A recent estimate was that there are about two billion air passengers per year set to double by 2020. Prolonged immobility such as with a long journey increases the risk of DVT, STP and PE. This was first recognized in people sheltering in the London underground during bombing raids in World War II, The risk is much the same for travel by car, bus, train or plane and the incidence is the same for plane travel in Economy and Business Class. DVT is now known to be a risk for other causes of immobility such as sitting at a computer (E-thrombosis). Studies show that the overall incidence of unrecognized asymptomatic DVT without prevention as detected by ultrasound is about 5%. Far fewer are symptomatic while the risk of death from PE is about 1-2 per million passengers - about the same as being struck by lightning. The thromboses are approximately equally distributed between below-knee deep veins, above-knee deep veins and superficial veins.
There is an exponential increased risk for flights of more than 4 hours and the risk is higher for sequential flights or the return journey. The time of onset is more than 24 hours after completing the flight for 80% of DVTs and 30% of PEs. The risk peaks during the first week and is no longer apparent after two weeks.
Most passengers who develop thrombosis have a risk factor, in order of frequency:
- Thrombophilia
- Oral contraceptive pill
- Previous DVT
- Recent injury or surgery
- Varicose veins
- Obesity
Two or more combined markedly increases the risk. There is no evidence to support an increased risk from:
- Age or gender
- Alcohol consumption
- Sleeping pills
- Seating arrangements
- Cabin environment
- Physical activity
This is not to say that they should be ignored. Since some of the risk factors such as thrombophilia may not be known or are common, it is advisable to attempt to prevent thrombosis during any long trip. Class I support stockings have been shown to virtually eliminate the risk. In passengers known to be at high risk, there is strong protection from injections of Low Molecular Weight Heparin before and after each leg of the journey. Aspirin has been shown to be of no value.