CANBERRA, AUSTRALIA. Australian researchers have just released a study relating hospital admissions for deep vein thrombosis (DVT) to air travel. They found that 46 out of 4.8 million Australian airline passengers arriving in Western Australia during 1981-1999 had been admitted with DVT within 2 weeks of arrival. Out of 4.6 million non-Australians arriving over the same period 200 were admitted. Thus total hospital admissions for DVT was 26.5 per million passengers. The corresponding mortality from pulmonary embolism (a possible end result of DVT) was 1 per 2 million passengers or 2% of passengers diagnosed with DVT. The DVT rate was fairly low until about age 40 years, after which it increased quite dramatically to reach 140 per million at age 75 years and older. The main predisposing factors for DVT are blood clotting disorders, cancer, heart failure, obesity, leg fracture, recent surgery, infection, and pregnancy. The risk is also increased by conditions common during air travel such as sitting still for long periods of time, dehydration, smoking, and alcohol consumption.
Kelman, CW, et al. Deep vein thrombosis and air travel: record linkage study. British Medical Journal, Vol. 327, November 8, 2003, pp. 1072-76

Editor's comment: International air travel now accounts for about 1.56 billion person trips each year. Applying the Australian findings to this number would result in a total worldwide incidence (based on hospital admissions) of DVT related to air travel of some 41,000 passengers per year and a total mortality of about 780 passengers. Of course, it is entirely possible that some passengers may have developed DVT beyond the 2-week survey period or, in the case of foreign travellers, may have left Australia before DVT was diagnosed.

Other researchers have provided very different estimates of DVT risk related to air travel. Scurr et al at the Royal Free and University College in London, UK estimate that as many as 10% of all air travellers develop symptomless DVT in the calf[1]. American researchers observed that 17% of patients with DVT admitted to hospital developed their symptoms during or after a flight[2]. Considering that the overall annual incidence of DVT (including air travel) is about 48 per 100,000 people, the total number of air travellers suffering DVT would be about 75,000 per year[3]. The LONFLIT study carried out recently found an overall DVT risk of 1.5% among 744 long distance air travellers[4]. All passengers affected by DVT in the LONFLIT study had either cardiovascular disease or used pharmaceutical drugs. Assuming that even as little as 5% of all airline flights are long distance, the LONFLIT data would indicate that over 1 million passengers are at risk for DVT every year. Airhealth.org, an organization dedicated to research into the risk of DVT related air travel, estimates that there are 100,000 deaths in the US alone every year from DVT associated with air travel[5].

Professor Gianni Belcaro of G d'Annunzio University in Italy, who has conducted a number of travel-related DVT studies, led a team of experts who examined 568 passengers flying between the UK and Italy from May to September this year. The passengers, aged between 25 and 65 years, were screened by ultrasound for blood clots both before and after their flight. The researchers discovered clots in 4.3% of the high-risk subjects after the flight, with two passengers going on to develop pulmonary embolisms possibly related to their trip. Those with an increased risk of DVT are women on the birth control pill and HRT, people who have recently had surgery, and pregnant women. Alcohol also increases the risk. Prof. Belcaro said, "The results show passengers are at risk of developing blood clots even on short flights. In fact, our research suggests most blood clots develop in the first 2 to 3 hours of a journey and grow larger and more dangerous with time". Two of the 568 passengers involved in Dr. Belcaro's study went on to develop thromo embolism so extrapolating his numbers to the 1.56 billion annual passengers would mean that 15 million travellers would develop a clot and over 600,000 would develop DVT.

It is clear that estimates of DVT vary substantially and that there is as yet no consensus as to the magnitude of the problem. Nor does there appear to be a clear understanding of how many clots detected by ultrasound actually end up producing clinical DVT or pulmonary embolism. Until authorities get a better handle on the situation and can provide a realistic risk estimate it would seem prudent to follow the preventive measures outlined in the November 2003 issue of IHN.

References

1. Scurr, JH, et al. Frequency and prevention of symptomless deep-vein thrombosis in long-haul flights: a randomised trial. The Lancet, Vol. 357, May 12, 2001, pp. 1485-89
2. Eklof, B, et al. Venous thromboembolism in association with prolonged air travel. Dermatol Surg, Vol. 22, July 1996, pp. 637-41
3. Silverstein, MD, et al. Trends in the incidence of deep vein thrombosis and pulmonary embolism: a 25-year population-based study. Archives of Internal Medicine, Vol. 158, March 23, 1998, pp. 585-93
4. Belcaro, G, et al. Venous thromboembolism from air travel: the LONFLIT study. Angiology, Vol. 52, June 2001, pp. 369-74
5. Reynolds, M. Deep-vein thrombosis in long-haul flights. The Lancet, Vol. 358, September 8, 2001, p. 838 (letter to the editor)