to the new Vascular Assessment Unit at the Royal Sussex County Hospital. All the equipment in the Unit has been provided by The Sussex Stroke & Circulation Fund through donations from the public. The most recent purchase is a “state of the art” Duplex ultrasound machine – this will diagnose a DVT immediately and painlessly without any unpleasant injections. If the test is positive treatment can be started immediately and further problems avoided.
By Paul Hurst BSc, MS, MRCP, FRCS
Consultant Vascular Surgeon
Royal Sussex County Hospital