A deep vein thrombosis (DVT) is a blood clot that forms in the veins of the leg. A pulmonary embolism is a life-threatening complication of DVT. Long-distance flights may contribute to the risk of DVT or ‘economy-class syndrome’. Other risk factors for DVT include coronary heart disease, being overweight or obese, cigarette smoking, pregnancy, taking a high-dose combined oral contraceptive pill, family history of DVT or recent surgery or injury.

A thrombosis is a blood clot. The clot may block a blood vessel, causing potentially serious health effects. A deep vein thrombosis (DVT) is a blood clot that forms in the deep veins of the leg. A deep vein thrombosis in the thigh carries a risk of pulmonary embolism.

This occurs when the clot, or thrombus, loses its attachment to the inside of the vein, leaves the leg and lodges in the pulmonary artery, the main blood vessel to the lungs. If the clot is large enough, it can completely block that artery and cause death.

Blood flow through the leg veins generally requires some mechanical help, since it ‘flows’ up instead of down. Working calf muscles act as a pump. The contracting muscles compress the veins and force the blood in these veins upwards to the heart. This process is aided by valves in the veins, which direct the flow of blood and counteract the effects of gravity.

Anything that slows the flow of blood through the deep veins can cause DVT. This includes injury, surgery or long periods of sitting or lying. There is debate over whether or not the confinement of long-distance international flights may contribute to the risk of DVT. This condition is known as ‘economy-class syndrome’.

Blood clotting and DVT
Blood contains platelets and compounds called clotting agents. Platelets are sticky and form the basis of the blood’s ability to thicken (coagulate). If a blood vessel is cut, platelets collect at the site of the injury. In conjunction with clotting agents, the platelets produce a web or mesh, which traps platelets and creates a plug to seal off the wound. The ability of the blood to clot is essential for survival, but it can also lead to the formation of a thrombus.

Risk factors for DVT
Some of the risk factors that may contribute to the formation of a thrombus include:

• coronary heart disease
• being overweight or obese
• cigarette smoking
• pregnancy
• a high-dose combined oral contraceptive pill
• a susceptibility to ‘stickier’ blood and a family history of DVT
• blood clotting disorders
• sitting still for long periods of time
• recent surgery or injury
• some types of cancer
• congestive cardiac failure
• previous thrombosis
• hormone therapy.