New guidelines for travellers to avoid DVT on long flights

By Dr. Ananya Mandal, MDFeb 7 2012

New guidelines for travellers have been issued. Experts warn that those with a higher risk of serious blood clots who opt for the window seat on flights longer than four hours are potentially putting themselves in danger. People who should be most concerned are those who have recently had surgery, cancer patients, those with limited mobility, the severely obese, and travellers over 70. However, healthy women on the pill and those who are pregnant should also be careful, according to the guidelines by the American College of Chest Physicians.

The guidelines come from a team of physicians who studied 14 previous DVT research reports from around the world. They found no definitive evidence to support that dehydration and alcohol intake in-flight increase risk. The panel, which reviewed existing evidence, found the risk of developing deep vein thromboses - blood clots that usually form in the legs - was “strongest for flights over eight to 10 hours”. They added, “For those on flights over four hours, immobility during the flight and window seating (especially for obese persons) also increase the risk.”

Normally, blood in veins that run through calves and thighs is kept flowing by muscular contractions, which force it back towards the heart. However, during long periods of immobility the blood can pool in these deep veins. A clot can then form, partially or totally blocking a vein. When part of this breaks off, it can become lodged in a blood vessel serving a lung - an event known as a pulmonary embolism (PE). A big clot can cause collapse or even sudden death, as the body becomes deprived of oxygenated blood.

The panel noted that the absolute risk of developing a DVT or a resultant PE due to a long flight - collectively known as venous thromboembolism (VTE) - was “very small”. One person in 4,600 experiences a symptomatic VTE in a month following a flight of four hours or longer, and only a minority of those are serious.

Dr Mark Crowther of McMaster University in Ontario, Canada, said that “remaining immobile for long periods of time” will increase one's risk. He added, “Long-distance travellers sitting in a window seat tend to have limited mobility, which increases their risk for DVT. This risk increases as other factors are present.” They found no evidence that people in economy class were more prone to DVT than those in first or economy, debunking the myth of so-called “economy class syndrome”.

The new guidelines, published in the journal Chest, recommend all long-haul passengers should get up and walk regularly around the aircraft, and stretch their calf muscles while standing up or sitting down. Only those in higher risk categories - such as the elderly and pregnant women - should wear below-the-knee compression stockings (‘flight socks’), they advised.

Based on previous studies, the researchers concluded that the use of various brands of below-knee compression stockings reduced the rate of asymptomatic DVT detected by screening study participants after flights from 3.6% (47 of 1,323 control subjects) to 0.2% (three of 1,314 stocking users). The recommendations say stockings used should provide 15-30 mm Hg compression at the ankle and need to be fitted properly -– not too tight up near the knee. Researchers suggested travellers wear them around the house a few days prior to a long trip to ensure a good comfortable fit. The guidelines stressed, however, that people without risk factors shouldn’t wear compression stockings as a DVT preventive measure.

The doctors also recommended against the use of aspirin or other anti-coagulants as a preventative measure. More powerful anti-thrombotic drugs should only be considered “on an individual basis” among those at highest risk, “because the adverse effects may outweigh the benefits”.

Dr Gordon Guyatt, chair of the panel, said, “There has been a significant push in health care to administer DVT prevention for every patient, regardless of risk. As a result, many patients are receiving unnecessary therapies that provide little benefit and could have adverse effects.”