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Study results released on travel and blood clots

WHO project finds VTE risk higher after long travel, but still relatively low

WHO today released results from Phase I of the WHO Research Into Global Hazards of Travel (WRIGHT) project. Findings indicate that the risk of developing venous thromboembolism (VTE) approximately doubles after travel lasting four hours or more. However, the study points out that even with this increased risk, the absolute risk of developing VTE, if seated and immobile for more than four hours, remains relatively low at about 1 in 6000.

The two most common manifestations of VTE are deep vein thrombosis and pulmonary embolism. Deep vein thrombosis (DVT) is a condition in which a blood clot, or thrombus, develops in a deep vein - usually in the lower leg. Symptoms of DVT are principally pain, tenderness and swelling of the affected part. DVT can be detected through medical testing and can be treated. It can be life-threatening when associated with thromboembolism.

Thromboembolism occurs when a blood clot (from a deep vein thrombosis) in a leg vein breaks off and travels through the body to the lungs where it becomes lodged and blocks blood flow. This is known as pulmonary embolism, and symptoms include chest pain and breathing difficulties. VTE can be treated, but if it is not, it can lead to death.

The study showed that plane, train, bus or automobile passengers are at higher risk of VTE when they remain seated and immobile on journeys of more than four hours. This is due to a stagnation of blood in the veins caused by prolonged immobility, which can promote blood clot formation in veins.

One study within the project examining flights in particular found that those taking multiple flights over a short period of time are also at higher risk. This is because the risk of VTE does not go away completely after a flight is over, and the risk remains elevated for about four weeks.

Other factors of influence

The report shows that a number of other factors increase the risk of VTE during travel, including obesity, being very tall or very short (taller than 1.9 meters or shorter than 1.6 meters), use of oral contraceptives, and inherited blood disorders leading to increased clotting tendency.

"The study does confirm that there is an increased risk of venous thromboembolism during travel where the passenger is seated and immobile for over four hours, whether in a plane, train, bus or car. However, it is important to remember that the risk of developing VTE when travelling remains relatively low," says Dr Catherine Le Galès-Camus, WHO Assistant Director-General for Noncommunicable Disease and Mental Health.

This study did not investigate effective preventive measures against DVT and VTE. However, experts recognize that blood circulation can be promoted by exercising the calf muscles with up-and-down movements of the feet at the ankle joints. Moving feet in this manner encourages blood flow in the calf muscle veins, thus reducing blood stagnation. People should also avoid wearing tight clothing during travel, as such garments may promote blood stagnation.

Phase I of the research project concludes that there is a need for travellers to be given appropriate information regarding the risk of VTE by transport authorities, airlines, and medical professionals. Further studies will be needed to identify effective preventive measures. This will comprise Phase II of the project, which requires additional funding before it can begin.

Individuals with questions regarding prevention of VTE should consult their physicians before travelling.

Background of WRIGHT project

In 2000, media and public attention was focused on the risk of thrombosis in long-haul travellers, following the death from pulmonary embolism of a young English woman who returned on a long-haul flight from Australia. In the same year, a report from the Select Committee on Science and Technology of the United Kingdom House of Lords recommended research into the risk of DVT. Following a consultation of experts convened by WHO in March 2001, the WRIGHT project was initiated. Phase 1 was funded by the UK Government (Department for Transport and Department of Health) and the European Commission.

The objectives of Phase I were to confirm whether the risk of VTE is increased by air travel and to determine the magnitude of risk.

The studies were conducted under the auspices of WHO and performed by an international collaboration of researchers from the Universities of Leiden, Amsterdam, Leicester, Newcastle, Aberdeen and Lausanne. There were five studies:

  • a population-based case control study to investigate the risk factors of VTE;
  • two retrospective cohort studies among employees of international organizations and Dutch commercial pilots to investigate the actual risk of VTE related to air travel; and
  • two pathophysiological studies to investigate the influence of immobility on VTE related to travel and the influence, if any, of low oxygen and low pressure in the cabin of aircraft on VTE related to travel.
For further information journalists may contact:

Shanthi Mendis
Senior Adviser
Cardiovascular diseases, Chronic Disease and Health Promotion
WHO, Geneva
Tel.: +41 22 791 3141
Mobile: +41 79 505 7455
E-mail: mendiss@who.int

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