Venous thromboembolism, a term used to describe deep vein thrombosis (DVT; blood clots forming in the veins) and pulmonary embolism (clots passing to the lungs where they may obstruct the blood flow), has been associated with long-haul air travel, but it has been unclear whether this is due to the effects of sitting for a long time, or whether there is a relationship with some other specific factor in the aeroplane environment, according to background information in the article. One hypothesis has been that hypoxia (reduced oxygen in the blood), associated with the decreased cabin pressure that occurs at altitude, produces changes in the blood that increase the risk for blood clots.
Dr William D. Toff, from the University of Leicester's Department of Cardiovascular Sciences and colleagues -including in the UK Professor Mike Greaves, Head of the University of Aberdeen's School of Medicine -conducted a study, from September 2003 to November 2005, to assess the effects of hypobaric (reduced air pressure) and hypoxia, similar to that which might be encountered during commercial air travel, on a variety of markers of activation of the haemostatic (blood clotting) system. The study included 73 healthy volunteers who spent 8 hours seated in a hypobaria chamber and were exposed to hypobaric hypoxia, similar to the conditions of reduced aeroplane cabin pressure that might occur during a long-haul flight (cabin pressure may be reduced to the equivalent of that at an altitude of about 8,000 feet). Blood was drawn before and after exposure to assess activation of factors associated with haemostasis (blood clotting). Similar measurements were taken of the same volunteers, on a separate occasion, before and after they spent 8 hours seated in a controlled environment, equivalent to atmospheric conditions at ground level (normobaric exposure).
The study was a collaboration between the Universities of Leicester and Aberdeen. The chamber studies were performed at the RAF Centre of Aviation Medicine at Henlow in Bedfordshire and Aberdeen. Analysis of blood samples was performed at Leicester and Aberdeen, and also by colleagues at the Academic Medical Center at the University of Amsterdam.
The researchers found that when comparing the results between the normobaric and the hypobaric exposures, there was no significant difference in the overall change for markers of coagulation activation (clot formation), fibrinolysis (the normal breakdown of small, naturally occurring blood clots), activation of platelets (small cells in the blood that clump together when stimulated to promote clot formation), and activation of endothelial cells (the cells that line the interior surface of blood vessels).
Dr Toff said: "Our study provides, for the first time, a carefully controlled assessment of the effects on blood clotting of the low air pressure and low oxygen level that might be found during a long-haul flight. We found no evidence that these conditions cause activation of the blood clotting mechanism.
"In our study we predominantly used healthy young people without known risk factors for thrombosis. We included some users of the combined oral contraceptive pill and some older subjects (aged over 50 years), both of which are factors that increase the risk of thrombosis. Although we found no difference in the results in these groups, the numbers of each were relatively small. We plan to extend the study to look at more people with these and other risk factors to see if they respond differently.
"Our volunteers abstained from alcohol during the study, to avoid any possible confounding of results. Air travellers are often advised to moderate their alcohol consumption in-flight, as excessive alcohol may induce somnolence and decrease mobility. Prolonged seated immobility is probably the most significant factor predisposing to thrombosis during long-haul travel. For the same reason, hypnotics (sleeping tablets) are sometimes also discouraged."
"In conclusion, our findings do not support the hypothesis that hypobaric hypoxia of the degree that might be encountered during long-haul air travel is associated with prothrombotic alterations in the haemostatic system in healthy individuals at low risk of venous thromboembolism," the authors write.
(JAMA. 2006;295:2251-2261. Available pre-embargo to the media at www.jamamedia.org)
Editor's Note: This study was funded by the UK Department for Transport, the UK Department of Health, and the European Commission.
The aim of this study was to learn more about the mechanisms that might lead to thrombosis after air travel.
Dr Toff said: "Although we found no evidence that the low pressure and low oxygen activate blood clotting, we know from many other studies that prolonged sitting, such as during long journeys by air, road or rail, does increase the risk of deep vein thrombosis (DVT). That link is quite clearly established.
"The overall risk of thrombosis after a long-haul flight is estimated to be about 1 in 2,000. For people with known risk factors, the risk may be higher but for those without other risk factors it is likely to be very low and should be kept in perspective.
"People undertaking long-haul travel should take sensible precautions appropriate to their own inherent level of risk. For everyone, that should include regular leg exercises and getting up to walk around from time to time when conveniently possible. People with known risk factors should take advice about other preventive measures, as described, for example, on the Department of Health website.
"Other preventive measures might include wearing graduated compression stockings. In people at the highest risk (e.g. a person who has previously had a flight-related thrombosis), the use of anti-coagulant drugs, such as low-molecular weight heparin, might be considered." Other risk factors for deep vein thrombosis (DVT) include:
- Increasing age
- Previous venous thrombosis
- Family history of venous thrombosis
- Suffering from or having had treatment for cancer
- Certain blood diseases
- Being treated for heart failure and circulation problems
- Recent surgery especially on the hips or knees
DVT is also more common in women who:
- Are pregnant
- Have recently had a baby
- Are taking the contraceptive pill
- Are on hormone replacement therapy or HRT.
Combinations of risk factors may be synergistic - that is to say, the combined risk is more than additive.
Dr Toff concluded: "Long-haul travel, whether by air or other modes, is associated with an increased risk of thrombosis and travellers should consider taking appropriate steps to minimise that risk, as described, for example, on the Department of Health website."
University of Leicester: Ather Mirza-0116 252 3335 email: firstname.lastname@example.org
University of Aberdeen: Jennifer Phillips -01224 273174 email: email@example.com
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