Behind The Medical Headlines

Flight-related DVT

• Dr D Dawson, Emergency Department, Manchester Royal Infirmary, Manchester, England
• Dr K Mackway-Jones, Emergency Department, Manchester Royal Infirmary, Manchester, England

Summary

As long-haul travel becomes more popular and affordable, public concern has increased about the possible connection between long-haul travel and the development of potentially-fatal deep vein thromboses (DVTs). In this article Dr Debbie Dawson and Dr Kevin Mackway-Jones examine the evidence in relation to flight-related DVT.

Declaration of interests: No conflict of interests declared

Earlier this year, a BBC news item¹ brought to our attention, once again, the issue of DVT and its association with travel, in particular long-haul air travel. It has yet to be fully established that air travel is a risk factor for DVT, though an association between the two is generally accepted and conceivable despite the limited evidence available to us. Most research in this field has been carried out using case controlled studies where selection bias reduces reliability of results. Randomised controlled trials (RCTs) are not a useful means of assessing the incidence of DVT from air travel due to the impracticalities of randomising, the large number of passengers that would be required to provide accurately powered results,² and also because occurrences of flight-related DVT are low and often asymptomatic. In spite of the limited evidence available, a systematic review³ investigating DVT in relation to air travel found the incidence of symptomatic DVT ranged from 0% to 0.28%; and the incidence of asymptomatic DVT ranged from 0% to 10.34%.

The condition and its potential to evolve into pulmonary embolism (PE), has led to a great deal of media attention, with phrases such as ‘economy class syndrome’ and ‘travellers’ thrombosis’ being coined. Consequently, it is a cause for widespread concern. In addition, researchers are still trying to ascertain why flight-acquired DVT occurs. While prolonged immobilisation has been assumed to be the reason for the condition, research indicates that it may not be the only mechanism involved. What is also unclear is exactly who is at risk and what (if any) advice and preventative measures should be recommended by clinicians.

Air travel vs immobility

It is inevitable during the course of a long-haul flight for passengers to experience periods of prolonged immobilisation. Immobility is certainly a recognised risk factor associated with DVT as lack of muscle motion leads to blood stasis which in turn can lead to thrombus formation. Other situations where mobility is decreased, such as surgery, hospitalisation or limb paralysis, are also acknowledged for their potential to precipitate a diagnosis of DVT. As a result, it is generally accepted that immobilisation during long-haul air travel is the foundation of flight-related DVT.

It is unclear whether the same risk applies to other modes of prolonged travel or sitting, for example, long-distance coach journeys. This question complicates the whole flight-related DVT issue and has led to theories that there are additional underlying risk factors specific to air travel that increase the risk of DVT.

A recent study in the Lancet⁴ attempts to provide further insight into this particular uncertainty. The aim of the study was to discover whether flying leads to a hypercoagulable state. The researchers measured activated coagulation markers, thrombin-antithrombin complex (TAT), prothrombin fragment 1 and 2 (F1 and F2) and D-dimer, in a sample of 71 healthy volunteers aged between 18 and 40 years; some of this number included had the factor V Leiden mutation and/or took the oral contraceptive pill, both of which are recognised risk factors for venous thrombosis. Blood samples were taken before, during and after an eight-hour flight; the volunteers then had th