What is deep vein thrombosis?

Deep vein thrombosis (DVT) is the formation of a blood clot in the major veins, usually occurring in the calf or thigh. DVT can be dangerous if the clot moves through the bloodstream and lodges in the heart or lung (pulmonary embolism). Symptoms of DVT involve pain and/or swelling of the leg. Causes of DVT include older age, smoking, being overweight, pregnancy, immobility (eg, after surgery) or long periods of inactivity (eg, long bus, car journeys or long-haul flights – ‘economy class syndrome’).

What causes DVT?

Blood clots are formed when blood stops moving and coagulates. This natural mechanism ensures blood flow will stop following minor accidents and cuts. If a clot (known as a thrombus) forms in the deep veins of the leg, DVT occurs.

If the clot is dislodged and begins to circulate around the body, it can cause a major obstruction in the narrower vessels of the heart or lungs – known as venous thromboembolism (VTE) or pulmonary embolism if in the lungs. Pulmonary embolism can sometimes be fatal.

What are the symptoms of DVT?

The main symptoms of DVT are pain and/or swelling of the leg. There may also be tenderness, warmth and redness of the affected part of the leg. In some cases DVT can occur without symptoms.

DVT can also occur in the hours or days after a long journey, so seek medical advice if you develop symptoms.

What are the risk factors for DVT?

Risk factors for developing DVT, and VTE, include:

• blood clotting disorders (can run in families)
• being overweight
• smoking
• getting older (risk increases with age)
• previous history of DVT 
• pregnancy
• childbirth
• varicose veins
• cancer
• heart failure
• some contraceptive pills or hormone therapy (HT/HRT)
• recent surgery
• injury (eg, fracture) 
• immobility
• inactivity (eg, on long distance journeys).

Factors related to flights

In addition to not moving around much while on a flight, other factors thought to be related to DVT risk are the cabin pressure and dehydration.

At high altitude, the reduced cabin pressure causes fluid to pass from the blood vessels into the surrounding tissue, causing thickening of the blood. Normal movement of the calf muscle when walking helps to pump blood from the legs to the heart, but with the loss of fluid while sitting for long periods, the blood can thicken, coagulate and form a clot in the deep veins of the leg. In most cases the clots do not dislodge from the leg and they merely cause pain, or they may be symptomless.

Dehydration can occur in-flight due to dry air in the cabin, drinking too much alcohol on the flight, and not drinking enough water.

Talk to your doctor about your risk

If you are planning a long trip you could mention this to your doctor at your next visit – your doctor can then assess any risk factors you may have.

If you have health conditions or risk factors for DVT, you may be advised to take preventive medicine and/or wear compression socks or stockings for the flight/journey. 

Prevention tips for long journeys

Steps to help prevent DVT on long-haul airline fights and during long car, train or bus journeys include:

• do ankle and knee exercises every half hour while seated
• walk down the aisles regularly, or if travelling by car, stop frequently, get out and take a walk
• wriggle toes frequently
• drink plenty of water (or non-alcoholic fluids)
• go easy on alcohol and caffeine
• consider wearing knee-high compression stockings for the journey
• if at high risk, take preventive medication (as advised by your doctor).

Compression stockings

Compression stockings are favoured by many long-distance travellers to reduce the risk of blood clots. Wearing a compression stocking counteracts the loss of fluid into the tissue of the leg, and so reduces the risk of clotting. Compression stockings are not all the same - there are different levels of compression. For most travellers, Class 1 hosiery is all that's needed to prevent swelling of the legs. This class of compression stocking is also suitable for mild varicose veins, during pregnancy and for anyone with tired, aching legs - ask your pharmacist for advice.

Travellers with existing health problems, such as heart problems or mild to severe varicose veins, or those who have recently undergone surgery, should see their doctor first to ensure they get the right class of compression. All Class 2 and Class 3 compression stockings should only be used on the recommendation of a doctor.

Original material provided by Pharmacy Today and New Zealand Doctor. Reviewed and edited by everybody, December 2011.