Risks

People who are very fit tend to have low resting pulse and so are even more prone during prolonged periods of inactivity. If you have just run a marathon, you probably have at least minor bruising that can trigger clotting.

Factors that can contribute to poor or sluggish blood flow through the veins & hence increase your risks of DVT include:- standing or sitting still for long periods without exercising your leg muscles, prolonged bed rest or wearing clothing that restrict your blood flow.

Classic DVT risks are:-
1. Recent surgery or injury, especially on the hips or knees. Airhealth.org advises to avoid surgery for at least 30 days after air travel.
2. Personal or family history of DVT.
3. Cancer, heart disease, diabetes, liver disease.
4. Obesity is often cited as a risk factor, but probably ranks much lower than the preceding diseases.
5. Women who are pregnant or taking estrogen as birth control or hormone replacement.
6. Age over 40 is often cited as a risk factor; sometime age over 60 is cited. However, in the Airhealth.org registry over 80% are under age 60, the majority are under 50.

Heart Attacks (More info from Airhealth.org)
A '92 study by Teenan (see bibliography) said that air travel doesn't just produce DVT. It also causes peripheral arterial thrombosis, most commonly resulting in myocardial ischemia, heart failure. Dr. Omer Iqbal presented a paper on this at the International Angiology Union World Congress in March 2002. More than half of heart attacks are caused by clots. No one has studied how often these follow modern long range travel.

Strokes
Arterial clotting can also lead to strokes, about 80% of which are caused by clots. No one has studied how many of these are caused by air travel. About 7% of air travel thrombosis cases in the registry of AirHealth.org suffered strokes. Fifty years ago stroke was the 11th leading cause of death. Today it is fourth and rising. We do not know how much of the increase is due to air travel.

e-DVT
Lack of activity whilst seated is the basic cause of DVT. We may be at risk by being at our desk for too long. Remember that being very fit is an additional risk, as it gives you a low resting pulse rate.
Silicon.com highlighted the risk recently & pointed out the lack of research. Many DVTs are silent (without symptom) so after a prolonged period without excersise at your desk, you may have blood clots, that could break free, in your deep veins for several days.
http://www.silicon.com/news/500008/1/2669.html?et=search