Airlines probe low oxygen-DVT link

Deep vein thrombosis (DVT) on flights may be caused by poor air quality, new research has suggested.

Until now, the potentially-fatal condition was thought to be brought on by sitting still in cramped conditions for long periods.

But research from the World Health Organisation (WHO) published in The Lancet suggests "the low pressure and low oxygen environment during air travel may contribute to the development of DVT in some susceptible individuals".

British Airways said: "We welcome research into deep vein thrombosis. We look forward to reading the paper

 in full. The health and well-being of our passengers remains of paramount importance."

Sir Richard Branson's airline, Virgin Atlantic, said: "Clearly, the research again offers interesting findings but there are conflicting views on just how long-haul flying affects travellers. No conclusions can be drawn and more research needs to be done."

The WHO research involved 71 health volunteers who were tested for possible blood clotting before, during and after an eight-hour flight.

The same volunteers were tested during eight hours of sitting in a cinema and during eight hours of regular activities.

The testers measured the concentrations of markers of clotting activation in blood samples taken from the volunteers.

The Lancet said that "the authors (of the research) found increased concentrations in markers during flight compared to the other two situations".

Professor Frits Rosendaal, of Leiden University Medical Centre in Holland, which carried out the research, said: "Activation of coagulation (clotting) occurs in some individuals after an eight-hour flight, indicating an additional mechanism to immobilisation underlying air travel-related thrombosis."

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Long flights:  Cabin air quality linked to blood clots, says study

Low cabin air pressure and poor oxygenation enhance the risk of deep-vein thrombosis (DVT), a study says, also identifying a minority of people who are the most prone to developing these dangerous blood clots.

DVT occurs when a clot forms in leg veins during periods of sedentary activity. The clot can then migrate to the heart, lung or brain, sometimes days or weeks later, and inflict a heart attack or stroke.

Even though the phenomenon has been known since World War II, it has in recent years become a major issue for the airline industry, through lawsuits in which DVT victims placed the blame on cramped economy seating in long flights.

The airlines retort that DVT can occur long after a flight, which thus makes it impossible to establish a link, and point out that clotting can occur in other forms of sedentary activity, even from sitting and reading a book.

The latest research, published in next Saturday's issue of The Lancet, puts the ball back into the airlines' court, pointing the finger at cabin air quality as a potential risk factor.

A team led by Frits Rosendaal, a professor of clinical epidemiology at Leiden University Hospital in the Netherlands, monitored levels of a key clotting protein called thrombin-antithrombin (TAT) complex among 71 healthy men and women aged 20-39.

Forty percent of the guinea pigs were selected as they took the birth control pill and/or had a gene varation called factor V Leiden, both of which are known to greatly increase blood clotting.

The volunteers were taken onboard a specially-chartered Boeing 757 for a non-stop eight-hour flight, which cruised at at around 11,000 metres (35,750

feet) with a cabin pressure that corresponded to an altitude of 1,800-2,100 metres (2,600-6,825 feet).

Blood samples were taken before, during and after the flight.

A couple of weeks later, the same group underwent the same tests, but this time at a movie marathon, watching comedy and action films for eight hours.

They were not allowed to drink alcohol, take aspirin or wear elastic stockings during the flight or the movies, and were asked to remain seated as much as possible.

The group was monitored for the final time during eight hours of regular daily activity.

After the flight, the concentration of TAT complex rose by 30.1 percent on average across the group. It fell by 2.1

 percent after the cinema, and by7.9 percent after the "daily life" experiment.

The post-flight rise was overwhelmingly concentrated in just 11 of the 71 individuals, especially those who took oral contraceptives and also had factor V Leiden.

The findings suggest that "flight-associated factors" are the cause behind increased clot formation after a long trip, say the team.

They put the ring of suspicion around hypobaric hypoxia -- a combination of low cabin pressure and a low oxygen level compared with terrestrial air conditions.

The data also provide a useful pointer for medical help to those most at risk.

There already exists an array of simple techniques or over-the-counter drugs for combatting DVT.

These include cutting out alcohol on a flight, stretching one's legs, taking an aspirin and wearing compressive stockings that improve blood circulation.

There is also a powerful anti-clotting prescription drug called heparin.

This medication has side-effects, which means it should be targeted at those most at risk, such as women on the Pill and with factor V Leiden, cancer patients and recent surgery patients, said Hans Stricker, a doctor at the Charity Hospital in Locarno, Switzerland, in a commentary.

Previous studies have found a two-to-fourfold risk of DVT after air travel, while a study in 2003 estimated that DVT may occur among one in every 100 frequent long-haul air travellers, in business class as well as economy.

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