Are YOU at risk of developing a shock blood clot from flying?

Travel-related deep vein thrombosis (DVT) has remained a hot topic ever since the link with long-haul flights was first recognized. 

Like Russian Roulette, it seems to strike at random, in otherwise healthy, fit adults – many of whom are young with no obvious risk factors apart from the fact they have recently flown by air. 

Here, Dr Sarah Brewer, medical director of Healthspan, explains why it strikes and how to protect yourself. 

What is travel-related thrombosis?

A thrombosis is an unwanted clot that forms within the circulation. 

A deep vein thrombosis (DVT) is the formation of a blood clot within a deep lying vein, most commonly in the thigh or calf. 

DVT occurs when blood flow becomes sluggish or constricted so that blood cells more easily clump together. 

If the clot remains where it formed, then local symptoms occur, such as swelling, pain and discoloration. 

If the clot breaks off and travels within the circulation to the lungs, this is known as pulmonary embolus (PE) and is a life-threatening condition requiring urgent hospital treatment.

Guidelines state that a DVT or PE that occurs within eight weeks of travel may be attributed to the journey and classed as travel-related.

Deep vein thrombosis – also known as ‘economy class syndrome’ – can be life-threatening if the clot breaks off and travels up to the lungs

Why does travel increase the risk of DVT?

DVT mostly results from prolonged inactivity during travel, which causes blood to pool in the legs. 

Although the problem is often referred to as Economy Class Syndrome – because cheaper seats in a plane have less leg room and encourage minimal movement – this is misleading, as the problem is not just confined to Economy Class, or to long haul flights.

People flying in Business and First Class – even on flying beds – have also been affected, and some deaths have been linked with short haul flights. 

It seems it is not just inactivity or cramped space that’s a problem – other risk factors are also involved, such as reduced oxygen saturation in the cabin, dehydration, and reduced cabin pressure which causes fluid to leak from the circulation into the lower legs to cause swelling.

DVT can also result from other forms of travel which involve prolonged immobility. 

Any long distance journey lasting 4 hours or more doubles your risk of a venous thrombosis, however you travel – whether by plane, car, coach or train. 

Some studies show that for healthy people, the risk of DVT is quadrupled with air travel, with longer flights carrying the greatest risk.

For flights lasting less than four hours, the risk of a DVT is low, with one estimated DVT for every 106,667 flights. 

For flights lasting over four hours, the risk of a DVT dramatically increases to one event per 4,656 flights. For flights lasting 16 hours or more, the risk is one event per 1,264 flights.

Who is most at risk of travel related thrombosis?

For most people, the risk of a travel-related DVT is low, but it’s important to know if you are at increased risk so you can take steps to reduce that risk. 

Many women don’t realize, for example, that if you board a plane while taking HRT or the contraceptive pill, your risk of a travel thrombosis is significantly increased as estrogen hormone increases blood stickiness.

Another risk factor that is not widely recognized is that people who are extremely tall (over 1.9 meters) or short (less than 1.6 meters) are also at increased risk. 

If you are short, the back of the seat will press more against the back of your legs, while if you are very tall, blood has to flow further through the veins and is more likely to become sluggish.

People at low risk

According to the National Institute of Health and Care Excellence (NICE), your risk of a travel-related DVT (for those aged 16 and above) is low if you have no history of DVT or pulmonary embolism, and if you have not undergone surgery in the previous four weeks or have no other risk factors to indicate moderate or high risk.

People at moderate risk

You are at moderate risk of a travel-related DVT if you:

  • Have a previous history of DVT or PE (although the risk is low if you are still on anticoagulant treatment). 
  • Are a smoker – smoking, which makes blood stickier. 
  • Recently had surgery under a general anesthetic which lasted more than 30 minutes during the previous two months (risk is high if this was within the last four weeks). 
  • Are pregnant or recently had a baby. 
  • Have had a recent heart attack, or have uncontrolled heart failure or other recent major illness such as pneumonia. 
  • Are taking the combined oral contraceptive pill or hormone replacement therapy. 
  • Are obese (body mass index greater than 30 kg/m2). 
  • Have varicose veins with phlebitis (inflammation of the veins). 
  • Have a family history of DVT or PE in a first degree relative (mother, father, brother, sister). 
  • Have a condition called polycythaemia in which you make more red blood cells than normal. 
  • Have a lower-limb fracture that is still in plaster. 

The more of these risk factors that you have, then the higher your risk of developing a travel-related DVT. 

For example, being overweight doubles the risk of thrombosis, but in woman who are both overweight and take the oral contraceptive pill, the risk increases tenfold.

People at high risk

You are at high risk of a travel-related DVT if you have undergone surgery within the previous four weeks in which you were under general anesthesia for more than 30 minutes. 

You are also at high risk if you have a blood clotting disorder (thrombophilia, such as antithrombin III deficiency).

You are also at higher risk if you have cancer, whether untreated or currently on treatment. 

What are the symptoms to look out for?

The symptoms of DVT can include:

  • Tenderness and redness in the affected area e.g. calf 
  • Swollen, painful legs, especially if one is more affected than the other 
  • Discoloration of the affected area due to poor blood flow 
  • Fever

Sometimes the first indication that you have a travel-related DVT is when one or more bits of the clot break off and travel in the blood stream to your lungs to cause a pulmonary embolus (PE).

A PE blocks blood flow to a part of the lung and, as well as interfering with oxygen uptake, can cause the lung to collapse or lead to heart failure. Symptoms such as stabbing chest pain and shortness of breath may not occur for several days or even weeks after the DVT initially formed.

The symptoms of a PE can include:

  • Rapid heart beat 
  • Sudden, unexplained cough 
  • Stabbing chest pain 
  • Shortness of breath 
  • Collapse 

If you think you could have symptoms of a DVT or PE (whether or not you have travelled within the previous 8 weeks) seek immediate medical advice from your doctor or the nearest A&E department. 

How to prevent a travel-related DVT

  1. Before flying, consider whether you are fit to travel – if you are at a higher risk of DVT you should consult a doctor before flying. Ensure you have adequate medical insurance before travelling.
  2. If you are taken any prescribed medications, check with a doctor or pharmacist whether you need to take any additional precautions.
  3. Take pycnogenol (150mg) starting three days before you travel, and for three days afterwards but check with GP if on any other medication prior
  4. If you are already taking aspirin for a health condition, then continue to take it (don’t increase your dose) but otherwise, NICE does not recommend aspirin for the prevention of travel-related DVT as there is no evidence that it is effective, and it is not clear that the benefits outweigh the risks. If anti-clotting measures are needed, then an anticoagulant may be prescribed.[viii]
  5. Book an aisle seat if possible, so you can get up regularly to walk around on the plane. Aim to walk up and down the aisle at least once an hour during a long flight.
  6. Travel light, so you can put hand luggage in the overhead locker rather than under the seat in front of you, which will restrict your foot room.
  7. Wear loose-fitting, comfortable clothing and drink plenty of water before and during the trip.
  8. Avoiding alcohol – even if it’s free – as alcohol is dehydrating and also encourages inertia. Avoiding sleeping pills which can increase immobility
  9. Recline your seat as much as possible and don’t cross your legs or ankles
  10. While sitting, bend and straighten your legs, feet and toes regularly, at least every half-hour. Push your heels, and then the balls of your feet down hard against the floor or footrest, lifting them up and down to increase the blood flow through your legs. Flight exerciser cushions are available to make this easier – studies show that in-flight exercisers can increase blood flow in the femoral vein as much as five-fold. Don’t forget your upper body – exercise your arms and do deep breathing exercises, too
  11. Wear graduated compression stockings or flight socks to reduce leg swelling but have them properly measured and fitted by a pharmacist so they do not constrict the circulation by being too tight. Class 1 stockings and flight socks which exert a pressure of 14mmHg to 17mmHg at the ankle are usually sufficient to prevent swelling if you are generally healthy. Higher ratings are available if have existing problems with leg swelling (due to varicose veins, for example)
  12. During any refuelling stops, get up and walk around and, if it’s possible to leave the plane, take advantage of this