preview.tinyurl.com/2lpeo8 -> medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0040290&ct=1&SESSID=5bc813e48f8414ce704b553737d86b73
References RESEARCH ARTICLE Open Access Everything we publish is freely available online throughout the world, for you to read, download, copy, distribute, and use (with attribution) any way you wish.
The objective of this study was to assess the absolute risk of venous thrombosis after air travel. Methods and Findings We conducted a cohort study among employees of large international companies and organisations, who were followed between 1 January 2000 and 31 December 2005. The occurrence of symptomatic venous thrombosis was linked to exposure to air travel, as assessed by travel records provided by the companies and organisations. A long-haul flight was defined as a flight of at least 4 h and participants were considered exposed for a postflight period of 8 wk. A total of 8,755 employees were followed during a total follow-up time of 38,910 person-years (PY). The total time employees were exposed to a long-haul flight was 6,872 PY. This rate was equivalent to a risk of one event per 4,656 long-haul flights. The risk increased with exposure to more flights within a short time frame and with increasing duration of flights. The incidence was highest in the first 2 wk after travel and gradually decreased to baseline after 8 wk. The risk was particularly high in employees under age 30 y, women who used oral contraceptives, and individuals who were particularly short, tall, or overweight. Conclusions The risk of symptomatic venous thrombosis after air travel is moderately increased on average, and rises with increasing exposure and in high-risk groups. Funding: This study was funded by grant number 2002B53 from The Netherlands Heart Foundation and sponsored by the UK government and the European Commission. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Academic Editor: Eduardo L Franco, McGill University, Canada Citation: Kuipers S, Cannegieter SC, Middeldorp S, Robyn L, Bller HR, et al.
This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. PY, person-year * To whom correspondence should be addressed. Blood normally flows smoothly throughout the human body, supplying the brain and other vital organs with oxygen and nutrients. When an injury occurs, proteins called clotting factors make the blood gel or coagulate at the injury site. The resultant blood clot (thrombus) plugs the wound and prevents blood loss. Sometimes, however, a thrombus forms inside an uninjured blood vessel and partly or completely blocks the blood flow. A clot inside one of the veins (vessels that take blood to the heart) deep within the body is called a deep vein thrombosis (DVT). Symptoms of DVT (which usually occurs in the deep veins of the leg) include pain, swelling, and redness in one leg. DVT is usually treated with heparin and warfarin, two anticoagulant drugs that stop the blood clot growing. If left untreated, part of the clot (an embolus) can break off and travel to the lungs, where it can cause a life-threatening condition called pulmonary embolism (PE). Fortunately, DVT and PE are rare but having an inherited blood clotting disorder, taking an oral contraceptive, and some types of surgery are all risk factors for them. In addition, long-haul plane travel increases the risk of DVT and PE, known collectively as venous thrombosis (VT) 2- to 4-fold, in part because the enforced immobilization during flights slows down blood flow. Although the link between air travel and VT was first noticed in the 1950s, exactly how many people will develop DVT and PE (the absolute risk of developing VT) after a long flight remains unknown. This information is needed so that travelers can be given advice about their actual risk and can make informed decisions about trying to reduce that risk by, for example, taking small doses of anticoagulant medicine before a flight. In this study, the researchers have determined the absolute risk of VT during and after long-haul air travel in a large group of business travelers. The researchers enrolled almost 9,000 employees from several international companies and organizations and followed them for an average of 44 years. The details of flights taken by each employee were obtained from company records, and employees completed a Web-based questionnaire about whether they had developed VT and what risk factors they had for the condition. Out of 53 thrombi that occurred during the study, 22 occurred within eight weeks of a long-haul flight (a flight of more than four hours). From this and data on the total time employees spent on long-haul flights, the researchers calculated that these flights tripled the risk of developing VT, and that the absolute risk (the probability of something occurring in a certain time period) of a VT occurring shortly after such travel was one event per 4,656 flights. They also calculated that the risk of VT was increased by exposure to more flights during a short period and to longer flights and was greatest in the first two weeks after a flight. In addition, the risk of VT was particularly high in young employees, women taking oral contraceptives, and people who were short, tall or overweight. The main finding of this study is that the absolute risk of VT after of a long-haul flight is low--only one passenger out of nearly 5,000 is likely to develop VT because of flying. However, the study included only healthy people without previous VT whose average age was 40 years, so the absolute risk of VT after long-haul flights might be higher in the general traveling population. Even so, this finding strongly suggests that prophylactic (preventative) use of anticoagulants by all long-haul travelers may not be justified because these drugs have potentially dangerous side effects (for example, they can cause uncontrolled bleeding). Subgroups of travelers with additional risk factors for VT might, however, benefit from the use of this and other prophylactic measures, but randomized trials are needed to find out who would benefit most from which prophylactic measure.
Hence, the absolute risk of symptomatic venous thrombosis after long-haul travel must lie between these extremes. Knowledge of the absolute risk of symptomatic thrombosis after air travel is needed to provide travellers with solid advice regarding their actual risk and to evaluate the utility of prophylactic measures.
In addition to estimating the absolute risk of symptomatic deep vein thrombosis or pulmonary embolism after long haul air travel, we assessed the effects of exposure to several flights within a short time frame, duration of travel, and the occurrence of venous thrombosis in relation to the time passed after air travel. Finally, we determined the effect of air travel within high-risk groups. Methods Study Design We performed a cohort study among employees of large international companies and organisations. During the follow-up period, thrombotic events were linked to exposure to air travel. Participating Companies and Organisations Participating companies and organisations were Nestl (Vevey, Switzerland), General Mills (Minneapolis, Minnesota, US), the US Centers for Disease Control and Prevention (Atlanta, Georgia, US), the World Bank and the International Monetary Fund (Washington, D C, US), Shell Companies based in The Hague (The Netherlands) and London (UK), Shell Exploration and Production (SIEP) based in Rijswijk (The Netherlands), Sakhalin Energy Investment Company Ltd (SEIC) based in Sakhalin (Russia), and TNT NV (Thomas Nationwide Transport, Hoofddorp, The Netherlands). All organisations and companies had a central database with records of employees' business travel. Start of follow-up varied per company, between 1 January 1998 and 1 January 2001 or at start of the employment if later. Follow-up ended between 1 December 2002 and 1 January 2006, when venous thrombosis was diagnosed or at the end of empl...
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