Health news - No link between low air pressure and DVT on long-haul flights
26 May 2006 - written by Kate Nichols for Bupa's health information team
Reduced cabin pressure and oxygen levels associated with long-haul flights don't increase the risk of developing deep vein thrombosis (DVT), according to a study published this month by the Journal of the American Medical Association.
How was the study carried out?
Seventy-three healthy volunteers took part in the study, which took place over a two-year period. They each spent eight hours seated in a hypobaric chamber, which simulates the effects of reduced air pressure and blood oxygen levels associated with flying at altitude. Their blood was examined before and after the test to see if there was any sign of blood-clot formation.
The volunteers also underwent a second test, similar to the first, except that the atmospheric conditions were changed to those found at ground level. As before, tests were taken before and after the test.
What did the study show?
The researchers compared the results of the two tests and found that there were no significant differences in the overall changes for markers of blood-clot formation. They concluded that there was no evidence that low cabin pressure and correspondingly low oxygen levels increase the risk of DVT or pulmonary embolisms (blood clots in the lungs) to healthy individuals on long-haul flights.
How conclusive were the findings?
The number of participants used was relatively small, so more extensive tests may be needed. Most of the participants were healthy and young, although some people with an increased risk of thrombosis were also used - for example, older people and some women who were taking the contraceptive pill. The researchers said they wanted to look at more people with these and other risk factors to see if they responded differently to the tests.
Did the study contradict other findings?
The risk of developing DVT on long-haul flights has been known for a number of years. However, it has been unclear whether this is due to the effects of sitting for a long time, or whether there is another specific factor in the aeroplane environment (such as low cabin pressure) that plays a part. Past studies have concluded that prolonged sitting (often experienced on long journeys by road, rail or air) remains the common underlying risk factor in relation to the onset of DVT. The current study seems to contradict the theory that reduced cabin pressure on flights is a significant risk factor as well.
What is DVT?
DVT is a condition where blood clots form in the deep veins of the legs. If part of the blood clot breaks off, it could travel to an artery in the lungs and get stuck - this is called a pulmonary embolism and is a dangerous complication that can be fatal.
Blood clots are a normal part of the body's healing mechanism. They form naturally in the body to stop severe loss of blood from veins and arteries after an injury. But DVT is different, since these blood clots form within the blood vessel - not outside it. Doctors say that some people are at greater risk of DVT than others. You are more at risk if you:
- have just had an operation
- sit or lie for long periods
- are over 40 years of age
- are obese
- have had blood clots before
- smoke
- have cancer
- are pregnant
- are taking the contraceptive pill that contains oestrogen or having hormone replacement therapy (HRT)
How common is DVT?
The exact number of people who get DVT is hard to quantify, since many people don't seek medical help for the condition, which can resolve by itself. However, studies suggest that DVT occurs in around 1-3 in 1000 people of the general population in the UK. That figure drops to less than 1 in 3000 for people under the age of 40, but rises to one in a few hundred for people over 80. Around one in 100 of those who are diagnosed die of the condition.
What are the symptoms of DVT?
A common symptom of DVT is swelling and/or pain in the calf or thigh on one leg. The leg can also feel hot and go red or purple in places. But some people with DVT don't get any symptoms at all.
If the blood clot has travelled to an artery in your lung, you could feel dizzy, breathless, have chest pains, cough up blood and feel your heart racing. However, some of these symptoms are associated with other conditions.
If you are worried you might have DVT, see your doctor immediately. You may be sent to have an ultrasound scan, a blood test or a venography - an X-ray where dye is injected into your vein. You may also have your lungs scanned to check for a pulmonary embolism.
Why do long-haul flights pose a risk of DVT?
A lot of research has been done on DVT and long-haul flights. Many doctors agree that DVT on long-haul flights may happen because people are sitting still for long periods and doing very little activity. However, the research is not conclusive and it may be that other factors cause certain people to develop DVT on long journeys.
How can you minimise the risk of getting DVT on flights or long journeys?
Department of Health guidelines suggest that you could reduce the risk of getting DVT on long-haul flights (and other long journeys) in the following ways:
- try to sit comfortably in your plane seat
- wear comfortable clothes
- bend and straighten your legs, feet and toes every 30 minutes or so
- press your feet hard against the floor to help increase the blood flow in your legs
- do upper-body and breathing exercises to further improve circulation
- take occasional walks (even if only down the aisle)
- drink enough water
- consider wearing elastic compression stockings
- don't drink alcohol, which can lead to dehydration and immobility
- don't take sleeping pills, which also cause immobility
Should I consult my doctor before I travel?
Consulting you doctor before a long trip is sensible if you have:
- ever had a DVT or pulmonary embolism
- ever had a stroke
- ever had cancer or cancer treatment
- had major surgery in the previous three months
- had a hip or knee replacement in the previous three months
- any family history of blood-clotting conditions
- an inherited tendency to clotting problems
Can you get DVT on other long journeys where movement is limited?
Yes. DVT could develop on any form of long-distance travel - whether by air, car, coach or train. Research from the Department of Health says that the risk of blood clots developing in the legs increases significantly after four hours of travel time.
Bupa information
- Bupa factsheet: Deep vein thrombosis
Sources and further information
- Department of Health
www.dh.gov.uk - Circulation Foundation
www.circulationfoundation.org.uk - Toff WD, Jones CI, Ford I, Pearse RJ, Watson HG, Watt SJ et al. Effect of hypobaric hypoxia, simulating conditions during long-haul air travel, on coagulation, fibrinolysis, platelet function, and endothelial activation. JAMA 2006; 295 (19): 2251-2261
