Long flights linked to blood clots

Being stuck in your seat on a long flight isn't the only factor that can trigger "traveller's thromobosis" in people who are prone to the blood clots, a study suggests.

Deep-vein thrombosis or DVT is thought to be caused by sitting rigidly for too long.

• INDEPTH: Traveller's thromobosis

A blood clot forms in leg veins and travels to the lungs or heart or brain days or weeks later, where it can lead to a heart attack or stroke.

There appears to be an increase in occurrence among intercontinental air travellers, but airlines say the clots can form from other sedentary activities. How the clots form during air travel remains unclear.

Dutch researchers designed an experiment to disentangle the effects of the low-oxygen, low-pressure conditions encountered on flights from simple immobilization and daily activities.

Frits Rosendaal of Leiden University Medical Centre, Netherlands and his team measured levels of a key clotting protein in 71 healthy volunteers, 15 men and 56 women.

Concentrations of those markers increased during flight compared with results from the two other activities, especially in those with other risk factors for thrombosis, the researchers found.

Of the participants, almost 40 per cent took oral contraceptives or had a genetic risk factor called factor V Leiden two known risk factors for increased blood clotting.

Seating study

The volunteers rode on a chartered Boeing 757 for a non-stop, eight-hour flight. Blood samples were taken before, during and after the trip.

A few weeks later, the same people had blood drawn as they watched an eight-hour movie marathon to see whether immobilization alone made a difference. Seats on the plane and in the cinema offered similar amounts of leg room.

In the final part of the experiment, participants were monitored for eight hours as they went about their regular daily activities.

During all parts of the study, volunteers were not allowed to drink alcohol, take ASA or wear elastic stockings, variables that could have affected the results. They were also asked to remain seated as much as possible during the flight and films.

Concentrations of the marker rose by about 30 per cent on average after the flight, while they fell by 2.1 per cent after the movies and 7.9 per cent during daily activities.

The post-flight increases were found in 11 of the participants, especially those using oral contraceptives and with factor V Leiden. The team estimated about five per cent of women taking oral contraceptives have the blood mutation.

The findings suggest a combination of low cabin pressure and low oxygen levels during long flights can add to the effects of immobilization in susceptible individuals, the team reports in the March 11 issue of the medical journal The Lancet.

For those at risk, the powerful anti-clotting drug heparin could be prescribed, Hans Stricker of the department of internal medicine in Locarno, Switzerland said in a journal commentary. He noted heparin shouldn't be used indiscriminately because of the drug's side-effects.

Other recommendations include:

• Wear compression stockings to improve circulation during flights.
• Stretch your legs occasionally.
• Take ASA.
• Avoid alcohol.

The study was conducted under the framework of the WHO Research Into Global Hazards of Travel or WRIGHT initiative, and was funded by the UK government and the European Commission.