Deep Vein Thrombosis or DVT for short refers to a blood clot in the deep veins of the legs (or less commonly arms). This disease has also been called "Economy Class Syndrome" because of its propensity to occur when people sit in cramped spaces for an extended period of time without moving around. Symptoms of DVT include pain, swelling and redness in the leg.
To understand how DVT relates to travel one must first understand how blood circulates in the body. Blood in our arteries moves away from the heart to the rest of the body and is propelled by the force of our heart beating. Blood in our veins on the other hand drains back towards our heart. The force of our heart contracting does not affect the movement of blood in our veins. Instead the venous blood is propelled by the surrounding muscles contracting and squeezing against the vessel walls. There are one-way valves in our veins which prevent the back flow of blood. When we sit in one place for an extended period of time, our muscles are not contracting and the blood in our veins does not circulate. Blood staying in the same place for an extended period of time is called venous stasis. This blood has a greater tendency to form clots than blood that is circulating.
A blood clot in the leg however is not where the real concern about DVT arises. The reason why DVT is a potentially life threatening condition is because the blood clots (thromboses) in the legs can break off or dislodge and travel to the lungs. Once in the lungs the clot is called a Pulmonary Embolism or PE. If a PE is large enough death could result while smaller PEs may only present with shortness of breath or rapid heart rate. DVTs are diagnosed with an ultrasound of the legs which looks at blood flow and PE is diagnosed by a CT of the chest which looks at blood flow in the pulmonary arteries. Both DVT and PE are treated with blood thinners such as coumadin (warfarin) or heparin.
There are several things that you can do to help prevent DVT. If you find yourself in a cramped seat in coach on a nonstop flight to Asia (or any other similar situation) it is important that you periodically get up and move around. You can also do calf raises and stretches while in your seat. The goal is to get your muscles contracting and your blood flowing.
Disclaimer: The views expressed above represent my personal opinion, they are not intended to be taken as medical advice and should not replace a visit to your primary care doctor. While DVT is a serious condition, it is uncommon in otherwise healthy people and should not stop you from enjoying your next great adventure!