No link between low air pressure on long haul flights and DVT, says new study

No link between low air pressure on long haul flights and DVT, says new study

Researchers simulating conditions of reduced cabin pressure and reduced oxygen levels, such as those which may be encountered during an 8-hour aeroplane flight, found no increase in the activation of the blood clotting system among healthy individuals, according to a study in the May 17 issue of JAMA (Journal of the American Association).

Venouss 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), have 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.

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.

Professor Mike Greaves, Head of the University of Aberdeen’s School of Medicine and Chair in Haemotology, and Dr William Toff from the University of Leicester’s Department of Cardiovascular Sciences, conducted a study, from September 2003 to November 2005, to examine whether reduced air pressure and oxygen comparable to that on an airliner increases the ability of blood to form clots.

The study included 73 healthy volunteers who spent eight hours seated in a 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 Aberdeen and Leicester. The chamber studies were performed at the RAF Centre of Aviation Medicine at Henlow in Bedfordshire and the University chamber in 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 changes in a very extensive and sophisticated range of tests of blood clotting.

Professor Mike Greaves said: “Our study provides reassurance regarding previous concerns about the effect of low air pressure and oxygen on clotting in healthy people. The carefully conducted study closely simulated the relevant conditions experienced during lengthy air travel and the results indicate that blood clotting is not adversely affected.

“It is known that immobility is an important risk factor for deep vein thrombosis, due to the reliance of blood flow in the veins of the legs on muscle contraction and relaxation. It is likely, therefore, that in healthy people it is the sedentary state imposed by long distance mechanised travel, including air travel, which is a major contributor to the risk of deep vein thrombosis.

“However, although deep vein thrombosis related to long distance travel is a reality, it should be remembered that the risk is extremely low for the average traveller.”

Dr Toff added: “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.”

(JAMA. 2006;295:2251-2261. Available pre-embargo to the media at www.jamamedia.org)

· This study was funded by the UK Department for Transport, the UK Department of Health, and the European Commission.

ENDS

Notes to Editors:

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), have 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.

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.

Professor Mike Greaves, Head of the University of Aberdeen’s School of Medicine and Chair in Haemotology, and Dr William Toff from the University of Leicester’s Department of Cardiovascular Sciences, conducted a study, from September 2003 to November 2005, to examine whether reduced air pressure and oxygen comparable to that on an airliner increases the ability of blood to form clots.

The study included 73 healthy volunteers who spent eight hours seated in a 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 Aberdeen and Leicester. The chamber studies were performed at the RAF Centre of Aviation Medicine at Henlow in Bedfordshire and the University chamber in 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 changes in a very extensive and sophisticated range of tests of blood clotting.

Professor Mike Greaves said: “Our study provides reassurance regarding previous concerns about the effect of low air pressure and oxygen on clotting in healthy people. The carefully conducted study closely simulated the relevant conditions experienced during lengthy air travel and the results indicate that blood clotting is not adversely affected.

“It is known that immobility is an important risk factor for deep vein thrombosis, due to the reliance of blood flow in the veins of the legs on muscle contraction and relaxation. It is likely, therefore, that in healthy people it is the sedentary state imposed by long distance mechanised travel, including air travel, which is a major contributor to the risk of deep vein thrombosis.

“However, although deep vein thrombosis related to long distance travel is a reality, it should be remembered that the risk is extremely low for the average traveller.”

Dr Toff added: “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.”

JAMA. 2006;295:2251-2261. Available pre-embargo to the media at www.jamamedia.org

· This study was funded by the UK Department for Transport, the UK Department of Health, and the European Commission.

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