Immobility during long flights may increase risk

An Australian government-backed study says that the risk of DVT to those with no extra 'risk factors' is very small.

People who are overweight, taking the Pill or HRT, or recovering from an operation face a far higher risk.

Blood clots can form in the lower limbs during long periods of immobility - and could kill if they travel to the lungs.

DVTs which cause physical symptoms are rare events in healthy adults, and doctors are looking for ways to calculate who is at risk - and how worried they should be by long-haul travel.

The problem has been dubbed "economy-class syndrome", because it was originally believed that there was a linked between restricted legroom - but it can arise in business class and on other long trips where passengers sit for hours on end.

Low risk

The message of the Australian research, commissioned by the Department of Health and Ageing, is that the risk to healthy individuals is low.

Researchers looked at data from people admitted to Western Australian hospitals betwen 2001 and 2003 with an obvious DVT.

Some had developed pulmonary embolisms - where a piece of the clot had broken off and moved to the lungs - and died as a result.

Chief Medical Officer John Horvath said: "For an average middle-aged traveller, this means that a DVT would occur only once in 40,000 flights, with a death about once in two million flights."

"For young people the average risk is much smaller."

No guarantee

This "individual risk" doesn't mean that it will never happen - your one in 40,000 chance could happen on the 40,000th flight you take - or the first one.

And many passengers would fall into the "high risk" group, as they have medical conditions which make DVT more likely.

It may be very difficult to predict the risk for many people as DVTs are so infrequent Mr John Scurr, vascular surgeon

Women taking the contraceptive pill increase their risk of DVT by approximately three or four times, and there is an increased risk for HRT users.

Women who are pregnant, or who have recently given birth, as well as heart and cancer patients, and those who have undergone surgery recently, are also at greater risk.

Some experts remain concerned that long-haul travelling could be leading to small DVTs - which don't cause symptoms, but which increase the subsequent risk of a bigger, more dangerous clot.

Mr John Scurr, a vascular surgeon based at the Lister Hospital in London, who has conducted research into DVTs, said that there were still many "unknowns" about the risk of an unwanted clot.

He said: "It may be very difficult to predict the risk for many people as DVTs are so infrequent.

"However, we know that if you have one DVT you are more likely to have another.

"Advice such as drinking plenty of water during the flight and moving about regularly is sensible, regardless of your age or health."

The World Health Organization is coordinating further research into the incidence of symptomless DVTs, using ultrasound to scan the legs of passengers before and after long-haul flights.