11 July 2007

WHO study results on travel and venous thromboembolism

On 29 June 2007, the World Health Organization (WHO) released results from Phase I of the WHO Research Into Global Hazards of Travel (WRIGHT) project [1].

The WRIGHT project was initiated in March 2001 following a consultation of experts convened by WHO. The objectives of Phase I were to confirm that the risk of venous thromboembolism (VTE) is increased by air travel, to determine the magnitude of risk, and to assess the effect of other factors on the risk. Phase I consisted of five studies analysing different aspects of the association between travel and VTE. The project was funded by the United Kingdom’s Department of Transport and Department of Health, and the European Commission.

The findings of Phase I indicate that the risk of VTE approximately doubles after a flight of four hours or longer. The risk also doubles with other forms of travel including train, bus and automobile where travellers are exposed to prolonged immobility while seated. The risk increases with duration of travel and with multiple flights within a short period of time. It is important to note, however, the absolute risk of developing VTE if seated and immobile for more than four hours remains relatively low at 1 case in 6,000 healthy individuals. 

The report also found that a number of other factors increase the risk of VTE during travel. These include obesity, extremes of height (over 1.9 metres or under 1.6 metres (a risk seen with air travel only)), use of oral contraceptives and the presence of genetic blood disorders which result in increased clotting tendency such as factor V Leiden mutation.

Phase II of the study will aim to identify effective preventive measures. 

Advice for travellers

The two most common manifestations of VTE are deep vein thrombosis (DVT) and pulmonary embolism (PE). A DVT is the formation of a clot or thrombus which develops in the deep veins, usually of the lower leg. A PE is a complication of DVT and occurs when the thrombus dislodges and travels to the lungs.

The evidence for an association between DVT and long-haul travel has been reviewed previously [2-4] and is summarised in the NaTHNaC Health Information Sheet: Travel Related Deep Vein Thrombosis.

Although Phase I of the WRIGHT study did not review preventive measures for VTE, there is general consensus on the following practical measures to decrease the risk of VTE:

• Exercising the calf muscles while seated by flexing and extending the feet and ankles.
• Taking regular deep breaths which will increases venous return.
• Avoiding dehydration and excessive consumption of alcohol.
• Not wearing constrictive clothing around the waist or lower extremities.
• Walking around the plane cabin at regular intervals.
• Avoiding stowing hand luggage under the seat if it restricts movement.

Wearing properly fitted compression stockings will reduce the risk of DVT and their use should be considered by travellers who are at increased risk [5].

Low molecular weight heparin (LMWH) may be appropriate for travellers at high risk of developing DVT, for example a history of previous DVT [6,7]. This should be discussed with a haematologist.

Travellers who have other risk factors for DVT are advised to discuss their travel plans with their doctor or practice nurse. 

References

1. World Health Organization press release. Study results released on travel and blood clots. WHO project finds VTE risk higher after long travel, but still relatively low. 29 June 2007. [Accessed 11 July 2007] Available at http://www.who.int/mediacentre/news/releases/2007/pr35/en/

index.html

2. Ferrai E, Chevallier T, Chapelier A, Baudouy M. Travel as a risk factor for venous thromboembolic disease: a case control study. Chest 1999; 115: 440-44

3. Cannegieter SC, Doggen CJM, van Houwelingen HC, Rosendaal FR. Travel-related venous thrombosis: Results from a large population-based case control study (MEGA study). PLoS Med. 2006; 3: 1258-1265.

4. Kelman CW, Kortt MA, Becker NG et al. Deep vein thrombosis and air travel: record linkage study. BMJ. 2003; 327: 1072-1075

5. Clarke M, Hopewell S, Juszczak E, Eisinga A, Kjeldstrom M. Compression stockings for preventing deep vein thrombosis in airline passengers. Cochrane Database Syst Rev 2006;CD004002. [Accessed 11 July 2007] Available at: http://www.mrw.interscience.wiley.com/cochrane/clsysrev/

articles/CD004002/pdf_fs.html

6. Geerts WH, Pineo GF, Heit JA et al. Prevention of venous thromboembolism. The seventh ACCP conference on antithrombotic and thrombolytic therapy. Chest 2004; 126: 338S-400S.

7. Board of Science and Education of the British Medical Association. The impact of flying on passenger health: a guide for healthcare professionals. 2004; BMA.

Links

World Health Organization. WRIGHT project, final report from Phase I.

http://www.who.int/cardiovascular_diseases/wright_project/

phase1_report/en/index.html

NaTHNaC Health Information Sheets. Travel related deep vein thrombosis.

http://www.nathnac.org/pro/factsheets/trav_dvt.htm