There are many published anecdotal reports which link VTE with prolonged travel, particularly air travel, but there are only three published case-control studies,^203,204,205 and some studies of consecutive patients which are small, prone to bias, and gave contrasting and imprecise results.^206,207,208

The risk appears higher in patients with known risk factors^209,210 (see Table 1) and with flights over 3000 miles.²¹¹

Possible mechanisms include: immobility; cramped position; dehydration (augmented by drinking alcohol and coffee) ; compression of popliteal vein by edge of seat; and seated posture, especially when sleeping.^206,207 The relative risk remains to be established in further case-control studies, and the absolute risk remains to be established in large, prospective studies.

Small RCTs have observed reductions in the incidence of asymptomatic DVT by GECS.^212,214  Stockings used in hospital are designed for use in recumbent patients and are not suitable for use in flight.²¹³  Patients should be provided with the correct type and size of stocking, and should be instructed how to wear them correctly. Stockings also reduce leg oedema after long flights. They may precipitate superficial thrombophlebitis in people with varicose veins. ²¹⁴ 

One small RCT observed reduction in the incidence of asymptomatic DVT by a single dose of LMWH heparin 2-4 hours before flight, but not by aspirin (400 mg daily for three days, starting 12 hours before flight).

To minimise the risk of thrombosis when travelling long distances (e.g. over four hours) , especially by air, all travellers should be advised to: ensure good hydration restrict alcohol and coffee intake regularly carry out simple leg exercises and take occasional walks during travel.

In patients at high risk of thrombosis (e.g. previous DVT or PE; known thrombophilia; recent major trauma, surgery or immobilising medical illness, pregnancy), the following prophylactic methods should be considered: GECS a single dose of aspirin (150 mg) before travel (±GECS) a single injection of a LMWH before travel in prophylactic dose (see section 3) (±GECS) patients already receiving warfarin should continue to take it (±GECS). INR should be checked one week before long distance travel and the dose adjusted to within the target therapeutic range.

The risks of bleeding should be considered (e.g. increased risk of major bleed with aspirin or heparins, which is difficult to treat on a long haul flight) , and the balance of risks and benefits should be discussed with the individual patient.

At present, the balance of risk and benefit is difficult to estimate because the absolute risks of symptomatic VTE with long distance travel, and the absolute risks of bleeding with single-dose aspirin or heparin, are undefined. Further research is required to define these risks.