GP and hospital practitioner in cardiology Dr Matt Hughes looks at the evidence behind the recommendations on travel-related deep vein thrombosis
Every year airlines carry two billion people across the world and the average flight takes two hours – with passengers largely immobile.
This extended immobility – together with discomfort and dehydration – may contribute to an increased risk of deep vein thrombosis and venous thromboembolism. But the decades after this was first suggested in 19541 proved how difficult it was to establish a clear association, with studies that appeared to confirm it being vigorously challenged by the airline industry.
The evidence to date
In 2007 the World Health Organisation published a review of the data on air travel and VTE. It concluded that the risk of VTE approximately doubles after a long-haul flight of longer than four hours, as it does with other forms of travel involving prolonged immobility.
But the report concluded the absolute risk of VTE in a healthy individual was very small – one in 6,000 for a flight of at least four hours.2
Hypobaric hypoxia does not seem to increase the risk of thrombosis, so it seems immobility is the culprit.
The risk increases significantly in the presence of other known risk factors for VTE: obesity, extremes of height, use of oral contraceptives, the presence of prothrombotic blood abnormalities, recent trauma, pregnancy and cancer.
Every year airlines carry two billion people across the world and the average flight takes two hours – with passengers largely immobile.
This extended immobility – together with discomfort and dehydration – may contribute to an increased risk of deep vein thrombosis and venous thromboembolism. But the decades after this was first suggested in 19541 proved how difficult it was to establish a clear association, with studies that appeared to confirm it being vigorously challenged by the airline industry.
The evidence to date
In 2007 the World Health Organisation published a review of the data on air travel and VTE. It concluded that the risk of VTE approximately doubles after a long-haul flight of longer than four hours, as it does with other forms of travel involving prolonged immobility.
But the report concluded the absolute risk of VTE in a healthy individual was very small – one in 6,000 for a flight of at least four hours.2
Hypobaric hypoxia does not seem to increase the risk of thrombosis, so it seems immobility is the culprit.
The risk increases significantly in the presence of other known risk factors for VTE: obesity, extremes of height, use of
oral contraceptives, the presence of prothrombotic blood abnormalities, recent trauma, pregnancy and cancer.
The effect of travel class
‘Economy class' syndrome has been the focus of much of the media attention around travel-related DVT. But the evidence only supports an association between thrombosis risk and length of flight – not the class of cabin.
One of the largest studies, published in 2001, followed up just over 135 million passengers arriving at Charles de Gaulle airport in Paris over seven years, 56 of whom suffered a pulmonary embolism.
The incidence of VTE was higher in those who had travelled more than 5,000km, compared with those whose flights were shorter.3 But no conclusions could be made about the class of travel.
Two other studies around the same time identified a significantly increased risk of isolated calf muscle thrombosis following a long-haul flight – but again no evidence that sitting in economy was a particular problem.4,5
Despite this, a perception persists that there is a problem and in 2008 the House of Lords science and technology committee recommended the UK Civil Aviation Authority should require a minimum seat pitch of 72cm and this issue is now with the European Aviation Safety Agency.
It is worth highlighting that the airline industry's efforts to counter the idea that sitting in economy is dangerous paid off.
In 2003 the Aerospace Medical Association proposed the term ‘travellers' thrombosis' be used and this has largely supplanted economy class syndrome in the media and on the internet.
Stratifying and managing risk
The following recommendations are based on the 2010 report from a working group of the British Cardiovascular Society on fitness to fly for passengers with cardiovascular disease.6
Low risk
The risk of travel-related DVT is low for people for whom the following all apply:
• no history of DVT or VTE
• no surgery in the previous four weeks
• no other risk factors indicating moderate or high risk.
People at low risk should be encouraged to keep as mobile as possible and try the in-seat exercises found at the back of airline magazines. They should keep well hydrated the day of the flight and the day before, avoiding alcohol and caffeine.
Moderate risk
The risk of travel-related DVT is moderate for people who:
• have a previous history of DVT/VTE
• have had surgery lasting longer than 30 minutes in the past two months but not in the last four weeks
• have thrombophilia
• are pregnant
• have a BMI over 30kg/m2.
These patients should receive the advice outlined above but should also be advised to wear compression stockings (see below).
High risk
The risk of travel-related DVT is high for people who:
• have within the past four weeks had surgery under general anaesthetic lasting longer than 30 minutes
• have had a previous DVT with one or more known additional risk factors, including a diagnosis of cancer.
These patients should be managed in line with the guidance for moderate risk but should also be referred to haematology to be considered for low molecular weight heparin (LMWH). The recommended regime is subcutaneous enoxoparin 40mg on the morning of the flight and the following day.
Although the risk of bleeding and thrombocytopenia with LMWH is much lower than with unfractionated heparin, its use should be reserved for those at high risk. Anyone already taking warfarin should continue with it.
The evidence for compression stockings
Much like the whole issue of travel-related DVT, the evidence for use of compression stockings is far from clear.
In 2006 a Cochrane review looked at trials in which passengers on flights lasting at least four hours were randomised either to wear compression stockings on both legs or not at all. It also included a small trial in which passengers were randomised to wear a stocking on one leg only.7
It found a large reduction in symptomless DVT among those allocated to wear compression stockings compared with those who did not. But there were no deaths, pulmonary embolism or symptomatic DVTs – possibly because numbers were too small or because participants were followed up and had symptomless DVTs spotted and managed.
So it remains unproven whether these outcomes are affected by compression, but there were no reported problems and people who wore the stockings were less likely to suffer leg oedema. So there seems little need to discourage those who choose to use them.
There is also no strong evidence for their use in travellers at moderate or high risk of DVT. Despite this their use is included in guidelines – but on the basis of expert opinion, rather than a solid evidence base.
There is no evidence to support the use of aspirin in the prevention of travel-related DVT. One study showed it had no effect and its use was associated with gastrointestinal symptoms in 13% of those who used it.9
Dr Matt Hughes is a GP and hospital practitioner in cardiology in Cardiff
+key points• Travelling for longer than four hours doubles the risk of VTE compared with not travelling.
• The risk is highest in the first week after travel but persists for two months.
• Most of the research has been done on air travel, but the risk appears to be similar for car, bus or train travel over a similar period.
• The relative risk of VTE is 3.45 for a four-hour flight and is greater for multiple and prolonged flights.
• The absolute risk of VTE in a fit person is around one in 6,000 for a flight of over four hours.
• Pilots do not appear to be at higher risk of VTE than the general population.
• Stratifying travellers on the basis of their DVT
risk and managing them accordingly is the recommended strategy.
