A deep vein thrombosis (DVT) is a blood clot that usually occurs in a leg vein deep in the calves. If a fragment of this clot breaks off, it can travel through the circulation and into the lungs, causing a potentially fatal pulmonary embolism.
Experts do not know exactly why blood clots start to form in the deep veins of the legs when the circulation slows down, only that they do. Other than the genetic abnormalities and three original conditions identified more than 100 years ago, risk factors that increase the likelihood of a deep vein thrombosis include having had one before, having relatives who have had one, having had recent surgery, pregnancy, cancer and hormone therapy.
Most people who develop a DVT are over the age of 40 and with poor circulation. A history of congestive heart failure, obesity, multiple pregnancies and trauma all increase risk. Those with pre-diagnosed health conditions such as heart disease, hypertension or diabetes are at risk. Then there are those with a diminished general health status such as individuals who are still weak or recuperating from recent illness or alcoholics and the drug addicted. Poor diet and nutritional deficiencies also contributes. Women of all ages are more at risk because of the likelihood of hormonal fluid retention, and factors involved with taking contraceptive pills and hormone replacement therapy - any women taking artificial hormones has already substantially increased her likelihood of developing DVT. The elderly are at risk too, because older bodies do not handle the stresses of air travel as well as younger bodies. Short-term risk factors include prolonged periods of immobility such as being bedridden or during travel. Immobility can cause blood pooling in the lower limbs, thereby raising the risk of clot formation. Signs include aching legs, pins and needles and problems bearing weight on the legs. If the clot has moved to the lungs, chest pain is often a sign.
Deep vein thrombosis is common, affecting about 1 in 1,000 people. It is the fourth-leading cause of death in the USA.
Deep-vein thrombosis is common complication of major operations, such as hip replacement. The clots can be deadly if they break off and travel through the bloodstream to the lungs, forming a pulmonary embolism. Acute DVT may be demonstrated in 10-13% of all general medical patients placed at bed rest for a week, though is usually not clinically recognized. One-half or more of the patients with DVT can also be shown to have suffered a PE, even though the majority will have had none of the classic symptoms of PE. Gynaecologic surgery patients, major trauma patients and patients with indwelling venous catheters may have deep vein thrombi that start at any location. For other patients, lower extremity venous thrombosis nearly always starts in the calf veins.
Between 3 and 5 percent of people have thrombophilia – the tendency to form blood clots. When these vulnerable people are put in a provocative situation, like surgery, a car accident, coma, an extra-long flight or even sitting at a desk for a prolonged time, they are more prone to forming a blood clot.
Although deep vein thrombosis in the air has been dubbed "economy class syndrome" because of the cramped seating, not everyone who dies was travelling economy. Sitting in one place for a long time is the risk and applies to long-haul flights (greater than 6 hours, in economy or otherwise) and long bus trips. Risk factors are the pressurized cabins on aircraft, which slightly reduce oxygen in the blood, the constricted space and the fact that airline passengers frequently suffer dehydration.
In Australia, where many arriving passengers have been on long-haul flights, a surgeon estimated that up to 400 people may be arriving at Sydney airport every year suffering with deadly clots. A minimal estimate is one in 100,000 of the 14 million passengers who pass through Sydney airport every year. A study of passengers arriving at a Paris airport suggested that the rate of symptomatic pulmonary emboli was about 5 in a million (one in 200,000) for those passengers who had travelled more than 10,000 km. The authors of this airport study concede that this is almost certainly a significant underestimate, as they had data only on those presenting with symptomatic pulmonary disease soon after arrival.
Experts on the British Parliamentary Select Committee have estimated the possible increased incidence of DVT due to long-distance travel to be 0-40 per 100,000 travelers making 1 long-distance journey per year (with 20 per 100,000 being cited as the most likely figure).
Nobody knows what the true incidence of deep vein clots on airplanes is, or how long people have to stay still to get one. Scientists have not established that blood clots are more frequent on airplanes than anywhere else. There are multiple contributing factors. Part of the reason reports of such clots seem to be increasing could be that more sick people who are most likely to get a blood clot anyway are traveling these days because better medicine lets them lead more normal lives. Also, airplanes are now one of the few places where people sit still for long periods of time. We may all be forming blood clots all the time. There's something different about somebody who goes on to develop a clinical clot and someone whose clot is transient. The most important thing to do to avoid a clot is to move around.