Key Point:

• Among air travelers at low to moderate risk of VTE, the risk that VTE will develop during a very long flight is about one in 100. This is roughly twice the risk seen in nontravelers.

WELLINGTON, NEW ZEALAND—Air travel has been portrayed by the lay media as putting passengers at risk for venous thromboembolism (VTE). Long-haul flights, in particular, have been targeted. However, the true incidence and clinical relevance of VTE after long-haul flights is open to debate and highly dependent on study design. “The main problem in assessing VTE in air travel is that the incidence is low,” explained Richard Beasley, MD, a Professor of Medicine at the Medical Research Institute of New Zealand in Wellington. The studies are also difficult to conduct because they require close following of extremely large populations, he added.

Recently, Dr. Beasley and colleagues were able to conduct a prospective study in 878 travelers to determine the frequency of VTE in low- to moderate-risk travelers flying long distances. They found an association between VTE and long-haul flights—even among those at low risk.[1]

All participants were planning trips that included flights of at least four hours’ duration, with a scheduled return to New Zealand within six weeks. Before the flight, participants completed a brief questionnaire about VTE risk factors, and blood was taken for assessment of plasma D-dimer levels. No patients with a high baseline risk for VTE were included in the final analysis.

Within 72 hours of their return, participants completed a questionnaire asking about symptoms suggestive of VTE. Clinical assessments were also performed, and the D-dimer measurement was repeated. Two weeks later, D-dimer was measured again. All participants were contacted after three months and asked about VTE symptoms, medical interventions, and additional long-distance travel. During the follow-up period, any patient with a positive D-dimer value or high clinical probability of VTE underwent radiological evaluation, including bilateral lower-limb Doppler ultrasonography and CT pulmonary angiography.

INCREASED RISK OF VTE

The mean duration of total air travel (outbound and return flights) was 39.4 hours per person. Of 878 passengers, nine were found to have radiologically confirmed VTE during follow-up. Four of the patients had pulmonary embolism; the other five had deep venous thrombosis.

All nine of the patients had an elevated D-dimer measurement at the initial examination after the return flight, although their baseline D-dimer level had been normal. There was no difference in duration of flight between those with VTE and those without.

Only five of the nine patients had clinical findings that suggested an increased clinical probability for VTE, such as a history of stroke or hormone replacement use. Furthermore, six of the patients had used aspirin or compression stockings during their flights to lower their VTE risk.

“In terms of the individual traveler,” said Dr. Beasley, “an incidence of VTE of one in 100 is quite a low risk, but in terms of the number of people who travel on long-haul flights, it becomes quite substantial. One of the reasons for our study was to give doctors information about the baseline risk in a person who is really just a member of the general traveling public.”

The fact that this study excluded those with major risk factors for VTE attenuates the assumption that flight-associated VTE occurs only in those at high risk. In addition, VTE occurred among travelers in all classes, and thus the investigators suggested that the term economy-class syndrome be replaced with traveler’s thrombosis.

“VTE can also occur in other forms of travel and [may even affect] people who have been sitting at computers for a very long time without getting up,” observed Dr. Beasley. “It’s cramped, seated immobility that causes problems. This is less likely to occur with train, car, or bus travel because you tend to move your feet more and are unlikely to go for 14 hours without getting up.”

DIFFERENT STUDY, COMPARABLE RISK

Support for Dr. Beasley’s findings comes from another prospective study that involved 964 travelers on long-haul flights and 1,213 nontraveling controls matched for age and sex.[2] The travelers were examined during the week before their outbound flight and again within 48 hours after their return. The baseline evaluation included blood coagulation tests and thrombophilia screening.

During their flights, the travelers were encouraged to perform lower-limb stretching during flight, to walk about the cabin frequently, and to stay well hydrated. At their initial postflight examination, they were asked about symptoms of deep venous thrombosis or pulmonary embolism. Four weeks later, they were again questioned about symptoms of VTE, deep venous thrombosis, isolated calf muscle venous thrombosis (ICMVT), and pulmonary embolism.

The control group underwent two evaluations several weeks apart; blood coagulation tests and thrombophilia screening were performed during their initial examination. In addition, the controls were questioned again about symptoms four weeks after the second evaluation.

ICMVT was diagnosed in 2.1% of the travelers and 0.8% of the controls. All but one of the travelers with ICMVT were asymptomatic. The incidence of deep venous thrombosis was 0.7% among the travelers and 0.2% among the controls. One of the travelers with deep venous thrombosis developed a pulmonary embolism.

In this study, the overall incidence of VTE was higher than that found in Dr. Beasley’s investigation. However, this study did not exclude patients with major risk factors for thromboembolic disease, and it did include ICMVT in its definition of VTE. Furthermore, the flight durations in this study (8.5 to 15 hours) were shorter than those in the New Zealand investigation. Despite these differences, the 0.7% incidence of deep venous thrombosis in this study is similar to the 1% incidence estimated by Dr. Beasley’s group.

The authors of this study concluded that in the overall population, asymptomatic ICMVT has an incidence of one in 1,000 per month and that this incidence is doubled by exposure to long-haul flights. They added that simple preventive measures could probably reduce the risk for flight-related venous thrombosis.

RECOMMENDATIONS FOR PATIENTS

What, then, should patients who are planning long flights do to lower their VTE risk? “General precautionary advice [ie, getting up, walking, stretching, and staying hydrated] is worth taking; having said that, I think that it will reduce the risk, but it won’t completely eliminate it.” For the concerned long-haul traveler, Dr. Beasley recommended wearing compression stockings as well. “If you’re a high-risk patient, however, this is something for which you really need to seek medical advice,” he noted.

—Gale Jurasek

References
1. Hughes RJ, Hopkins RJ, Hill S, et al. Frequency of venous thromboembolism in low to moderate risk long distance air travellers: the New Zealand Air Traveller’s Thrombosis (NZATT) study. Lancet. 2003;362:2039-2044.
2. Schwarz T, Siegert G, Oettler W, et al. Venous thrombosis after long-haul flights. Arch Intern Med. 2003;163:2759-2764.

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