DEEP VEIN THROMBOSIS
Background
The term "economy class syndrome" is used to describe a consequence of a medical condition known as deep vein thrombosis (DVT) that occurs when people develop blood clots in the deep veins of their legs. It can happen when the blood does not move through the vessels adequately, for example, after sitting through long flights.
Symptoms can appear several weeks after flights as short as three hours. Such clots have been reported after automobile trips and even after evenings at the theatre, but long flights seem to pose a greater risk. The syndrome can result in anything from minor body pains and shortness of breath, to heart attacks and strokes.
A young English woman who died after a long return flight from the Sydney Olympics put the risk of deep vein thrombosis in the headlines. However, it should be noted that this problem is not new and has been evident for many years. It has come to prominence recently with the ever-increasing number of air travellers.
What is DVT?
Deep vein thrombosis is a blood clot in one of the deep veins usually in the calf or thigh. In this part of the body, blood moves quite slowly and when a clot occurs, it can get stuck in the vein blocking the blood supply. It isn't always immediately obvious (pain and swelling in the leg may be the first symptoms), and is not necessarily fatal. However, these blood clots can become dislodged and travel to the lungs or other areas, causing strokes, severe organ damage, or death.
Likely Cause
When you sit for a long time without contracting the muscles in your legs, blood can pool in the veins resulting in DVT. Tightly packed seating, such as that found in the economy class section of airlines, can restrict movement. It doesn't allow for a lot of room to move. However, it is the lack of movement rather than the lack of space that is the probable cause of DVT. Hence, business/first class travel need not necessarily provide the solution to this problem.
Doctors suspect that cramped leg room combined with dehydration interrupts the blood flow which causes clots, cutting off the supply of oxygen to various parts of the body. This may account for the results of one study which showed that 18% of sudden deaths on airplanes were due to blood clots in the lungs.
source: http://www.minerals.csiro.au/safety/ecs.htm